First-order Magnetic Phase Transition Research Articles

Fine tuning of Mn/Co vacancies for optimized magnetocaloric performance in MnCoGe alloys

As a family of rare-earth-free magnetocaloric materials, MnCoGe-based alloys have attracted intensive attention in the past few years. In this study, we reveal that the deficiency of either Mn or Co reduces the valence electron concentration of MnCoGe system, thereby lowering its structural transition temperature (Ttr) by pushing the Fermi level away from the Mn-Mn antibonding electronic states. Moreover, Ttr is found to be more sensitive to Co content. By systematically analyzing the temperature dependent magnetization plots and Arrott curves, it is proven that the magnetic and structural degrees of freedom are coupled in the Mn0.97CoGe alloy, resulting in a first-order magnetic phase transition. On the contrary, MnCo0.98Ge undergoes a second-order transition. As a consequence, the Mn0.97CoGe alloy exhibits a maximum magnetic entropy change of 27.6 J kg−1K−1 under a magnetic field of 5 T near the Curie temperature of 265 K, corresponding to a refrigeration capacity of 212.1 J kg−1. Our work demonstrates Mn/Co vacant MnCoGe alloy as a robust candidate of magnetic substance for room temperature magnetocaloric refrigeration.

Metamagnetic multiband Hall effect in Ising antiferromagnet ErGa2

Frustrated rare-earth-based intermetallics provide a promising platform for emergent magnetotransport properties through exchange coupling between conduction electrons and localized rare-earth magnetic moments. Metamagnetism, the abrupt change of magnetization under an external magnetic field, is a signature of first-order magnetic phase transitions; recently, metamagnetic transitions in frustrated rare earth intermetallics have attracted interest for their accompanying nontrivial spin structures (e.g., skyrmions) and associated nonlinear and topological Hall effects (THE). Here, we present metamagnetism-induced Hall anomalies in single-crystalline ErGa2, which recalls features arising from the THE but wherein the strong Ising-type anisotropy of Er moments prohibits noncoplanar spin structures. We show that the observed anomalies are neither due to anomalous Hall effect nor THE; instead, can be accounted for via 4f-5d interactions which produce a band-dependent mobility modulation. This leads to a pronounced multiband Hall response across the magnetization process-a metamagnetic multiband Hall effect that resembles a topological-Hall-like response but without nontrivial origins. The present findings may be of general relevance in itinerant metamagnetic systems regardless of coplanar/noncoplanar nature of spins and are important for the accurate identification of Hall signals due to emergent magnetic fields.

The large magnetocaloric effect in GdErHoCoM (M = Cr and Mn) high-entropy alloy ingots with orthorhombic structures

High-entropy alloys (HEAs) with significant magnetocaloric effects (MCEs) have attracted widespread attention due to their potential magnetic refrigeration applications over a much more comprehensive temperature range with large refrigerant capacity (RC). However, most of them are metallic glasses (MGs) with problems of limited size, resulting in the difficulty of further applications. Therefore, research on HEAs with crystalline structures and giant MCE is urgently needed. In this paper, GdErHoCoM (M = Cr and Mn) rare-earth HEA ingots with orthorhombic structures are developed, and their magnetic behavior and MCE are studied in detail. Phase investigations find that the main phase of GdErHoCoM ingots is probably (GdErHo)Co with an orthorhombic Ho3Co-type structure of a space group of Pnma. The secondary phases in GdErHoCoCr and GdErHoCoMn are body-center-cubic Cr and Mn-rich HoCo2-type phases, respectively. Magnetic investigations reveal that both ingots undergo a first-order magnetic phase transition below their respective Neel temperatures. Above their respective Neel temperatures, a second-order transition is observed. The Neel temperatures are 40 and 56 K for GdErHoCoCr and GdErHoCoMn, respectively. Additionally, the GdErHoCoCr and GdErHoCoMn ingots exhibit maximum magnetic entropy changes and RC values of 12.29 J/kg/K and 746 J/kg and 10.13 J/kg/K and 606 J/kg, respectively, under a magnetic field of 5 T. The ingots GdErHoCoM (M = Cr and Mn) show excellent MEC properties and can be manufactured easily, making them promising for magnetic refrigerant applications.

