First-order Magnetostructural Phase Transition Research Articles

Inverse Magnetocaloric Effect in Heusler Ni44.4Mn36.2Sn14.9Cu4.5 Alloy at Low Temperatures

Direct measurements of the magnetocaloric effect were performed in a Heusler Ni44.4Mn36.2Sn14.9Cu4.5 alloy at cryogenic temperatures in magnetic fields up to 10 T. The maximum value of the inverse magnetocaloric effect in a 10 T field was ∆Tad = –2.7 K in the vicinity of the first-order magnetostructural phase transition at T0 = 117 K. Ab initio and Monte Carlo calculations were performed to discuss the effect of Cu doping into a Ni-Mn-Sn compound on the ground-state structural and magnetic properties. It is shown that with increasing Cu content the martensitic transition temperature decreases and the Curie temperature of austenite slightly increases. In general, the calculated transition temperatures and magnetization values correlated well with the experimental ones.

Anomalous magnetic entropy changes of bulk and powder Mn0.5Fe0.5-xNi1+xSi0.94Al0.06 intermetallic system

The multi-component MnTX (T = Ni, Co; X = p-block elements) intermetallic system have attracted intensive attention due to their tunable magneto-structural phase transition and associated giant magnetocaloric effect. Here, we report an experimental study on the magnetic, magnetocaloric, and electrical properties of alloys that belong to the MnTX family- Mn0.5Fe0.5-xNi1+xSi0.94Al0.06. For all values of x (0≤ x ≤0.10), the materials exhibited a first-order magneto-structural phase transition from a low-temperature ferromagnetic orthorhombic phase to a high-temperature paramagnetic hexagonal phase. The transition was accompanied by large magnetic entropy changes of up to –22 and −57 J kg−1K−1 at T = 322 K for field changes of 2 T and 5 T, respectively. Most noticeably, a sharp jump in electrical resistivity was observed at T = 320 K for x = 0.1, which confirmed that the first-order phase transition preserved the mechanical stability of the material. The alloy with x = 0.1 was further crushed into powder and the magnetic properties were compared to its bulk counterpart. The enhanced mechanical stability and nearly zero magnetic hysteresis loss along with the fact that the system is constituted of cheap, non-toxic elements make this system worthy of consideration for near-room temperature magnetic refrigeration applications.

Температурно-барические особенности магнитных характеристик в системах со структурными переходами типа смещения

Using pnictides MnAs and germaindes Mn0.89Cr0.11NiGe as an example, the transformation of thermobaric features of their magnetic characteristics at high pressures is considered. A unified approach is used to describe paramagnetic (PM) structural transitions of the displacement type with a change in the symmetry PM(P63/mmc)-PM(Pnma) from hexagonal to orthorhombic. It is shown that the competition between the parameters of the structural and magnetic orders in both systems manifests itself differently in the stabilization and alternation of the so-called high-spin and low-spin magnetically ordered states initiated by pressure. As a consequence, the structural contribution in these systems weakens (MnAs) or enhances (Mn0.89Cr0.11NiGe ) the giant magnetocaloric effect in the temperature-baric region of the first-order magnetostructural phase transitions.

Temperature-baric features of magnetic characteristics in systems with structural transitions of the displacement type

Using pnictides MnAs and germaindes Mn0.89Cr0.11NiGe as an example, the transformation of thermobaric features of their magnetic characteristics at high pressures is considered. A unified approach is used to describe paramagnetic (PM) structural transitions of the displacement type with a change in the symmetry PM(P6_3/mmc)-PM(Pnma) from hexagonal to orthorhombic. It is shown that the competition between the parameters of the structural and magnetic orders in both systems manifests itself differently in the stabilization and alternation of the so-called high-spin and low-spin magnetically ordered states initiated by pressure. As a consequence, the structural contribution in these systems weakens (MnAs) or enhances Mn0.89Cr0.11NiGe the giant magnetocaloric effect in the temperature-baric region of the first-order magnetostructural phase transitions. Keywords: structural phase transitions of the displacement type, magnetostructural phase transitions, helimagnetism, ferromagnetism, high spin-low spin states, direct magnetocaloric effect.

