First-order Magnetic Transition Research Articles

Structure, Microstructure and Magnetocaloric/Thermomagnetic Properties at the Early Sintering of MnFe(P,Si,B) Compounds

Minimizing the sintering time while ensuring high performances is an important optimization step for the preparation of magnetocaloric or thermomagnetic materials produced by powder metallurgy. Here, we study the influence of sintering time on the properties of a Mn0.95Fe1P0.56Si0.39B0.05 compound. In contrast to former reports investigating different annealing temperatures during heat treatments of several hours or days, we pay special attention to the earliest stages of sintering. After ball-milling and powder compaction, 2 min sintering at 1100 °C is found sufficient to form the desired Fe2P-type phase. Increasing the sintering time leads to a sharper first-order magnetic transition, a stronger latent heat, and usually to a larger isothermal entropy change, though not in all cases. As demonstrated by DSC or magnetization measurements, these parameters present dissimilar time evolutions, highlighting the existence of various underlying mechanisms. Chemical inhomogeneities are likely responsible for broadened transitions for the shortest sinterings. The development of strong latent heat requires longer sinterings than those for sharpening the magnetic transition. The microstructure may play a role as the average grain size progressively increases with the sintering time from 3.5 μm (2 min) to 30.1 μm (100 h). This systematic study has practical consequences for optimizing the preparation of MnFe(P,Si,B) compounds, but also raises intriguing questions on the influence of the microstructure and of the chemical homogeneity on magnetocaloric or thermomagnetic performances.

Controlling phase transitions in MnNiGe using thermal quenching and hydrostatic pressure

Abstract The phase transitions in MnNiGe compounds were explored by manipulating the heat treatment conditions and through hydrostatic pressure application. As the quenching temperature increased, both the first-order martensitic structural transition temperatures and magnetic transition temperatures decreased relative to those in the slowly-cooled samples. When the samples were quenched from 1200 ℃, the first-order martensitic structural transition temperature lowered by more than 200 K. The structural transitions also shifted to lower temperature with the application of hydrostatic pressure during measurement. Temperature-dependent X-ray diffraction results reveal
that the changes of the cell parameters resulting from the structural transitions are nearly identical for all samples regardless of the extensive variation in their structural transition temperatures. In addition, neutron scattering measurements confirm the magnetic structure transition between simple and cycloidal spiral magnetic structures.

Magnetic properties and large cryogenic magnetocaloric effects in the RE2SiO5 (RE = Gd, Dy, Ho, and Er) silicates

We herein investigated four rare earth (RE)-based oxides, the RE2SiO5 (RE = Gd, Dy, Ho, and Er) silicates, regarding their structural and cryogenic magnetic properties. All the RE2SiO5 silicates are found to crystallized in a monoclinic structure. X-ray photoemission spectroscopy spectra illustrate that RE, Si and O elements in RE2SiO5 silicates are present as RE3+, Si4+, and O2−, respectively. The Dy2SiO5 reveals a first-order magnetic transition around 4.5 K. The magnetic phase transition temperatures are probably below 2 K for the RE2SiO5 (RE = Gd, Ho, and Er) silicates. Large cryogenic magnetocaloric effects (MCE) and good magnetocaloric performances were observed in the RE2SiO5 silicates. The MCE parameters in terms of the maximum magnetic entropy changes, the temperature-averaged entropy change (5 K-lift) and refrigerant capacity under a magnetic field change of 0–5 T are determined to be 35.5, 30.4 J/kgK, and 215.7 J/kg for Gd2SiO5, to be 13.3, 12.9 J/kgK, and 190.5 J/kg for Dy2SiO5, to be 15.3, 15.1 J/kgK, and 274.1 J/kg for Ho2SiO5 and to be 17.9, 16.5 J/kgK, and 181.14 J/kg for Er2SiO5, respectively. These derived values of MCE parameters of the RE2SiO5 silicates, especially for Gd2SiO5, are at similarly high level with or better than those of the updated candidate materials, making them also of potential for practical magnetic refrigeration applications.

