We investigated the structural, magnetic, magnetocaloric, and magnetotransport properties of Ni50Mn35In15 Heusler alloys via partial substitution of Ni with 3 at.% Bi (Ni47Bi3Mn35In15) and 3 at.% Si (Ni47Si3Mn35In15) synthesized by arc melting. X-ray diffraction confirms a predominantly L21 cubic structure (space group Fm-3m), while SEM/EDX analysis verifies compositional homogeneity. Temperature-dependent magnetization measurements reveal that the Bi-substituted alloy exhibits a first-order magnetostructural transition associated with the martensitic transformation, followed by a second-order magnetic phase transition from ferromagnetic to paramagnetic behavior near the Curie temperature. In contrast, the Si-substituted alloy shows a single second-order transition with negligible thermal hysteresis, indicating suppression of the martensitic phase. The Curie temperature decreases from 324 K for the parent alloy to 313 K and 286 K for the Bi- and Si-substituted alloys, respectively. A maximum magnetic entropy change of 6.0 Jkg−1K−1 and 4.5 Jkg−1K−1 is observed for the Bi- and Si-substituted alloys, respectively, under an applied magnetic field change of 50 kOe, with corresponding relative cooling power values of 303 Jkg−1 and 345 Jkg−1. These results demonstrate that lattice expansion (Bi) and contraction (Si) distinctly modify Mn–Mn exchange interactions, enabling tunable magnetocaloric performance in Ni–Mn–In Heusler alloys.

The phase transitions of a series of Co-doped Heusler alloys, Ni2Mn1-xCoxGa ( 0 <= x <= 0.2), were investigated experimentally using the magnetization measurements, x-ray diffraction, and calorimetric measurements up to their respective melting points. With increasing Co concentration, the structural transition temperatures, Curie temperatures, and melting points, were observed to increase, while the order-disorder transition temperatures decreased. Temperature-dependent x-ray diffraction experiments revealed two different crystal structures in the low-temperature martensite phase for different Co concentrations. However, above their respective structural transitions, both low-temperature crystal structures transformed into the L2( 1) cubic structure. These findings enabled the construction of a complete magnetic and structural phase diagram for Ni2Mn1-xCoxGa, spanning from cryogenic temperatures to the melting points. The temperature-dependent XRD results revealed the abrupt changes in interatomic Mn-Mn distances, which validates the crucial role of Mn-Mn interatomic distance and the effect of the magnetic coupling competition in the structural stability between the martensite phase and austenite phase.

In this study, phase transitions (structural and magnetic) and associated magnetocaloric properties of stoichiometric MnCoGe have been investigated as a function of annealing pressure. Metastable phases were generated by annealing at 800 °C followed by rapid cooling under pressures up to 6.0 GPa. The x-ray diffraction results reveal that the crystal cell volume of the metastable phases continuously decreases with increasing thermal processing pressure, leading to a decrease in the structural transition temperature. The magnetic and structural transitions merge and form a first-order magnetostructural transition between the ferromagnetic orthorhombic and paramagnetic hexagonal phases over a broad temperature range (>80 K) spanning room temperature, yielding considerable magnetic entropy changes. These findings demonstrate the utility of thermal processing under high pressure, i.e., high-pressure annealing, to control the magnetostructural transitions and associated magnetocaloric properties of MnCoGe without altering its chemical composition.

The phase transitions in MnNiGe compounds were explored by manipulating the heat treatment conditions and through hydrostatic pressure application. As the quenching temperature increased, both the first-order martensitic structural transition temperatures and magnetic transition temperatures decreased relative to those in the slowly-cooled samples. When the samples were quenched from 1200 circle C, the first-order martensitic structural transition temperature lowered by more than 200 K. The structural transitions also shifted to lower temperature with the application of hydrostatic pressure during measurement. Temperature-dependent x-ray diffraction results reveal that the changes of the cell parameters resulting from the structural transitions are nearly identical for all samples regardless of the extensive variation in their structural transition temperatures. In addition, neutron scattering measurements confirm the magnetic structure transition between simple and cycloidal spiral magnetic structures.