Corrosion behavior and magnetocaloric property of pure phase La1-xCexFe11.5Si1.5 (x=0.1,0.2,0.3) compounds

To further study the corrosion mechanism of pure phase La(Fe,Si)13, Ce were added into the LaFe11.5Si1.5 compounds to facilitate phase formation and improve corrosion resistance and magnetic properties. The corrosion test conducted on the pure phase La1-xCexFe11.5Si1.5 (x = 0.1, 0.2, 0.3) revealed a significant number of minute corrosion pits with irregular shapes such as round, square and triangular after only 1 h of corrosion in tap water. However, in the case of La0.7Ce0.3Fe11.5Si1.5 compound, the number of corrosion pits was significantly lower. Potentiodynamic polarization curves showed that Ecorr goes up from −0.76 V to −0.59 V, and the Icorr goes down from 4.92 × 10−6 A/cm2 to 2.46 × 10−6A/cm2 as a result of Ce addition from 0.1 to 0.3. With the increase of Ce content, the corrosion resistance of La1-xCexFe11.5Si1.5 compounds was improved. Adding Ce element in the LaFe11.5Si1.5 compounds can also enhance the magnetocaloric effect. Significant extra spike-shaped peaks were observed on the M-H curves of La0.8Ce0.2Fe11.5Si1.5 and La0.7Ce0.3Fe11.5Si1.5, indicating a strong first-order magnetic phase transition. Furthermore，-△SM values improved from 21.2 J/kg K to 51.6 J/kg K by increasing Ce from 0.1 to 0.3.

Temperature-induced mixed magnetic states in FeRh@FeO composite nanoparticles

Reducing the size of a material with a first-order magnetic phase transition to the nanoscale results in a significant change in its physical properties. An example of this behaviour is the FeRh alloy. According to magnetometry results, the nanoparticles of this alloy do not show a transition from the antiferromagnetic to the ferromagnetic state near room temperature. In this work we have measured the magnetic properties and the Mössbauer effect for FeRh@FeO composite nanoparticles at different temperatures. An analysis of the results allows us to conclude that superparamagnetic relaxations dominate the formation of the magnetic structure of the nanoparticles. For particle sizes near 10 nm, the antiferromagnetic state is possible near helium temperatures. Further heating contributes to the formation of superparamagnetic behaviour.

Reconstruction of thermomagnetic and phase transformation loops in magnetostructural transitions using selective integration of temperature first-order reversal curves (TFORC) distributions

The temperature variant of the first-order reversal curves (TFORC) analysis technique is emerging as a useful tool for the study of the thermal hysteresis in magnetocaloric alloys undergoing first-order phase transitions. This work demonstrates the capabilities of the TFORC technique for reconstructing and deconvoluting the reversible and irreversible components of thermomagnetic transitions through the integration of the TFORC distributions. Through numerical simulations, the influence of applying smoothing protocols, the presence of noise in the magnetization curves or the distance between the hysteretic first-order phase transition and the Curie transitions of the phases have been tested. By employing the appropriate integration protocol of TFORC distributions, the transformed phase fraction can be separated from the total thermomagnetic behavior. The results obtained from the numeric simulations have been confirmed by applying the proposed analysis methods to the experimental TFORC results of a Ni-Mn-In Heusler alloy sample undergoing magnetic first-order phase transition. In this way, the TFORC technique can be stablished as a valuable characterization method for magnetostructural phase transition materials.

Magnetization and Magnetostriction of LaFe<sub>11.2 – <i>х</i></sub>Mn<sub><i>x</i></sub>Co<sub>0.7</sub>Si<sub>1.1</sub> (<i>x</i> = 0.1, 0.2, 0.3) Alloys In Pulsed Magnetic Fields

Abstract—The magnetization and magnetostriction of polycrystalline alloys LaFe11.2 – хMnxCo0.7Si1.1 (x = = 0.1, 0.2, 0.3) were measured in pulsed magnetic fields up to 180 kOe in the temperature range 80–270 K. The replacement of Fe atoms by Mn atoms shifts the TC towards low temperatures and does not affect the value of saturation magnetization. The observed field dependence of the magnetization M(H) near the TC is characteristic of second-order phase transitions, while the temperature dependence of the magnetization M(T) above the TC in strong magnetic fields indicates a first-order magnetic phase transition. The magnetic volume effect ΔV/V reaches 0.81% in a field of 180 kOe. The asymmetry of magnetostriction relative to the temperature of the maximum effect, manifested in strong magnetic fields, and the magnetic field hysteresis of magnetostriction bear signs of a first-order phase transition

