Magnetostructural Transition Temperature Research Articles

Effect of cobalt doping on the structural, magnetic, optical, and electronic properties of LiFeCr4O8

Cobalt doping's influence on the structural, magnetic, and electronic properties of the double spinel LiFeCr4O8 was systematically investigated. Rietveld refinement revealed a consistent increase in lattice parameter a and unit cell volume with cobalt incorporation. Magnetic susceptibility measurements indicated three distinct transitions: an increase in the ferrimagnetic transition temperature (TN), a decrease in the magnetostructural transition temperature (TMS), and an increase in the spin gap transition by cobalt doping. Through diffuse reflectance spectroscopy the direct optical band gap (Eog) was obtained with values of 1.169 eV, 1.303 eV, 1.396 eV, and 1.230 eV for x = 0.0, 0.1, 0.2, and 0.5, respectively. The observed increase in Eog with cobalt doping suggests a widening of the band gap, which could be beneficial for optoelectronic applications. By means of X-ray photoelectron spectroscopy (XPS) the core levels of Li 1s, Co 3p, Fe 3p, Cr 3p, Fe 2p, Cr 2p, and O 1s were identified. XPS valence band (VB) analysis revealed a decreasing intensity at binding energy BE = 0 eV with respect to x = 0.0. Density functional theory calculations with Hubbard U correction (DFT + U) confirmed a ferrimagnetic semiconductor behavior with a direct electronic band gap (Eeg) of 1.085 eV.

Tuning of the magneto-caloric effects in Ni43Mn46In11 magnetic shape memory alloys by substitution of boron

In this study, we report the structural, magnetic, and magnetocaloric properties of B substitution on the Mn site in Ni43Mn46−x B x In11(x = 0.5, 1.0) Heusler alloys. Crystal structure analysis using room-temperature x-ray diffraction data reveals both samples have mixed phases composed of cubic and tetragonal phases. The structural and magnetic phase transition characteristic temperatures are determined using differential scanning calorimetry, isothermal magnetization (MT), and isofield magnetization (MH) measurements. Both alloys exhibit inverse and direct magnetocaloric effects in the vicinity of their magnetostructural transition and Curie temperature (T C), respectively. For Ni43Mn45.0B1.0In11 a maximum magnetic entropy change of 25.06 J kg−1 K−1 is observed at 250 K for a magnetic field change of 5 T.

Effect of heat treatment on coupled magnetostructural transition and magnetocaloric effect of MnNiGe system

Material with coupled magnetic and structural transition is of significant interest in the field of energy-efficient technology. In this work, a sharp first-order magneto-structural transition (MST) from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni2In-type austenite phase is observed in hexagonal Mn0.85Fe0.15NiGe compound through the heat treatment technique. Through heat treatment, the first-order MST for Mn0.85Fe0.15NiGe sample is observed to decrease drastically from 279.1 K to 211.8 K for the elevated annealing temperatures from 873 K to 1173 K. Furthermore, total entropy change (ΔS) is synergistically associated with the magnetic and structural entropy change in this system. This synergic feature governs a large ΔS ∼ 62.7 Jkg-1K−1, while magnetic entropy change is about 31.3 Jkg-1K−1 (∼52.1%) and the lattice part contributes the remaining ∼48.7% for the sample annealed at 1073 K due to the field change of 5T. In addition, a large effective refrigerant capacity reaches 226.7 Jkg−1 at 226 K for the sample due to the field change of 5T. Henceforth, this work shows an effective and efficient scheme to control the MST temperature of hexagonal phase-transition families using heat treatment method without changing the chemical composition of the sample. Our results based on heat treatment methodology provide a feasible platform for tuning MST and the associated caloric properties.

