Search

In the last few years, the quantum superparamagnetic (QSP) phenomenon has been reported in isolated ultrasmall nanoparticles. In this work, a QSP state and superparamagnetic (SP) state in nanostruc...

The critical behavior of single phase Y2Fe17 melt-spun ribbons with the Th2Fe17-type crystal structure has been studied around the transition temperature (TC). This alloy undergoes a second-order ferromagnetic (FM)–paramagnetic (PM) phase transition at TC = 301 K. Various techniques, such as the modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis, were used to determine the critical exponents that were found to be β = 0.226(3), γ = 1.296(2), and δ = 6.804(5). The universality class of the critical phenomenon in the Y2Fe17 ribbons can be explained with the help of the renormalization group theory approach, in which the magnetic properties show a feature changing from three (3D)- to two-dimensional (2D) Ising model. The first-principles calculations based on density functional theory qualitatively explain the experimental results, confirming the strong correlation between lattice atoms and critical behavior in magnetic intermetallic Y2Fe17.

ErNi2 ribbons were produced by rapid solidification using the melt spinning technique. Their structural, magnetic and magnetocaloric properties in the as-solidified state were studied by X-ray diffraction, scanning electron microscopy, magnetization and specific heat measurements. Samples are single phase with the MgCu2-type crystal structure, a Curie temperature TC of 6.8 K and a saturation magnetization at 2 K and 5 T of 124.0 A⋅m2/kg. For a magnetic field change μ0ΔH of 5 T (2 T) ribbons show a maximum magnetic entropy change |ΔSMpeak| of 24.1 (16.9) J/(kg⋅K), and an adiabatic temperature change ΔTadmax of 8.1 (4.4) K; this is similar to the previously reported literature for bulk alloys that were processed through conventional melting techniques followed by prolonged thermal annealing. In addition, the samples also show slightly wider ΔSM(T) curves with respect to bulk alloys leading to a larger refrigerant capacity.

A novel superparamagnetic state has been observed in high quality La0.7Sr0.3MnO3 (LSMO) thin films directly grown by rf-sputtering on SiOx/Si(100) substrates. The films are nanostructured without grain boundaries, constituted by locally epitaxial nanoregions grown layer-by-layer with out-of-plane (012) preferential orientation, induced by the constrain of the native silicon oxide. Low magnetic field ZFC-FC magnetization curves show a cross-over from superparamagnetic to ferromagnetic state dependent of the thickness. The thicker film (140 nm) exhibits typical ferromagnetic order. The thinner films (40 and 60 nm) exhibit superparamagnetic behavior attributed to interacting ferromagnetic monodomain nanoregions with critical size, random in-plane oriented, where the inter-monodomain boundaries with surface spin-glass structure regulate the blocking of magnetization depending on the magnetic field intensity. M(H) hysteresis loops showed noticeable coercive fields in all samples, larger than those reported for LSMO. Such properties of half-metal LSMO film foresee potential integration in new Si-technology nanodevices in Spintronics.

We fabricated a Ni45Co6.4Mn37In11.6 polycrystalline sample with a specific texture by directional solidification. The alloy is characterized by wide successive structural and magnetic phase transitions that considerably extend their temperature intervals with the increase in the applied magnetic field. Under a magnetic field change μoΔH of 5 T, the working temperature window obtained from the magnetic entropy change curve for the structural and magnetic transition is as large as 66 and 52 K, respectively. As a result, the corresponding refrigeration capacity RC reached high values of 118 and 95 J kg−1, respectively, irrespective of the considerably small magnetic entropy changes.

The influence of Fe-alloying on the structural transition temperatures, microstructure, and mechanical and magnetocaloric properties of polycrystalline Ni41Co9−xFexMn40Sn10 (at. %) (x = 0–3) alloys has been studied. The substitution of Fe for Co introduced the so-called γ phase when the Fe content exceeds 1 at. %. The transformation temperatures decreased almost linearly with increasing Fe content without alteration of the transformation sequence. The compressive strength and maximum compressive strain increased to 1270 MPa and 21.3 % for Fe content of 3 at. %. In addition, the fracture type changed gradually from typical intergranular cracking to transgranular cracking with increasing Fe content. Magnetic field-induced reverse martensitic transformation was modified by increasing the Fe alloying, leading to the reduction of magnetic hysteresis loss (∼ 82.5 % drop after addition of 3 at. % Fe) but at the expense of a significant reduction in the maximum magnetic entropy change value.

The structural, magnetic and magnetocaloric properties of glass-coated magnetic microwires, composed of Ni2MnGa and produced with the Taylor-Ulitovski technique, have been investigated with XRD, EBSD, SEM and magnetization measurements. At room temperature, the microwires exhibit a monocrystalline phase with a Cu2MnAl-type crystal structure (space group Fm-3m; cell parameter a = 5.832 A). The microwires in this study show a magnetocaloric effect with a sharp martensitic transformation in the range of 185–195 K, as well as a magnetocaloric effect owing to the magnetic phase transition at the Curie temperature. Due to the different strength of anisotropy and easy magnetization axis, the magnetocaloric effect during the martensitic transformation exhibit a maximum at low fields (0.5 T), and high efficiency (defined as a ratio between refrigerant capacity and applied magnetic field) at 0.2 T.

