Search

An off-stoichiometric Fe 2 MnSn alloy was prepared by vacuum arc melting and homogenized by vacuum annealing. The structural analysis indicated that the alloy exhibits a D0 19 hexagonal structure. Isothermal magnetization analysis at room temperature showed soft ferromagnetic behavior and exchange magnetic anisotropy. Furthermore, the isofield magnetization study revealed a Curie temperature ( T c ) of 455 K with a second-order magnetic phase transition. A similar trace of the transition point was observed in the differential scanning calorimetric analysis. We estimated a magnetic entropy change ( Δ S M = − 4 J kg - 1 K - 1 ) and refrigerant capacity (RC = 116 J kg - 1 ) from M-T measurements, highlighting its potential for thermomagnetic generators and magnetic refrigerator applications. [ABSTRACT FROM AUTHOR]

In magnetism literature, usually the applied or external field H0 is taken to present the results of an investigation. But for certain purposes, it is advantageous to work with the internal magnetic field H that occurs inside a magnetic body. It is well-known that the susceptibility of the material and the demagnetization effect, given by the geometry of a body, link the two usually different fields under consideration. If a probe is a long needle with the external magnetic field and the magnetization in the probe parallel to the axis of the slender body, the two fields are identical. But when building thermomagnetic machines, other demands may require also other shapes (porous materials, particle beds, wavy structures, etc.) of the magnetized material and then a correct distinction of these fields becomes important and in some cases also laborious if one of them must be theoretically determined from the other. This article shows how--from a theoretical point of view--the most important physical properties of thermomagnetism/magnetocalorics, namely, the adiabatic entropy change, the effective specific heat capacity, and the adiabatic temperature change must be transformed. Furthermore, this theory reveals the invariants of magnetocalorics, which are combinations of these three most important properties. [ABSTRACT FROM AUTHOR]

The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5-400 K). Direct adiabatic temperature change (DTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H=0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD=2.5K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. [ABSTRACT FROM AUTHOR]

Measurements of the adiabatic temperature change (ΔT) and the specific heat transfer (ΔQ) of Ni2.18Mn0.82Ga Heusler alloy were taken in order to quantify the direct giant magnetocaloric effect of the alloy when it is in the vicinity of magneto-structural phase transition (PT) from paramagnetic austenite to ferromagnetic martensite, and their results are presented. A new vacuum calorimeter was used to simultaneously measure ΔT and ΔQ of magnetocaloric materials with a Bitter coil magnet in fields of up to H=140 kOe. Other thermomagnetic properties of this alloy were investigated using standard differential scanning calorimetry and PPMS equipment. The maximal values of magnetocaloric effect in H=140 kOe were found to be ΔT=8.4K at initial temperature 340K and ΔQ=4900 J/kg at 343 K. Using this direct method, we show that the alloy indeed demonstrates the largest value of ΔQ as compared with previously published results for direct measurements of magnetocaloric materials, even though at 140 kOe the magnetic field-induced magnetostructural PT is still not complete. [ABSTRACT FROM AUTHOR]

Recent studies have pointed out that first-order magnetic transitions can be affected by defects. It however remains unclear which precise microstructural features are involved and how to pragmatically control their formation. Here, we show that a genuine technical parameter such as the compaction pressure employed prior to the sintering can induce subtle differences on magnetization data and yet correspond to a large evolution of the magnetocaloric performances in MnFe 0.95 P 0.575 Si 0.36 B 0.065 compounds prepared by powder metallurgy, resulting in an enhancement of the isothermal entropy change (Δ S) in 1 T up to ∼40 %. This outcome is unexpected as Δ S normalized per mass should not be affected by what is at first glance a matter of compaction or density. Our detailed structural and microstructural investigations indicate that the sharpening of the transition for the highest compaction pressures originate from different distributions of the secondary phase at the grain boundaries. This work not only bring into light an interesting fundamental question how microstructural details can affect the development of a magnetic transition, it has also important practical consequences to ensure a reproducibility of large magnetocaloric or thermomagnetic effects in MnFe(P,Si,B) materials for possible applications. [Display omitted] • Magnetocaloric performances of MnFe(P,Si,B) increase with higher green compaction. • Final density not affected by green pressure with common 24 h high temperature sintering. • Increase in ΔS is not related to secondary phase content but rather to its distribution. [ABSTRACT FROM AUTHOR]

