Magnetoelastic Transition Research Articles

Phase analysis of near equiatomic bulk FeRh alloys: X-ray versus neutron diffraction and magnetization measurements

The information obtained by X-ray diffraction (XRD) by impinging the X-ray beam onto the flat surface of bulk Fe–Rh alloys is contrasted with that provided by the thermomagnetic analysis curves measured under a magnetic field of 5 mT and 2 T, and the neutron diffraction (ND) patterns. The experiments were performed on slices cut from bulk Fe50Rh50 and Fe49Rh51 alloys prepared by induction melting followed by 48 h of thermal annealing at 1273 K. Whereas ND patterns and magnetization curves, directly and indirectly point out that the samples are single-phase with a CsCl-type crystal structure undergoing a magnetoelastic transition that changes the magnetic structure from antiferromagnetic to ferromagnetic on heating, multiple phases are identified in the XRD patterns, which modify upon polishing or etching the surface with a mixture of hydrochloric (HCl) and nitric acid (HNO3) at a volume ratio of 90:10 from 30 to 210 min. The limitation of XRD for accurately characterizing the phase constitution within the bulk of Fe–Rh alloys is underlined.

Exploring low-temperature dynamics in triple perovskite ruthenates using nonlinear dielectric susceptibility measurements

We report the nonlinear dielectric properties of three triple-perovskite ruthenates: Ba3CoRu2O9, Ba3BiRu2O9, and Sr3CaRu2O9. These compounds exhibit notable correlations among their spin, charge, lattice, and polar degrees of freedom. Ba3CoRu2O9 displays a pronounced frequency-dependent relaxation in χ2,3 just above the magnetoelastic transition, occurring around 100 K, followed by an abrupt loss of polarization within the ordered phase. In Ba3BiRu2O9, we encounter the possibility of multiple coexisting relaxation behaviors, indicating a complex phase strongly influenced by the spin-gap opening at 175 K. Lastly, Sr3CaRu2O9 displays anomalies with strong dispersion effects close to its magnetic transitions, indicating a robust coupling between magnetic and dipolar orders in the system. These measurements highlight the significance of higher-order (hyper-)susceptibilities in providing profound insight into the dynamics of a system, offering information otherwise inaccessible through the linear polarization response alone.

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.

Emergent evolution of first-order phase transitions from magneto-structural to magneto-elastic in MnCo1−y Fe y Ge1−x Si x alloys

The emergent evolution of first-order phase transitions from magneto-structural to magneto-elastic and magnetocaloric effect (MCE) have been investigated by X-ray diffraction, differential scanning calorimetry and magnetization measurements. Applying the isostructural alloying principle, the martensitic transition temperature (T M) increases effectively and the Curie temperatures of the two phases increase slightly by substituting the Si content (x). With an appropriate amount of Fe and Si content, an emergent first-order antiferromagnetic–ferromagnetic magnetoelastic transition with thermal hysteresis in the martensitic state occurs for MnCo0.7Fe0.3Ge1–x Si x (x = 0.15–0.40) alloys, which results from the decrease in the nearest-neighbor Mn–Mn distance. Moreover, the values of magnetic entropy change (ΔS M), refrigeration capacity (RC) and temperature-averaged entropy change (TEC, 10 K) with ΔH = 50 kOe reach −12.2 J kg−1 K−1, 112.8 J kg−1 and 11.4 J kg−1 K−1 for MnCo0.7Fe0.3Ge0.8Si0.2 undergoing the ferromagnetic magneto-structural transition in the Curie temperature window. The results facilitate the magnetocaloric/magnetoelastic performance and tunability of multiple phase states in a wider temperature range.

Magnetoelastic transition and negative thermal expansion of Fe2Hf0.83Ta0.17 ribbons

In this work, the magnetocaloric effect and negative thermal expansion in melt-spun Fe2Hf0.83Ta0.17 Laves phase alloys were studied. Compared to arc-melted alloys, which undergo a first-order magnetoelastic transition from the ferromagnetic to the antiferromagnetic phase, melt-spun alloys exhibit a second-order transition. For Fe2Hf0.83Ta0.17 ribbons, we observed a large volumetric coefficient of negative thermal expansion of −19 × 10−6 K−1 over a wide temperature range of 197 – 297 K and a moderate adiabatic temperature change of 0.7 K at 290 K for a magnetic field change of 1.5 T. The magnetic field dependence of the transition temperature (dTt/dµ0H = 4.4 K/T) for the melt-spun alloy is about half that of the arc-melted alloy (8.6 K/T). The origin of second-order phase transition of the melt-spun alloy is attributed to the partially suppressed frustration effect, which is due to the atomic disorder introduced by the rapid solidification.