Excellent thermomagnetic power generation for harvesting waste heat via a second-order ferromagnetic transition.

Thermomagnetic generation (TMG), a promising technology to convert low-grade waste heat to electricity, utilizes high performance TMG materials. However, the drawbacks of large hysteresis, poor mechanical properties and inadequate service life hinder the practical applications. For the first time, we evaluated the effect of different phase transitions on the TMG performance by systematically comparing the TMG performance of three typical Heusler alloys with similar composition but different phase transitions. Ni2Mn1.4In0.6 exhibits second-order magnetic transition (SOMT) from the ferromagnetic (FM) to paramagnetic (PM) state around TC = 316 K without thermal hysteresis. It presents highly comprehensive TMG performance, which is not only better than those of other two Heusler alloys with different phase transitions, but also better than those of most typical TMG materials. The maximum power density (1752.3 mW m-3), cost index (2.78 μW per €), and power generation index PGI (8.91 × 10-4) of Ni2Mn1.4In0.6 are 1-5, 1-4, and 1-7 orders of magnitude higher than those of most typical reported materials, respectively. In addition, Ni2Mn1.4In0.6 with SOMT also shows some advantages that first-order magnetic transition (FOMT) materials do not have, such as zero hysteresis and a long-term service life. In contrast to the short lifetime of a few minutes for the materials with FOMT, Ni2Mn1.4In0.6 with SOMT can serve for one month or even longer with excellent cycling stability. Consequently, we conclude that the SOMT Ni2Mn1.4In0.6 Heusler alloy with good TMG performance as well as zero hysteresis and long service life can be a better candidate than FOMT materials for practical applications of TMG.

Enhanced magnetocaloric effect accompanying successive magnetic transitions in TbMn2Si2-xGex compounds

The lack of magnetic refrigeration (MR) materials with high magnetocaloric effect (MCE) and large relative cooling power (RCP) in the temperature range required for hydrogen liquefaction (20 K–77 K) is a bottleneck for practical applications of MR cooling systems. The present investigation of TbMn2Si2-xGex compounds (x = 0.1, 0.2) by variable temperature neutron and synchrotron X-ray diffraction, magnetization and heat capacity measurements, establish that substitution of Si with Ge in TbMn2Si2 leads to a significant enlargement of the unit cell and modification of the magnetic properties. Two consecutive ferromagnetic first-order transitions occur below 77 K with the third transition from paramagnetism to a collinear antiferromagnetic state being determined around 500 K. The resultant plateau-like MCE with large RCP below 77 K in these designed compounds offers scope for application for hydrogen liquefaction. Detailed neutron investigation confirm that four magnetic states exist within the temperature range 5 K to 500 K, with two successive first-order magnetic transitions below 77 K responsible for the large MCE. Our specific heat studies provide evidence of strong contributions from the nuclear specific heat and the corresponding nuclear specific heat coefficients of A = 430 ± 50 mJ mol−1 K and A = 418 ± 60 mJ mol−1 K have been determined for TbMn2Si2-xGex with x = 0.1 and x = 0.2, respectively. The overlapping entropy curves near these successive transitions lead to a plateau-like magnetothermal effect as well as a large reversible MCE for both samples (e.g. ΔSMmax = 14.0 J/kg K and ΔTmax = 7.6 K; RCP = 379 J/kg for TbMn2Si1.9Ge0.1 for an applied field of 5 T) indicating that the material can operate over a wide temperature range – particularly for hydrogen liquefaction.

