Magnetoelastic Phase Transition Research Articles

Giant magnetocaloric effect for (Mn, Fe, V)2(P, Si) alloys with low hysteresis

The Fe2P type Mn–Fe–P–Si alloys exhibit a giant magneto-elastic first-order transition, but the large hysteresis limits their performance. Crystal structure evolution and magnetocaloric performance were investigated by varying the Mn and Fe contents at a constant V substitution of 0.02 in Fe2P-type (Mn1.17-xFe0.73-yV0.02) (P0.5Si0.5) (where x + y = 0.02). The V substitution of Fe content shows a larger reduction of hysteresis compared with the same substitution amount of Mn content. During magnetoelastic phase transition, V-substitution reduces the volume change and the volumetric stresses, providing a superior mechanical stability. Compound with the V substitution of Fe (y = 0.02) shows the best magnetocaloric effect with a low thermal hysteresis of 0.6 K. Our developed Mn1.17-xFe0.73-yV0.02P0.5Si0.5 alloys are excellent materials for room-temperature magnetic heat-pumping applications by using a permanent magnet.

Anomalous dielectric response in the triple perovskite ruthenate Ba3BiRu2O9

We have investigated the magnetic, dielectric and thermal properties of the S = 1 magnetic dimer Ba3BiRu2O9, which is known to exhibit a spin-gap opening in conjunction with a first-order magneto-elastic phase transition at ∼175 K. Above the spin-gap temperature, the temperature dependence dielectric constant shows a peak like feature with pronounced frequency dependence. The critical slowing down behavior of this frequency dispersion suggests that a ferroelectric relaxor like electrical glassy state exists above the spin-gap opening temperature regime. The extermination of frequency dispersion—right at the magneto-elastic phase transition, is suggestive of a strong coupling between the lattice and charge-spin degrees of freedom in this triple perovskite system.

Reduction of hysteresis in (La1-xCex) y(MnzFe11.4-z)Si1.6 magnetocaloric compounds for cryogenic magnetic refrigeration

(La,Ce)(Mn,Fe,Si)13-based compounds which show a giant magnetocaloric effect are potential materials for the cryogenic magnetic refrigeration. However, large hysteresis originating from the first order magneto-elastic phase transition deteriorates cyclic performance of these materials, hindering their practical applications. In this work, Curie temperature of (La1-xCex)y(MnzFe11.4-z)Si1.6 compounds was tuned to the cryogenic temperatures below 77 K and hysteresis was successfully reduced to 1.5 K by tuning first order magneto-elastic transition to the critical point of second order magnetic phase transition. Based on detail microstructure characterizations, the reason for the reduction of hysteresis is ascribed to the change of a secondary phase from a paramagnetic LaFeSi phase to ferromagnetic Ce2Fe17 and α-Fe phases. Cryogenic Lorentz microscopy observations and micromagnetic simulations showed the α-Fe ferromagnetic phase produces a large stray field of ∼0.7 T at their interface. This causes the magnetic field assisted paramagnetic/ferromagnetic phase transition in the NaZn13-type phase. Cryogenic X-ray diffraction analysis indicated the energy barrier of magneto-elastic transition was reduced, resulting in an enhancement of their mechanical stability during the cyclic performance. This work has shown that the hysteresis in the magnetocaloric materials with first order magneto-elastic transition can be tuned by engineering the size, distribution, and magnetism of the secondary phases.

Magnetoelastic transition and giant anisotropic magnetostriction effect in textured Mn<sub>1.9</sub>Co<sub>0.1</sub>Sb polycrystal

Mn2Sb-based alloys have extremely promising practical applications in many fields owing to their abundant and controllable magnetofuntional properties around the first-order magnetoelastic transition, as well as their low costs. However, only a few studies on the magnetostrain of these alloys have been reported. Herein, a highly textured Mn1.9Co0.1Sb polycrystal is fabricated by using the arc-melting technique, which produces a strong temperature gradient induced by rapid cooling during the arc-melting process, resulting in the highly selective orientation of crystal growth. X-ray diffraction and thermomagnetic measurements show that the grains grow in certain crystallographic directions. Metamagnetic transition is revealed by the isothermal magnetization curve measurements, which is observed to be more obvious at lower temperatures. The standard resistive strain-gauge method is adopted to measure the magnetostriction effect. Giant anisotropic magnetostrain values of up to 0.246% and −0.11% are obtained parallel and vertical to the texture of the alloy, respectively. This magnetostriction effect mainly originates from the anisotropic lattice distortion across the first-order magnetoelastic phase transition.