Влияние долговременного воздействия циклических полей на магнитокалорические свойства сплава Гейслера Ni-=SUB=-43.18-=/SUB=-Mn-=SUB=-45.15-=/SUB=-In-=SUB=-11.67-=/SUB=-

The thermal expansion (Delta l/l0), magnetocaloric effect (Delta Tad) and the magnetostriction (λ) of a polycrystalline sample of the Ni43.18Mn45.15In11.67 Heusler alloy were studied. A correlation has been established between the magnetocaloric effect and magnetostriction. The smaller value of the MCE in the cooling run can be explained by the smaller contribution of the lattice subsystem to the total measured MCE. It is found that the long-term action of the cyclic magnetic field results in a decrease in the value of Delta Tad in the region of the first-order magnetostructural phase transition. The virgin properties of the alloy can be restored by transformation the sample to the austenite phase. The Heusler alloy Ni43.18Mn45.15In11.67 can be used as a working body in magnetic cooling technology, provided that the alloy periodically transforms to the austenite phase.

Simulation of a Hybrid Thermoelectric-Magnetocaloric Refrigerator with a Magnetocaloric Material Having a First-Order Transition

A simple hybrid thermoelectric-magnetocaloric (TE-MC) system is analytically and numerically simulated using the working parameters of commercial Peltier cells and the properties of a material with a first-order and low-hysteresis magneto-structural phase transition as La(Fe,Mn,Si)13H1.65. The need for a new master equation of the heat diffusion is introduced to deal with these materials. The equation is solved by the Crank–Nicolson finite difference method. The results are compared with those corresponding to a pure TE system and a pure MC system with ideal thermal diodes. The MC material acts as a heat “elevator” to adapt its temperature to the cold or hot source making the TE system very efficient. The efficiency of the realistic hybrid system is improved by at least 30% over the pure Peltier system for the same current supply and is similar to the pure MC with ideal diodes for the same cooling power.

Magnetocaloric properties of a ribbon sample of Heusler alloy Ni-=SUB=-45-=/SUB=-Co-=SUB=-5-=/SUB=-Mn-=SUB=-31-=/SUB=-Al-=SUB=-19-=/SUB=-: experimental and theoretical studies

The results of experimental and theoretical studies of magnetocaloric properties of a ribbon sample of alloy Ni45Co5Mn31Al19 in the range of T=80-350 K in magnetic fields of up to 8 T are given. This alloy demonstrates a first-order magnetostructural phase transition (MSPT) in the temperature range of 270 K, as well as a second-order transition --- at the Curie temperature of 294 K. The magnetocaloric effect (MCE) was studied both by the direct method of magnetic field modulation in cyclic fields up to 8 T and by the classical extractive method. Field dependence of MCE have a different nature for first- and second-order phase transitions. A reverse MCE near MSPT is irreversible, i.e. the final sample temperature is 0.75 K below the initial one. Theoretical studies of magnetic properties and MCE of the studied sample were performed by ab initio calculations and Monte Carlo modeling. Theoretical temperature dependences of MCE are characterized by a similar interval of effect manifestation in the region of martensitic transformation and a narrower interval in the region of austenite Curie temperature as compared to the experiments, which is conditioned by the presence of a heterogeneous mixed state of austenite in the experimental sample. On the whole, theoretical data qualitatively and quantitatively reproduce the experimental dependences. Keywords: magnetocaloric effect, cyclic fields, Heusler alloy, Monte Carlo method.

Significant reduction of phase-transition hysteresis for magnetocaloric (La1-xCex)2Fe11Si2Hy alloys by microstructural manipulation

First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change in La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope, three-dimensional atom probe and geometric phase analysis. For the LaFe13-xSix system via co-doping of Ce and H atoms, we have observed the appearance of nanograins in size range of 5 – 50 nm that is totally different from the widely reported compositions. Such refinement can be ascribed to the release of internal stress caused by the inhomogeneous distribution of hydrogen atoms. With the formation of the nanocrystals in (La1-xCex)2Fe11Si2Hy alloys, the value of hysteresis loss can be monotonously reduced from 48.3 to 0.6 J kg−1. More importantly, the magnetostructural transition keeps an obvious first-order type, which leads to a large adiabatic temperature change of 2.03 K in 1.3 T upon 105 magnetic cycles, as well as a high reversible refrigeration capacity of 89.4 J kg−1 for (La0.6Ce0.4)2Fe11Si2Hy.

Significant Reduction of Phase-Transition Hysteresis for Magnetocaloric (La 1-xCe x) 2Fe 11Si 2H y Alloys by Microstructural Manipulation

First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change for La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope (HAADF-STEM), three-dimensional atom probe (3DAP) and geometric phase analysis (GAP). For the LaFe13-xSix system via co-doping of Ce and H atoms, we have observed the appearance of nano-sized grains (5 ~ 50 nm) that is totally different from the conventional compositions. Such refinement can be ascribed to the release of internal stresses caused by the inhomogeneous distribution of hydrogen atoms. With the increase of Ce content for (La1-xCex)2Fe11Si2Hy alloys, the value of hysteresis losses can be significantly reduced from 48.3 to 0.6 J kg-1. More importantly, the magnetostructural transition keeps the obvious first-order type, which leads to a large adiabatic temperature change of 2.2 K in 1.3 T upon 105 magnetic cycles, as well as a high refrigeration capacity of 89.4 J kg-1.