Metastable phases were formed in Mn1−xCoxNiGe (x=0.05 and 0.08) by annealing at 800 °C followed by rapid cooling, i.e., quenching, at ambient pressure (P=0) and under a pressure of P=3.5 GPa, and their phase transitions and associated magnetocaloric properties were investigated. The crystal cell volumes of the metastable phases decreased, and their structural transitions significantly shifted to lower temperatures relative to those of the slow-cooled compounds, with a greater reduction observed in the samples where the rapid cooling occurred under high pressures. The magnetic and structural transitions coupled to form a magnetostructural transition in the metastable phases, resulting in large magnetic entropy changes up to −79.6 J kg−1 K−1 (x=0.08) for a 7-T field change. The experimental results demonstrate thermal quenching and high-pressure annealing as alternative methods to create magnetostructural transitions, without modifying the compositions of the materials.

The magnetic, structural, and thermal behaviors of the Cu-doped Heusler alloy Ni2Mn1-xCuxGa (0 <= x <= 0.4) were studied as a function of concentration x. As the Cu concentration increased, the structural transition temperatures increased, whereas the chemical order-disorder transitions and melting points decreased. The experimental results from temperature dependent X-ray diffraction reveal different crystal structures of the martensite phase at low temperatures for samples with different x, but all the samples ultimately crystallized in the L2(1) cubic crystal structure upon heating above their respective structural transitions. The experimental data were used to construct a comprehensive magnetic and structural phase diagram as a function of x from below their respective structural transition temperatures to their melting temperatures. The XRD analysis shows that the observed volume reduction is associated with the increasing structural transition temperature. Therefore, one of the samples was annealed under high pressure to permanently reduce its volume, and the correlation between the increasing structural transition temperatures and volumes was confirmed. (C) 2021 Elsevier B.V. All rights reserved.

The effects of doping, hydrostatic pressure, and thermal quenching on the phase transitions and magnetocaloric properties of the Mn1−xCoxNiGe system have been investigated. Cobalt doping on the Mn site shifted the martensitic structural transition toward lower temperature until it was ultimately absent, leaving only a magnetic transition from a ferromagnetic (FM) to a paramagnetic (PM) state in the high-temperature hexagonal phase. Co-occurrence of the magnetic and structural transitions to form a first-order magnetostructural transition (MST) from the FM orthorhombic to the PM hexagonal phase was observed in samples with 0.05 < x < 0.20. An additional antiferromagnetic–ferromagnetic-like transition was observed in the martensite phase for 0.05 < x < 0.10, which gradually vanished with increasing Co concentration (x > 0.10) or magnetic field (H > 0.5 T). The application of external hydrostatic pressure shifted the structural transition to lower temperature until an MST was formed in samples with x = 0.03 and 0.05, inducing large magnetic entropy changes up to −80.3 J kg−1 K−1 (x = 0.03) for a 7-T field change under 10.6-kbar pressure. Similar to the effects of the application of hydrostatic pressure, an MST was formed near room temperature in the sample with x = 0.03 by annealing at high temperature (1200 °C) followed by quenching, resulting in a large magnetic entropy change of −56.2 J kg−1 K−1. These experimental results show that the application of pressure and thermal quenching, in addition to compositional variations, are effective methods to create magnetostructural transitions in the MnNiGe system, resulting in large magnetocaloric effects.