Synthesis and characterization of LaFe11.57Si1.43 and LaFe11.57Si1.43H alloys for magnetic refrigeration applications

This study is based on the need to consolidate environmentally friendly technology to overcome pollution problems caused by conventional refrigerators. Compounds based on La(Fe,Si)13 have attracted much attention in many fields of industry as low cost materials as well as bright prospects for practical applications. The alloys obtained in this study are based on the LaFe11.57Si1.43 and LaFe11.57Si1.43H systems. These alloys were produced by melt-spinning of samples initially melted in an electric arc furnace and homogenized by heat treatment at 1050 °C for 2 h. LaFe11.57Si1.43 alloys to be hydrogenated with very little amount of 0.9 MPa of 5% H2/Ar gas and heat-treated at 723 K for one hour. An analysis of the crystal structure using X-ray diffraction measurements revealed that all alloys showed La-Fe-Si (1:13 phase) as the majority one with 85.8 wt% and 83.1 wt% for LaFe11.57Si1.43 and LaFe11.57Si1.43H respectively. The hyperfine and magnetic behavior of the alloys were studied by means of Mössbauer spectroscopy, and it was observed that the LaFe11.57Si1.43 alloy was fitted with a singlet and a doublet corresponding to 1:13 phase with a total spectral area of 72.5%, and in the LaFe11.57Si1.43H alloy exhibited a ferromagnetic behavior at room temperature with a hyperfine field of 19.4 T. Using vibrating sample magnetometry, it was demonstrated that the essential magnetic parameters to optimize the magnetocaloric effect, Curie temperature, and saturation magnetization at room temperature, significantly improved from 228.23 K to 308.1 K and 37.03 A m2kg−1 to 120 A m2kg−1, respectively, with the addition of hydrogen. Endothermic peaks possibly associated with first-order magnetic phase transition were found in each of the La-Fe-Si alloys using differential scanning calorimetry, coinciding with its TC value of 315.1 K. These results demonstrate that hydrogenation under a poor hydrogen gas atmosphere is an effective and safer method for improve the magnetocaloric properties of the LaFe11.57Si1.43system.

Magnetoresistance Features at the Magnetic Field-Induced Phase Transition in FeRh Thin Films

The causes of the appearance of first-order magnetic phase transitions remain a mystery. FeRh alloy is a classical material where a first-order magnetic phase transition occurs. The authors of this article studied the phase transition from the antiferromagnetic state to the ferromagnetic state in FeRh alloy. Comparison of the magnetometry and transport properties results allowed us to determine a number of differences in the mechanisms of the phase evolution during magnetic field and temperature induced transition. This article notes the priority of the rearrangement of the micromagnetic structure of the ferromagnetic phase as a result of the induction of a phase transition by a magnetic field. The main feature of the magnetic field induced phase transition compared to the temperature induced one is the change in the micromagnetic structure of the ferromagnetic phase. The growth of a ferromagnetic phase with less scattering fields leads to asymmetric behavior when a phase transition is induced near the metastable state. We also focused on the importance of taking into account the effect of magnetostriction when analyzing the evolution of the phase transition, which leads to the irreversibility of the phase transition near a zero magnetic field.

Borderline first-order phase transition and large cryogenic magnetocaloric effect in PrNdIn

We report a large cryogenic magnetocaloric effect stemming from an unconventional borderline first-order magnetic phase transition with negligibly small thermomagnetic hysteresis in a rare-earth-based intermetallic compound PrNdIn. The sample exhibits maximum magnetic field-induced entropy change as large as −10 J/Kg K (for 20 kOe magnetic field change) near the boiling point of oxygen. Magnetocaloric properties of PrNdIn are comparable to those of other known potential magnetocaloric materials with operating temperatures ranging between 50 and 125 K. The magnetic properties of the present sample are qualitatively reminiscent of those of the binary Pr2In and Nd2In, including the emergence of a second low-temperature anomaly in the temperature dependence of magnetization.

