The Effects of Pressure and Magnetic Field on Phase Transitions and Related Physical Properties in Solid State Caloric Materials

Solid-state caloric effects, such as the magnetocaloric (MCE) and barocaloric (BCE) effects, may be utilized in future cooling technologies that are more efficient and environment-friendly. Large caloric effects often occur near phase transitions, especially near coupled first-order magnetostructural transitions (MST), and are initiated by external parameters, such as magnetic field or hydrostatic pressure. In this dissertation, the effects of pressure, temperature, and magnetic field on the phase transitions in three material systems are studied in order to elucidate how the respective caloric effects are affected. In the first study, the realization of a coupled MST in a MnNiSi-based system through isostructural alloying is explored, which resulted in a giant conventional MCE. The MST shifts towards lower temperature with increasing applied hydrostatic pressure, whereas it shifts towards higher temperature with an increase in magnetic field. The strong pressure dependence along with a large volume change during the MST suggested the possibility of pressure-induced BCE in this material. In a subsequent study, we observed a giant hydrostatic pressure induced inverse BCE through pressure-dependent calorimetric measurements. The multiple caloric effects in the same material for the same phase transition qualify this material as a multicaloric material. In second study, we investigated the hydrostatic pressure dependence of the metamagnetic transitions in DyRu2Si2, which shows multiple metamagnetic transitions at atmospheric pressure. With the application of moderate hydrostatic pressure, the metamagnetic transitions disappeared, but then reappeared with increasing pressure. We discuss the pressure-induced magnetostrictive behavior, the variation of the entropy changes with pressure, and a possible origin of the pressure-dependent behavior in light of the variation of the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interactions. For x = 0.25 in the Ni2Mn1-xCuxGa Heusler alloy series, the structural and magnetic transitions coincide to create a coupled first-order MST. Since giant MCE was reported for this system, it is useful to understand the underlying physics driving the coupling of transitions. Although first-order transitions cannot be investigated through the critical behavior analysis, the structural and magnetic transitions in Ni2MnGa (parent alloy) and Ni2Mn0.85Cu0.15Ga are not coupled. In this case, investigating the critical behaviors of the two alloys near their second-order phase transitions will provide insight as to how the magnetism in these materials evolve with increasing copper doping. In this study, through the calculated critical exponent values, we identified the universality classes which best described the parent and Cu-doped (x = 0.15) alloys. The exponent values shed light into the range of the magnetic interactions, and the evolution of the interactions due to the non-magnetic Cu substitution scheme. This type of analysis can be performed in other material systems to get a picture of the systematic trends, through doping or other processes, such as applied pressure, that lead to first-order phase transitions

Critical behavior in Ni2MnGa and Ni2Mn0.85Cu0.15Ga

The critical behaviors of polycrystalline Ni2MnGa and Ni2Mn0.85Cu0.15Ga have been examined through high-resolution bulk magnetization measurements. The critical exponents, β and γ, were derived from modified Arrott plots using the Kouvel-Fisher method. The values of the extracted critical exponents satisfied the scaling equation of state and associated exponent relations, indicating self-consistency of the extracted values. In Ni2MnGa, the critical exponents (β = 0.401 ± 0.003, γ = 1.27 ± 0.02) indicate a deviation from the 3D-Heisenberg values toward the mean-field values, likely due to the presence of long-range Ruderman-Kittel-Kasuya-Yosida interactions. However, the critical exponents of Ni2Mn0.85Cu0.15Ga (β = 0.389 ± 0.004, γ = 1.39 ± 0.02) are closer to the 3D-Heisenberg values. This indicates a weakening of the long-range exchange interactions due to the substitution of Cu in the Mn site

Controlling the microstructure and associated magnetic properties of Ni0.2Mn3.2Ga0.6 melt-spun ribbons by annealing

Here we report on the structural and magnetic properties of Ni0.2Mn3.2Ga0.6 meltspun ribbons. The as-spun ribbons were found to exhibit mixed cubic phases that transform to non-cubic structure upon annealing. Additionally, an amorphous phase was found to co-exist in all ribbons. The SEM images show that minor grain formation occurs on the as-spun ribbons. However, the formation of extensive nano-grains was observed on the surfaces of the annealed ribbons. While the as-spun ribbons exhibit predominantly paramagnetic behavior, the ribbons annealed under various thermal conditions were found to be ferromagnetic with a Curie temperature of about 380 K. The ribbons annealed at 450 ◦C for 30 minutes exhibit a large coercive field of about 2500 Oe. The experimental results show that the microstructure and associated magnetic properties of the ribbons can be controlled by annealing techniques. The coercive fields and the shape of the magnetic hysteresis loops vary significantly with annealing conditions. Exchange bias effects have also been observed in the annealed ribbons

Controlling the microstructure and associated magnetic properties of Ni0.2Mn3.2Ga0.6 melt-spun ribbons by annealing

