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The quest for sustainable, efficient energy systems is more pressing than ever as we step into the 21st century. Finding the suitable materials for a specific technology is a very challenging task. The required properties can be unique to particular chemical compositions and structures. Furthermore, achieving high performance often demands the precise alignment of multiple interacting properties, which can have competing effects. Additional economic constraints, such as the cost of raw materials and the manufacturing process, further add to the challenges. Traditionally, this journey has been arduous, relying on slow and laborious experimental techniques. One pivotal approach that has revolutionized the pace and efficiency of this discovery process is High-throughput screening (HTS). HTS is a powerful and systematic methodology that enables the rapid evaluation of a large number of materials in a relatively short period by leveraging large databases, accurate and efficient computational techniques including ab initio calculations and machine learning. When large libraries of materials are screened based on specific properties or descriptors, only a limited amount of the most promising ones pass to the final stage for further validation through expensive calculations and experiments. This thesis aims to develop HTS pipelines on finding promising candidates for two energy applications: thermoelectric materials that convert heat into electricity, allowing for waste heat recovery, and magnetocaloric materials that serve an environmental friendly cooling technology. The integration of predictive models and screening pipelines significantly reduces the time and cost associated with material discovery, transforming the traditional methods of discovering and developing novel materials.In the pursuit of innovative solutions for sustainable energy, thermoelectric materials hold undeniable potential to transform energy generation. They possess the unique ability to convert the

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

This paper presents a systematic investigation of the microstructure and magnetocaloric properties of melt-extracted Sm20Gd20Dy20Co20Al20 high-entropy microwires. The fabricated wires exhibited an amorphous structure, and the temperature interval of the undercooled liquid Delta T was 45 K. The microwires underwent a second-order magnetic transition from a ferromagnetic to a paramagnetic state near the Curie temperature (T-C=52 K). The maximum magnetic entropy change (-Delta S-M(max)), the relative cooling power and the refrigeration capacity reached 6.34 J.kg(-1).K-1, 422.09 J.kg(-1) and 332.94 J.kg(-1), respectively, under a magnetic field change of 5 T. In addition, the temperature-averaged entropy changes with two temperature lifts (3 and 10 K) were 6.32 and 6.27 J.kg(-1).K-1, respectively. The good magnetocaloric performance highlights the significant potential for the Sm20Gd20Dy20Co20Al20 microwires to be used as magnetic refrigerant materials in low-temperature region applications. This work will serve as a valuable reference for future investigations on low-temperature high-entropy magnetocaloric materials., QC 20240715

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

One important issue associated to magnetocaloric materials that hinders its technological application is the poor processability and structural integrity of those with the highest performance, usually intermetallics undergoing first-order magnetic phase transitions. Additionally, the performance of these magnetocaloric materials highly depends on the structural stability of the magnetocaloric phase, which is, in many cases, very sensitive to temperature and mechanical processes. Additive manufacturing via the extrusion of polymer-based composites is regarded as a promising way to overcome these issues. A recently presented manufacturing method of encapsulating functional fillers into polymer capsules has been used to produce a composite filament with a large load of magnetocaloric off-stoichiometric Ni2MnGa Heusler alloy fillers with a uniform distribution throughout the polymer matrix as demonstrated by x-ray tomography characterization. The incorporation of these metallic particles causes changes in the thermal behavior of the polymer as well as an increase in the flowability of the composite with respect to the polymer at the same temperature. The increased flowability of the composites found during manufacturing can be compensated by lowering the extrusion temperatures, making this technique even more convenient for preserving the filler properties, which is an important concern when additive manufacturing magnetocaloric materials. This is confirmed by the magnetic and magnetocaloric behavior of the composites, with responses proportional to the fraction of fillers.

One important issue associated to magnetocaloric materials that hinders its technological application is the poor processability and structural integrity of those with the highest performance, usually intermetallics undergoing first-order magnetic phase transitions. Additionally, the performance of these magnetocaloric materials highly depends on the structural stability of the magnetocaloric phase, which is, in many cases, very sensitive to temperature and mechanical processes. Additive manufacturing via the extrusion of polymer-based composites is regarded as a promising way to overcome these issues. A recently presented manufacturing method of encapsulating functional fillers into polymer capsules has been used to produce a composite filament with a large load of magnetocaloric off-stoichiometric Ni2MnGa Heusler alloy fillers with a uniform distribution throughout the polymer matrix as demonstrated by x-ray tomography characterization. The incorporation of these metallic particles causes changes in the thermal behavior of the polymer as well as an increase in the flowability of the composite with respect to the polymer at the same temperature. The increased flowability of the composites found during manufacturing can be compensated by lowering the extrusion temperatures, making this technique even more convenient for preserving the filler properties, which is an important concern when additive manufacturing magnetocaloric materials. This is confirmed by the magnetic and magnetocaloric behavior of the composites, with responses proportional to the fraction of fillers.

The magnetocaloric properties of Mn5Si1-xPxB2 (0 ≤ x ≤ 1) compounds were studied for energy harvesting applications. The crystal structure and the magnetic structure were characterized by powder X-Ray Diffraction and powder Neutron Diffraction. The results indicate that these magnetocaloric materials crystallize in the tetragonal Cr5B3-type crystal structure. The introduction of P causes a stretching of the c axis and compression of the a-b plane, leading to a decrease in the unit-cell volume V. In the ferromagnetic state the magnetic moments align within the a-b plane, and the magnetic moment of the Mn1 atom on the 16 l site is larger than that of the Mn2 atom on the 4c site. The Curie temperature TC can be adjusted continuously from 305 K (x = 1) to 406 K (x = 0) by replacing Si with P. The corresponding magnetic entropy change varies from 1.90 Jkg−1K−1 (x = 0) to 1.35 Jkg−1K−1 (x = 1) for a magnetic field change of 1 T. The PM-FM transition in these compounds corresponds to a second-order phase transition. Mn5Si1-xPxB2 compounds exhibit a magnetization difference of 28.1 - 31.3 Am2kg−1 for a temperature span of 30 K around TC in an applied magnetic field of 1 T. The considerable change in magnetization, the tunable TC near and above room temperature and the absence of thermal hysteresis make these compounds promising candidates for magnetocaloric energy harvesting materials., RST/Fundamental Aspects of Materials and Energy