Bulk Mn2-xFe1+xGe (0.0 <= x <= 1.0) alloys have been synthesized by arc-melting followed by a low temperature homogenization thermal annealing, whereas for comparison purposes the Mn2FeGe alloy was also produced in ribbon form by rapid solidification. A study of the structural and magnetic properties is presented. Contrary to theoretical predictions, Mn2FeGe crystallizes in a hexagonal DO19 crystal structure (space group P63/mmc) and orders ferromagnetically with a saturation magnetization (M-S) value of similar to 1.7 mu(B)/f.u. in the ground state. With the substitution of Fe for Mn in bulk Mn2-xFe1+xGe, we observed an increase in the FM interactions with a maximum M-S value of 5.1 mu(B)/f.u. for x = 1.0, and a significant progressive increase in the Curie temperature (T-C) in a wide range spanning similar to 200 K to over 400 K.

In this work, the phase transitions of stoichiometric MnCoGe alloys were studied by systematically varying the annealing conditions and applying hydrostatic pressure. First-order martensitic structural transitions from the Ni2In-type hexagonal austenite phase to the TiNiSi-type orthorhombic martensite phase spanned a wide temperature window (>200 K) as a result of quenching the samples at temperatures ranging from the solid phase at 700 °C to the liquid phase at 1150°C. Despite the large variation in their structural transition temperatures, the changes in cell parameters across the structural transitions and the Curie–Weiss temperatures of the martensite/austenite phase were relatively small. For the sample quenched from 800°C, coupled magnetostructural transitions were observed, and the largest maximum magnetic entropy change was found to be −ΔSmax=33.6 J/kg K for a 7-T field change. The coupled magnetostructural transitions and the corresponding magnetic entropy enhancements were found to also be achievable by applying hydrostatic pressures. Meanwhile, as the quenching temperatures or hydrostatic pressures increased, the first-order martensitic structural transition shifted toward lower temperature until it was ultimately absent, in which case only the crystal structure and magnetic transition of the Ni2In-type hexagonal austenite phase were present.

Three temperature-induced phase transitions at T=T1, TM/TA, and TC, related to the ferromagnetic order of the martensitic phase (FMMP), martensitic (structural) transitions (MT), and the ferromagnetic order of the austenitic phase (FMAP), respectively, have been observed in the off-stoichiometric Heusler alloys, Ni50-xCoxMn35In15 (x=1, 2.5) and Ni45Co5Mn37In13. The phase transitions temperatures are found to be depended on alloy composition. A kinetic arrest of the AP was observed for Ni47.5Co2.5Mn35In15 in the magnetization measurements during field-cooling cycle (FCC) at 50 kOe. Depending upon the cooling protocols, ZFC and FCC (at H = 50 kOe), two different ground states of the alloys can be found in Ni47.5Co2.5Mn35In15 and Ni45Co5Mn37In13 alloys. The ground states (T=4.2 K and external field H=0) of the alloys was found to be characterized by three main line: two, partially overlapping, at higher frequencies (300-450 MHz), most likely corresponding of manganese resonance lines and one at lower frequency at about 200 MHz. A significant shift in the spectrum of Ni45Co5Mn37In13 by about 100 MHz to higher frequencies was observed. The correlation of magnetizations obtained from magnetic moment and NMR studies is discussed.

Search for low-moment magnetic materials with high spin-polarization is important for emerging spintronics applications. In this work, we have conducted detailed growth and characterization along with complementary first-principles calculations to investigate the structure and magnetism of Mn2FeSi, which is a prospective inverse-Heusler material. We confirm that Mn2FeSi adopts a cubic inverse-Heusler structure, in excellent agreement with theory. The magnetic and resistivity measurements show an antiferromagnetic behavior with a Neel temperature of 48 K, which is consistent with prior experimental reports. We find that a low-moment state with higher ordering temperature (150-200 K) can be stabilized under certain growth conditions. Supporting calculations show that Neel-type antiferromagnetic states are energetically very close to the ferrimagnetic ground state. Our work provides evidence that Mn2FeSi may be interesting for exploring newer applications with low-moment materials, but the ordering temperatures are low for viable practical applications. (C) 2020 Elsevier B.V. All rights reserved.