Burst-like features observed from different techniques at the first-order magnetic transition of La(Fe,Co,Si)13

La(Fe,Si)13 alloys are prone to display burst-like phenomena at their first-order ferromagnetic transition, such as a latent heat decomposed into a succession of heat flux avalanches. For some extreme cases, these phenomena are so pronounced that they can yield recalescence/antirecalescence events. It turns out that these features are also highly sensitive to the technical specificities of the employed measurement systems. Here, we compare the thermal events at the first-order magnetic transition of LaFe11.4Si1.31Co0.29 using magnetization, two differential scanning calorimeters and a thermometry probe specially designed for the present study. These techniques are complementary to each other, and allow to make a distinction between the properties influenced by thermal coupling and those more intrinsic to the compound or to the sample. A simple thermal model is built to account for the thermal lags in the different devices, and to provide insights into the dependence of the duration of the transition on the thermal sweep rate. Looking in closer detail at the development of the transition, we found evidence of a rate independent initial regime, followed by a second one along which the same shape of temporal profile is spread over a more or less long duration depending on the sweep rate. Possible underlying mechanisms corresponding to these regimes are discussed.

Influence of cold compaction pressure on intergranular secondary phase distribution and magnetocaloric/thermomagnetic performances of MnFe(P,Si,B) compounds

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 MnFe0.95P0.575Si0.36B0.065 compounds prepared by powder metallurgy, resulting in an enhancement of the isothermal entropy change (ΔS) in 1 T up to ∼40 %. This outcome is unexpected as ΔS 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.

Itinerant ferromagnetism and room temperature giant magnetocaloric effect in (Mn0.6Fe0.4) (Ni0.5Co0.5) Si shape memory alloy

In this article, the feasibility of obtaining a magneto-structural coupling over a wide working temperature range of about 127 K (from 206.5 K to 333.5 K) with negligible thermal hysteresis in combination with the structural, microstructural, magnetic, and magnetocaloric properties of a 50 % Co-doped (at the Ni site) polycrystalline (Mn0.6Fe0.4) (Ni0.5Co0.5) Si (MFNCS-0.5) magnetic shape memory alloy (MSMA) has been thoroughly investigated. Along with leaf-like microstructure, at room temperature, the system shows a mixed phase of TiNiSi type low-temperature high-magnetic (ferromagnetic) orthorhombic martensite and Ni2In type high-temperature low-magnetic (paramagnetic) hexagonal austenite, with the dominance of the second one. To estimate the magnetocaloric effect (MCE) across the phase transition, the isothermal magnetic measurement technique has been employed. The merging of heating and cooling iso-field thermo-magnetization (M-T) curves, the presence of negligible thermal hysteresis in the MH curves, universal magnetic entropy curves, a proposed phenomenological model (across the Curie temperature), and the positive slopes of the Arrott plot (Banerjee criterion) show that the first-order structural and magnetic transition are coupled along with a second-order ferromagnetic to paramagnetic (FM → PM) phase transition (SOPT). By extrapolating the Arrott plot to the H/M = 0 axis the spontaneous magnetization value very close to 0 K, Ms(0), is found to be 71.92 emu/g (∼1.83 µB/f.u.). The linear dependency of MS2T on T2 below SOPT and the RWR (Rhodes–Wolhfarth ratio) = 4.29 > 1 indicate the long-range itinerant ferromagnetism in this alloy. A giant magnetic entropy change (ΔSm) of 25.77 J kg−1 K−1 and a refrigerant capacity (RC) of 472.83 J kg−1 are obtained at 6 T field. This observed magnetic refrigerant's key characteristics, including a giant magnetocaloric effect (MCE) near room temperature (RT), negligible thermal hysteresis loss, a wide operating temperature range of 127 K, and the use of cost-effective, non-toxic constituent elements, make it a suitable and very promising member for applications in solid-state refrigeration technology near RT.