Optimizing magnetocaloric and thermoelectric performance of MnCoGe/BiSbTe composites by regulating magnetostructural transition and element diffusion

Composites consisted of MnCoGe magnetocaloric alloy and BiSbTe thermoelectric alloy were prepared to explore their potential use in all-solid-state cooling. It was found that in the composites the phase transition of MnCoGe from orthorhombic to hexagonal phase causes a large drop of magnetostructural transition temperature from room temperature to 270 K. The diffusion of Mn into BiSbTe leads to the formation of interfacial phase Mn(Bi,Sb)2Te4 and doping effect that depresses the thermoelectric performance. By adopting a low-temperature sintering strategy, the phase transition was restricted and the doping effect due to Mn diffusion was alleviated. As a result, the magnetostructural transition temperature around 300 K is maintained and the room-temperature magnetocaloric and thermoelectric properties are simultaneously optimized. The maximum magnetic entropy change of 4.3 × 10−3 J kg−1 K−1 at 2.0 T and maximum ZT value of 0.81 at 384 K have been obtained on the composite incorporated with 1 wt% MnCoGe. This work demonstrates that the control of phase transition and the doping effect of MnCoGe is crucial in optimizing the thermo-electro-magnetic properties of MnCoGe/BiSbTe composites.

Magnetocaloric and Magneto-transport Properties in Polycrystalline Ni<sub>56</sub>Mn<sub>20</sub>Ga<sub>24</sub> Heusler Alloy

Polycrystalline ingots of Ni-rich Ni56Mn20Ga24 alloy were prepared by conventional arc melting procedure. The magnetocaloric and magnetotransport properties of the alloy were investigated within a temperature range from 4.2-300 K and up to a magnetic field of 8 T. A large magnetocaloric effect (∆SM ~ 29 J/Kg K) was observed in the alloy in the vicinity of magneto-structural transition temperature at a change of 5 T magnetic field. From magnetic measurements, it was observed that the martensite to austenite transition was accompanied by higher to lower magnetization in higher magnetic fields. The large entropy change was explained by considering the first-order magneto-structural transition in this alloy. A high negative magnetoresistance (⁓7 %) associated with magnetic field induced first order magneto-structural transition was also observed near room temperature due to a change of 8 T magnetic field for this alloy.

Density functional theory study of energetics, local chemical environment and magnetic properties in a high-entropic MnNiSi0.2Ge0.2Sn0.2Al0.2Ga0.2 intermetallic magnet

Rare-earth-free magnetostructural MnNiSi-based solid solutions are considered as promising candidates for solid-state cooling applications. In this paper, we use density functional theory calculations to study the energetics, variations in atomic displacements and bond length, and magnetic properties of high-entropic, intermetallic MnNi-X (X = Si0.2Ge0.2Sn0.2Al0.2Ga0.2) magnet in both the low-symmetry Pnma and high-symmetry structures, where we confine the large configurational entropy to the non-magnetic X-site of the compound. Our calculations reveal that the high-entropic chemical substitution of Si0.2Ge0.2Sn0.2Al0.2Ga0.2 in the X-site carry fingerprints that favor a reduction in magnetostructural transition temperature with minimal impact of total magnetization. These results motivate a promising path of high-entropic X-site substitutions to tune the magnetostructural properties of MnNiSi-based solid solutions.

Large magnetocaloric effect near room-temperature by tuning the magneto-structural transition in (Mn, Fe)(Ni, Fe)Si alloy

The observation of near room-temperature coupling of both magnetic and structural transitions in Fe substituted Mn-Ni-Si alloy leads to an interesting magneto-structural transition (MST) phenomenon which intensifies the required magnetocaloric parameters for magnetic cooling applications. The crystal structure analysis reveals that temperature as well as Fe substitution work as the driving forces for deforming the orthorhombic unit cell and to stabilize the hexagonal phase. The correlation between the unit cell parameters of both phases confirms that a large change in unit cell volume (∼2.9%) is mainly responsible for the structural transition. The simultaneous substitution of Fe across Ni and Mn sites helps to tune the MST temperature. Both magnetization and calorimetric studies confirm the existence of first-order type structural transition across MST. The [Mn0.64 Ni0.64 Fe0.72 Si] alloy exhibits a large isothermal magnetic entropy change (ΔS M ) of −19.2 0.4 J kg−1 K−1 accompanied by a large value of effective refrigerant capacity (RC effe ) of 181.8 3.7 J kg−1 due to a magnetic field change of 0–30 kOe. The estimated total entropy change associated with the complete structural transition from the calorimetric studies sets an upper limit for the entropy change of the system. The low cost and non-toxic nature of constituent elements, composition-dependent tunability of the transition temperature, and near room-temperature large magnetocaloric parameters make this alloy potentially suitable to be used as a refrigerant in solid-state magnetic cooling technology.