The Fe2P type Mn–Fe–P–Si alloys exhibit a giant magneto-elastic first-order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe2P-type (Mn1.17-xFe0.73-yV0.02) (P0.5Si0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V-substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn1.17-xFe0.73-yV0.02P0.5Si0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet., Electrical Engineering, Mathematics and Computer Science, RST/Fundamental Aspects of Materials and Energy

The purpose of work presented in this thesis is to investigate whether M5XB2 (M = Fe, Mn,V, Cr and X = Si, P) material system has potential as a room-temperature magnetocaloric application material by studying its crystal and magnetic structure, and magnetocaloric properties., RST/Fundamental Aspects of Materials and Energy

Amorphous precursors with the composition MnCo0.8Fe0.2Ge1-xSix, where x = 0.2, 0.3, 0.4, and 0.6, were obtained through mechanical alloying. These precursors were subsequently subjected to different thermal treatments, resulting in the formation of the austenite Ni2In-type structure for x ≤ 0.4. For the highest Si content (x = 0.6), this phase coexists with the martensite TiNiSi-type structure without apparent partitioning of the elements at the nanoscale. These structures remain stable up to their melting points, but subsequent melting processes exhibit differences compared to the initial solidification at a cooling rate of − 20 K/min. The magnetocaloric effect was investigated through thermomagnetic and heat capacity measurements and simulated using the Arrott-Noakes equation of state, revealing the presence of a distribution of Curie temperatures. Despite the absence of a martensitic transformation, these alloys exhibit a significant magnetocaloric response, which increases as the precursor system's amorphization (i.e., homogenization) is enhanced.

This study investigates the magnetocaloric potential of the Al50Cr21-xMn17+xCo12 (x=0, 4, 8 at%) high-entropy alloy (HEA) series using integrated experimental and theoretical approaches. Structural analysis by X-ray diffraction and scanning electron microscopy indicate a dual phase containing B2 and body-centered cubic (BCC) structures. Magnetic characterization shows an approximately linear decrease in saturation magnetization and Curie temperature with increasing Cr content. Curie temperatures calculated by Monte Carlo simulations suggest that the measured magnetic properties originate from the B2 phase rather than the BCC phase. The enhanced magnetocaloric effect with decreasing Cr content highlights the attractiveness of HEAs in magnetocaloric applications.

We study magnetic multilayers, incorporating dilute ferromagnetic spacers between strongly-ferromagnetic layers exhibiting a proximity-enhanced magnetocaloric effect (MCE). Using magnetometry and direct measurements of the adiabatic temperature change based on a nanomembrane-calorimetry, we find that the MCE in the studied multilayer is indeed enhanced compared to that in the bulk spacer material. We develop a phenomenological numerical model of the studied trilayer and find that a long-range exchange interaction through the weakly-ferromagnetic spacer is required to adequately describe the magnetic and magnetocaloric properties of the system.

RENiX2 compounds, where RE = rare-earth element and X = p-block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi2-type, NdNiGa2-type, or MgCuAl2-type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl2-type aluminides show larger values than those of the CeNiSi2-type silicides and the NdNiGa2-type gallides due to the favored ferromagnetic ground state. However, RENiGa2 gallides can crystallize in either NdNiGa2- or MgCuAl2-type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl2-type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆Sisomax∣) values are 6.2, 10.4, and 11.4 J kg−1 K−1 (0–5 T) for DyNiGa2, HoNiGa2, and ErNiGa2, respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa2 and ErNiGa2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.

The magnetocaloric effect (MCE) is the temperature change in a magnetic material due to a change in an applied magnetic field. How the MCE behaves in different magnetic materials and at different phase transitions is fundamental to understand. The driver of the MCE is a change in entropy which has multiple contributions: magnetic, lattice, and electron. In this thesis the MCE is studied in a simple antiferromagnetic (AFM) model andin a realistic noncollinear spin glass Neodymium model using Monte Carlo and Atomistic Spin Dynamics simulations. For the simple AFM system, no clear results were achieved, indicating that MCE in AFM materials is not due to a change solely in the magnetic entropy. For the complex magnetic material Nd, a more clear result is seen, indicating that frustration in the system might be important to the MCE in noncollinear materials. Nd results also signify more phase transitions than previously reported.

The binary GdNi adopting the orthorhombic CrB-type (oS8, Cmcm) dissolves up to 21 at% Co (GdCo0.42Ni0.58), but when the Co content reaches 23.7 at% the crystal structure of the GdCoxNi1−x phase changes from the orthorhombic CrB-type to the tetragonal MoB-type (tI16, I41/amd). The latter remains stable between 23.7 and 36.0 at% Co (0.47 ≤ xCo ≤ 0.72), and there is a two-phase region compositional gap of ≈ 2.5 at% Co between the two structures. One of the two 8e sites available in the MoB-type structure is filled by Gd atoms, while Co and Ni atoms are randomly occupying the other 8e site. The lattice parameter a increases from 3.9485(1) Å to 3.9591(1) Å, while c decreases from 21.3286(1) Å to 21.2404(1) Å as xCo increases, both following a quadratic trend. As a result, the unit cell volume increases from 332.53(2) Å3 to 332.93(2) Å3 for, respectively, the Co-poor (GdCo0.47Ni0.53) and the Co-rich (GdCo0.72Ni0.28) sides of the solid solution, also following a quadratic trend with a negative deviation of the Zen’s law. The title compound becomes ferromagnetic with TC increasing linearly from 150 K for GdCo0.50Ni0.50 to 171 K for GdCo0.70Ni0.30; linear interpolation leads to TC = 127 K and 174 K for the two ends of the solid solution with xCo = 0.47 and 0.72, respectively. The magnetic susceptibility follows the Curie-Weiss law, indicating a trivalent state of Gd ions, with an effective magnetic moment slightly increasing with the Co content. The magnetocaloric effect, calculated in terms of the isothermal magnetic entropy change, decreases slightly as xCo increases, remaining large in the range 127–174 K.