We report the influence of the Mn atomic concentration (at.%) on the nanostructures formation and magnetic properties of GaAs:Mn layers grown by Molecular Beam Epitaxy at a relatively high substrate temperature of 530 °C varying the nominal Mn at.% content from 0.01 to 0.2. It is shown that by modifying the Mn at.% different kind of nanostructures, ranging from 2D (such as islands and surface corrugation) to 3D microleave- and nanowire-like arrays, form on the surface layer. Samples produced with Mn contents ranging from 0.02 to 0.20 at.% show a significant room temperature ferromagnetic response that is attributed to the formation of MnAs nanocrystals as confirmed from X-ray diffraction analysis and magnetization measurements. The influence of MnAs clusters on the formation of the nanostructures observed is discussed.

International audience; For NiMn-based Heusler alloys, simultaneously achieving high magnetization difference and low thermal hysteresis across the magnetostructural transformation is of crucial importance to obtain large low-field actuated magnetocaloric properties. Here, the fifth element Si was introduced to replace In for reducing the thermal hysteresis in Ni-Co-Mn-In alloys while keeping a large magnetization change through the martensitic transformation. Consequently, a giant maximum entropy change ΔSMmax of 33.6 J kg−1 K−1 under the low field change of 2 T was achieved in a Ni45Co5Mn37In12Si1 alloy, which is among the highest values reported in NiMn-based alloys.

In this work, crystallographic, microstructural and magnetocaloric investigations were performed on textured Ni50Mn35.25In14.75 melt-spun ribbons with low thermal (6 K) and magnetic-field induced hysteresis (−0.73 J kg−1 at 2 T) and moderate maximum magnetic entropy change ΔSMmax (11 J kg−1 K−1 at 5 T) at room temperature (302 K). The austenite in the ribbons crystallizes into a L21 structure, whereas martensite has a monoclinic incommensurate 6 M modulated structure as determined with the superspace theory. By means of electron backscatter diffraction technique, the morphological and crystallographic features of microstructure were systematically characterized. Ribbons possess a fine microstructure with an average grain size (initial austenite phase) of around 10 μm, whereas the 6 M martensite has a self-accommodated microstructure with 4 kinds of twin-related martensite variants. During inverse martensitic transformation, the austenite prefers to nucleate at the grain boundaries of initial austenite. By means of cofactor conditions and crystallographic orientation analyses, the good geometrical compatibility between austenite and martensite was confirmed. Based on the crystal structure and microstructure information obtained, the reason of the low thermal hysteresis was discussed.

In this study, bimetallic nanoparticles (BMNPs) with different mass ratios of Cu and Fe were evaluated. The influence of the morphology on the removal of pollutants was explored through theoretical and experimental studies, which revealed the best structure for removing arsenate (As(V)) in aqueous systems. To evidence the surface characteristics and differences among BMNPs with different mass proportions of Fe and Cu, several characterization techniques were used. Microscopy techniques and molecular dynamics simulations were applied to determine the differences in morphology and structure. In addition, X-ray diffraction (XRD) was used to determine the presence of various oxides. Finally, the magnetization response was evaluated, revealing differences among the materials. Our cumulative data show that BMNPs with low amounts of Cu (Fe0.9Cu0.1) had a non-uniform core-shell structure with agglomerate-type chains of magnetite, whereas a Janus-like structure was observed in BMNPs with high amounts of Cu (Fe0.5Cu0.5). However, a non-uniform core-shell structure (Fe0.9Cu0.1) facilitated electron transfer among Fe, Cu and As, which increased the adsorption rate (k), capacity (qe) and intensity (n). The mechanism of As removal was also explored in a comparative study of the phase and morphology of BMNPs pre- and post-sorption.

The crystal structure, and magnetic and magnetocaloric properties of rapidly solidified Ni0.895Cr0.105MnGe1.05 melt-spun ribbons is reported. The ribbon samples crystallize into a single-phase hexagonal Ni2In-type structure at room temperature. The as-quenched ribbons showed a second order magnetic transition at 192 ± 1 K at μoH = 5 mT. A magnetic-field-induced transition from an antiferromagnetic (AFM)-like to a ferromagnetic (FM) state of martensite structure was observed in annealed ribbons below the temperature of the martensitic transformation (TM ∼ 245 ± 1 K). The annealed ribbons undergo a first-order magnetostructural transition (MST) with a large maximum reversible magnetic entropy change of ΔSM = 16.1 J kg−1 K−1 (this is about a four-fold increase compared to the ΔSM observed for the bulk sample of the same nominal composition) and RC = 144 J kg−1 for μoΔH = 5 T at temperature T = TM ∼ 245 ± 1 K. The increase in the ΔSM peak value leads to an improved RC compared to that of the bulk sample (122 J kg−1). The observed MCE and quasi-reversible character of ΔSM at the MST illustrates the potential of Ni0.895Cr0.105MnGe1.05 ribbons for magnetic cooling technology.