Abstract It is usually claimed that structural and magnetic transitions should coincide to improve the total entropy change. Here we show that overlapping structural and magnetic transitions can compete with each other, which gives rise to some signature of the competition misinterpreted as an experimental artifact of the measurements as observed in Ni 49+x Mn 36-x In 15 (nominally x = 0, 0.5, 1.0, 1.5 and 2.0) investigated in this work. Their various magnetic phase transitions show a low temperature conventional magnetocaloric effect (MCE), an inverse MCE followed by another high temperature conventional MCE. For concurrent phase transitions with their respective transition temperatures close to one another, an observed spike embedded in the high temperature conventional MCE region is found arising from the martensitic transformation. We show that a crossover of the magnetic field dependence of the transition temperatures is obtained for concurrent phase transitions occurring in a narrow temperature range. As an evidence of the complexity of overlapping phase transitions, a recently proposed quantitative method for determining the order of phase transition is applied to this compositional series, showing its applicability even to cases when first- and second-order phase transitions occur in a narrow temperature range. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The adiabatic temperature change (ATad) in Ni45 Mn43CoSn11 has been measured by a direct method in magnetic field changes up to 14 T. Large reversible magnetocaloric effects resulting in ΔTad of about = -11 and 5 K have been observed for magnetic field changes of 14 T at the magnetostructural (TA ~ 260 K) and magnetic transitions (TC ~ 320 K), respectively. The impact of the thermomagnetic history on ΔTad at high magnetic fields has been reported. The significant observed changes in the relaxation time of ΔTad, depending on the type of the phase transitions, magnetization, and demagnetization cycle are discussed. [ABSTRACT FROM AUTHOR]

In this paper, the crystallographic, magnetic, thermomagnetic, and magnetocaloric properties of the quenched NiMn1−xCrxGe (x = 0.04, 0.18, and 0.25) half-Heusler alloys have been studied by x-ray diffraction, differential scanning calorimetry and magnetization measurements. An influence of partial substitution of Cr for Mn and quenching of the samples on the character of structural and magnetic phase transitions is presented. Quenching of the alloys results in the formation of two phase (orthorhombic and hexagonal) crystal structure. The magnetic properties were investigated by means of magnetization measurements over a wide temperature (5–400 K) and magnetic field (up to 60 kOe) ranges. The experimental data indicate that at quenching and with increasing Cr content the magnetic order changes from antiferromagnetic to ferromagnetic. Hardened compounds exhibit a thermal hysteresis in the vicinity of the magnetic phase transition, what is characteristic for a first-order magnetic phase transition. The magnetic phase transition temperatures are decreased as a result of quenching of the samples. The magnetic entropy changes were calculated using the field dependences of isothermal magnetization in terms of the thermodynamic Maxwell relation. The magnetic entropy changes, |ΔSMmax|, obtained for the hardened alloys with x = 0.25 have the maximum value equal to 23 J/(kg·K) near the magnetic phase transition for a field change of ΔH = 0–60 kOe. [ABSTRACT FROM AUTHOR]

Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS2, which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW→NCCDW transition. [ABSTRACT FROM AUTHOR]

Advances in nanoscale material characterization on two-dimensional van der Waals layered materials primarily involve their optical and electronic properties. The thermal properties of these materials are harder to access due to the difficulty of thermal measurements at the nanoscale. In this work, we create a nanoscale magnetothermal device platform to access the basic out-of-plane magnetothermal transport properties of ultrathin van der Waals materials. Specifically, the Nernst effect in the charge density wave transition metal dichalcogenide 1T-TaS2 is examined on nano-thin flakes in a patterned device structure. It is revealed that near the commensurate charge density wave (CCDW) to nearly commensurate charge density wave (NCCDW) phase transition, the polarity of the Nernst effect changes. Since the Nernst effect is especially sensitive to changes in the Fermi surface, this suggests that large changes are occurring in the out-of-plane electronic structure of 1T-TaS2, which are otherwise unresolved in just in-plane electronic transport measurements. This may signal a coherent evolution of out-of-plane stacking in the CCDW→NCCDW transition. [ABSTRACT FROM AUTHOR]