Mechanically amorphized MnCo1−xFexGe alloys (x = 0.20, 0.06 and 0.03) were used as precursors to obtain hexagonal austenite single phase samples. Combining thermomagnetic and magnetocaloric analysis and in situ X-ray diffraction, we observed that the presence of a distribution of transition temperatures jeopardizes the first order character of the magnetoelastic transition from the ferromagnetic to paramagnetic state. Both magnetothermal and in situ X-ray diffraction identify the presence of such distribution and the first order character of the magnetoelastic transition.

RENiX2 compounds, where RE = rare-earth element and X = p-block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi2-type, NdNiGa2-type, or MgCuAl2-type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl2-type aluminides show larger values than those of the CeNiSi2-type silicides and the NdNiGa2-type gallides due to the favored ferromagnetic ground state. However, RENiGa2 gallides can crystallize in either NdNiGa2- or MgCuAl2-type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl2-type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆Sisomax∣) values are 6.2, 10.4, and 11.4 J kg−1 K−1 (0–5 T) for DyNiGa2, HoNiGa2, and ErNiGa2, respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa2 and ErNiGa2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.

All-d-metal Ni(Co)-Mn-Ti Heusler alloys show high magnetocaloric/barocaloric effects ascribed to the occurrence of a martensitic transformation together with excellent mechanical properties. However, high magnetic fields are needed to fully drive the transformation and to obtain their maximum responses. To further tune the martensitic transition and the associated magnetocaloric response, we systematically investigate the role of partial Mn substitution by Fe or Cr on the parent composition Ni36Co14Mn35Ti15. On the one hand, Cr doping increases the entropy change of the transformation but causes a tighter overlap of both martensitic and Curie transitions. This significantly reduces the magnetization difference between austenite and martensite and, consequently, strongly decreases the magnetocaloric response. On the other hand, Fe doping reduces the entropy change of the transformation and separates both martensitic and Curie transitions while keeping the magnetization difference among both phases. These two combined features reduce the magnetic field needed to completely drive the martensitic transformation and leads to higher and broader isothermal entropy change peaks for moderate magnetic field changes, reaching up to 25% enhancement for 2 T when compared to the undoped alloy.

Cryogenic magnetic refrigeration is a highly efficient and environmentally friendly technique for gas liquefaction. However, refrigerant materials undergoing large magnetocaloric response at the interesting cryogenic range are dominated by critical elements (mainly rare-earth elements) which impedes practical applicability of such refrigeration systems. Therefore, there is a need for dedicated investigations on optimization of magnetocaloric response at cryogenic range by utilizing compositions that are rare-earth free. In this work, we synthesize the mechanically stable and rare-earth free, all-d-metal Ni35Co15Mn35Ti15 Heusler alloys and investigate the role of varying Mn:Ni ratio on the magnetostructural and magnetocaloric properties of the alloy system. The results of the microstructural characterization indicate homogenous composition for the investigated alloy series. As the Mn:Ni ratio increases from 1.01 to 1.10, the martensitic transition shifts from near-room temperature down to cryogenic region (120–140 K) while the magnetization of the austenitic phase remains unaltered. Isothermal entropy change as high as ∼ 13 J kg−1 K−1 at 1.5 T is achieved for the sample with the highest Mn:Ni ratio at the temperature region for natural gas liquefaction, which significantly surpasses the values previously reported in the literature for similar alloys. In addition to large magnetocaloric response, the martensitic transformation falls in an interesting temperature of the cryogenic region, paving the way for various low-temperature magnetocaloric applications.

Optimizing the performance of magnetocaloric materials is facilitated by understanding the thermomagnetic transitions they undergo, including the order of these transitions and their strength. Those exhibiting strong first-order phase transitions (FOPT) are accompanied by large heating and cooling responses but with relatively small cyclic responses, while materials with second-order (SOPT) character exhibit moderate heating and cooling responses. However, the lack of hysteresis could partially compensate for the lower magnitudes with a more cyclic response. One way to effectively maximize the cyclic response, combining the advantages of FOPT and SOPT, is to fine tune the transition towards the borderline of FOPT-SOPT, which can minimize hysteresis. For the well-known La(Fe,Si)13 family, it is challenging to identify and/or evaluate the critical point where FOPT crossovers to SOPT based on conventional techniques. To address these ambiguities, in this work, we apply the field dependence exponent n criteria to a series of lowly hysteretic and high-performance La(Fe,Mn,Si)13H magnetocaloric materials with compositions close to the critical one. Even if the sample with the lowest hysteresis resembles characteristics of SOPT, it is evidently identified as undergoing FOPT from the n criteria: (1) existence of n > 2 overshoot and (2) n at the transition temperature, ntransition, is 0.37. This proximity to the critical composition (ntransition=0.4) further explains the low hysteresis observed. This FOPT character of the series is confirmed by temperature-dependent 57Fe Mössbauer spectrometry studies, fitting the hyperfine field to the Bean-Rodbell model instead of the usual Brillouin function. As it is a zero-field method, the confirmation by Mössbauer spectrometry gives further strength to the n-criterion.