In the present contribution, the formation of nanoscrolls after a hydrazine treatment in iron oxide-decorated graphene oxide was observed. After a HCl treatment, the nanoscrolls lead to the formation of nanoribbons. The proposed mechanism of nanoscroll formation includes the decoration onto the epoxy/hydroxyl moieties and the reduction of carboxyl and carbonyl groups at the end ot the graphene sheet that cause the sheet rolling. The nanoribbons form upon iron oxide dissolution and beaking of the iron-carbon bonds causing the sheet ripening. Both materials are superparamagnetic, with maximum saturation magnetization of 30 and 20 A.m2.kg−1 respectively, measured in the 10–400 K range by vibrating sample magnetometry. The formation of these structures seem to be observed, yet not described in other reports, suggesting the nanoscroll formation in FeOx-GO could be a more general phenomenon.

The kinetic arrest (KA) of martensitic transformation (MT) observed in Ni 45 Co 5 Mn 36.8 In 13.2 melt-spun ribbons has been studied. These alloy ribbons show an ordered columnar-like grain microstructure with the longer grain axis growing perpendicular to ribbon plane and transform martensitically from a single austenitic (AST) parent phase with the L2 1 -type crystal structure to a monoclinic incommensurate 6 M modulated martensite (MST). Results show that the volume fraction of austenite frozen into the martensitic matrix is proportional to the applied magnetic field. A fully arrest of the structural transition is found for a magnetic field of 7 T. The metastable character of the non-equilibrium field-cooled glassy state was characterized by introducing thermal and magnetic field fluctuations or measuring the relaxation of magnetization. The relaxation of magnetization from a field-cooled kinetically arrested state at 5 and 7 T follows the Kohlrausch–Williams–Watts (KWW) stretched exponential function with a β exponent around 0.95 indicating the weak metastable nature of the system under the strong magnetic fields. The relationship between the occurrence of exchange bias and the frozen fraction of AST into the MST matrix was studied.

The magnetic, structural, thermal, and magnetocaloric properties of Ni50Mn35In14.5B0.5 melt-spun ribbons have been studied through X-ray diffraction (XRD), differential scanning calorimetry, and magnetization measurements. A comparison of magnetic properties and magnetocaloric effects (MCE) of Ni50Mn35In14.5B0.5 melt-spun and annealed ribbons to its bulk form has been shown in detail. We have observed that a short time vacuum annealing (1073 K-10 min) on ribbon sample can restore the properties of the bulk material. Significant changes in magnetic and magnetocaloric properties have been observed between Ni50Mn35In14.5B0.5 ribbons in the as-solidified state and after thermal annealing. The MCE parameters of annealed ribbons were found to be comparable to those observed in the bulk alloy. The maximum value of relative cooling power of 150 J/kg for a magnetic field change of 5 T was found at the martensitic transition for annealed ribbons. The working temperature range of the magnetic entropy change (ΔSM) for annealed ribbons has been significantly enlarged in comparison to melt-spun ribbons. The role of the magnetic and structural changes on the transition temperatures of the ribbons is discussed.

Bulk Mn2-xFe1+xGe (0.0 ≤ x ≤ 1.0) alloys have been synthesized by arc-melting followed by a low temperature homogenization thermal annealing, whereas for comparison purposes the Mn2FeGe alloy was also produced in ribbon form by rapid solidification. A study of the structural and magnetic properties is presented. Contrary to theoretical predictions, Mn2FeGe crystallizes in a hexagonal DO19 crystal structure (space group P63/mmc) and orders ferromagnetically with a saturation magnetization (MS) value of ~1.7 µB/f.u. in the ground state. With the substitution of Fe for Mn in bulk Mn2-xFe1+xGe, we observed an increase in the FM interactions with a maximum MS value of 5.1 µB/f.u. for x = 1.0, and a significant progressive increase in the Curie temperature (TC) in a wide range spanning ~200 K to over 400 K.

The present work was undertaken to investigate the effect of microstructure on the magnetic entropy change of Ni 50 Mn 37 Sn 13 ribbon alloys. Unchanged sample composition and cell parameter of austenite allowed us to study strictly the correlation between the average grain size and the total magnetic field induced entropy change (Δ S T ). We found that a size-dependent martensitic transformation tuning results in a wide temperature range tailoring (>40 K) of the magnetic entropy change with a reasonably small variation on the peak value of the total field induced entropy change. The peak values varied from 6.0 J kg −1 K −1 to 7.7 J kg −1 K −1 for applied fields up to 2 T. Different tuning efficiencies obtained by diverse MCE tailoring approaches are compared to highlight the advantages of the herein proposed mechanism.

We have fabricated melt-spun ribbons of nominal composition ErMn2 and studied their phase constitution and magnetocaloric (MC) properties by X-ray diffraction, scanning electron microscopy (SEM), magnetization and heat capacity measurements. The major phase formed shows the MgZn2-type hexagonal structure (C14-type); however, both XRD and SEM analyses revealed the formation of impurity phases (i.e., Er6Mn23 and ErMn12). Ribbons exhibit a saturation magnetization of 149 Am2kg−1 at 2 K and a Curie temperature of TC = 15 K. A field-induced metamagnetic transition at very low critical magnetic fields was observed below 8 K that leads to a change of sign in the magnetic entropy change ΔSM below this temperature (ΔSMpeak = 2.5 Jkg−1K−1 at 2 K and 5 T). For a magnetic field change of 5 T (2 T) applied along the ribbon length, the samples show a large peak value of the magnetic entropy change ΔSMpeak of −20.5 (−10.8) Jkg−1K−1, a full-width at half-maximum δTFWHM for the ΔSM(T) curve of 20 (12) K, and a maximum adiabatic temperature change ΔTadmax of 7.4 (3.6) K. The obtained results are compared with the reported in literature by other authors for bulk alloys.