Room-temperature magnetocaloric properties of (La1-xNdx)2Fe10.7Mn0.3Si2Hy

Magnetocaloric properties of (La1-xNdx)2Fe10.7Mn0.3Si2Hy (x = 0.1 and 0.2) are evaluated to develop a room-temperature magnetic refrigerant. While Nd which is introduced to reduce the thermal hysteresis has a negligible effect on magnetic properties, the partial replacement of Fe with Mn converted the first-order magnetic transition to a second-order and TC is reduced by 50 K to near room temperature. The peak entropy change, ΔSM|pk values are 10.4 J kg−1 K−1 (at 290 K) for x = 0.1 and 10.6 J kg−1 K−1 (at 286 K) for x = 0.2 with μ0∆H = 3 T. The maximum values of the adiabatic temperature change, ΔTad are 4.11 K (at 283 K) for x = 0.1 and 4.05 K (at 289 K) for x = 0.2 with μ0∆H = 1.4 T. Because TC can be shifted by the addition of Mn to room temperature, this series of compounds can be used as an efficient magnetic refrigerant or a combination as a composite refrigerant for an active magnetic refrigeration cycle.

Magnetization steps at the ferromagnetic transition of (Mn,Fe)2(P,Si) single crystals

Single crystals are highly desirable to study intrinsic properties of phase transitions, unfortunately due to an embrittlement at the transformation, (Mn,Fe)2(P,Si) studies were so far limited to polycrystalline samples. Here, we report the growth of (Mn,Fe)2(P,Si) single crystals which can be cycled across their first-order ferromagnetic transition. In single crystals with optimized composition, detailed magnetization measurements reveal a ferromagnetic transition whose shape is significantly different from polycrystalline samples, with unusual discontinuous magnetization steps induced by temperature or magnetic field leading to a stair-like transition. In contrast to the “virgin effect” of (Mn,Fe)2(P,Si) samples, this behavior is reproducible. This unique magnetic response of some (Mn,Fe)2(P,Si) crystals can be correlated to a microstructure very different from that of the bulk and preserving the clamping/release role played by strains and stresses at the ferro-paramagnetic transformation. On top of offering more details on how the first-order magnetic transition develops in time, space and as a function of the driving parameter, which is of importance for designing functional magnetocaloric materials, the growth of cyclable crystals opens new possibilities for future experiments.

A multi-stage, first-order phase transition in LaFe11.8Si1.2: Interplay between the structural, magnetic, and electronic degrees of freedom

Alloys with a first-order magnetic transition are central to solid-state refrigeration technology, sensors and actuators, or spintronic devices. The discontinuous nature of the transition in these materials is a consequence of the coupling between the magnetic, electronic, and structural subsystems, and such transition can, in principle, cross several metastable states, where at one point, the transition takes place within the magnetic subsystem, while at another, the changes occur in the structural or electronic subsystems. To address this issue, we conducted simultaneous measurements of the macroscopic properties—magnetization, temperature change of the sample, longitudinal, and transversal magnetostrictions—to reveal the rich details of the magneto-structural, first-order transition occurring in the prototypical alloy LaFe11.8Si1.2. We found that the transition does not complete in one but in two distinct stages. The presence of the intermediate state changes the potential-energy landscape, which then impacts strongly on the width of the hysteresis associated with the first-order transition. We complement these findings with experiments on the atomistic scale, i.e., x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and Mössbauer spectroscopy, and then combine them with first-principles calculations to reveal the full complexity and two-stage nature of the transition. This new approach can be successfully extended to a large class of advanced magnetic materials that exhibit analogous transformations.

Evidence of a first-order magnetic transition in HoPtSn

Evidence of a first-order magnetic transition in HoPtSn

Hall effect of RCo2 (R= Er and Lu) compounds

The Hall effect of RCo2 with R = Er and Lu has been studied. LuCo2 is an exchange-enhanced Pauli paramagnet. ErCo2 is ferrimagnetic below TC = 32 K and undergoes a first-order magnetic transition at TC. The Hall resistivity ρxy was measured as a function of the magnetic field B at various temperatures. We estimated the normal (ordinary) Hall coefficient RN and the anomalous Hall coefficient RA from the analyses of the ρxy – B curves and the magnetization curves. For LuCo2, ρxy increases linearly with increasing field and its slope decreases rapidly with increasing temperature below 70 K. This is attributable to strong temperature dependence of RN. The ρxy – B curve of ErCo2 depends on a magnetic state. In the ferrimagnetic state, both RN and RA have positive values, while RN is positive and RA is negative in the paramagnetic state. We calculated RN's of nonmagnetic and ferromagnetic LuCo2 from the energy band structures and compared them with the experimental values. The origin of a sign change in RA of ErCo2 is briefly discussed.