The investigation is focussed on phase generation and magnetocaloric properties of a series of Ni 77-X Mn X Ga 23 (x = 22, 23, 24, 25, 27) alloys prepared through arc melting furnace. With increase in Mn content, the alloys showed systematic transition from a non-modulated martensite (NM) to a fully austenitic parent phase through an appearance and coexistence of modulated (M) structure. Intermediate Mn containing alloy (#Mn 24 ) not only displayed high magnetic entropy change (ΔS M ) of −7.7 J/kg −1 K −1 but also large Refrigerant Capacity (RC) of 169 J.Kg −1 at magnetic field change of 3 T compared to other alloys. The coexisting martensite (NM, M) and parent austenite as well as overlapping thermomagnetic and structural transformation was deliverable through tuning of alloy chemistry wherein Ni was systematically substituted by Mn. Transmission electron microscopy (TEM) supported the proposition with existence of martensite plates of different morphology in Mn 24 alloy exhibiting superior magnetocaloric properties. [ABSTRACT FROM AUTHOR]

We estimate the efficiency and power of a thermal energy harvesting thermodynamic cycle using a magnetocaloric material as active substance. The thermodynamic cycle is computed using an equation of state, either extrapolated from experimental data or deduced using a phenomenological Landau model. The magnetic work is then compared to the maximum work. Afterwards power is estimated using a simple thermal exchange model. Simulations of different cycles for different working points illustrate the tradeoff between power and efficiency. [ABSTRACT FROM AUTHOR]

Refractory borides are good candidates for thermomagnetic energy conversion due to their high chemical stability and good magnetic properties. A systematic study focusing on the crystallographic, magnetic and magnetocaloric properties of cobalt and iron substituted MnB was conducted. The pure MnB sample shows an orthorhombic structure with a spontaneous magnetization ( M s ) of 156 Am 2 kg −1 and a sharp magnetic transition ( T C ) of 567 K that yields a large magnetic entropy change of 7.5 J kg −1 K −1 in a 2T field change at this temperature. Both the substitution of Co and Fe elements for Mn were found to effectively modify T C accompanied by a lower magnetization ( M s ) leading to a reduced magnetocaloric effect (MCE). The differences in magnetic properties with substitution are described by calculation of the density of states and interatomic distances. It is found that in contrast to the Co atoms the Fe atoms develop large and stable moments similar to the Mn atoms which is consistent with the experimental findings. The sharp and significant change of M s (T), the very stable nature of these refractory borides and abundant availability makes some of these compositions suitable for thermomagnetic power generation applications. [ABSTRACT FROM AUTHOR]

The martensitic and magnetic phase transitions are investigated in Ni50Mn35–xIn15Cux ( $x = 0$ , 1, 2, 3, 4, and 5) ferromagnetic shape memory alloys using X-ray diffraction, differential thermal analysis, and magnetization measurements. Ascribed to the increase of valence electron concentration, the martensitic transformation temperatures increase to over room temperature with the increase of Cu-substitution, both in bulks and ribbons. Curie temperature of martensitic phase, however, decreases with Cu-substitution. Thermomagnetic curves show that the magnetic and structural transitions are coupled from $x = 0$ to 3. Cu-substitution improves magnetocaloric effect (MCE). The comparison between bulks and ribbons shows that the martensitic transformation temperatures and the MCE of ribbons are slightly lower than those of the bulks, and are suggested to be related to Mn-volatilization in annealed bulks, small and inhomogeneous grain sizes in ribbons. [ABSTRACT FROM AUTHOR]

The effect of the substitution of Co, Ni or (CoCr) for the part of Fe atoms on the thermal stability and some magnetic properties, i.e. magnetization and magnetocaloric effect of Fe86-xZr7MxNb2Cu1B4 (M = Co, Ni, (CoCr); x = 0 or 6) alloys was investigated. From X-ray studies it was stated that the investigated alloys in the as-quenched state and after the accumulative annealing for 10 min at 600 K and then 700 K were fully amorphous. The amorphous as-quenched Fe80Zr7Ni6Nb2Cu1B4 alloy shows the lowest thermal stability and the onset of its crystallization temperature is equal to 775 K. The Arrott plots are almost linear and show positive slope near the Curie temperature. The maximum magnetic entropy change is very sensitive to the chemical composition and annealing conditions of the alloys. [ABSTRACT FROM AUTHOR]

