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.

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.

The influence of hydrostatic pressure (up to 5 kbar) on the magnetic structure of ${\mathrm{Ce}}_2{\mathrm{Fe}}_{17}$ was investigated using neutron diffraction in the temperature range from 2 to 300 K. The existence of a collinear ferromagnetic phase below 95 K with a magnetic moment of Fe, $m_{\mathrm{Fe}}{\text{=2.0\hspace{0.17em}μ}}_B,$ was confirmed at ambient pressure. Magnetic peaks present between 95 and 205 K correspond to an incommensurate antiferromagnetic structure with a wave vector changing its value from ${\tau }_1{\text{=0.026\hspace{0.17em}Å}}^{\text{−1}}$ at 100 K to ${\tau }_1{\text{=0.034\hspace{0.17em}Å}}^{\text{−1}}$ at 205 K. A helical model is used to describe the magnetic structure. Application of high pressures leads to significant changes of the magnetic structure. The ferromagnetic phase, suppressed in the studied temperature range by pressures higher than 3 kbar, gets substituted by a new incommensurate antiferromagnetic phase. This phase can be described as a superposition of the helical structure with a second antiferromagnetic coupling with propagation vector ${\tau }_2{\text{≈0.078\hspace{0.17em}Å}}^{\text{−1}}$ at 40 K under pressures above 3 kbar. The correlation with recent results of magnetization studies under pressure is analyzed and discussed.