Magnetic Refrigeration: An Environment-friendly Cooling Technology

Magnetic Refrigeration: An Environment-friendly Cooling Technology

Competing magnetic interactions, structure and magnetocaloric effect in Mn3Sn1-xZnxC antiperovskite carbides

Structural, magnetic and magnetocaloric properties of Mn3Sn1-xZnxC antiperovskite carbides have been studied. With increasing Zn content the first-order magnetic transition (FOMT) is weakened. The Curie temperature (TC) reduces first from 273 to 197 K and when x > 0.3, TC increases, reaching its maximum of 430 K for x = 1.0. An increase in TC is accompanied by pronounced changes in magnetic behaviour and a significant rise in magnetization from 21.82(4) to 76.2(2) Am2kg−1 for x = 0.8 in the maximum applied magnetic field of 5 T. Neutron powder diffraction (NPD) was employed to study the magnetic structure of Mn3Sn1-xZnxC compounds. The refinement of the NPD data for x = 0.3 revealed a magnetic structure with propagation vector k = (½,½,0) with a decrease in the canted antiferromagnetic (AFM) moment, which results in a reduction of the negative volume change at the magnetic transition and a decrease in the magnetocaloric effect (MCE). For x = 0.4, the magnetic structure is described by a propagation vector k = (½,½,½) for the AFM moment which dominates at low temperature, with the presence of a minor ferromagnetic (FM) component with a k = (0, 0, 0) propagation vector, which confirms the presence of the ferrimagnetic (FiM) state. For a higher Zn content (x = 0.6), the magnetic moment originates mainly from the FM component found on three independent Mn positions and an additional AFM moment oriented in the a-b plane. The results presented confirm the presence of competing AFM-FM interactions in Mn3Sn1-xZnxC antiperovskite carbides.

Landau theory-based relaxational modeling of first-order magnetic transition dynamics in magnetocaloric materials

The magnetocaloric effect is often largest within the neighborhood of a first-order phase transition. This effect can be utilized in magnetocaloric refrigeration, which completely eliminates the need for the greenhouse gases utilized in conventional refrigeration. However, such transitions present unique dynamical effects and are accompanied by hysteresis, which can be detrimental for such refrigeration applications. In this work, a Landau theory-based relaxational model is used to study the magnetic hysteresis and dynamics of the first-order magnetic transition of LaFe13−x Si x . Fitting the experimental magnetization data as a function of applied magnetic field under different field sweep rates with this model provided the Landau parameters (A, B, and C) and the kinetic coefficient of the studied material. We demonstrate the tendency of the magnetic hysteresis to increase with the magnetic field sweep rate, underlining the importance of studying and minimizing the magnetic hysteresis in magnetic refrigerants at practical field sweep rates. While evaluating the temperature dependence of the time required for a complete transition to occur, a nonmonotonic behavior and a sharp peak were found for temperatures near the transition temperature. Such peaks occur at the same temperature as the peak of the magnetic entropy change for low fields, whereas for higher fields the two peaks decouple. This information is critical for technological applications (such as refrigerators/heat pumps) as it provides guidelines for the optimization of the magnetic field amplitude in order to reduce the transition timescale and consequently maximize the machine operational frequency and amount of heat that is pumped in/out per second.