Low-field reversible magnetocaloric effect in Co-dopedMn-Cr-Sb alloys

The magnetic phase transition and magnetocaloric effect in the Mn1.89− x Cr0.11Co x Sb ( x =0, 0.04, 0.06) alloys were studied in this work. Results show that the alloys undergo the first-order magneto-elastic phase transition from ferrimagnetic phase to antiferromagnetic phase on cooling, with very narrow thermal hysteresis, i.e., 3–4 K. Co doping cannot only enhance the transformation temperatures of magneto-elastic phase transition, but also avoid the formation of unfavorable MnSb phase. Direct measurements on the adiabatic temperature change ∆ T ad are employed to evaluate the magnetocaloric properties for the studied alloys. Under the field change of 1.5 T, the maximum ∆ T ad up to −1.4 K and the reversible ∆ T ad of −1.2 K can be achieved in the Mn1.85Cr0.11Co0.04Sb alloy. The combination of very small thermal hysteresis and large low-field reversible magnetocaloric effect makes the present alloys very attractive for the potential applications in magnetic refrigeration.

Charge redistribution and the magnetoelastic transition across the first-order magnetic transition in (Mn,Fe) 2 (P,Si,B)

We used temperature dependent high-resolution x-ray powder diffraction and magnetization measurements to investigate structural, magnetic and electronic degrees of freedom across the ferromagnetic magneto-elastic phase transition in Mn1Fe1P0.6-wSi0.4Bw (w = 0, 0.02, 0.04, 0.06, 0.08). The magnetic transition was gradually tuned from a strong first-order (w = 0) towards a second-order magnetic transition by substituting P by B. Increasing the B content leads to a systematic increase in the magnetic transition temperature and a decrease in thermal hysteresis, which completely vanishes for w = 0.08. Furthermore, the largest changes in lattice parameter across the magnetic transition occur for w = 0, which systematically becomes smaller approaching the samples with w = 0.08. Electron density plots show a strong directional preference of the electronic distribution on the Fe site, which indicates the forming of bonds between Fe atoms and Fe and P/Si in the paramagnetic phase. On the other hand, the Mn-site shows no preferred directions resembling the behaviour of a free electron gas. Due to the low B concentrations (w = 0 - 0.08), distortions of the lattice are limited. However, even small amounts of B strongly disturb the overall topology of the electron density across the unit cell. Samples containing B show a strongly reduced variation in the electron density compared to the parent compound without B.

Nanocalorimeter platform for in situ specific heat measurements and x-ray diffraction at low temperature.

Recent advances in electronics and nanofabrication have enabled membrane-based nanocalorimetry for measurements of the specific heat of microgram-sized samples. We have integrated a nanocalorimeter platform into a 4.5 T split-pair vertical-field magnet to allow for the simultaneous measurement of the specific heat and x-ray scattering in magnetic fields and at temperatures as low as 4 K. This multi-modal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the x-ray beam, and fast acquisition of x-ray scans over a wide temperature range without the need for time-consuming re-centering and re-alignment. Furthermore, on an YBa2Cu3O7-δ crystal and a copper foil, we demonstrate a novel approach to x-ray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the first-order magneto-elastic phase transition of Fe2P, a material that is of interest for magnetocaloric applications.

Kinetic-arrest-induced phase coexistence and metastability in(Mn,Fe)2(P,Si)

Neutron diffraction, M\"ossbauer spectroscopy, magnetometry, and in-field x-ray diffraction are employed to investigate the magnetoelastic phase transition in hexagonal ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds. ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds undergo for certain compositions a second-order paramagnetic (PM) to a spin-density-wave (SDW) phase transition before further transforming into a ferromagnetic (FM) phase via a first-order phase transition. The SDW-FM transition can be kinetically arrested, causing the coexistence of FM and untransformed SDW phases at low temperatures. Our in-field x-ray diffraction and magnetic relaxation measurements clearly reveal the metastability of the untransformed SDW phase. This unusual magnetic configuration originates from the strong magnetoelastic coupling and the mixed magnetism in hexagonal ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds.

Efficient Room-Temperature Cooling with Magnets

Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential ...

Latent heat of the first-order magnetic transition ofMnFeSi0.33P0.66

The latent heat of a magnetoelastic phase transition is used as a measure of the magnetocaloric effect since it is directly proportional to the entropy change. Taking ${\mathrm{MnFeSi}}_{0.33}{\mathrm{P}}_{0.66}$ as a model magnetocaloric material, density functional theory calculations in addition to the phonon calculations based on the density functional perturbation theory were performed in order to calculate the latent heat of the magnetoelastic phase transition. The Curie temperature (${T}_{C}$) was determined by taking into account the quasiharmonic approximation and the configurational entropy. The material exhibits a first-order magnetic transition accompanied by a large latent-heat (19.97 kJ/kg) near-room-temperature operation.