Magnetostructural phase transitions and magnetocaloric effect in Mn-Fe-Ni-Ge-B alloys

The search for room-temperature high-performance magnetocaloric materials is highly stimulated towards magnetic refrigeration techniques. Here we report the boron doping effect on the magnetostructural transition and magnetocaloric effect of the Mn 0.89 Fe 0.11 NiGe alloy. Our studies reveal that the addition of only 0.2 at% boron induces a large magnetic entropy change (∆ S m ) of − 69.4 J kg −1 K −1 at 298.5 K under μ 0 H up to 7 T, which is ~ 34.0% higher than that of − 51.8 J kg −1 K −1 in the host alloy Mn 0.89 Fe 0.11 NiGe obtained at a much lower temperature of 267.5 K. With the increase of boron content, the phase transition temperature M s showed a nonlinear change that first increased and then decreased. The investigation of the microstructure by transmission electron microscopy suggests that the minor B atoms will get into the interstitial site of Mn 0.89 Fe 0.11 NiGe alloy, increase the cell volume, and stabilize the orthorhombic structure of the alloy. This could be the main reason for the increase in magneto-structural transition temperatures. Otherwise, the excessive B atoms will introduce precipitates, and cause the reduction of the magnetostructural transition temperature. It suggests that proper doping point defects are effective to tailor magneto-structural transition toward achieving the magnetocaloric effect. • B causes a nonlinear change of the magneto-structural transition temperature of Mn 0.89 Fe 0.11 NiGe. • Adding 0.2 at% B enhances magnetic entropy change by~ 34.0%. • The minor boron atoms will increase the cell volume and stabilize the martensitic phase. • Nano-precipitates at high B contents hinder the phase transition and decrease the transition temperature.

Large magnetocaloric effect and magnetoresistance in Fe-Co doped Ni50-(FeCo) Mn37Ti13 all-d-metal Heusler alloys

The effect of substituting FeCo in Ni site maintaining their same ratio on structural, magnetocaloric, and magneto-transport properties of all- d -metal Ni 50- x (FeCo) x Mn 37 Ti 13 ( x = 16, 18, and 20) Heusler alloys are investigated. These alloys undergo a first-order magnetostructural transition (MST) between low temperature weak magnetic martensite (monoclinic or orthorhombic structure) to a high temperature ferromagnetic (FM) austenite (cubic structure) phase around room temperature. The MST temperature is observed to shift towards the lower temperature with increasing (FeCo) x content. These alloys with x = 16, 18, and 20 exhibit isothermal magnetic entropy change (∆S M ) as large as about ~ 13.8 , ~ 12.7 and ~ 11.8 JkgK −1 across their MST associated with an effective refrigerant capacity (RC eff ) of about 119.9 , ~ 126.2 and ~ 194.4 J/kg respectively due to an application of 5 T magnetic field. It is interesting to note that ∆S M remains almost identical over a large temperature region with (FeCo) x content from 16 to 20 at%. In addition, the maximum values of large magnetoresistance (MR)~ − 26.1 % for x = 16, − 25.4 % for x = 18, and − 24.3 % for x = 20 are achieved at the field of 5 T. These materials with large magnetocaloric response in an extended temperature window around room temperature as well as large negative MR can be considered as potential candidates for multifunctional application. • We report the co-doping effect on magnetocaloric effect and magnetoresistance in all- d -metal Heusler alloys. • The magnetostructural transition (MST) shifts towards lower temperature with doping contents. • We observe large isothermal entropy change ( ∆ S M ) , and large magnetoresistance near room temperature.