In this work, the phase transitions and magnetocaloric properties of Mn1−xAuxCoGe (0≤x≤0.025) alloys were studied as a function of concentration x and applied hydrostatic pressure. The increasing substitution of Au for Mn results in the decrease of the first-order martensitic transition temperature, and this first-order martensitic transition was ultimately converted to a second-order magnetic transition when the Au substitution (x) reached 0.025. The magnitudes of the maximum magnetic entropy changes increased when the magnetic and structural transitions were coupled, which occurred for 0.005≤x≤0.020. The largest maximum magnetic entropy change for a field change of μ0ΔH=7 T was 33.1 J/kg K for the sample with x=0.020. Similar to the effect of Au substitution, the first-order martensitic transition temperature initially decreased, and then converted to second order, when the applied hydrostatic pressure reached a large enough value. Interestingly, both Au substitution and pressure application cause a volume reduction and, in both cases, the first-order martensitic transition temperature initially reduced and then converted to second-order. These results suggest two different methods of tuning the transition temperatures in these magnetocaloric materials. One can either apply hydrostatic pressure and temporarily adjust the transition temperatures or modify the composition chemically and permanently change the transition temperatures.

The structural, magnetic, and magnetocaloric properties of the Bi-doped Heusler alloy Ni49BiMn35In15 have been investigated using room temperature X-ray diffraction (XRD) and magnetization measurements in a temperature interval of 5-400 K. The alloy at room temperature was found to be in a mixture of a high temperature austenite phase (AP) and a low temperature martensite phase (MP). A drastic shift in the martensitic transition temperature at the rate of 16 K/T from 197 K to lower temperatures was observed. A kinetic arrest phenomenon of the AP was observed in the magnetization and electrical resistivity measurements during field-cooled (FC) measurements at 5T. A metamagnetic behavior characterized by a jump in magnetization in the isothermal M(H) curves near TM was observed. The maximum value of the magnetic entropy change and refrigerant capacity at Curie temperature were found to be 5.5 Jkg-1K-1 and 312 Jkg-1 for μoΔH = 5T, respectively. A large magnetoresistance value of -56% was found near the martensitic transition.

Magnetization, electrical resistivity, magnetoresistance, and Hall resistivity of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5 Heusler alloys were studied in a temperature range T = 80-400 K in magnetic fields up to 20 kOe. Both alloys exhibit a martensitic transformation from a high-temperature ferromagnetic austenite phase to a low-temperature, low-magnetization martensitic phase. The electrical resistivity nearly doubles as a result of the martensitic transformation, reaching 180 and 100 mu Omega cm in the martensitic states of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5, respectively. The temperature dependence of the electrical resistivity does not corresponded with the Mooij correlation. The magnetoresistance is negative with a narrow negative peak at the martensitic transition. Normal and anomalous Hall effect coefficients were determined by fitting the field dependences of the Hall resistivity using magnetization data. The coefficients of the normal Hall effect for both compositions were found to decrease with temperature from positive values in the austenite to negative values in the martensite phase. None of the known correlations between the anomalous Hall effect coefficient and resistivity were satisfied. Significant changes in the values of the anomalous Hall coefficients during the martensitic transformation are explained by the difference in spin-up and spin-down state occupations in the martensite and austenite phases. First-principles calculations of the electronic structures confirm this explanation.

The relaxation processes of the adiabatic temperature changes (Delta T-ad) at the phase transitions in Ni45Mn43CoSn11, Ni50Mn36.5In13.5, and Ni50Mn35In14.25B0.75 Heusler alloys with different magnetic structures have been studied using a direct extraction method in magnetic fields up to 14 T. It has been found that Delta T-ad exhibits short relaxation times (less than 10(-1) (s)) in the vicinity of the second order phase transitions at the Curie temperatures. The relaxation times of the first order martensitic transitions strongly depend on the latent heat of the transition and can be characterized by a logarithmic law. (C) 2019 Elsevier B.V. All rights reserved.