Magnetostructural Phase Transition Research Articles

Strain-induced high ferromagnetic transition temperature of MnAs epilayer grown on GaAs (110).

MnAs films are grown on GaAs surfaces by molecular beam epitaxy. Specular and grazing incidence X-ray diffractions are used to study the influence of different strain states of MnAs/GaAs (110) and MnAs/GaAs (001) on the first-order magnetostructural phase transition. It comes out that the first-order magnetostructural phase transition temperature Tt, at which the remnant magnetization becomes zero, is strongly affected by the strain constraint from different oriented GaAs substrates. Our results show an elevated Tt of 350 K for MnAs films grown on GaAs (110) surface, which is attributed to the effect of strain constraint from different directions.PACS: 68.35.Rh, 61.50.Ks, 81.15.Hi, 07.85.Qe

Monte Carlo simulations of the magnetocaloric effect in magnetic Ni–Mn–X (X = Ga, In) Heusler alloys

Monte Carlo simulations were used for a detailed description of magnetic, martensitic and magnetocaloric properties of Ni2+xMn1−xGa (0.18 ⩽ x ⩽ 0.24) and Ni50Mn34In16 Heusler alloys, which undergo a first-order magnetostructural phase transition. In the simulations we made use of magnetic exchange parameters which were obtained by ab initio calculations. Results of magnetic and lattice contributions to the total specific heat as well as the change in the isothermal magnetic entropy ΔSmag and the adiabatic temperature ΔTad changes around the magnetic and magnetostructural transitions in an external magnetic field agree fairly well with available experimental data.

Antiferromagnetic critical fluctuations inBaFe2As2

Magnetic correlations near the magneto-structural phase transition in the bilayer iron pnictide parent compound, BaFe$_2$As$_2$, are measured. In close proximity to the antiferromagnetic phase transition in BaFe$_2$As$_2$, a crossover to three dimensional critical behavior is anticipated and has been preliminarily observed. Here we report complementary measurements of two-dimensional magnetic fluctuations over a broad temperature range about T$_N$. The potential role of two-dimensional critical fluctuations in the magnetic phase behavior of BaFe$_2$As$_2$ and their evolution near the anticipated crossover to three dimensional critical behavior and long-range order are discussed.

The origin of large overestimation of the magnetic entropy changes calculated directly by Maxwell relation

By investigating the nature of the first-order magnetostructural phase transition, we point out the origin of large overestimation of magnetic entropy changes calculated by directly applying the Maxwell relation. Furthermore, if the mass variations in ferromagnetic and paramagnetic phases on temperature are taken into consideration in two-phase region, taking MnAsCx, (Mn, Al)As, and Mn0.994Fe0.004As compounds as example, based on the modified Maxwell relation, the large overestimation and discrepancy of magnetic entropy changes disappear. The magnetic entropy changes are equivalent with the values by Clausius–Claperyon equation, indicating the validity of this modified approach.

Giant magnetocaloric effects by tailoring the phase transitions

The MnCoGe alloy can crystallize in either the hexagonal Ni2In- or the orthorhombic TiNiSi-type of structure. In both phases MnCoGe behaves like a typical ferromagnet with a second-order magnetic phase transition. For MnCoGeBx with B on interstitial positions, we discover a giant magnetocaloric effect associated with a single first-order magnetostructural phase transition, which can be achieved by tuning the magnetic and structural transitions to coincide. The results obtained on the MnCoGe-type alloys may be extensible to other types of magnetic materials undergoing a first-order structural transformation and can open up some possibilities for searching magnetic refrigerants for room-temperature applications.

Phonon Mechanism of the Magnetostructural Phase Transition in MnAs

Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.

First-principles and Monte Carlo study of magnetostructural transition and magnetocaloric properties ofNi2+xMn1−xGa

First-principles calculations of magnetic exchange parameters of the austenitic and martensitic phases of Ni-Mn-Ga allow one to characterize this Heusler system across the phase diagram in agreement with experimental trends. The ab initio investigations have been combined with Monte Carlo simulations for a detailed description of the magnetic, martensitic, and magnetocaloric properties of ${\text{Ni}}_{2+x}{\text{Mn}}_{1\ensuremath{-}x}\text{Ga}$ $(0.18\ensuremath{\le}x\ensuremath{\le}0.27)$ Heusler alloys, which undergo a first-order magnetostructural phase transition. For these alloys, the calculated temperature dependence of the magnetic and lattice contributions to the total specific heat as well as the evaluation of the isothermal magnetic entropy $\ensuremath{\Delta}{S}_{mag}(T,{H}_{ext})$ and adiabatic temperature $\ensuremath{\Delta}T(T,{H}_{ext})$ changes around the magnetostructural transition in an external magnetic field agree fairly well with the experimental data. In particular, results for $\ensuremath{\Delta}{S}_{mag}(T,{H}_{ext})$ and $\ensuremath{\Delta}T(T,{H}_{ext})$ may be used to speculate about designing new magnetic Heusler alloys with better magnetocaloric properties, i.e., larger $\ensuremath{\Delta}T(T,{H}_{ext})$ values.