RENiX2 compounds, where RE = rare-earth element and X = p-block element, have been highly regarded for cryogenic magnetocaloric applications. Depending on the elements, they can crystallize in CeNiSi2-type, NdNiGa2-type, or MgCuAl2-type crystal structures, showing different types of magnetic ordering and thus affect their magnetic properties. Regarding the magnetocaloric effect, MgCuAl2-type aluminides show larger values than those of the CeNiSi2-type silicides and the NdNiGa2-type gallides due to the favored ferromagnetic ground state. However, RENiGa2 gallides can crystallize in either NdNiGa2- or MgCuAl2-type structures depending on the RE element. In this work, we select heavy RE (HRE) elements for exploring the microstructure, magnetic ordering and magnetocaloric performance of HRENiGa2 (HRE = Dy, Ho or Er) gallides. They all crystallize in the desired MgCuAl2-type crystal structure which undergoes a second-order transition from ferro- to para-magnetic state with increasing temperature. The maximum isothermal entropy change (∣∆Sisomax∣) values are 6.2, 10.4, and 11.4 J kg−1 K−1 (0–5 T) for DyNiGa2, HoNiGa2, and ErNiGa2, respectively, which are comparable to many recently reported cryogenic magnetocaloric materials. Particularly, the excellent magnetocaloric properties of HoNiGa2 and ErNiGa2 compounds, including their composite, fall in the temperature range that enables them for the in-demand hydrogen liquefaction systems.

All-d-metal Ni(Co)-Mn-Ti Heusler alloys show high magnetocaloric/barocaloric effects ascribed to the occurrence of a martensitic transformation together with excellent mechanical properties. However, high magnetic fields are needed to fully drive the transformation and to obtain their maximum responses. To further tune the martensitic transition and the associated magnetocaloric response, we systematically investigate the role of partial Mn substitution by Fe or Cr on the parent composition Ni36Co14Mn35Ti15. On the one hand, Cr doping increases the entropy change of the transformation but causes a tighter overlap of both martensitic and Curie transitions. This significantly reduces the magnetization difference between austenite and martensite and, consequently, strongly decreases the magnetocaloric response. On the other hand, Fe doping reduces the entropy change of the transformation and separates both martensitic and Curie transitions while keeping the magnetization difference among both phases. These two combined features reduce the magnetic field needed to completely drive the martensitic transformation and leads to higher and broader isothermal entropy change peaks for moderate magnetic field changes, reaching up to 25% enhancement for 2 T when compared to the undoped alloy.

Cryogenic magnetic refrigeration is a highly efficient and environmentally friendly technique for gas liquefaction. However, refrigerant materials undergoing large magnetocaloric response at the interesting cryogenic range are dominated by critical elements (mainly rare-earth elements) which impedes practical applicability of such refrigeration systems. Therefore, there is a need for dedicated investigations on optimization of magnetocaloric response at cryogenic range by utilizing compositions that are rare-earth free. In this work, we synthesize the mechanically stable and rare-earth free, all-d-metal Ni35Co15Mn35Ti15 Heusler alloys and investigate the role of varying Mn:Ni ratio on the magnetostructural and magnetocaloric properties of the alloy system. The results of the microstructural characterization indicate homogenous composition for the investigated alloy series. As the Mn:Ni ratio increases from 1.01 to 1.10, the martensitic transition shifts from near-room temperature down to cryogenic region (120–140 K) while the magnetization of the austenitic phase remains unaltered. Isothermal entropy change as high as ∼ 13 J kg−1 K−1 at 1.5 T is achieved for the sample with the highest Mn:Ni ratio at the temperature region for natural gas liquefaction, which significantly surpasses the values previously reported in the literature for similar alloys. In addition to large magnetocaloric response, the martensitic transformation falls in an interesting temperature of the cryogenic region, paving the way for various low-temperature magnetocaloric applications.

Optimizing the performance of magnetocaloric materials is facilitated by understanding the thermomagnetic transitions they undergo, including the order of these transitions and their strength. Those exhibiting strong first-order phase transitions (FOPT) are accompanied by large heating and cooling responses but with relatively small cyclic responses, while materials with second-order (SOPT) character exhibit moderate heating and cooling responses. However, the lack of hysteresis could partially compensate for the lower magnitudes with a more cyclic response. One way to effectively maximize the cyclic response, combining the advantages of FOPT and SOPT, is to fine tune the transition towards the borderline of FOPT-SOPT, which can minimize hysteresis. For the well-known La(Fe,Si)13 family, it is challenging to identify and/or evaluate the critical point where FOPT crossovers to SOPT based on conventional techniques. To address these ambiguities, in this work, we apply the field dependence exponent n criteria to a series of lowly hysteretic and high-performance La(Fe,Mn,Si)13H magnetocaloric materials with compositions close to the critical one. Even if the sample with the lowest hysteresis resembles characteristics of SOPT, it is evidently identified as undergoing FOPT from the n criteria: (1) existence of n > 2 overshoot and (2) n at the transition temperature, ntransition, is 0.37. This proximity to the critical composition (ntransition=0.4) further explains the low hysteresis observed. This FOPT character of the series is confirmed by temperature-dependent 57Fe Mössbauer spectrometry studies, fitting the hyperfine field to the Bean-Rodbell model instead of the usual Brillouin function. As it is a zero-field method, the confirmation by Mössbauer spectrometry gives further strength to the n-criterion.

Mechanically amorphized MnCo1−xFexGe alloys (x = 0.20, 0.06 and 0.03) were used as precursors to obtain hexagonal austenite single phase samples. Combining thermomagnetic and magnetocaloric analysis and in situ X-ray diffraction, we observed that the presence of a distribution of transition temperatures jeopardizes the first order character of the magnetoelastic transition from the ferromagnetic to paramagnetic state. Both magnetothermal and in situ X-ray diffraction identify the presence of such distribution and the first order character of the magnetoelastic transition.

The magnetocaloric effect (MCE) is the temperature change in a magnetic material due to a change in an applied magnetic field. How the MCE behaves in different magnetic materials and at different phase transitions is fundamental to understand. The driver of the MCE is a change in entropy which has multiple contributions: magnetic, lattice, and electron. In this thesis the MCE is studied in a simple antiferromagnetic (AFM) model andin a realistic noncollinear spin glass Neodymium model using Monte Carlo and Atomistic Spin Dynamics simulations. For the simple AFM system, no clear results were achieved, indicating that MCE in AFM materials is not due to a change solely in the magnetic entropy. For the complex magnetic material Nd, a more clear result is seen, indicating that frustration in the system might be important to the MCE in noncollinear materials. Nd results also signify more phase transitions than previously reported.