Accelerated design of [formula omitted] alloys with targeted magnetostructural properties through interpretable machine learning

Data-driven machine learning (ML) models are developed to rapidly predict the magnetostructural transition temperature (Tt) and thermal hysteresis in the vast search space of MnNiSi-MTX solid solutions. Each alloy is represented in three unique ways based on: (1) chemical compositions, (2) descriptors describing average elemental properties, and (3) crystal structure and magnetic descriptors derived from density functional theory. The trained ML models are experimentally validated through two newly synthesized alloy compositions. While the Tt predictions show good agreement with experimental measurements, the thermal hysteresis predictions were inconclusive. The global and local behaviors of the trained models are then examined using novel post hoc model interpretability techniques. These techniques show that the ML models have learned the salient features that are known to govern the Tt of MTX compounds and give unique insights into how the model behaves for different alloying elements. The outcome of this work has major implications in the design of novel MTX-solid solutions with targeted Tt, in addition to demonstrating new techniques that make the ML results more interpretable. An interactive web application is built, where the researchers can query our trained ML models and design previously unexplored alloys with the desired Tt and thermal hysteresis properties.

Enhancing reversible entropy change of all-d-metal Ni37.5Co12.5Mn35Ti15 alloy by multiple external fields

The multiple external fields have been introduced to enhance the reversible entropy change of all-d-metal Ni37.5Co12.5Mn35Ti15 alloy. Our studies reveal that the reversible entropy change can be largely increased from 8.9 J kg−1K−1 to 24.1 J kg−1K−1 for a magnetic field change of 0.01 T to 5 T. The combined investigations of structural transition by transmission electron microscopy and magneto-strain reveal a strong coupling between magnetism and volume in the alloy, due to which, the magneto-structural transition temperature of Ni37.5Co12.5Mn35Ti15 decreases with the magnetic field by a rate of -1.4K T−1, and increases with the hydrostatic pressure by a rate +5K kbar−1, thus providing the condition for the promotion of reversible entropy change under multiple fields cycling.

Anomalous Thermoelectric Transport Properties of Fe‐Rich Magnetic FeTe

The interplay between magnetism and quantum effects has motivated several thermoelectric studies on iron‐telluride yet with little insight on the anomalous features in transport properties near magnetostructural transition temperature (≈70 K). A detailed investigation is carried out on Fe1.1Te by characterizing magnetic, heat capacity, galvanomagnetic, and thermoelectric transport properties to understand the electronic, magnetic, and structural origin of those anomalies. The magnetic susceptibility indicates a bicollinear stripe and short‐range ordering in the antiferromagnetic and paramagnetic domains, respectively. Hall conductivity and transverse magnetoresistance reveal a multicarrier transport impacted by spin fluctuations and magnons. Contributions from phonon‐drag and magnon‐drag are evaluated to understand the origin of the broad peak in antiferromagnetic thermopower. The peak at ≈50 K and the insignificant entropy contribution from the magnonic heat capacity support the phonon‐drag as the origin. The field‐dependent enhancement of thermal conductivity must be associated with field‐dependent spin‐phonon coupling modification. The field‐induced thermopower reduction can be attributed to the suppression of magnons or paramagnons, as evidenced by the magnetic susceptibility data. Above 70 K, the thermal conductivity drops sharply due to the structural change modifying phonon modes. Understanding these properties originated from the spin, and quantum effects are instrumental for designing high‐performance spin‐driven thermoelectrics.