Here we report on the structural and magnetic properties of Ni0.2Mn3.2Ga0.6 melt- spun ribbons. The as-spun ribbons were found to exhibit mixed cubic phases that transform to non-cubic structure upon annealing. Additionally, an amorphous phase was found to co-exist in all ribbons. The SEM images show that minor grain forma- tion occurs on the as-spun ribbons. However, the formation of extensive nano-grains was observed on the surfaces of the annealed ribbons. While the as-spun ribbons exhibit predominantly paramagnetic behavior, the ribbons annealed under various thermal conditions were found to be ferromagnetic with a Curie temperature of about 380 K. The ribbons annealed at 450 ◦C for 30 minutes exhibit a large coercive field of about 2500 Oe. The experimental results show that the microstructure and associated magnetic properties of the ribbons can be controlled by annealing techniques. The coercive fields and the shape of the magnetic hysteresis loops vary significantly with annealing conditions. Exchange bias effects have also been observed in the annealed ribbons

Tuning martensitic transitions in (MnNiSi)0.65(Fe2Ge)0.35 through heat treatment and hydrostatic pressure

A first-order magneto-structural transition from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni 2In-type austenite phase was observed in (MnNiSi) 0.65(Fe 2Ge) 0.35. 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. The first-order transition temperature was reduced by over 100 K as the annealing temperature went from 600 to 900 °C, and this first-order transition was converted to second order when the sample was annealed at 1000 °C. The maximum magnetic-induced isothermal entropy change with μ0ΔH=7 T reaches -58 J/kg K for the sample annealed at 600 °C, and the relative cooling power reaches 558 J/kg for the sample annealed at 700 °C. Similar to the influence of annealing temperatures, the first-order martensitic transition temperatures were reduced as the application of hydrostatic pressure increased until they were converted to second order. Our results suggest that the (MnNiSi) 0.65(Fe 2Ge) 0.35 system is a promising platform for tuning magneto-structural transitions and the associated magnetocaloric effects. Furthermore, a similar heat treatment methodology or application of hydrostatic pressure can be applied to MnNiSi-based shape memory alloys to tailor their working transition temperatures

The influence of hydrostatic pressure on the magnetic and magnetocaloric properties of DyRu2Si2

We report the magnetic and magnetocaloric properties of the tetragonal rare-earth compound DyRu2Si2 under applied hydrostatic pressure. The isothermal entropy change (ΔS) and the adiabatic temperature change (ΔTad) were calculated from magnetization data collected at different applied pressures and from heat capacity measurements conducted at atmospheric pressure, respectively. The application of hydrostatic pressure significantly modified the multi-step magnetization curve and the saturation magnetization. A suppression of the magnetization was observed for P = 0.588 GPa and P = 0.654 GPa whereas, at about P ≈1 GPa, the saturation magnetization increased and the magnetization isotherms again resembled the curves measured at atmospheric pressure. A small thermal hysteresis was observed between the heating and cooling M(T) curves at Tt=3.4 K, with an applied magnetic field of H = 0.1 T. This thermal hysteresis indicates a first-order like transition which was also supported by the Arrott plot analysis. The volume magnetostriction was estimated from the pressure-dependent magnetization measurements using a Maxwell relation

Asymmetric switchinglike behavior in the magnetoresistance at low fields in bulk metamagnetic Heusler alloys

A novel physical phenomenon has been observed that resembles a large asymmetric switchinglike magnetoresistance at low applied fields in bulk metamagnetic Heusler alloys. A thermally activated isothermal forward metamagnetic transition with a signature of a pronounced time-dependent relaxation was observed in a bulk B-substituted off-stoichiometric Ni-Mn-In Heusler alloy, whereas the reverse metamagnetic transition exhibited the usual athermal behavior with no thermal activation. The asymmetry between the forward and reverse metamagnetic transitions resulted in a large switchinglike, low-field magnetoresistance (∼16% for a field change of B = 0 → 0.25 T at T = 304 K) in the bulk Heusler alloys, Ni50Mn35In15-xBx (1 ≤ x ≤ 2). © 2014 American Physical Society

Evidence for topological semimetallicity in a chain-compound TaSe3

Among one-dimensional transition-metal trichalcogenides, TaSe3 is unconventional in many respects. One is its strong topological semimetallicity as predicted by first-principles calculations. We report the experimental investigations of the electronic properties of one-dimensional-like TaSe3 single crystals. While the b-axis electrical resistivity shows good metallicity with a high residual resistivity ratio greater than 100, an extremely large magnetoresistance is observed reaching ≈7 × 103% at 1.9 K for 14 T. Interestingly, the magnetoresistance follows the Kohler’s rule with nearly quadratic magnetic field dependence, consistent with the electron–hole compensation scenario as confirmed by our Hall conductivity data. Both the longitudinal and Hall conductivities show Shubnikov-de Haas oscillations with two frequencies: Fα ≈ 97 T and Fβ ≈ 186 T. Quantitative analysis indicates that Fα results from the two-dimensional-like electron band with the non-trivial Berry phase [1.1π], and Fβ from the hole band with the trivial Berry phase [0(3D) − 0.16π(2D)]. Our experimental findings are consistent with the predictions based on first-principles calculations