A novel ferromagnetic state coexisting with ferroelectric ordering at room temperature in strained BiFeO 3 (BFO) thin films grown using a sputtering technique on La 0.7 Sr 0.3 MnO 3 /SiO 2 /Si(100) (LSMO/SOS) substrates is reported. The properties of BFO films with different thicknesses ( t BFO = 15, 50, 70, 120, and 140 nm) on 40 nm LSMO layers are explored. [012] out-of-plane highly textured BFO/LSMO stacks grew with rhombohedral structures. LSMO layers are nanostructured in nature, constituted by ferromagnetic single-domain nanoregions induced by the constrain of the SiO 2 surface, with T C ∼200 K and high coercive field ( H C ) of ∼1100 Oe at 2.5 K. BFO films grew epitaxially nanostructured on LSMO, exhibiting ∼4 nm spherical nanoregions. The BFO layers show typical antiferromagnetic behavior (in a greater volume fraction) when made thicker ( t BFO > 70 nm). The thinner films ( t BFO T C > 400 K, H C ∼ 1600 Oe for 15 nm and ∼1830 Oe for 50 nm. We propose that such ferromagnetic behavior is originated by the establishment of a new magnetic configuration in the Fe 3+ O Fe 3+ sublattice of the BFO structure, induced by strong hybridization at the interface as consequence of superexchange coupling interactions with the ferromagnetic Mn 3+ O Mn 3+ /Mn 4+ sublattice of LSMO. All BFO layers show excellent ferroelectric and piezoelectric properties (coercive field ∼ 740 kV/cm, and d 33 = 23 p.m./V for 50 nm; ∼200 kV/cm and 55 p.m./V for 140 nm), exhibiting 180° and 109° DWs structures depending on the thickness. Such multiferroic properties predict the potential realization of new magneto-electronic devices integrated with Si technology.

Were fabricated melt-spun ribbons of the quinary alloy Mn0.5Fe0.5NiSi0.945Al0.055. After an annealing time of 4 h at temperatures between 1073 K and 1223 K, samples underwent a martensitic-like magnetostructural transformation around room temperature from a paramagnetic Ni2In-type hexagonal structure to a ferromagnetic TiNiSi-type one. The phase transformation practically remained unchanged with the annealing temperature, whereas the first cooling process induced a massive cracking in the samples. In comparison to the reported in literature for bulk alloys, the melt-spun ribbons showed a lower maximum magnetic field-induced entropy change but a well-broader working temperature range. A relatively low magnetic hysteresis loss was obtained across the martensitic transformation.

Magnetoelastic transitions (METs) in bulk in nearly equiatomic Fe-Rh alloys produced by arc melting may show poor reproducibility related to insufficient chemical homogeneity and presence of impurity phases in variable concentrations. To better understand the synthesis conditions that reliably yield bulk FeRh materials with reproducible MET characteristics, Fe100-xRhx alloys with x = 50, 50.5 and 51 at. % were prepared by induction melting and thermal annealing under identical conditions. The fabricated samples were cut into several slices, followed by characterization of METs in each of the slices using isothermal and isofield magnetization measurements, differential scanning calorimetry, and direct measurements of the magnetocaloric effect. All of the slices exhibit METs between the AFM and FM states, but the transitions are abrupt with nearly the same change of magnetization, ΔM, when x = 50.5 and 51, whereas for the x = 50 alloy the transition spreads over a wide temperature interval and ΔM may fluctuate by as much as 10 % from one specimen to another. A comparison of the magnetocaloric responses of x = 50 and 51 materials is presented. The clearly different effect of the magnetic field on the transition in both directions leads to significant differences in the reversibility and maximum values of the magnetic field-induced entropy and adiabatic temperature changes, as well as average hysteresis losses. In terms of reproducibility, our results suggest that induction melting is a more appropriate technique to prepare these binary alloys.

Magnetic microcomposites were fabricated by emulsification of a mixture of polydimethylsiloxane (PDMS) and nickel microparticles. The composites were obtained in a temperature-controlled water-surfactant media with and without the influence of an external magnetic field. The presence of a moderate external magnetic field of 80 G (8 mT) during the polymerization stage leads to the arrangement of nickel microparticles into chains that form the magnetic core of the synthesized composites. The method allows controlling the shape of the composite particles by applying a magnetic field and varying the stirring speed. Three shapes of composite particles, namely spherical, teardrops, and ellipsoidal, were obtained and magnetically characterized. Room temperature hysteresis loops and dM/dH versus H curves in the second-to-third quadrants show that spherical particles are isotropic while non-spherical particles show an induced uniaxial magnetic anisotropy which depends on the shape of the resulting composite particles.