Isothermal martensitic transformation in metamagnetic shape memory alloys

We show that in metamagnetic shape memory alloys exhibiting a magnetostructural first order phase transition the direct transition from ferromagnetic austenite to nonmagnetic martensite is isothermal. In contrast to the direct transformation, the reverse one (nonmagnetic martensite–ferromagnetic austenite) is athermal, just as are athermal both direct and reverse martensitic transformations in conventional ferromagnetic shape memory alloys. The observed asymmetry of properties of the direct and reverse phase transitions in metamagnetic alloys, together with the data on entropy change during the magnetostructural transition, evidences that the magnetostructural transition is driven by the first order lattice modification. The change in magnetic ordering is an effect accompanying the lattice modification, opposing the direct transformation and promoting the reverse one. It has been shown that relaxation effects in metamagnetic shape memory alloys are intrinsic in the direct transformation itself and do not require the “arrest” of the transformation.

The modeling of phase diagrams and premartensitic effects in Heusler Ni–Mn–Ga alloy by Monte Carlo method

In this work the coupled magnetostructural and premartensitic phase transitions in Heusler Ni–Mn–Ga alloy have been modeled by Monte Carlo method using Ising and Blume–Emery–Griffiths models. The phase diagrams in coordinates (Temperature–Magnetoelastic constant) were obtained. For the first time it is shown that the magnetoelastic interaction account in austenitic and martensitic phases allows to describe the coupled magnetostructural phase transition in Ni-Mn-Ga alloys. The area of the premartensitic phase transition in the temperature–magnetoelastic constant diagram has been found.

Monte Carlo study of magnetocaloric properties of Ni-Mn-Ga Heusler alloys

A three-dimensional Monte Carlo model has been used for theoretical description of magnetic and magnetocaloric properties of Ni2.18Mn0.82Ga Heusler alloys undergoing first order magnetostructural phase transition. For this alloy, calculated temperature dependence of magnetization, magnetic contribution to the total specific heat and magnetic entropy change ΔSmag are in good qualitatively agreement with available experimental data.

Theoretical model of the coupled magnetostructural phase transitions in Heusler Ni-Mn-In alloys by Monte Carlo simulation

On the basis of classical Monte Carlo simulations, we investigate the temperature and magnetic field dependencies of magnetization, strain order parameter, specific heat and entropy of Ni-Mn-In Heusler alloy, in which the part of the Mn atoms interact antiferromagnetically. It is shown that this antiferromagnetic exchange is responsible for existence of several magnetic phase transitions in the Heusler alloys. It is shown that with the help of the model Hamiltonian we obtain coupled magnetostructural phase transition. Results of simulations are in good qualitatively agreement with the experimental data.

Electric properties of erbium cobaltites

The temperature and electric, including magnetic-field, characteristics of ceramic samples Er1−xSrxCoO3−δ at temperatures ranging from room to liquid-helium temperatures and in magnetic fields to 4kOe applied perpendicular to the direction of the transport current have been investigated. Current-induced nonlinear features of percolation transport which are characteristics for a metal-insulator transition have been observed for weak currents. Regular behavior such as a decrease of the breakdown field with increasing concentration of divalent Sr and ordering effects with decreasing temperature have been observed for the first time. An anomalous increase of the conductivity and a large magnetoresistance effect in narrow Er and Sr concentration ranges, narrower and shifted more in the direction of the metallic phase than in lanthanum manganites, has been observed. The special behavior of the conductivity and the magnetoresistance, including nonlinear effects, are explained from a unified point of view—magnetostructural phase transitions, induced by a corresponding dopant concentration, temperature, or magnetic field, as well as the presence of a spin-dependent contribution, associated with double-exchange of delocalized electrons between heterovalent cobalt ions, to the mechanism of electron correlation.

Monte Carlo calculations of the phase transformations and the magnetocaloric properties in Heusler Ni–Mn–Ga alloys

The three-dimensional model for the theoretical description of phase transformations and the magnetocaloric effect in shape–memory Heusler Ni 2+ x Mn 1− x Ga alloys ( x=0.18–0.27) with a coupled magnetostructural phase transition by the classical Monte Carlo method is proposed. The isothermal magnetic entropy change upon magnetic field variations from 0 to 5 T, and other magnetic properties are in good qualitative agreement with the available experimental data.

Entropy determinations and magnetocaloric parameters in systems with first-order transitions: Study of MnAs

We present a study of the giant magnetocaloric effect in MnAs produced by a magnetostructural first-order phase transition. Results deduced from magnetization, M, and heat capacity, Cp,B(T), are compared and discussed. Some spurious effects are explained, and especially a spike in the isothermal entropy change, ΔST, occurring at TC when obtained via the Maxwell relation (∂S/∂B)T=(∂M/∂T)B. Alternative determination methods are given to circumvent this problem. The spike is explained as an artifact due to the incorrect application of the Maxwell relation to path dependent thermodynamic functions that are not state functions. The added wrong contribution to ΔST has been calculated using calorimetric data, giving a good agreement with the result from the magnetization measurements.