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.

On the order of magnetic transition in MnCo1−xFexGe (x = 0.20, 0.06 and 0.03) mechanical alloys

Mechanically amorphized MnCo1−xFexGe alloys (x = 0.20, 0.06 and 0.03) were used as precursors to obtain hexagonal austenite single phase samples. Combining thermomagnetic and magnetocaloric analysis and in situ X-ray diffraction, we observed that the presence of a distribution of transition temperatures jeopardizes the first order character of the magnetoelastic transition from the ferromagnetic to paramagnetic state. Both magnetothermal and in situ X-ray diffraction identify the presence of such distribution and the first order character of the magnetoelastic transition.

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.

Bulk-like first-order magnetoelastic transition in FeRh particles

Near-equiatomic, chemically-ordered iron-rhodium (FeRh) alloy is a fundamentally interesting material that may become useful in niche applications making use of its unique magneto functional phenomena, for example, the giant inverse magnetocaloric effect near room temperature that is associated with a sharp first-order magnetic phase transition. The nearly discontinuous antiferromagnetic-ferromagnetic phase transformation in bulk FeRh is well-known; however, the transition broadens considerably in fine particles and films with thickness less than 50 nm, precluding their potential applications. Here, we report an abrupt, bulk-like first-order magnetoelastic transformation in powders consisting of sub-micron particles of nearly equiatomic FeRh compound synthesized via solid-state mechanochemical co-reduction of FeF2 and RhCl3 and subsequent heat treatments. We demonstrate that annealing at temperatures ranging from 600 ̊C to 800 ̊C enables tailoring phase content, particle size, and magnetic properties of the powders. A maximum magnetic-field-induced entropy change of ~10 J/kg K at μ0ΔH = 1 T has been achieved in powders annealed at 800 ̊C. The retention of extraordinary responsiveness in sub-micron particles of FeRh is likely to open doors for system component fabrication using additive manufacturing methods, along with new opportunities to employ FeRh in theranostics.

Large magnetoresistance and magnetostrain in the batch-processed (Mn2Sb)1-xBix crystals (x = 0.06, 0.08, 0.10, 0.12)

We investigate the effects of Bi substitution on the structure, magnetization, magnetoresistance, and magnetostrain in single crystals of (Mn2Sb)1-xBix (x = 0.06, 0.08, 0.1, and 0.12). It is found that the element Bi can be partially substituted for Sb in the tetragonal Mn2Sb main phase, and partially forms the MnBi and Bi-rich secondary phases. The introduction of Bi causes the simultaneous lattice shrinkages in a and c for the main phase, and also enhances the transition temperature of the first-order magnetoelastic ferrimagnetic-antiferromagnetic transition. Such an enhancement in transition temperature due to contraction of the unit-cell volume contraction is also experimentally proven based on the thermomagnetic curves under hydrostatic pressure for a crystal with × = 0.12. The volume contraction rate due to hydrostatic pressure is deduced to be −0.0386 Å3/GPa. Moreover, the highest values of in-plane magnetoresistance (∼-30%) and magnetostrain (∼−1448 ppm) with good reversibility are attained at 120 K under 30 kOe, which was proven to be the optimal magnetic field. Our experimental results show that Mn2Sb-based intermetallic compounds can be considered promising candidates for low-cost, multifunctional magnetic materials.