An accurate calculation of the different magnetocaloric-related magnitudes derived from the temperature dependence of the magnetic entropy change in materials exhibiting first-order magnetocaloric effect is imperative to correctly estimate the true potential of a specific material for refrigeration purposes. In this contribution, we present a meticulous study of two different thermal procedures to measure the set of isothermal magnetization curves from which the total field induced magnetic entropy change, ΔST, is calculated using the adequate Maxwell relation. If the accurate determination of ΔST for any temperature is pursued the thermal and magnetic history of the materials must be taken into account, and then, the unidirectional measurement of reversible isothermal magnetization curves after a thermal cycle is required. The analysis was conducted on MnCoGeB0.01 alloy ribbons that show a giant ΔST at the coupled magneto-structural transition, from a ferromagnetic (TiNiSi-type) phase to a paramagnetic (NiIn2-type) one, owing to the concomitant abrupt magnetization change. We suggest that the conclusions reached can be applicable to any other system displaying magnetocaloric effect originated at a first-order phase transition.

We report a step-like martensitic transformation and its effect on magnetocaloric properties in Ni 40.6 Co 8.5 Mn 40.9 Sn 10 unidirectional crystal grown using the Bridgman-Stockbarger technique followed by vacuum annealing at 1173 K for 72 h. This alloy undergoes successive first-order phase transitions which are attributed to the monotonic compositional variation along the crystal growth direction; the latter results in the increase of the full-width at half-maximum of the magnetic entropy change as a function of temperature (δ T FWHM ). For a magnetic field change of 2 and 5 T, the maximum magnetic entropy change are 9.7 and 16.0 J kg −1 K −1 with an extended δ T FWHM of 11 and 23 K, respectively. The average hysteresis loss for a field change of 2 T is 19 J kg −1 reducing the refrigerant capacity by 17%.

The magnetic properties and reversibility of the magnetocaloric effect of Ni 50 Mn 35 In 15 have been studied in the vicinity of the phase transition using magnetization and direct adiabatic temperature change (ΔT ad ) measurements in magnetic fields up to 14 T. The magnetostructural phase transitions (MSTs) between a martensitic phase (MP) with low magnetization (paramagnetic or antiferromagnetic) and a nearly ferromagnetic austenitic phase were detected from thermomagnetic curves, M(T,H), at the applied magnetic fields up to 5 T. The MST temperature was found to be nearly independent of magnetic field for H ad ∼ −11 К for a magnetic field change of ΔH = 14 T was observed in the vicinity of the MST. The irreversibility of ΔT ad was found to be 1 K. A direct ΔT ad of +7 K for ΔH = 14 T was detected at the second order ferromagnetic-paramagnetic phase transition. The obtained results have been discussed in terms of the suppression of antiferromagnetic correlations with the application of a strong magnetic field, and a reversibility of the initial magnetic state of the MP with applied magnetic field when the MST coincides with T C .

The inspection of simplified fabrication and/or processing routes in order to produce materials with attractive magnetocaloric properties is of paramount importance for the development of environmentally friendly magnetic cooling technology. In this work, we have made use of the melt-spinning technique to obtain directly single-phase Y2Fe17 polycrystalline ribbons avoiding any high-temperature annealing for phase consolidation and homogenization. The melt-spun ribbons, with hexagonal Th2Ni17-type crystal structure, exhibit a moderate maximum value of the magnetic entropy change, |ΔSMpeak| = 2.4(4.4) J kg−1 K−1 under an applied magnetic field change of 2(5) T. Although these values are similar to those for the bulk alloy, the ΔSM(T) curves are manifestly broader, thus giving rise to an expansion of the working temperature range and the enhancement of about 15% in the refrigerant capacity. We also show that the magnetic field dependence of |ΔSM peak| at T = TC follows a H2/3 power-law.

The SEM micrographs of Figures 1(a) and (b) were obtained by Ana Iris Pe?a Maldonado and Jes?s Antonio Garc?a Carrillo in an FEI Helios NanoLab 600 system. Jes?s Antonio Garc?a Carrillo and P.J. Ibarra Gayt?n prepared the melt-spun ribbon samples processed by Spark Plasma Sintering in the present study.

The support received from the following organizations is gratefully acknowledged: (a) Laboratorio Nacional de Nanociencias y Nanotecnología (LINAN, IPICyT); (b) Consejo Potosino de Ciencia y Tecnología (Copocyt); (c) Banco Santander Central Hispano; (d) Spanish MCIU and AEI and European FEDER (MCIU-19-RTI2018-094683-B-C52), and; (e) Principado de Asturias (IDI/2018/000185), Spain. Authors are also indebted to M.Sc. B.A. Rivera-Escoto, M.Sc. A.I. Pena Maldonado, and Dr. G.J. Labrada-Delgado (from LINAN)and D. Martínez Blanco (from scientific and technological resources of the University of Oviedo) for the technical support given. J. Zamora thanks to CONACYT-Mexico for supporting his postdoctoral position at IPICYT. C.F. Sanchez-Valdés is grateful to DMCU-UACJ for supporting his research stays at IPICyT (program PFCE and academic mobility grant); also, for the financial support received from SEP-Conacyt, Mexico (Grant No. A1-S-37066).

Here, the anisotropy between the longitudinal and transverse directions of multi-filamentary Bi 2 Sr 2 Ca 2 Cu 3 O 10 + x tapes (“in plane ” anisotropy) is studied by transport and magnetization measurements. By minimally manipulating experimental data, we quantify it using one number associated with transport measurements, and two numbers based on magnetization curves. We propose that the simplest and most reliable number is that based on the magnetization loop width. We propose that the best way to evaluate the in-plane anisotropy is to measure the vertical width of the M vs. H loop, where H is applied along two mutually perpendicular directions lying on the wide face of the tape.