The growing demand for energy highlights the need for developing more efficient and environmentally friendly technologies. Magnetic cooling based on the magnetocaloric effect is a promising alternative for compressor-based refrigeration. Among various magnetocaloric materials, Mn-based alloys appear especially attractive due to their low cost, high abundance and the potentially large magnetic moment density. We aimed to obtain a large magnetocaloric effect at room temperature in various non-toxic and inexpensive magnetocaloric materials. Thus, this thesis contains the study on the magnetism, magnetocaloric effect and (micro)structure of three families of Mn-based magnetocaloric materials: (Mn,Fe)2(P,Si), Mn3Sn1-xAxC antiperovskite carbides, MnNiSi half Heusler alloys. A particular part of this dissertation focuses on the elucidation of magnetic structures in antiperovskite carbides., RST/Fundamental Aspects of Materials and Energy

The transition-metal based Laves phase materials represent an extended family of alloys with rich and fascinating physical properties. In this work, we have investigated the negative thermal expansion and magnetocaloric effect in arc-melted and melt-spun Fe2Hf1-xTix (x = 0.15, 0.27, 0.30, 0.33, 0.36, 0.40) alloys. For x = 0.30–0.40, two hexagonal phases with different compositions share the same P63/mmc lattice symmetry, but have slightly different lattice parameters. The saturation magnetization and Curie temperature both follow a decreasing trend with the average unit-cell volume. For Fe2Hf0.6Ti0.4 melt spinning improves the saturation magnetization from 48.7 to 59.6 Am2/kg and the magnetic entropy change from 0.46 to 0.54 J/kgK at a magnetic field change of 2 T. These enhanced values are attributed to an improved homogeneity caused by a suppression of phase segregation during rapid solidification. We have utilized neutron powder diffraction and Mössbauer spectroscopy to illustrate the correlation between the magnetic order and the negative thermal expansion in single-phase Fe2Hf0.85Ti0.15. The magnetic moments of Fe align below 400 K in the a-b plane and a moment change for the Fe atoms is responsible for the large volumetric coefficient of thermal expansion of −25 × 10−6 K−1 over a wide temperature range of 300–400 K., RST/Fundamental Aspects of Materials and Energy, RID/TS/Instrumenten groep

In this work, the magnetocaloric effect and negative thermal expansion in melt-spun Fe2Hf0.83Ta0.17 Laves phase alloys were studied. Compared to arc-melted alloys, which undergo a first-order magnetoelastic transition from the ferromagnetic to the antiferromagnetic phase, melt-spun alloys exhibit a second-order transition. For Fe2Hf0.83Ta0.17 ribbons, we observed a large volumetric coefficient of negative thermal expansion of −19 × 10−6 K−1 over a wide temperature range of 197 – 297 K and a moderate adiabatic temperature change of 0.7 K at 290 K for a magnetic field change of 1.5 T. The magnetic field dependence of the transition temperature (dTt/dµ0H = 4.4 K/T) for the melt-spun alloy is about half that of the arc-melted alloy (8.6 K/T). The origin of second-order phase transition of the melt-spun alloy is attributed to the partially suppressed frustration effect, which is due to the atomic disorder introduced by the rapid solidification., RST/Fundamental Aspects of Materials and Energy, RID/TS/Instrumenten groep

Motivée par leur grand potentiel d’applications technologiques dans plusieurs domaines, la recherche sur les matériaux et composants à base d’oxydes de type pérovskite comme les manganites et les doubles pérovskites demeure très active et d’actualité. Dans la présente thèse, la spintronique et la réfrigération magnétique sont les deux cibles principales. Le manganite semi-métallique ferromagnétique La2/3Sr1/3MnO3 (LSMO) et le double pérovskite isolant ferromagnétique La2NiMnO6 (LNMO) sont d’intérêt puisqu’ils peuvent être utilisés en réfrigération magnétique et pour des jonctions à effet tunnel magnétiques grâce à leur grande température de transition magnétique, leur grande aimantation et leur grande polarisation magnétique. Dans la première partie de cette thèse, nous étudions l’effet magnétocalorique (EMC) d’hétérostructure de LSMO etLNMOsimulant des composites. Les buts de cette première étude sont d’étendre l’intervalle de température avec un effet magnétocalorique significatif et d’améliorer leurs performances en modifiant les propriétés magnétiques de multicouches grâce à des effets de contraintes structurales entre les couches et le substrat. Les propriétés magnétiques et magnétocaloriques des couches de LSMO et LNMO incluses dans les hétérostructures peuvent être contrôlées et même ajustées en les soumettant à différents champs de contraintes, tout particulièrement pour le double pérovskite LNMO pour lequel les propriétés magnétiques sont très sensibles au niveau d’ordre cationique sur les sites B/B’. Nous démontrons comment la proportion d’ordre cationique dansLNMOpeut être ajusté dans les hétérostructures simplement en changeant l’ordre de succession des couches provoquant des effets différents des contraintes structurales. La présence de plusieurs transitions magnétiques dans le LNMO multi-domaine permet le design de structures artificielles présentant un EMC de type "table-top" (constant sur une large gamme de température). Cependant, la présence de plu, There has been an active, ongoing research on perovskite-structured oxides such as manganites and double perovskites due to their great potential for a wide range of technological applications in many fields. In the present thesis, spintronics and magnetic refrigeration are the main targets. Half-metallic ferromagnetic manganite La2/3Sr1/3MnO3 (LSMO) and ferromagnetic insulating double perovskite La2NiMnO6 (LNMO) are of particular interest as they can be used in magnetic refrigeration and magnetic tunnel junctions due to their high and tunable transition temperature, large magnetization and large spin polarization. In the first part of this thesis, we study the magnetocaloric effect (MCE) of composite-like heterostructures of LSMO and LNMO. The goal is to extend the MCE and improve its performance by altering the magnetic properties in multilayers using epitaxial strain. The magnetic and magnetocaloric properties of LSMO and LNMO layers can be controlled and tuned by subjecting them to different strain fields, especially for LNMO double perovskite where its magnetic properties are very sensitive to the level of cationic ordering at the B/B’ sites. We demonstrate how the ratio of cation ordering in LNMO can be adjusted in heterostructures simply by changing the layout of the layers triggering contrasting strain effects. The presence of multiple transitions in a multi-domain LNMO allows the design of artificial structures with table-top-like MCE. The presence of several domains with different properties is often difficult to perceive using only magnetization. Other approaches, like tunneling through LNMO, may be an interesting avenue to explore its multi-domain aspect. In the second part, we exploit the insulating nature of LNMO double perovskite to study the effect of a ferromagnetic barrier on the tunnel magnetoresistance (TMR) of a magnetic tunnel junction (MTJ). We show that the TMR does not depend only on the relative orientation of the magnetization in