Enhanced reversibility of the magnetoelastic transition in (Mn,Fe)2(P,Si) alloys via minimizing the transition-induced elastic strain energy

• Generalized relationship: hysteresis v.s. transition-induced elastic strain energy. • Hysteresis reduced by 91% after replacement of Fe by 4 at.% Mo in (Mn,Fe)2(P,Si). • Reversible adiabatic temperature change enhanced by 500 via 4 at.% Mo substitution. • Spin-lattice-electron coupling dominate magnetoelastic transition in (Mn,Fe)2(P,Si). Magnetocaloric materials undergoing reversible phase transitions are highly desirable for magnetic refrigeration applications. (Mn,Fe) 2 (P,Si) alloys exhibit a giant magnetocaloric effect accompanied by a magnetoelastic transition, while the noticeable irreversibility causes drastic degradation of the magnetocaloric properties during consecutive cooling cycles. In the present work, we performed a comprehensive study on the magnetoelastic transition of the (Mn,Fe) 2 (P,Si) alloys by high-resolution transmission electron microscopy, in situ field- and temperature-dependent neutron powder diffraction as well as density functional theory calculations (DFT). We found a generalized relationship between the thermal hysteresis and the transition-induced elastic strain energy for the (Mn,Fe) 2 (P,Si) family. The thermal hysteresis was greatly reduced from 11 to 1 K by a mere 4 at.% substitution of Fe by Mo in the Mn 1.15 Fe 0.80 P 0.45 Si 0.55 alloy. This reduction is found to be due to a strong reduction in the transition-induced elastic strain energy. The significantly enhanced reversibility of the magnetoelastic transition leads to a remarkable improvement of the reversible magnetocaloric properties, compared to the parent alloy. Based on the DFT calculations and the neutron diffraction experiments, we also elucidated the underlying mechanism of the tunable transition temperature for the (Mn,Fe) 2 (P,Si) family, which can essentially be attributed to the strong competition between the covalent bonding and the ferromagnetic exchange coupling. The present work provides not only a new strategy to improve the reversibility of a first-order magnetic transition but also essential insight into the electron-spin-lattice coupling in giant magnetocaloric materials.

Nonlinear influence of excess Mn on the magnetoelastic transition in (Mn,Cr)2Sb

The influence of excess Mn on the magnetoelastic ferromagnetic-to-antiferromagnetic transition Tt in the magnetocaloric compound (Mn,Cr)2Sb has been studied. With increasing excess Mn the magnetoelastic transition temperature for (Mn,Cr)2Sb initially increases and then decreases. This trend is accompanied by a strong reduction of the (Mn,Cr)Sb secondary phase. With increasing excess Mn a higher Cr content was found in the (Mn,Cr)Sb secondary phase in comparison to the matrix phase. This competition for Cr leads to a nonlinear dependence of Tt with increasing excess Mn at a fixed nominal Cr content. However, we observed that Tt depends linear on the c/a ratio for a wide range of temperatures from 170 to 350 K. A compositional diagram of the c/a ratio was constructed to assist the selection of (Mn,Cr)2Sb alloys with a desired transition temperature.

Large Low-Field Reversible Magnetocaloric Effect in Itinerant-Electron Hf1-xTaxFe2 Alloys.

First-order isostructural magnetoelastic transition with large magnetization difference and controllable thermal hysteresis are highly desirable in the development of high-performance magnetocaloric materials used for energy-efficient and environmental-friendly magnetic refrigeration. Here, we demonstrate large magnetocaloric effect covering the temperature range from 325 K to 245 K in Laves phase Hf1−xTaxFe2 (x = 0.13, 0.14, 0.15, 0.16) alloys undergoing the magnetoelastic transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state on decreasing the temperature. It is shown that with the increase of Ta content, the nature of AFM to FM transition is gradually changed from second-order to first-order. Based on the direct measurements, large reversible adiabatic temperature change (ΔTad) values of 2.7 K and 3.4 K have been achieved under a low magnetic field change of 1.5 T in the Hf0.85Ta0.15Fe2 and Hf0.84Ta0.16Fe2 alloys with the first-order magnetoelastic transition, respectively. Such remarkable magnetocaloric response is attributed to the rather low thermal hysteresis upon the transition as these two alloys are close to intermediate composition point of second-order transition converting to first-order transition.

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.