We report the magnetocaloric properties of two Fe-containing quinary bulk polycrystalline alloys of nominal compositions Ni41Co7Fe2Mn40Sn10 and Ni41Co6.5Fe2.5Mn40Sn10 that were determined by indirect and direct methods. Both samples showed a large refrigeration capacity RC and maximum magnetic entropy change ΔSMpeak around room temperature. For a magnetic field change of 2 T (5 T), a large magnetic entropy change of 18.9 (22.4) J kg−1 K−1 and 11.8 (16.8) J kg−1 K−1 and a refrigeration capacity of 128 (396) J kg−1 and 99 (313) J kg−1 were found in Ni41Co7Fe2Mn40Sn10 and Ni41Co6.5Fe2.5Mn40Sn10 alloys, respectively. RC for the alloy Ni41Co7Fe2Mn40Sn10, is among the largest value reported so far for Ni–Mn based Heusler alloys. Under a 1.5 T field change, the direct measurements showed the maximum adiabatic temperature changes ΔTadmax of −0.8 K and −1.5 K for these two alloys, respectively. The present findings point out the potential of Fe-alloyed Ni41Co9Mn40Sn10 Heusler alloys as room-temperature magnetic refrigerants.

The structural, magnetic and magnetocaloric properties of HoNi2 melt-spun ribbons were investigated by X-ray diffraction, magnetization and specific heat measurements. The as-solidified ribbons samples produced are single phase with the cubic MgCu2-type crystal structure of the Laves phase (C15; space group Fd-3m), and a Curie temperature TC of 13.9 K. For a magnetic field change μoΔH of 5 T (2 T) applied along the ribbon length, the produced ribbon samples show a maximum magnetic entropy change ΔSMpeak of −27.2 (−16.9) J kg−1 K−1, a full-width at half-maximum δTFWHM for the ΔSM(T) curve of 19 (11) K, and a refrigerant capacity, determined from the area below δTFWHM the ΔSM(T) curve, of 388 (145) Jkg−1; the adiabatic temperature change ΔTad at 2 T is 6.7 K. Whereas crystal structure and intrinsic magnetic properties are in good agreement with the experimental data previously reported in literature for bulk polycrystalline alloys, the magnetocaloric parameters at 2 T are markedly superior owing to the favorable combination of partial texture with the anisotropic behavior of magnetization.

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K.

We report the effect of low temperature vacuum annealing (823 K; 550 °C) on the elemental chemical composition, structural phase transition temperatures, phase structure, and magnetic properties of Ni 50.6 Mn 36.3 Sn 13.1 as-solidified ribbons. Their elemental chemical composition, highly oriented columnar-like microstructure and single-phase character (L2 1 -type crystal structure for austenite) remain unchanged after this low temperature annealing. Annealed ribbons show a reduction of interatomic distances which lead to a small change in the characteristic phase transition temperatures (~3–6 K) but to a significant rise of ~73 and 63% in the saturation magnetization of the martensite and austenite phases, respectively, that can be strictly ascribed to the strengthening of ferromagnetic interactions due to the change in interatomic distances.

The phase transformation and the magnetocaloric effect of Ni53Mn22Ga25 and Ni52Mn26Ga22 ribbons were studied. It is shown that in melt-spun Ni53Mn22Ga25 ribbons the magnetic transition and the martensitic transformation are well separated. For this sample, the austenite grains are columnar in shape with a typical preferential orientation of {001}A//ribbon plane. For a magnetic field change of 5 T, the maximum entropy change ${\Delta {S_{T}^{\textrm {max}}}}$ of −4.7 Jkg−1K−1 is achieved at the vicinity of the martensitic transformation. For the Ni52Mn26Ga22 ribbons, the structural transformation and the magnetic transition occurred simultaneously in both melt-spun and annealed ribbons. Owing to the coupled magnetostructural transformation, a ${\Delta {S_{T}^{\textrm {max}}}}$ of −11.4 Jkg−1K−1 is reached for a magnetic field change of 5 T in melt-spun Ni52Mn26Ga22 ribbons. After thermal annealing, ${\Delta {S_{T}^{\textrm {max}}}}$ increases to −30.0 Jkg−1K−1, which is almost three times higher as that of the initial melt-spun ribbons, due to the improved atomic ordering and the coupling of the intermartensitic transformation with the magnetostructural transformation.

The pulsed photoacoustic (PA) technique was used to study the synthesis by laser ablation of silver nanoparticles (Ag-NPs) in ethanol. PA technique allowed to determine the production rate per laser pulse and concentration of synthesized Ag-NPs. The samples were produced by using a pulsed Nd:YAG laser with 1064 nm of wavelength and 7 ns of pulse duration. The laser pulse energy varied from 10 to 100 mJ. Transmission electron microscopy micrographs demonstrated that the obtained nanoparticles were spherical with an average size close to 10 nm. The absorption spectra of the colloids showed a plasmon absorption peak around 400 nm. The PA analyses showed a significant reduction of the production rate of Ag-NPs during the first hundreds of laser pulses. For a higher number of pulses this rate was kept almost constant. Finally, we found that the root mean square (RMS) value of the PA signal was proportional to the laser pulse fluence on the target surface. Thus PA technique was useful to monitor the ablation process.