Large reversible magnetocaloric effect in antiferromagnetic Er3Si2C2 compound

The magnetic properties, magnetic phase transition and magnetocaloric effects (MCE) of Er3Si2C2 compound were investigated based on theoretical calculations and experimental analysis. Based on the first principles calculations, the antiferromagnetic (AFM) ground state type in Er3Si2C2 compound was predicted and its electronic structure was investigated. The experimental results show that Er3Si2C2 compound is an AFM compound with the Néel temperature (TN) of 7 K and undergoes a field-induced first-order magnetic phase transition from AFM to ferromagnetic (FM) under magnetic fields exceeding 0.6 T at 2 K. The magnetic transition process of Er3Si2C2 compound was investigated and discussed. The values of the maximum magnetic entropy change (−ΔSMmax) and the refrigeration capacity (RC) are 17 J/(kg∙K) and 193 J/kg under changing magnetic fields of 0–5 T, respectively. As a potential cryogenic magnetic refrigerant, the Er3Si2C2 compound also provides an interesting research medium to study the magnetic phase transition process.

Quantification of electronic and magnetoelastic mechanisms of first-order magnetic phase transitions from first principles: application to caloric effects in La(FeSi)13

and derived quaternary compounds are well-known for their giant, tunable, magneto- and barocaloric responses around a first-order paramagnetic-ferromagnetic transition near room temperature with low hysteresis. Remarkably, such a transition shows a large spontaneous volume change together with itinerant electron metamagnetic features. While magnetovolume effects are well-established mechanisms driving first-order transitions, purely electronic sources have a long, subtle history and remain poorly understood. Here we apply a disordered local moment picture to quantify electronic and magnetoelastic effects at finite temperature in from first-principles. We obtain results in very good agreement with experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic mechanisms, drives the first-order character and causes at the same time a huge electronic entropy contribution to the caloric response.

Improving work temperature span and reversibility of magnetoelastic transition of (Mn,Fe)2(P,Si) alloys by Mg doping

Magnetocaloric materials with wide working temperature span and reversible magnetic transition are beneficial for the magnetic refrigerator. Besides of giant magnetocaloric effect, (Mn,Fe)2(P,Si) alloys exhibit significant thermal/magnetic hysteresis and a narrow working temperature span. In this work, the impact of Mg doping on the phase structure and magnetocaloric effect of Mn1.05Fe0.9P0.5Si0.5-xMgx alloys has been studied. The results show that all samples crystallize into Fe2P-type main phase with little impurity phase. After Mg doping, the lattice parameter a of the main phase increases from 6.027(0) Å to 6.049(0) Å and c decreases from 3.488(6) Å to 3.480(6) Å, which leads to the increase of Curie temperature. Density functional theory calculation results confirm that the Mg atom located on the 2c site can increase the covalent bond length, and then enhance the ferromagnetic coupling between atoms leading to the increase of Curie temperature. Moreover, the first-order magnetic phase transition of Mn1.05Fe0.9P0.5Si0.5-xMgx alloys is inhibited with the increase of Mg content, which results in the decrease of thermal/magnetic hysteresis and the increase of the effective working temperature span. For Mn1.05Fe0.9P0.5Si0.48Mg0.02 alloy, it has a Curie temperature of about room temperature, effective refrigeration capacity of 291.7 J·kg−1, working temperature window of 46 K and magnetic entropy change of 9.4 J·kg−1·K−1. The alloy can be an excellent candidate in room temperature magnetic refrigeration devices with a wide temperature span.

Critical Point in the Curve of First-Order Magnetic Phase Transition

Critical Point in the Curve of First-Order Magnetic Phase Transition

Simulating the giant magnetocaloric effect-from mean-field theory to microscopic models

Magnetocaloric materials are recognized as one of the major classes of magnetic materials for energy applications, and can be either employed as refrigerants in heat-pumping devices, or in thermomagnetic generators for energy conversion/harvesting. For both applications, having a material that presents a first-order magnetic phase transition is advantageous, as this typically leads to enhanced values of magnetization change in temperature (relevant to energy conversion) and of the magnetocaloric effect (relevant to heat-pumping). We present a brief overview of selected models applied to the simulation of applied magnetic field and temperature-dependent magnetization and magnetic entropy change of first-order magnetic phase transition systems, covering mean-field models such as the Landau theory of phase transitions and the Bean-Rodbell model, up to more recent developments using a Ising-like microscopic model with magnetovolume coupling effects. We highlight the fundamental and practical limitations of employing these models and compare predicted thermodynamic properties.

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.