Capturing first- and second-order behavior in magnetocaloricCoMnSi0.92Ge0.08

Here we examine the constituent components that make a magnetocaloric material attractive for application. The field-temperature phase diagram is studied and using calorimetry, the 1 and 2 order components of the magnetic field driven magneto-structural phase transition in CoMnSi0.92Ge0.08 are extracted. It is demonstrated that below 262K the transition shows a latent heat component associated with first order behavior when the material changes from antiferromagnetic to ferromagnetic order. Such a transition is known as a metamagnetic transition. We identify 262K as a tricritical point and above this temperature Tcrit the transition shows only continuous, 2 order characteristics. Hall probe imaging that has a five micron pixel resolution is then used to study the striking differences in the spatial evolution of the transition above and below Tcrit. We demonstrate that the hysteresis of the transition is linearly related to the magnitude of the latent heat; an observation that has important implications for the use of this and other 1 order systems for application as magnetic refrigerants.

Monte-Carlo Calculation of the Magnetocaloric Effect in Ni-Mn-Ga Alloys

The three-dimensional model for the theoretical description of the phase transformations and the magnetocaloric effect in shape – memory Heusler Ni2+xMn1-xGa alloys (x=0.18 – 0.27) with a coupled magnetostructural phase transition by the classical Monte Carlo method is proposed. It is shown that the isothermal magnetic entropy change upon magnetic field variations from 0 to 5 T and other magnetic properties are in good agreement with the available experimental data.

Study of Magnetocaloric Properties of Ni-Mn-X (X = Ga, In) Heusler Alloys by Monte Carlo Technique

AbstractTwo theoretical Monte Carlo models have been presented for description of the positive and negative magnetocaloric effects of Heusler Ni-Mn-X (X = Ga, In) alloys undergoing first order coupled magnetostructural phase transition. For both models all quantities such as temperature dependence of the magnetic contribution to the total specific heat, magnetization and isothermal magnetic entropy change are in qualitative agreement with the available experimental data.

Comparative Study of the Magnetocaloric Properties in Ni-Mn-X (X = Ga, In, Sn) by Magnetization and Specific Heat Measurements

AbstractSome compositions of Ni-Mn-X (X = Ga, In, Sn) ferromagnetic shape memory alloys exhibit a first order magnetostructural phase transition. Magnetic entropy change ΔSm in the vicinity of this transition has been studied by magnetization and heat capacity measurements. Comparison of these results point to a large difference in magnitudes of ΔSm obtained from magnetization and heat capacity data. It is suggested that this discrepancy originates from overestimation of \Delta S_m determined from the magnetization measurements and underestimation of ΔSm obtained from the heat capacity measurements.

Magnetostructural phase transition and magnetocaloric effect in off-stoichiometric Mn1.9−xNixGe alloys

Phase transitions and magnetic entropy changes are studied in Mn1.9−xNixGe (x=0.85, 0.855) alloys. In these off-stoichiometric alloys, the crystallographic transition temperature decreases remarkably due to the deficiency of transition-metal atoms, and, consequently, a magnetostructural transition from the antiferromagnetic TiNiSi-type structure to the ferromagnetic Ni2In-type structure is observed. Owing to the abrupt change in magnetization, large magnetic entropy changes are obtained. The effect of transition-metal vacancies on the phase transition temperature is discussed.

Adiabatic temperature change at first-order magnetic phase transitions:Ni2.19Mn0.81Gaas a case study

A comprehensive study of the adiabatic temperature change $\ensuremath{\Delta}{T}_{\text{ad}}$ in the vicinity of a first-order magnetostructural phase transition has been carried out on a ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ Heusler alloy. It was found that the directly measured $\ensuremath{\Delta}{T}_{\text{ad}}\ensuremath{\sim}1\text{ }\text{K}$ is of order of magnitude smaller than that expected from reported in the literature isothermal magnetic entropy change and specific-heat data. A new feature of the adiabatic temperature change in materials with giant magnetocaloric effect, specifically an irreversible character of $\ensuremath{\Delta}{T}_{\text{ad}}$ when the sample is subjected to repeatable action of magnetic field at a constant temperature, has been observed. This effect has been attributed to the irreversible magnetic-field-induced structural transformation. It has been shown that the small value of $\ensuremath{\Delta}{T}_{\text{ad}}$ in ${\text{Ni}}_{2.19}{\text{Mn}}_{0.81}\text{Ga}$ is not due to the kinetics of the transformation but originates from other factors which are intrinsic to first-order magnetic phase transitions. Relevance of these factors to other giant magnetocaloric materials has been outlined.