The magnetostructural transition in the Mn 0.96 Co 1.04 GeB 0.02 ribbon alloy was investigated. Chemical, structural, microstructural, and magnetic studies were performed on the samples which were annealed at different temperatures. The resulting samples underwent a first-order phase transition in which the characteristic structural transition temperature shows a near-linear and inversely proportional dependence to the annealing temperature. The magnetostructural transition occurs through a simultaneous ferroelastic–magnetic transition between the ferroelastic–paramagnetic and paraelastic–ferromagnetic phases. Our results suggest that the crystal structure of the hexagonal high temperature phase allows the formation of ferroelastic domains, and the domain walls act as the natural nucleation site of the low temperature paraelastic–ferromagnetic phase.

We present a cost-effective and robust set-up designed to measure directly the magnetic field-induced adiabatic temperature change. The system uses a piston to introduce/remove the sample to/from the magnetic field (μ0∆His up to 1.7T) created by an ordinary electromagnet. The temperature of the sample is controlled by a double pipe heat exchanger operating by the electrical heater and air flow circulation from a Dewar with liquid nitrogen to the sample holder assembly.We have measured the adiabatic temperature change, ΔTad, of two polycrystalline samples: Gd and Ni50Mn35In15Heusler alloy. At the second-order magnetic phase transitions (18oC for Gd and 42oC for Ni50Mn35In15), ΔTadunder μ0∆H=1.7T are 3.8±0.1oC for Gd and 1.9±0.1oC for Ni50Mn35In15. The Heusler alloy shows an inverse magnetocaloric effect: ΔTadis-1.5±0.1oC on cooling and-1.6±0.1oC on heating at the martensitic transformation temperatures of ~24oC and ~29oC, respectively.

This review devoted to magnetocaloric effect (MCE) in specially designed materials is divided into three main sections. In the first section, the fundamental issues of magnetocaloric effect are explained, including the thermodynamics of MCE and magnetic phase transitions. In addition, there is a description of the different methods and experimental tools commonly utilized to measure the MCE in real materials, as well as the two most commonly used criteria for modeling the temperature dependence of the adiabatic temperature and magnetic entropy changes in any material, which are based on the Mean Field Theory. The second section focuses on materials with promising behavior for practical applications that are fabricated with unlike geometries, from powders to thin ribbons or composites, and making use of different processing techniques to optimize their magnetocaloric properties. In the third section, the technological applications of these materials to a new generation of refrigeration systems and magnetic regenerators are discussed.

We have investigated the effects of quenching rate on the thermal dependence of the magnetic entropy change ΔSM(T) and the magnetic field induced hysteresis loss through the antiferromagnetic (AFM) ↔ ferromagnetic (FM) transformation in bulk Fe49Rh51. Two nearly identical square-prism-shaped samples were subjected to two different temperature cooling regimes; one was rapidly quenched (FQ) in iced-water and another slow cooled (SC) to room temperature at a cooling rate of 2 K/min. The temperature of the AFM ↔ FM transition is similar for both samples, but the FQ sample shows much sharper temperature- and magnetic field-induced magnetization change; in addition, the total magnetization change is 14% larger. In FQ material, the magnetocaloric effect, i.e., ΔSM(T) quickly approaches saturation above 1 T and shows a large peak value at 2 T (13.9 versus 8.9 Jkg−1 K−1 in SC material), but a larger average hysteresis loss FWHM in the temperature range coinciding with of the full-width at half-maximum of the ΔSM(T) curve.

Gd60Fe20Al20 microwires with an average diameter of ∼44 μm were fabricated by the melt-extraction method. XRD, TEM and HRTEM confirmed the amorphous nature of the microwires. The dimensional and chemical uniformity of the microwires over their length were confirmed by SEM and EDS, respectively. Magnetization measurements revealed a broad paramagnetic to ferromagnetic phase transition at TC ∼202 K. For μ0ΔH = 5 T, the microwires exhibit a broad magnetic entropy change with its maximum value ΔSMmax of ∼4.8 J kg−1 K−1 and a large refrigerant capacity (RC) of ∼687 J kg−1 over a large temperature interval (150 K). This RC value of the microwires is larger than those of the previously reported bulk and ribbon counterparts. An analysis of critical exponents reveals that γ = 1.246 ± 0.017 is close to that predicted by the 3D Ising theoretical model, while β = 0.723 ± 0.011 does not agree with any of the critical exponents predicted by the existing models. This can be attributed to the antiferromagnetic coupling between the rare earth (Gd) and the transition metal (Fe). The origins of the broad magnetic phase transition, the broad magnetic entropy change, and the large refrigerant capacity are discussed.