Intense ferromagnetic fluctuations preceding magnetoelastic first-order transitions in giant magnetocaloric LaFe13−xSix

First-order magnetic transitions are of both fundamental and technological interest given that a number of emergent phases and functionalities are thereby created. Of particular interest are giant magnetocaloric effects, which are attributed to first-order magnetic transitions and have attracted broad attention for solid-state refrigeration applications. While the conventional wisdom is that atomic lattices play an important role in first-order magnetic transitions, a coherent microscopic description of the lattice and spin degrees of freedom is still lacking. Here, we present a comparative neutron scattering study on the lattice and spin dynamics in intermetallic $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ and $\mathrm{La}{\mathrm{Fe}}_{11.2}{\mathrm{Si}}_{1.8}$, which represent one of the most classical giant magnetocaloric systems and undergo first-order and second-order magnetic transitions, respectively. While their spin-phonon coupling effects are quite similar, $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ exhibits a much stronger magnetic diffuse scattering in the paramagnetic state preceding its first-order magnetic transition, corresponding to intense ferromagnetic fluctuations. These dynamic insights suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.

Absolute calibration of the latent heat of transition using differential thermal analysis.

We describe a simple and accurate differential thermal analysis setup to measure the latent heat of solid state materials undergoing abrupt phase transitions in the temperature range from 77K to above room temperature. We report a numerical technique for the absolute calibration of the latent heat of transition without the need for a reference sample. The technique is applied to three different samples-vanadium sesquioxide undergoing the Mott transition, bismuth barium ruthenate undergoing a magnetoelastic transition, and an intermetallic Heusler compound. In each case, the inferred latent heat value agrees with the literature value within its error margins. To further demonstrate the importance of absolute calibration, we show that the changes in the latent heat of the Mott transition in vanadium sesquioxide (V2O3) remain constant to within 2% even as the depth of supersaturation changes by about 10K in non-equilibrium dynamic hysteresis measurements. We also apply this technique for the measurement of the temperature-dependent specific heat.

Reversibility of the Magnetocaloric Effect in the Bean-Rodbell Model

The applicability of magnetocaloric materials is limited by irreversibility. In this work, we evaluate the reversible magnetocaloric response associated with magnetoelastic transitions in the framework of the Bean-Rodbell model. This model allows the description of both second- and first-order magnetoelastic transitions by the modification of the η parameter (η<1 for second-order and η>1 for first-order ones). The response is quantified via the Temperature-averaged Entropy Change (TEC), which has been shown to be an easy and effective figure of merit for magnetocaloric materials. A strong magnetic field dependence of TEC is found for first-order transitions, having a significant increase when the magnetic field is large enough to overcome the thermal hysteresis of the material observed at zero field. This field value, as well as the magnetic field evolution of the transition temperature, strongly depend on the atomic magnetic moment of the material. For a moderate magnetic field change of 2 T, first-order transitions with η≈1.3−1.8 have better TEC than those corresponding to stronger first-order transitions and even second-order ones.

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.

Magnetoelastic transition and magnetocaloric effect in induction melted Fe100−xRhx bulk alloys with x = 50, 51

Magnetoelastic transitions (METs) in bulk in nearly equiatomic Fe-Rh alloys produced by arc melting may show poor reproducibility related to insufficient chemical homogeneity and presence of impurity phases in variable concentrations. To better understand the synthesis conditions that reliably yield bulk FeRh materials with reproducible MET characteristics, Fe100-xRhx alloys with x = 50, 50.5 and 51 at. % were prepared by induction melting and thermal annealing under identical conditions. The fabricated samples were cut into several slices, followed by characterization of METs in each of the slices using isothermal and isofield magnetization measurements, differential scanning calorimetry, and direct measurements of the magnetocaloric effect. All of the slices exhibit METs between the AFM and FM states, but the transitions are abrupt with nearly the same change of magnetization, ΔM, when x = 50.5 and 51, whereas for the x = 50 alloy the transition spreads over a wide temperature interval and ΔM may fluctuate by as much as 10 % from one specimen to another. A comparison of the magnetocaloric responses of x = 50 and 51 materials is presented. The clearly different effect of the magnetic field on the transition in both directions leads to significant differences in the reversibility and maximum values of the magnetic field-induced entropy and adiabatic temperature changes, as well as average hysteresis losses. In terms of reproducibility, our results suggest that induction melting is a more appropriate technique to prepare these binary alloys.