The influence of off-stoichiometry and of doping with the 5d transition metal Ta has been studied in the quaternary (Mn,Fe)2(P,Si)-based compound, which is one of the most promising materials systems for magnetic refrigeration. It is found that Ta substitution can decrease the transition temperature Ttr, while the thermal hysteresis ∆Thys remains about constant. A low Ta doping enhances the magnetocaloric effect (MCE). For Mn0.6Fe1.27-yTayP0.64Si0.36 with y = 0.01 the magnetic entropy change ∆Sm shows and enhancement of 30.7% compared to the undoped material for a low magnetic field change of 1 T. The occupancy of substitutional Ta atoms is determined by XRD and DFT calculations. The Ttr shift and enhanced MCE upon Ta doping are ascribed to the competition between a weakening of the magnetic exchange interactions and a strengthening of the hybridization. Our studies provide a good strategy to further optimize the MCE of this material family., RST/Fundamental Aspects of Materials and Energy

The influence of doping with the 5d transition metal W has been studied in the quaternary (Mn,Fe)2(P,Si) based giant magnetocaloric compounds, which is one of the most promising systems for magnetic refrigeration. It is found that W substitution can separately decrease the Curie temperature TC and retain the thermal hysteresis ∆Thys at an almost constant level (∼5 K) for Mn0.6Fe1.27-xWxP0.64Si0.36 (x ≤ 0.02). Low-content W doping conserves the good magnetocaloric effect (MCE) without an obvious degradation. For x ≤ 0.02 the average magnetic entropy change |∆Sm| amounts to 11.4 Jkg−1K−1 for an applied magnetic field change of 2 T and the adiabatic temperature change ∆Tad amounts to 3.9 K for an applied magnetic field change of 1.5 T. The occupancy of substitutional W atoms is determined by XRD experiments and DFT calculations. Our studies provide a good strategy to further optimize the MCE of this material family., RST/Fundamental Aspects of Materials and Energy

Mechanically amorphized MnCo1−xFexGe alloys (x = 0.20, 0.06 and 0.03) were used as precursors to obtain hexagonal austenite single phase samples. Combining thermomagnetic and magnetocaloric analysis and in situ X-ray diffraction, we observed that the presence of a distribution of transition temperatures jeopardizes the first order character of the magnetoelastic transition from the ferromagnetic to paramagnetic state. Both magnetothermal and in situ X-ray diffraction identify the presence of such distribution and the first order character of the magnetoelastic transition.

Magnetic refrigeration technology based on the magnetocaloric effect can better meet the requirements of efficient energy consumption and conversion. From engineering perspective, preparing magnetocaloric composites is an effective and efficient approach to combine desirable magnetocaloric performance, large thermal conductivity and ameliorated mechanical properties. In the present work, Ni-Mn-Ga/Cu magnetocaloric composites were prepared by spark plasma sintering (SPS) and compared with their counterparts fabricated via conventional methods. Magnetic properties were studied in detail and the magnetocaloric effect of Ni-Mn-Ga/Cu composites was characterized to be better than that of their hot-pressed counterparts and that of many micro/nano-sized Ni-Mn-Ga alloys. Besides, the composites exhibited favorable thermal conductivity of 11.2 W mK−1. The Hassel-man-Johnson model was adopted and modified to relate the thermal conductivity to the microstructure of the composites. Compared with arc-melted alloys, these SPS magnetocaloric composites sintered at different temperatures exhibited largely enhanced mechanical performance with minimum fracture stress of 340 MPa and strain of 4%. Furthermore, the failure mechanism of Ni-Mn-Ga/Cu magnetocaloric composite was elucidated by finite element simulation based on an extended linear Drucker-Prager model. The above findings from both experiments and simulation advance the knowledge of magnetocaloric materials and promote the development of magnetic refrigeration technology towards practical applications.[Figure not available: see fulltext.]

The search for high-performance functional alloys with improved service life and reliability entails the optimization of their mechanical properties. Recently, the high-entropy alloy (HEA) design concept has found new alloys with excellent mechanical properties. It utilizes multiprincipal elements to yield high configurational entropy of mixing, entailing a large compositional freedom with wide window of opportunities for property exploration. Their functional properties are usually modest when compared to conventional materials. The discovery of HEAs with optimal combination of mechanical and functional properties would be a leap forward in the reliability of functional devices. This review article focuses on magnetocaloric HEAs, the design approaches, and the appropriate analysis methods for their performance. We will highlight the efficient strategic search within the vast HEA space, which has been instrumental for significantly enhancing MCE performance, closing the pre-existing gap between magnetocaloric HEAs and high-performance conventional magnetocaloric materials. Outlook for future directions will also be included. Graphical abstract: [Figure not available: see fulltext.]