Colossal Magneto Resistive (CMR) materials with the compositional formula, La 0.34 R 0.33 Ba 0.33 MnO 3 (R=Pr, Nd, Sm and Gd) were prepared by the sol gel technique. After characterizing the samples structurally, a systematic investigation of magnetization has been undertaken, over a temperature range 70–300 K at 0.05 T magnetic field. An effort has been made to calculate the magnetocaloric behavior of all the samples theoretically using the experimental magnetization data. Other important parameters of magnetocaloric effect (MCE) such as maximum entropy change, full width at half-maximum, relative cooling power and maximum adiabatic temperature change were also computed. The observed behavior has been explained qualitatively. [ABSTRACT FROM AUTHOR]

Effects of oxygen deficiency on the thermomagnetic properties of LaBaMnO polycrystalline perovskites have been predicted. By the help of the phenomenological model, the temperature dependences of the magnetization for LaBaMnO with δ=0.0, 0.02, 0.05, 0.08, and 0.1 upon 1 T magnetic field were simulated. The behavior of the temperature dependent magnetocaloric effect, in the vicinity of magnetic phase transitions, was investigated. The magnetic entropy change, specific heat, and adiabatic temperature change for several δ were obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, in 1 T magnetic field variation, were calculated. As the oxygen content increases, the magnetocaloric effect of LaBaMnO, decreases and shifts to room temperature. The results obtained show a strong dependence on the oxygen deficiency of the materials. The magnetocaloric effect of these materials is large and tunable, suggesting a possible technical application of the materials at moderate magnetic fields near room temperature. It is shown that for LaBaMnO, the magnetic entropy change and adiabatic temperature change follows a master curve behavior. [ABSTRACT FROM AUTHOR]

Magnetocaloric properties of the CeSrMnO system near a phase transition from a ferromagnetic to a paramagnetic state are investigated. The value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different external magnetic fields. The Δ S distribution is uniform, which is desirable for an Ericson-cycle magnetic refrigerator. The data show that CeSrMnO can be used as a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas from the temperature of liquid natural gas to nearly the boiling point of hydrogen. [ABSTRACT FROM AUTHOR]

Magnetic, magnetocaloric, and exchange bias properties have been systematically investigated in Ni46Co4Mn38Sb12 ribbon by applying magnetic field along in-plane (IP) and perpendicular out of plane (OP) to the ribbon plane. From the thermo-magnetization curves, the sharpness of the martensitic transition is observed to be nearly the same for both IP and OP ribbons. The thermomagnetic irreversibility region is found to be larger in the OP ribbon at 500 Oe, indicating that the magnetic anisotropy is larger in this case. The OP ribbon shows the Hopkinson maximum at 500 Oe, both for the field cooled cooled and zero field cooled modes. The magnetization curve for IP ribbon shows a faster approach to saturation, compared to the OP ribbon. Isothermal magnetic entropy change at 50 kOe has been found to be nearly same for both the ribbons. At 5 K, the coercivity and exchange bias values are larger for the OP ribbon. Crystallographic texturing of the ribbons and its effect in the easy magnetization direction are found to be the reason behind the differences between the two ribbons. [ABSTRACT FROM AUTHOR]

Thermomagnetics of a system of interacting particles with log normal distribution of particle size and anisotropy constant was studied self-consistently using a Monte Carlo calculation within the mean-field approximation. Characteristic hysteretic behavior and reversible transverse susceptibility (RTS) were investigated. The salient features are as follows: The hysteresis loops become sheared with increasing anisotropy distribution, /spl sigma//sub K/, while their slopes steepen with the mean field strength /spl alpha/. For a homogeneous systems RTS peaks increase sharply with /spl alpha/, both at the anisotropy field H/sub K/ and coercive field H/sub c/. For random systems, both peaks become shallower and blunted with increasing /spl sigma//sub K/. Mean field strength /spl alpha/ causes the peak positions of RTS at H/sub K/ to shift to lower values, while the shift at H/sub c/ depends subtly upon the interplay among /spl alpha/, /spl sigma//sub K/ and texture of the system. For random systems the shift is toward higher H/sub c/ values with increasing /spl alpha/. For systems with constant /spl alpha/, however, it shifts toward lower values with increasing /spl sigma//sub K/. [ABSTRACT FROM PUBLISHER]