X-ray diffraction analysis of the magnetoelastic phase transition in the Mn-Fe-P-Si magnetocaloric alloy

Structural characterization of the Mn1.3Fe0.65P0.5Si0.5 powder is reported. The rare-earth-free magnetocaloric material was prepared by ball milling and solid-state synthesis. X-ray diffraction data were collected in a wide temperature range across the magnetoelastic phase transition. The lattice parameters and volume fractions of the paramagnetic and ferromagnetic phases were determined as functions of temperature using Rietveld fitting. The virgin effect (a delay of the phase transition on first cooling) and associated variation of lattice parameters are analyzed on the assumption of elastic constraints imposed on the paramagnetic phase by the defect structure. A simple Landau model with magnetoelastic coupling illustrates the observed first-order behavior.

Tuning the phase transition in transition-metal-based magnetocaloric compounds

Neutron-diffraction experiments on the (Mn,Fe)2(P,Si)-type compounds have shown a site preference of Si atoms in the hexagonal structure. The degree of ordering of Si depends on the Si/P ratio, while it is independent of the Mn/Fe ratio. The ferromagnetic-paramagnetic magnetoelastic transition is closely related to the size of the magnetic moment on the 3f site. A preferred occupation of Si atoms on the 2c site stabilizes and decreases the magnetic moment on the 3f and 3g site, respectively, which is supported by our first-principles density functional theory calculations. This effect, together with the contribution from the Si substitution-induced changes in the interatomic distances, leads to a phase transition that is tunable in temperature and degree of first order in Mn1.25Fe0.70P1?xSix compounds. These results provide us with further insight into the relationship between the magnetoelastic phase transition and the local atomic coordination.

Magnetoelastic phase transitions in the LuFe4Ge2 and YFe4Si2 compounds: A neutron diffraction study

The magnetic structure of the tetragonal antiferromagnetic LuFe4Ge2 and YFe4Si2 compounds has been studied by high resolution neutron and X-ray diffraction above and below the magneto-elastic transition at TN, Tc=32K and 76K, respectively. Below TN,Tc the tetragonal high temperature (HT) phase transforms into the low temperature (LT) orthorhombic magnetic phase: P42/mnm (HT) → Pnnm (q=0) (LT) with a 2D canted moment arrangement of the Fe moments within the (001) plane with the magnetic space group P21n'21n'21m'(Sh58399). The Fe magnetic moment value is highly reduced in both compounds due to the presence of geometrical and exchange frustration relating to the underlying crystal structure comprising columns of Fe tetrahedra expanding along the c direction with antiferromagnetic interactions. The magnetic refinements were perturbed by the presence of partly unidentified impurity phases mainly in YFe4Si2.

Magnetoelastic Quantum Fluctuations and Phase Transitions in the Iron Superconductors

We examine the relevance of magnetoelastic coupling to describe the complex magnetic and structural behavior of the different classes of the iron superconductors. We model the system as a two-dimensional metal whose magnetic excitations interact with the distortions of the underlying square lattice. Going beyond the mean field, we find that quantum fluctuation effects can explain two unusual features of these materials that have attracted considerable attention: first, why iron telluride orders magnetically at a non-nesting wave vector (π/2,π/2) and not at the nesting wave vector (π,0) as in the iron arsenides, even though the nominal band structures of both these systems are similar, and second, why the (π,0) magnetic transition in the iron arsenides is often preceded by an orthorhombic structural transition. These are robust properties of the model, independent of microscopic details, and they emphasize the importance of the magnetoelastic interaction.

The magnetocaloric effect at the first-order magneto-elastic phase transition

This paper presents a study of the magnetocaloric effect at thefirst-order magneto-elastic phase transition. The entropy changeΔs at the transition temperature is given by the sum of the magnetic andthe structural contributions. By using a thermodynamic model, it isshown that the sign and amplitude of the structural contribution toΔs are determined by the dimensionless parameterζ (zeta) whichdepends on β, the steepness of the change of exchange forces with volume, and onαp, the thermal expansion coefficient of the structural lattice. Forζ < 0 thechange Δs can be larger than the magnetic contribution alone, because the structural entropy change and themagnetic entropy change have the same sign, giving rise to the giant magnetocaloric effect. For0 < ζ < 1 the two contributions partially cancel each other and the totalentropy change is lower than the magnetic entropy change. Forζ > 1 the structural entropy dominates and a transition occurs upon heating from a lowtemperature paramagnet to a high temperature ferromagnet.