Raman scattering study of lattice and magnetic excitations in CrAs

We report on the lattice and spin dynamics in the monopnictide CrAs using polarized Raman scattering. This system exhibits a first-order magneto-structural phase transition at T_N~ 265 K, below which the magnetic moments of Cr form an incommensurate double helical magnetic structure while the unit cell volume abruptly expands. Across the transition, the frequencies of the fully symmetric Ag phonons strongly renormalize, which along with the results of first-principle calculations suggest the presence of a sizeable coupling of the phonons to the magnetic degrees of freedom in this compound. In addition, we observe two broad modes around 350 and 1700 cm-1 in the magnetic phase, which we associate with two-magnon Raman scattering. This work provides one of the few examples of magnetic Raman scattering from a non-collinear magnet.

Giant and Reversible Inverse Barocaloric Effects near Room Temperature in Ferromagnetic MnCoGeB0.03.

Hydrostatic pressure represents an inexpensive and practical method of driving caloric effects in brittle magnetocaloric materials, which display first-order magnetostructural phase transitions whose large latent heats are traditionally accessed using applied magnetic fields. Here, moderate changes of hydrostatic pressure are used to drive giant and reversible inverse barocaloric effects near room temperature in the notoriously brittle magnetocaloric material MnCoGeB0.03 . The barocaloric effects compare favorably with those observed in barocaloric materials that are magnetic. The inevitable fragmentation provides a large surface for heat exchange with pressure-transmitting media, permitting good access to barocaloric effects in cooling devices.

Improved magnetostructural and magnetocaloric reversibility in magnetic Ni-Mn-In shape-memory Heusler alloy by optimizing the geometric compatibility condition

We report an improved reversibility of magnetostriction and inverse magnetocaloric effect (MCE) for the magnetic shape-memory Heusler alloy Ni$_{1.8}$Mn$_{1.8}$In$_{0.4}$. We show that the magnetostriction and MCE crucially depends on the geometrical compatibility of the austenite and martensite phases. Detailed information on the compatibility of both phases has been obtained from the transformation matrix calculated from x-ray diffraction data. The uniqueness of the lattice parameters results in an improved reversibility of the magnetostriction and the MCE. In the thermal hysteresis region of the martensitic transformation, the maximum relative length change is 0.3% and the adiabatic temperature change $\Delta T_{ad}\approx -10$ K in pulsed magnetic fields. Our results reveal that the approach of geometric compatibility will allow one to design materials with reversible magnetostriction and reversible inverse MCE at a first-order magnetostructural phase transition in shape-memory Heusler alloys.

Reversible adiabatic temperature change in the shape memory Heusler alloyNi2.2Mn0.8Ga: An effect of structural compatibility

The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based on the analysis of our detailed temperature dependent X-ray diffraction data, we demonstrate the geometric compatibility of the cubic austenite and tetragonal martensite phases. This finding directly relates the reversible MCE behavior to an improved geometric compatibility condition between cubic austenite and tetragonal martensite phases. The approach will help to design shape-memory Heusler alloys with a large reversible MCE taking advantage of the first-order martensitic phase transition.

On entropy determination from magnetic and calorimetric experiments in conventional giant magnetocaloric materials

In this work, we discuss measurement protocols for the determination of the magnetic entropy change associated with first-order magneto-structural transitions from both magnetization and calorimetric experiments. The Cu-doped Ni2MnGa Heusler alloy with a first-order magneto-structural phase transition is used as a case study to illustrate how commonly-used magnetization measurement protocols result in spurious entropy evaluations. Two magnetization measurement protocols which allow for the accurate assessment of the magnetic entropy change across first-order magneto-structural transitions are presented. In addition, calorimetric measurements were performed to validate the results from the magnetization measurements. Self-consistent results between the magnetization and calorimetric measurements were obtained when the non-equilibrium thermodynamic state was carefully handled. Such methods could be applicable to other systems displaying giant magnetocaloric effects caused by first-order phase transitions with magnetic and thermal hysteresis.