All-d-metal Ni(Co)-Mn-Ti Heusler alloys show high magnetocaloric/barocaloric effects ascribed to the occurrence of a martensitic transformation together with excellent mechanical properties. However, high magnetic fields are needed to fully drive the transformation and to obtain their maximum responses. To further tune the martensitic transition and the associated magnetocaloric response, we systematically investigate the role of partial Mn substitution by Fe or Cr on the parent composition Ni36Co14Mn35Ti15. On the one hand, Cr doping increases the entropy change of the transformation but causes a tighter overlap of both martensitic and Curie transitions. This significantly reduces the magnetization difference between austenite and martensite and, consequently, strongly decreases the magnetocaloric response. On the other hand, Fe doping reduces the entropy change of the transformation and separates both martensitic and Curie transitions while keeping the magnetization difference among both phases. These two combined features reduce the magnetic field needed to completely drive the martensitic transformation and leads to higher and broader isothermal entropy change peaks for moderate magnetic field changes, reaching up to 25% enhancement for 2 T when compared to the undoped alloy.

Optimizing the performance of magnetocaloric materials is facilitated by understanding the thermomagnetic transitions they undergo, including the order of these transitions and their strength. Those exhibiting strong first-order phase transitions (FOPT) are accompanied by large heating and cooling responses but with relatively small cyclic responses, while materials with second-order (SOPT) character exhibit moderate heating and cooling responses. However, the lack of hysteresis could partially compensate for the lower magnitudes with a more cyclic response. One way to effectively maximize the cyclic response, combining the advantages of FOPT and SOPT, is to fine tune the transition towards the borderline of FOPT-SOPT, which can minimize hysteresis. For the well-known La(Fe,Si)13 family, it is challenging to identify and/or evaluate the critical point where FOPT crossovers to SOPT based on conventional techniques. To address these ambiguities, in this work, we apply the field dependence exponent n criteria to a series of lowly hysteretic and high-performance La(Fe,Mn,Si)13H magnetocaloric materials with compositions close to the critical one. Even if the sample with the lowest hysteresis resembles characteristics of SOPT, it is evidently identified as undergoing FOPT from the n criteria: (1) existence of n > 2 overshoot and (2) n at the transition temperature, ntransition, is 0.37. This proximity to the critical composition (ntransition=0.4) further explains the low hysteresis observed. This FOPT character of the series is confirmed by temperature-dependent 57Fe Mössbauer spectrometry studies, fitting the hyperfine field to the Bean-Rodbell model instead of the usual Brillouin function. As it is a zero-field method, the confirmation by Mössbauer spectrometry gives further strength to the n-criterion.

In most cases, substitution studies that aim to optimize magnetic properties are performed at the magnetic atomic site. However, in the case of MnB, magnetic substitutions at the Mn site significantly decrease the once promising magnetocaloric and magnetic properties. This study employs computationally directed search to optimize the magnetocaloric properties of MnB where partial substitutions of boron atoms (Mn50B50−x Si x and Mn50B50−x Ge x where x = 3.125, 6.25, and 12.5) reveal new compounds with a greater magnetocaloric effect than pure MnB at the same Curie temperature. These new compounds were obtained by arc melting the pure elements and further characterized. The computationally driven screening process is based on density functional theory calculations that do not require large databases of known compounds. This work demonstrates that using simple computational screening procedures to search for new magnetocaloric materials with improved properties can be done quickly, cost-effectively, and while maintaining reliability.

In most cases, substitution studies that aim to optimize magnetic properties are performed at the magnetic atomic site. However, in the case of MnB, magnetic substitutions at the Mn site significantly decrease the once promising magnetocaloric and magnetic properties. This study employs computationally directed search to optimize the magnetocaloric properties of MnB where partial substitutions of boron atoms (Mn50B50−xSix and Mn50B50−xGex where x = 3.125, 6.25, and 12.5) reveal new compounds with a greater magnetocaloric effect than pure MnB at the same Curie temperature. These new compounds were obtained by arc melting the pure elements and further characterized. The computationally driven screening process is based on density functional theory calculations that do not require large databases of known compounds. This work demonstrates that using simple computational screening procedures to search for new magnetocaloric materials with improved properties can be done quickly, cost-effectively, and while maintaining reliability.

[EN] The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (X=Sb, Bi, Te) intermetallics (space group P 6 over line 2 m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129-370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with anti-ferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents beta,gamma,delta related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for 140 Delta H = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (140 Delta H = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie tem-perature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.

The novel all-d-metal Ni(Co)MnTi based magnetic Heusler alloys provide an adjustable giant magnetocaloric effect and good mechanical properties. We report that the second-order magnetic phase transition can be tailored in this all-d-metal NiCoMnTi based Heusler system by optimizing the Mn/Ti ratio, resulting in a reversible ferromagnetic-to-paramagnetic magnetic transition. A candidate material Ni33Co17Mn30Ti20 with a magnetic entropy change ∆Sm of 2.3 Jkg−1K−1 for a magnetic field change of 0–5 T, has been identified. The TC and saturation magnetization MS can be controlled by adjusting the Ni/Co concentration and doping non-magnetic Cu atoms. The compositional maps of TC and MS have been established. Density functional theory (DFT) calculations reveal a direct correlation between the magnetic moments and the Co content. By combining XRD, SQUID, SEM and DFT calculations, the (micro)structural and magnetocaloric properties have been investigated systematically. This study provides a detailed insight in the magnetic phase transition for this all-d-metal Ni(Co)MnTi-based Heusler alloy system., RST/Fundamental Aspects of Materials and Energy