Effect of Mn-site doping on the magnetofunctional behaviors of Mn5Si3 alloy

Present work reports a detailed investigation on the magnetoresistance and magnetocaloric behavior of Ni and Cr-doped Mn5Si3 alloys with general formula Mn5−xAxSi3 (where A = Ni/Cr; x = 0, 0.05, 0.1 and 0.2). Both pure (undoped) and doped alloys show a reasonably large amount of magnetoresistance (MR). Doping at Mn-site, both by Ni and Cr, results in a monotonic decrease in MR values. Magnetocaloric effect (MCE), on the other hand, is found to be interesting, and all the alloys show both conventional and inverse MCE around the magneto-structural transition temperature. Among the two types of MCE observed, the inverse MCE is found to decrease with increasing doping concentration and consistent with the MR behavior, whereas doping results in a significant increase in conventional MCE values.

Multicaloric Effects in (MnNiSi)1−x (Fe₂Ge) x Alloys

MnNiSi-based alloys, substituted with isostructural Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge as (MnNiSi) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , were prepared by arc-melting to examine their structural, magnetocaloric, and barocaloric properties. A simultaneous (coupled) first-order magnetic and structural, that is, magnetostructural, transition from a low-temperature ferromagnetic (FM) (TiNiSi-type) orthorhombic phase to a high-temperature paramagnetic (PM) (Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In-type) hexagonal phase was observed in all compositions, where 0.33 ≤ x ≤ 0.35, by magnetothermal and structural analyses. The magnetostructural transition temperature, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> , decreased from 350 to 256 K (for μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H = 0.1 T, on heating) by providing chemical pressure used by increasing the compositional variable, x, to 0.35. However, application of hydrostatic pressure overall decreased both T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> at dT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> /dP ~7.3 K/kbar and thermal hysteresis, ΔT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> , significantly. Indeed, in the x = 0.33 composition, ΔT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> was drastically reduced by >80% from 30 K at ambient pressure to 5 K at P = 7.6 kbar. A maximum magnetic entropy change, ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mag</sub> (μ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> H = 2 T), corresponding to 30 J/kg · K is noted at 270 K in (MnNiSi) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.67</sub> (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.33</sub> at an applied pressure of 7.9 kbar. For the x = 0.34 and 0.35 compositions, an application of P ~ 8 kbar resulted in the partial, and complete, decoupling of the magnetic (FM → PM) and structural (orthogonal → hexagonal) transitions, respectively, and consequently, a large drop in ΔS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mag</sub> . Application of P = 7.9 kbar on the x = 0.35 composition converted the first-order T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</sub> to the second-order Curie transition, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , and therefore loss of the structural transition indicating the stabilization of the high-temperature hexagonal structure. Overall, these features emphasize strong coupling between the magnetic spins and the lattice in MnNiSi-based alloys. Further fundamental and applied insight is obtained concerning pathways for optimizing the multicaloric response of MnNiSi-based alloys with isostructural substitution by Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ge for potential solid-state cooling applications.

Large Barocaloric Effect with High Pressure-Driving Efficiency in a Hexagonal MnNi0.77Fe0.23Ge AlloySupported by the National Natural Science Foundation of China (Grant No. 51722106), the National Key R&D Program of China (Grant No. 2019YFA0704904), Users with Excellence Program of Hefei Science Center CAS (Grant No. 2019HSC-UE009), and Fujian Institute of Innovation, Chinese Academy of Sciences.

The hydrostatic pressure is expected to be an effective knob to tune the magnetostructural phase transitions of hexagonal MM’X alloys (M and M’ denote transition metals and X represents main group elements). We perform magnetization measurements under hydrostatic pressure on an MM’X martensitic MnNi20.77Fe0.23Ge alloy. The magnetostructural transition temperature can be efficiently tuned to lower temperatures by applying moderate pressures, with a giant shift rate of –151 K/GPa. A temperature span of 30 K is obtained under the pressure, within which a large magnetic entropy change of –23 J⋅kg−1K−1 in a field change of 5 T is induced by the mechanical energy gain due to the large volume change. Meanwhile, a decoupling of structural and magnetic transitions is observed at low temperatures when the martensitic transition temperature is lower than the Curie temperature. These results show a multi-parameter tunable caloric effect that benefits the solid-state cooling.