Magnetic, thermal, and electrical properties of an Ni45.37Mn40.91In13.72 Heusler alloy

The magnetization, the electrical resistivity, the specific heat, the thermal conductivity, and the thermal diffusion of a polycrystalline Heusler alloy Ni45.37Mn40.91In13.72 sample are studied. Anomalies, which are related to the coexistence of martensite and austenite phases and the change in their ratio induced by a magnetic field and temperature, are revealed and interpreted. The behavior of the properties of the alloy near Curie temperature TC also demonstrates signs of a structural transition, which suggests that the detected transition is a first-order magnetostructural phase transition. The nontrivial behavior of specific heat detected near the martensite transformation temperatures is partly related to a change in the electron density of states near the Fermi level. The peculiar peak of phonon thermal conductivity near the martensitic transformation is interpreted as a consequence of the appearance of additional soft phonon modes, which contribute to the specific heat and the thermal conductivity.

Differences of the magnetic hysteresis behaviors in the magnetostructural transformation Mn1−xVxCoGe alloys

The magnetic hysteresis behaviors and the magnetocaloric properties of the Mn1−xVxCoGe (x=0.01 and 0.02) alloys in the heating and cooling runs have been investigated comparatively in the vicinity of the first-order magnetostructural phase transition. The magnetic hysteresis behaviors reveal an evident discrepancy between the heating and cooling processes. The average hysteresis losses estimated from the initial magnetization isotherms M(H) measured upon cooling are much larger than that upon heating. Considering the thermodynamics conditions and the potential barrier of phase transition, we analyzed the possible origin for this discrepancy and discussed the effects on the magnetocaloric properties for these two alloys.

Theoretical treatment and direct measurements of magnetocaloriceffect in Ni2.19−xFexMn0.81Ga Heusler alloys

Magnetocaloric effect in a series of Ni2.19−xFexMn0.81Ga (x=0.01–0.04) Heusler alloys has been studied experimentally by direct measurements of the adiabatic temperature change ΔTad and theoretically by means of classical Monte Carlo simulations. The direct measurements of ΔTad have shown that the adiabatic temperature change measured in the vicinity of the first-order magnetostructural phase transition and at the Curie temperature of austenite TC is practically the same, ΔTad≈1.6K, for the magnetic field change ΔHext=2T. On the basis of theoretical methods, the magnetic exchange interaction constants, the temperature dependencies of the magnetization and the adiabatic temperature change have been calculated. Results of the simulations are in a fair agreement with the experimental data.

Giant magnetocaloric effects near room temperature in Mn1 − xCuxCoGe

Magnetic and structural transitions were found to coincide by substituting Cu for Mn in Mn1−xCuxCoGe, leading to a giant magnetocaloric effects associated with the first-order magnetostructural phase transition from the ferromagnetic to paramagnetic state. Maximum magnetic entropy changes of −ΔSM = 52.5 and 53.3 J/kg K for a magnetic field change of ΔH = 5 T have been observed at 302 and 316 K, for x = 0.080 and 0.085, respectively. The giant magnetocaloric effects with tunable phase transition temperatures through subtle variations in composition make this system promising for room-temperature magnetic cooling applications.

Magnetization studies of first-order magnetostructural phase transition in polycrystalline FeRh thin films

The nucleation and growth of the transformed phase in the matrix of the original phase played an important role in the progress of magnetic transition. In spite of extensive investigations in B2 ordered FeRh alloy systems, until now few studies have been conducted for clarifying the nucleation and growth mechanism of the antiferromagnetic–ferromagnetic phase transition in FeRh alloys. In this work, B2 ordered polycrystalline FeRh thin films were fabricated on glass substrates by a sputtering technique and subsequent heat treatment. The as-deposited film shows a nonmagnetic property because of its face centred cubic structure. After annealing, the polycrystalline FeRh thin films show a clear first-order magnetostructural phase transition. The FeRh thin film shows an overall activation energy of about 228.6 kJ mol−1 for the entire first-order magnetostructural phase transition process. Results suggest that the first-order magnetostructural phase transition in ordered FeRh thin films follows the Johnson–Mehl–Avrami model with characteristic exponent n in the range 1–4, indicating that the phase transition process is a multi-step process characterized by different nucleation and growth mechanisms of the new ferromagnetic phase. The results obtained in this study will shed light on the underlying physics of the first-order magnetostructural phase transition of ordered FeRh alloys. The applicability of the concepts used in this study to the FeRh system shows universality and can be applied to other material systems where there is a first-order magnetostructural phase transition such as in manganites.

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature TC = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature TC is equal to ΔT ∼ 13 K. A model of the superparamagnetic state in MnAs above the temperature TC has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to TC, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.