Effects of temperature and pressure on magnetic, elastic, structural, and thermal properties of ${\mathrm{T}\mathrm{b}}_5{\mathrm{S}\mathrm{i}}_2{\mathrm{G}\mathrm{e}}_2$ have been studied by means of macroscopic (thermal expansion and magnetization) and microscopic (neutron powder diffraction) techniques. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order structural phase change into a single first-order magnetic-crystallographic transformation at and above a tricritical point in the vicinity of 8.6 kbar. This pressure-induced coupling has a remarkable effect on the magnetocaloric effect, transforming ${\mathrm{T}\mathrm{b}}_5{\mathrm{S}\mathrm{i}}_2{\mathrm{G}\mathrm{e}}_2$ from an ordinary into a giant magnetocaloric effect material.

The design of nonmagnetic CuBe and multilayer $(\mathrm{Cu}\mathrm{Be}+\mathrm{MP}35\mathrm{N})$ hydrostatic pressure cells for magnetization and magnetostriction measurements in a pressure range up to $\text{2\hspace{0.5em}}\mathrm{GPa}$ at temperatures down to $\text{5\hspace{0.5em}}\mathrm{K}$ and in fields up to $\text{14\hspace{0.5em}}\mathrm{T}$ is presented. The safety operation of the cells is based on destruction tests described in this work. An influence of the pressure cells on measured magnetic magnitudes and the temperature induced pressure changes in the clamped cells have been carefully studied in a temperature range from $\text{350\hspace{0.5em}}\mathrm{K}$ down to $\text{5\hspace{0.5em}}\mathrm{K}$ using different pressure transmitting media.

The giant magnetocaloric compound ${\mathrm{G}\mathrm{d}}_5{\mathrm{G}\mathrm{e}}_4$ is the only member of the ${\mathrm{G}\mathrm{d}}_5({\mathrm{S}\mathrm{i}}_x{\mathrm{G}\mathrm{e}}_{1-x}{)}_4$ family where three-dimensional exchange interactions between two-dimensional correlated layers of the crystallographic structure are so weak that spontaneous ferromagnetism does not set in at any temperature. In this Letter we explore the possibility to reach the ferromagnetic state by application of hydrostatic pressure. Linear thermal expansion and magnetic measurements under pressure reveal that the reduction of the unit cell volume induces a spatially phase-segregated ground state below 10 kbar.

Linear thermal expansion and magnetostriction measurements under a magnetic field up to 20 kOe, applied parallel and perpendicular to the measuring direction and in the temperature range of 10–300 K, have been performed on a Ni2MnGa polycrystal. The effects of zero-field cooling and field cooling on both the self-strain (H=0) and the magnetic-field-induced strain (MFIS) have been studied. We have found that the MFIS strongly depends on whether a magnetic field was applied during the cooling process. The applied magnetic field facilitates the growth of specific orientation variants along the field direction as the sample is cooled down through martensitic transformation. However, the application of a field in the martensitic phase induces a negligible motion of twin boundaries. On this basis, the singular point detection technique performed on polycrystalline specimens results in being the easiest and most direct way by which to determine the anisotropy field in this class of materials. The temperature behavior of the anisotropy constant and field has been determined for Ni2MnGa.

Magnetic and structural properties of single crystalline ${\mathrm{Gd}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ under hydrostatic pressure have been characterized by using magnetization, linear thermal expansion, and compressibility measurements. A strong dependence of Curie temperature on pressure, $d{T}_C∕dP=+4.8\mathrm{K}∕\mathrm{kbar}$, is observed in contrast with the smaller values of about $3\mathrm{K}∕\mathrm{kbar}$ found in polycrystalline specimens. This difference reflects the role the microstructure may play in pressure-induced magnetic-crystallographic phase changes, likely related to stress relaxation at the grain boundaries, domain pinning and/or nucleation of defects. The pressure dependence of the critical magnetic field, $d(d{H}_C∕dT)∕dP$, drops at the rate $-0.122\left(5\right)\mathrm{kOe}∕\mathrm{K}\mathrm{kbar}$, which points to an enhancement of the magnetoelastic coupling with pressure. The latter affects the magnetocaloric behavior of the material at the rate $d\left(\Delta S_M\right)∕dP\cong 1.8\mathrm{J}∕\mathrm{kg}\mathrm{K}\mathrm{kbar}$. The linear thermal expansion confirms the strongly anisotropic change of the lattice parameters through the orthorhombic to monoclinic crystallographic transformation with $\Delta a∕a=+0.94\%$, $\Delta b∕b=-0.13\%$, and $\Delta c∕c=-0.22\%$. The structural transition temperature varies with pressure synchronously with the Curie temperature, and the size and shape of the strain anomalies remain nearly unaffected by the hydrostatic pressure, indicating, respectively, that the structural and magnetic transformations remain coupled, and the anisotropic behavior of the lattice is preserved as pressure increases. The room temperature linear compressibility data show that the magnetostructural transformation can be triggered isothermally at $\sim 6\mathrm{kbar}$ and that the compressibility is anisotropic.