Influence of the particle size on a MnFe(P,Si,B) compound with giant magnetocaloric effect

How the microstructure affects first-order magnetic transitions (FOMT) in materials with giant magnetocaloric effect remains poorly understood. Here, we study the FOMT and giant magnetocaloric effect occurring near room temperature in MnFe0.95P0.575Si0.36B0.065 particles with sizes ranging from 300 μm down to less than 15 μm. While this materials system shows a volume preserving FOMT, large anisotropic lattice discontinuities make it particularly sensitive to particle size. Grinding and sieving may lead up to 80% difference on the isothermal entropy change (ΔS). Differential scanning calorimetric measurements reveal that the decrease in ΔS does not only originate from the broadening of the transition but also involves a sudden drop in latent heat when particles are reduced from 54 μm to 31 μm, a range corresponding to about the average grain size of the bulk (26 μm). Thermal hysteresis is the largest in large particles and decreases when reducing the particle size.

Magnetotransport and magnetic properties of Cr-modified Mn2Sb epitaxial thin films.

High-quality Mn2-xCrxSb (x = 0.01, 0.04, and 0.1) epitaxial thin films were grown on SrTiO3 (STO) (001) single-crystal substrates using molecular beam epitaxy. Magnetotransport and magnetic measurements reveal that the x = 0.01 sample undergoes a quasi-ferrimagnetic (I) [Q-FIM(I)]-to-ferrimagnetic (II) [FIM(II)] spin reorientation (SR) transition and a giant magnetoresistance (MR) associated first-order ferrimagnetic(II)-to-antiferromagnetic (AFM) phase transition upon cooling, resulting in the AFM ground state with a weak in-plane net moment. Upon increasing the doping level from x = 0.01 to 0.1, both the SR transition and the first-order magnetic transition are suppressed. For x = 0.1, the former transition is suppressed, leaving only the Q-FIM(I)-to-AFM transition within the whole temperature region. TAFM-FIM shows almost similar changes upon the application of either in-plane or out-of-plane magnetic fields. TAFM-FIM values of the x = 0.01 and 0.04 samples are much higher than those of the Mn2-xCrxSb bulk with similar doping levels, which can be understood by the clamping effect from STO substrates. For each thin-film sample, the MR effect is observed near TAFM-FIM and disappears in the high temperature Q-FIM(I) phase and low temperature AFM phase, indicating that MR is related to the spin-dependent electron scattering during the first-order magnetic phase transition. Based on the magnetotransport and magnetic data, a magnetic phase diagram is established for the Mn2-xCrxSb films in the low doping level region.

Correlation between deformation and total entropy change at the first-order magnetic transition of Fe2P-based magnetocaloric materials

While there is a growing consensus that lattice is an important source of entropy change at structural or elastic first-order magnetic transitions (FOMT) with finite volume change, the case of volume preserving transitions (ΔV = 0) remains more controversial. Here, the evolution of the total transition entropy change within the Fe2P family is presented as a function of the discontinuity on the ratio of the cell parameters. A striking correlation between ΔStr and Δ(c/a) is observed. This relationship is a first step towards a systematization of the FOMT properties in the Fe2P materials system. It also demonstrates that deformation can play a leading role in volume preserving FOMT.

Effect of magnetic phase coexistence on spin-phonon coupling and magnetoelectric effect in polycrystalline Sm0.5Y0.5Fe0.58Mn0.42O3

The various physical properties with their intercorrelations in polycrystalline samples of Sm0.5Y0.5Fe0.58Mn0.42O3 [SYFM (58–42)] were investigated. The dc magnetization measurements revealed a weak ferromagnetic (WFM) transition at TÑ361 K that is followed by an incomplete spin reorientation (SR) transition at TSR1∼ 348 K. A first order magnetic transition (FOMT) around TSR2 ∼ 292 K completes the spin reorientation transition and the material enters into a nearly collinear antiferromagnetic (AFM) state. Robust magnetodielectric (MD) is found to be present in material at room temperature. True ferroelectric transition with TFE = 108 K and having a value of saturation polarization (∼0.06 μC/cm2 at 15 K) have been found in the specimen. This spin phonon coupling (SPC) stabilizes the ferroelectric state and responsible for magnetoelectric coupling (ME). We argue that wave vector (q) dependence of the spin-pair correlation affects the electronic and magnetodielectric properties in a similar way.