We report the magnetic and magnetocaloric properties of rapidly solidified Ni0.895Cr0.105MnGe1.05 melt-spun ribbons studied by both direct (adiabatic temperature change) and indirect (isothermal magnetic entropy change) methods in intermediate and high magnetic fields up to 10 T. The maximum values of the adiabatic temperature changes (ΔTad) and magnetic entropy changes ( Δ S M ) were found to be ∼2.6 K (µoH = 10 T) and 4.4 J kg−1 K−1 (µoΔH = 5 T), respectively, near the Curie temperature (TC). The ΔTad curves and magnetization isotherms were found to be completely reversible, which indicates the high degree of reversibility of the MCEs in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ΔTad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC, in accordance with the Landau theory of phase transitions.

We report on the structural, magnetic, and magnetocaloric properties and critical behavior of high-quality amorphous melt-extracted Gd50(Co69.25Fe4.25Si13B13.5)50 alloy microwires with an average diameter of 42 μm. The microwires undergo a second-order paramagnetic to ferromagnetic (PM-FM) transition around the Curie temperature, TC ∼ 174 K. Around the PM-FM phase transition temperature, the magnetic entropy change ΔSM reaches a maximum value ΔSMmax of about 5.9 J/kg K for a magnetic field change of 5 T. An iterative Kouvel-Fisher method was used to analyze the critical behavior of the second-order phase transition. The critical exponent values are β = 0.493 ± 0.005, γ = 1.321 ± 0.011, and δ = 3.678 ± 0.073, which are close to those expected for the mean-field model with long-range interactions below TC and for the 3D-Heisenberg model with short-range interactions above TC. The magnetic and magnetocaloric behaviors of the Gd alloy microwires near the PM-FM phase transition are discussed.

Magnetization studies were carried out to characterize the magnetic properties of the Iron-rich metallic glasses (Fe1−xCrx)80Si6B14 with 0.02≤x≤0.14. The Curie temperature TC diminishes almost linearly with the increase in the Cr-content from 401 K (x=0.10) to 291 K (x=0.14), while the saturation magnetization MS at T=5 K also undergoes a linear reduction from 169 Am2 kg−1 (x=0.02) to 87 Am2 kg−1 (x=0.14). These results suggest that the system should become paramagnetic for x≈0.22. The magneto-caloric properties of samples with TC near room temperature, i.e., with x=0.12 and 0.14, were investigated up to a maximum magnetic field change of 8 T. Both ribbons are characterized by a very broad temperature dependence of the magnetic entropy change ΔSM(T) and moderate peak values of 2.9 Jkg−1 K−1 and 2.6 Jkg−1 K−1, respectively.

X-ray photoelectron spectroscopy (XPS) was used to investigate the binding energies and valence band of the Nb 2 SnC and Nb 2 SC compounds. The Nb 3d 5/2 , Sn 3d 5/2 , S 2p 3/2 and C 1s core levels associated with the chemical states of Nb 2 SnC and Nb 2 SC were identified. The spectra for Nb 2 SnC revealed Nb and Sn oxides on the surface of the sample, mainly Nb 2 O 5 and SnO 2 , while the Nb 2 SC only Nb 2 O 5 oxide. After Ar + ion etching the intensity of the oxides decreased in both samples. Comparing the Nb 3d, Sn 3d, S 2p and C 1s core levels with metallic Nb, Sn, S and C reference materials, we observed a positive chemical shift for Nb 3d 5/2 and a negative chemical shift for C 1s in both samples. These results suggest that the charge transfer model can be applicable to the Nb 2 SnC and Nb 2 SC compounds. Finally, the decrease in the T c in the Nb 2 SC compound respect to Nb 2 SnC might be associated to decrease in the density of states N ( E F ).

We report the effect of the simultaneous inclusion of Ag1+ and Sr2+ cations on the crystal structure and the magnetocaloric effect (MCE) of La0.80(Ag1−xSrx)0.20MnO3 manganites (x = 0.0, 0.25, 0.50, 0.75, 1.0) synthesized by the solid-state reaction method. X-ray photoelectron spectroscopy was used to verify chemical composition and the amount of Mn3+–Mn4+ cations, while the magnetic performance was evaluated by means of a Physical Property Measuring System. The progressive substitution of Ag1+ by Sr2+ causes the increase of the number of Mn4+ cations, together with increasing ionic radii for the A-site contents up to x = 0.75, both favoring the enhancement of the double-exchange interaction and hence, the MCE. Excellent values of magnetic entropy change (−4.6 J/kg K, μoΔH = 5.0 T) were observed for the x = 0.25 manganite, while for the x = 0.50 sample interesting refrigerant capacity (129 J/kg, μoΔH = 2.0 T) and a wide interval of temperature at the full-width at half maximum δTFWHM of the magnetic entropy change curve were recorded (101 K, μoΔH = 2.0 T) caused by two successive magnetic transitions. Magnetic performance was explained in terms of the effect of the cation substitution on the double-exchange interaction and the tolerance factor.

Single-phase MnCoGe ribbons with an NiIn2-type structure were produced by using the melt-spinning technique. We reduced the annealing time for stabilizing the lower-symmetry orthorhombic TiNiSi-type crystal structure by two orders of magnitude compared with that of the parent bulk alloy. Both phases exhibit a second-order magnetocaloric effect, with Δ S M peak ≈ - 2.8 ( - 4.0 ) J kg - 1 K - 1 and RC ≈ 238 (281) J kg−1 for the maximum isothermal magnetic entropy change and refrigerant capacity, respectively, for a magnetic field change of 5 T.