Magneto-elastic phase transition in a linear chain within the double and super-exchange model

In this work a magneto-elastic phase transition in a linear chain was obtained due the interplay between magnetism and lattice distortion in a double and super-exchange model. We consider a linear chain consisting of classical localized spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by the anti-ferromagnetic super-exchange interaction between neighbor localized spins. Additionally, the lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. The phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

Monte Carlo study of the double and super-exchange model with lattice distortion

In this work a magneto-elastic phase transition was obtained in a linear chain due to the interplay between magnetism and lattice distortion in a double and super-exchange model. It is considered a linear chain consisting of localized classical spins interacting with itinerant electrons. Due to the double exchange interaction, localized spins tend to align ferromagnetically. This ferromagnetic tendency is expected to be frustrated by anti-ferromagnetic super-exchange interactions between neighbor localized spins. Additionally, lattice parameter is allowed to have small changes, which contributes harmonically to the energy of the system. Phase diagram is obtained as a function of the electron density and the super-exchange interaction using a Monte Carlo minimization. At low super-exchange interaction energy phase transition between electron-full ferromagnetic distorted and electron-empty anti-ferromagnetic undistorted phases occurs. In this case all electrons and lattice distortions were found within the ferromagnetic domain. For high super-exchange interaction energy, phase transition between two site distorted periodic arrangement of independent magnetic polarons ordered anti-ferromagnetically and the electron-empty anti-ferromagnetic undistorted phase was found. For this high interaction energy, Wigner crystallization, lattice distortion and charge distribution inside two-site polarons were obtained.

Role of striction at magnetic and structural transitions in iron pnictides

We discuss the role of striction in the intertwined magnetic and structural phase transitions in the underdoped iron pnictides. The magnetoelastic coupling to acoustic modes is then derived and estimated in framework of the multiband spectrum for itinerant electrons with nesting features. We argue that the first-order character of the magnetoelastic phase transition originates from the lattice instabilities near the onset of spin-density wave order introducing, thus, a shear acoustic mode as another order parameter. Taking nonharmonic terms in the lattice energy into account may explain the splitting of the structural and magnetic transitions in some oxypnictides. Fluctuations of the magnetic order parameter show up in the precursory temperature dependence of the elastic moduli.

Magneto-elastic phase transitions in one-dimensional systems

The magneto-elastic phase diagram in one-dimensional systems relating to the interplaybetween magnetism and lattice distortion is studied in a double-exchange andsuper-exchange model considering classical localized spins and the limit of large Hund’scoupling. At low super-exchange interaction energy, a phase transition occurs betweenelectron-full ferromagnetic distorted and electron-empty antiferromagnetic undistortedphases via phase separation. In this case, all electrons and lattice distortions are foundwithin the ferromagnetic domain. For higher super-exchange interaction energy,phase separations consisting of two- or three-site distorted independent magneticpolarons separated by electron-empty undistorted antiferromagnetic links areobtained. In this regime, each polaron contains an electron, leading to a Wignercrystallization. The lattice distortion and charge distribution inside the polarons are alsocalculated.

Magneto-elastic properties and electronic structure analysis of the (Fe1−xNix)2P system

For the (Fe1−xNix)2P compounds with x=0.015, 0.02, 0.025 and 0.25, both X-ray diffraction and χac susceptibility experiments under pressure up to 1.5GPa were performed from 80 to 420K. For all the studied compounds, a clear cut metal atom distribution was found on the basis of the Fe2P structure-type (SG: P6̄2m), the 3g sites being fully occupied by Fe, while both Fe and Ni are randomly distributed on the 3f sites. The variation of the ordering temperature under pressure ∂TC/∂p increases from negative values for all studied compounds up to zero for the Ni-rich one (Fe0.75Ni0.25)2P. Additionally for the latter, neutron diffraction experiments were carried out in the 10–290K range to characterise the transformation between the ferromagnetic (F) and paramagnetic (P) states. An electronic structure analysis of the (Fe1−xNix)2P compounds performed using the Korringa–Kohn–Rostoker method with the coherent potential approximation (KKR-CPA) shows that the total energy calculation fully supports the Ni preference for the 3f occupancy. Accounting to this trend, the calculated total magnetic moment was found to decrease almost linearly with x, in very fair agreement with the experimental data when measured for x<0.25. However, the theoretical values of local magnetic moments computed in (Fe0.75Ni0.25)2P remain in satisfying agreement with the results of neutron diffraction. Hence, it is suggested that the strong electron polarisation at EF (P ∼ 90%) as established theoretically for the (Fe1−xNix)2P compounds having the smallest Ni content, can be responsible for the marked magneto-elastic phase transition as observed, e.g. in (Fe0.975Ni0.025)2P.