Multipeak quasielastic light scattering and high-frequency electronic excitations in honeycomb Li2RuO3

We measured the temperature dependence of low-frequency Raman spectra in Li$_2$RuO$_3$, and observed multipeak quasielastic scattering in the Ru honeycomb polarizations below and above the magnetostructural transition temperature. We attribute this scattering to the fluctuations of the energy density in the spin system. High-frequency electronic light scattering was observed at 2150 $cm^{-1}$. Its intensity increased significantly below the transition temperature, confirming substantial modification of electronic structure due to removal of degeneracy in $t_{2g}$-manifold of Ru$^{4+}$ ions.

D band filling and magnetic phase separation in transition metal-doped Mn3SnC

The structural and magnetic properties of transition metal-doped Mn3SnC are studied with an aim to understand the effect of transition metal atom on magnetostructural properties of the antiperovskite compound. The doped Mn2.8T0.2SnC (T = Cr, Fe, Co, Ni and Cu) compounds show a distinctly different magnetic behavior which can be related to electronic filling of the d band of the transition metal atom. While Cr and Fe doped Mn3SnC show properties similar to that of Mn3SnC, the Co, Ni and Cu doped compounds exhibit nucleation of secondary phases which are devoid of carbon and having Heusler and DO19 type hexagonal structure. A strong magnetic interaction is observed between the impurity phases and the major antiperovskite phase leading to a sharp decrease in magnetostructural transition temperature of the antiperovskite phase and a cluster glassy ground state.

Enhanced Ferromagnetism in Cylindrically Confined MnAs Nanocrystals Embedded in Wurtzite GaAs Nanowire Shells.

Nearly a 30% increase in the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry, and the first-principles calculations. The effect relies on defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires, where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs-MnAs granular system. Such a unique combination of two types of hexagonal lattices provides a possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.

Effects of heat treatments on magneto-structural phase transitions in MnNiSi-FeCoGe alloys

A first-order magneto-structural transition from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni2In-type austenite phase was observed in (MnNiSi)0.62(FeCoGe)0.38. In this work, we demonstrate that the first-order magneto-structural transition temperature for a given composition is tunable over a wide temperature range through heat treatment and hydrostatic pressure application. The first-order transition temperature decreased by over 150 K as the annealing/quenching temperature went from 700 to 1050 °C. The largest maximum magnetic-field-induced isothermal entropy change with μ0ΔH=7 T reached −36.2 J/kg-K for the sample quenched at 700 °C, and the largest effective refrigeration capacity reached 344.5 J/kg for the sample quenched at 800 °C. Similar to the influence of annealing temperatures, the first-order martensitic transition temperatures decreased as the applied hydrostatic pressure increased until they were completely converted to second order. Our results suggest that the class of MnNiSi-based alloys is a promising platform for tailoring working temperature ranges and associated magnetocaloric effects through heat treatment or application of hydrostatic pressure.

Magnetostructural transition in Co-Mn-Ge systems tuned by valence electron concentration

The effects of annealing process and chemical pressure on the structural and magnetocaloric properties of CoMn1−xVxGe (x = 0.05 and 0.10) alloys have been studied. Rietveld refinement of XRD patterns revealed that all samples are single phase and they crystallize in hexagonal structure with a space group (P63/mmc) at room temperature. The structural phase transition temperature Tstruct decreases with increasing vanadium (V) content drastically in compare to CoMnGe alloy which exhibit a first order structural transition at around 450 K. The decrease in Tstruct. by substituting fourth element in CoMnGe-based alloys are discussed in frame of changing its valence electron concentration (e/a) values. We found that, alloys exhibiting a first order magneto-structural transition (FOMT) around room temperature, have an e/a value of 6.62–6.67. The e/a dependence of the magnetostructural transition temperature can be used as a guide to obtain an alloy with FOMT around room temperature.