Magnetic field-dependent lattice entropy change in Gd5Ge4

The total entropy change in the first-order magnetic phase transition materials is mainly composed of lattice entropy change (LEC) and magnetic entropy change (MEC). Generally, the MEC is limited by the total angular momentum quantum number, while the LEC is related to the volume change associated with phase transition and it can be easily regulated. Therefore, maximizing LEC is considered as an effective means to improve the refrigeration performance. However, the current research methods only calculate the LEC between two stable phases after phase transition, whereas the LEC during phase transformation is not fully clarified. Here, the mixed Debye temperature (MDT) model and the mixed lattice entropy (MLE) model are proposed based on the two-phase coexistence mechanism for calculating the LEC during the magnetic field-driven phase transition in Gd5Ge4. Both models indicate that the lattice entropy strongly depends on the applied magnetic field during phase transformation. According to the MLE model, which shows better accuracy, a large LEC up to –22.31 Jkg-1K−1 at 25 K can be exerted in Gd5Ge4 by a relatively small magnetic field of 2.5 T. Our proposed theoretical models are helpful for designing high-performance magnetic refrigerants.

UI3 – 5f-electron magnetic van der Waals material

We grew high-quality single crystals of the 5f electron van der Waals compound UI3 and investigated them by measurements of specific heat and magnetization as function of temperature and magnetic field. UI3 behaves as an antiferromagnet with a first-order magnetic phase transition at the Neél temperature TN ​= ​2.65 ​K. It is characterized by a sharp symmetric specific-heat peak which is gradually shifted to lower temperatures by increasing magnetic field applied along either the a- or b-axis. The behavior in magnetic field reveals orthorhombic magnetocrystalline anisotropy with the hard magnetization direction along the c-axis. The 2-K a- and b-axis magnetization curves exhibit a metamagnetic transition at the critical field μ0Hc ​≈ ​3 ​T and ≈ 1.8 ​T, respectively. In higher fields, when the long-range antiferromagnetism is suppressed by a metamagnetic transition, signs of short-range magnetic ordering of antiferromagnetic correlations in the paramagnetic state show up both in specific heat and magnetization. The anomalous S-shape field dependence of a-axis magnetization well above Hc can be understood as the crossover from the correlated paramagnetic regime to the high-field polarized paramagnet. The magnetic phase diagrams for the magnetic field applied along the a- and b-axis, respectively, designed using the mentioned experimental results are presented. We have also found that the UI3 crystals are easily cleavable which predisposes this material for direct investigation of 5f electron magnetism in the 2D limit on exfoliated atomically thin samples.

Limitations of scaling laws for determining the order of magnetic phase transitions in antiperovskite materials

For materials with magnetic phase transition, researchers have tried to find a simple magnetic method that can replace the calorimetric technique for determining the type of phase transition. Recently, the researchers provide a quantitative characteristic of first-order magnetic phase transitions, which is believed to have wide application prospect. The maximum exponent n, calculated from field dependence of magnetic entropy change, is>2 only for first-order phase transitions. This method has been tested in Bean-Rodbell model and for various types of materials with first-order phase transitions from ferromagnetic to paramagnetic or antiferromagnetic to ferromagnetic state. However, in this work, the determination of the order of phase transitions based on field dependence of magnetic entropy change contradicts the results of calorimetric measurements in antiperovskite compounds Mn3SnC and MnCu0.97N. This discrepancy may be associated with a first-order magnetic transition from ferrimagnetic to paramagnetic state in antiperovskite materials.

Weak itinerant magnetic phases of La2Ni7

La2Ni7 is an intermetallic compound that is thought to have itinerant magnetism with a small moment ordering below 65 K. A recent study of single crystal samples by Ribeiro et. al. [Phys. Rev. B 105, 014412 (2022)] determined detailed anisotropic H-T phase diagrams and revealed three zero-field magnetic phase transitions at T1 ~ 61.0 K, T2 ~ 56.5K, and T3 ~ 42 K. In that study only the highest temperature phase is shown to have a clear ferromagnetic component. Here we present a single crystal neutron diffraction study determining the propagation vector and magnetic moment direction of the three magnetically ordered phases, two incommensurate and one commensurate, as a function of temperature. The higher temperature phases have similar, incommensurate propagation vectors, but with different ordered moment directions. At lower temperatures the magnetic order becomes commensurate with magnetic moments along the c direction as part of a first-order magnetic phase transition. We find that the low-temperature commensurate magnetic order is consistent with a proposal from earlier DFT calculations.