[EN] The structural, magnetic, magnetocaloric (MCE) and thermal properties of seven Fe2P-type Dy6(Fe,Mn)X2 (X=Sb, Bi, Te) intermetallics (space group P 6 over line 2 m, N 189, hP9) have been experimentally studied. They present a paramagnetic to ferromagnetic transition (in the range 129-370 K), followed, as temperature decreases, by a spin-reorientation one (from 52 to 170 K) and a ground magnetic state at 2 K with anti-ferromagnetic components. This state turns into a ferromagnetic state when a magnetic field is applied. The critical exponents beta,gamma,delta related to the PM-FM transition point to long range order interactions but in most compounds their values severely deviate from the Mean Field class, presenting an unconventional critical behavior, probably due to magnetocrystalline anisotropies. This magnetic complexity has the consequence that in every intermetallic three MCE effects arise: Two direct magnetocaloric effects (DMCE) with a table-like effect in between (from 40 K to more than 400 K), with moderate values of the magnetic entropy maxima (up to 6.9 J/kgK for 140 Delta H = 5 T, with the tableau in-between being around 4 J/kgK, for Dy6FeSb2 and Dy6FeSbTe). The calculation of the Thermal Average Entropy Change allows to place the properties of two compounds (Dy6FeSb2 and Dy6FeSbTe) close to other rare earth based high entropy alloys described in literature. The seven compounds present a relevant third MCE, inverse, below 25 K, with a value as high as 17.8 J/kgK (140 Delta H = 5 T) for Dy6FeSbTe. The maximum of the magnetic entropy change at the Curie tem-perature has been shown to scale with the critical exponents found and universal curves have been built. Finally, the thermal diffusivities in the range of the DMCE have been measured, with the result that they present good values (between 1 and 3 mm2/s) to be used in real magnetocaloric refrigeration systems.

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La0.8-xSr0.2-yMn1+x+yO3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at mu H-0 = 2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at mu H-0 = 2T).

We report on structural, magnetic properties of Na-doped La0.8Sr0.15Na0.05MnO3 (LSNMO) nanoparticles (NP) with size about 50 nm elaborated via sol-gel route. The chemical composition was verified using the energy dispersive X-ray analysis (EDAX) and by X-ray photoelectron spectroscopy (XPS). Magnetic characterizations demonstrate that LSMNO exhibits a coexistence of interacting superparamagnetic (ISPM) phase with blocking temperature T-B = 194 K and a ferromagnetic phase with Curie temperature T-C = 255.5 K. At low temperatures, the SPM state undergoes a collective freezing state at T-f = 46 K. the high-temperature regime (well above TC) reveals that NP-LSNMO has a strengthened Griffiths-like phase compared to their bulk counterpart. An itemized investigation of the critical behavior of the material was carried out in the vicinity of T-C. The critical exponents [beta = 0.546(7), gamma = 0.972(6), and delta = 2.94 (5)] were found to be in close agreement with of the mean-field theory. The maximum magnetic entropy change (-Delta(pk)(M)) is about 1.41 Jkg(-1) K-1 and the refrigeration capacity (RC) is 288 Jkg(-1) for a field change of 5 T at T = 215 K. This magnetocaloric response is reasonably high for nanomaterials and, together with its cost-effectiveness, makes NP LSMNO a potential candidate material for active magnetic refrigerators. Besides, the ISPM properties are desirable for hyperthermia applications. Our findings suggest that the magnetic inhomogeneity and the dipolar interaction between the SPM and FM phases in the range T-B < T < T-C are crucial factors in determining the magnetic properties of NP-LSNMO. (C) 2021 Elsevier B.V. All rights reserved.

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La0.8-xSr0.2-yMn1+x+yO3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at mu H-0 = 2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at mu H-0 = 2T).

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La0.8-xSr0.2-yMn1+x+yO3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at mu H-0 = 2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at mu H-0 = 2T).

The energy-efficient and environmentally friendly alternative cooling technology based on the magnetocaloric effect (MCE) is discussed in this thesis. The thesis has two major parts, one devoted to material characterization and the other to instrument development. Different magnetic oxides and intermetallic compounds with second-order and first-order magnetic transitions, respectively, were studied with the aim of finding materials suitable for magnetic refrigeration. For the application of the MCE, a high value of the isothermal entropy changes and the relative cooling power (RCP), along with minimal temperature hysteresis are required. The temperature hysteresis is negligible for all studied second-order compounds, while an almost ten times higher value of the isothermal entropy change has been observed for the first-order compounds. The highest value of isothermal entropy change (20 J/kgK at 2 T applied magnetic field) has been observed for the MnNiSi-type compounds exhibiting magneto-structural phase transitions, while the largest value of the RCP (176 J/kg at 2 T applied magnetic field) has been observed for the Fe2P-type compounds exhibiting magneto-elastic phase transitions. For the characterization of magnetocaloric properties, one important parameter is the adiabatic temperature change, which is often not reported in literature owing to the lack of experimental setups for direct measurements of the magnetocaloric effect. This thesis also includes the development of a setup for the direct measurement of the adiabatic temperature change upon a change in a magnetic field.

The energy-efficient and environmentally friendly alternative cooling technology based on the magnetocaloric effect (MCE) is discussed in this thesis. The thesis has two major parts, one devoted to material characterization and the other to instrument development. Different magnetic oxides and intermetallic compounds with second-order and first-order magnetic transitions, respectively, were studied with the aim of finding materials suitable for magnetic refrigeration. For the application of the MCE, a high value of the isothermal entropy changes and the relative cooling power (RCP), along with minimal temperature hysteresis are required. The temperature hysteresis is negligible for all studied second-order compounds, while an almost ten times higher value of the isothermal entropy change has been observed for the first-order compounds. The highest value of isothermal entropy change (20 J/kgK at 2 T applied magnetic field) has been observed for the MnNiSi-type compounds exhibiting magneto-structural phase transitions, while the largest value of the RCP (176 J/kg at 2 T applied magnetic field) has been observed for the Fe2P-type compounds exhibiting magneto-elastic phase transitions. For the characterization of magnetocaloric properties, one important parameter is the adiabatic temperature change, which is often not reported in literature owing to the lack of experimental setups for direct measurements of the magnetocaloric effect. This thesis also includes the development of a setup for the direct measurement of the adiabatic temperature change upon a change in a magnetic field.