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Samarium (Sm) has antiferromagnetic (AFM) ordering at the hexagonal site at ${T}_{\mathrm{N}}(\text{hex})=106$ K, and at the cubic site at ${T}_{\mathrm{N}}(\text{cub})=14$ K at ambient pressure. The structural transition from the so-called Sm-type structure to the double hexagonal close-packed (dhcp) structure occurs at approximately 6 GPa. According to electrical resistance measurements, the shift of ${T}_{\mathrm{N}}$(hex) toward the low-temperature side occurs simultaneously with the shifting of ${T}_{\mathrm{N}}$(cub) toward the high-temperature side with increasing pressure. We conducted dc magnetic measurements on Sm at high pressure up to 30 GPa using a SQUID-based vibrating-coil-magnetometer to pursue ${T}_{\mathrm{N}}$(cub) at high pressure: The magnetic measurements revealed stepwise anomalies below 2 GPa. A ferromagnetic (FM) anomaly was observed in the dhcp structure, suggesting that spins at both structural sites formed the FM magnetization. In the face-centered cubic (fcc) structure at above 12 GPa, a reduction in the magnetic signal occurred. In the distorted fcc structure at above 20 GPa, sufficient suppression of the FM moment was observed and afterward the diamagnetic signal suggesting the possibility of superconductivity was observed.

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations.

We investigated transport and magnetic properties of single crystal ${\mathrm{NdFeAsO}}_{0.9}{\mathrm{F}}_{0.1}$ under hydrostatic pressures up to 50 GPa. The ambient pressure superconductivity at ${T}_{c}\ensuremath{\sim}$ 45.4 K was fully suppressed at ${P}_{c}\ensuremath{\sim}21$ GPa. Upon a further increase of pressure, ferromagnetism associated with the order of the rare-earth subsystem was induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M(H$) loops and the strong increase in the field cooled data $M(T$) toward low temperatures. We also show that the temperature evolution of the electrical resistivity as a function of pressure is consistent with a crossover from a Fermi liquid to non-Fermi liquid to Fermi liquid. The Hall measurements suggest that the multiband electronic structures have changed with pressure, which should also affect the resistivity behavior. These results give access to the high-pressure side of the superconducting phase diagram in the 1111 type of materials.

We have performed ultrasonic measurements on the $\ensuremath{\alpha}$-pyrochlore spin compounds ${R}_{2}$Ti${}_{2}$O${}_{7}$, $R=$ Tb, Dy, and Ho. A distinct dip appears around 80 K, especially in transverse modes $({C}_{11}\ensuremath{-}{C}_{12})/2$ and $C$${}_{44}$ as a function of temperature in Dy${}_{2}$Ti${}_{2}$O${}_{7}$ and Ho${}_{2}$Ti${}_{2}$O${}_{7}$ compounds. This anomaly may correspond to a structural change detected by the temperature dependence of the permittivity and of the loss-angle tangent. No distinct anomaly was observed at lower temperatures in Dy${}_{2}$Ti${}_{2}$O${}_{7}$ and Ho${}_{2}$Ti${}_{2}$O${}_{7}$ compounds. On the other hand, a pronounced elastic softening toward low temperatures was observed in the temperature dependence of the principal elastic constants $C$${}_{11}$, $({C}_{11}\ensuremath{-}{C}_{12})/2$, and $C$${}_{44}$ in Tb${}_{2}$Ti${}_{2}$O${}_{7}$. A detailed discussion of the highly distinct behavior among three compounds is given in terms of the magnitude of a splitting energy between the ground state and the excited one formed by the crystal-field effect. It seems that geometrically structural frustration prohibits the formation of a long-range quadrupolar ordering as well as a magnetic one, even if the coupling constant between quadrupolar moments is rather large enough in Tb${}_{2}$Ti${}_{2}$O${}_{7}$ at low temperatures.

We report the pressure dependence of electrical resistance $R$($T$) of a frustrated pyrochlore oxide Nd${}_{2}$Ir${}_{2}$O${}_{7}$, which shows a second-order metal-insulator transition (MIT) at 36 K. This MIT is monotonically suppressed by the application of pressure. The insulating phase disappears at around 10 GPa. Then, in the pressure-induced metallic state above 10 GPa, a new pressure-induced phase transition emerges at around 3 K, which is characterized by a resistance drop. The new phase transition is due to a ferromagnetic (FM) ordering, which is suggested to be an ordered spin ice (two-in two-out) state of the Nd moment via the RKKY interaction. We succeeded in observing the phase competition between the MIT and the FM ordering by the application of high pressure.

The pressure effects on an $S=1/2$ Heisenberg two-leg ladder antiferromagnet (H2LLAF) ${\mathrm{Cu}}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Cl}}_{4}$ have been investigated through magnetic and thermal measurements under pressures up to 10 kbar. The exchange interactions along the rung and leg hardly change under pressures, but the pressurization induces paramagnetic spins and magnetic order. This magnetic order is a pressure-induced one observed in a quantum spin system with an energy gap. The amount of induced paramagnetic spins increases almost in accordance with the square of pressure. The magnetic field dependence of the pressure-induced Schottky-type heat capacity suggests that the induced paramagnetic spins are not completely free, but weakly correlate with the H2LLAF system. A magnetic anomaly of the heat capacity has been observed around 2.6 K for $Pg~8.5\mathrm{kbar},$ where more than 20% of the paramagnetic spins are induced. This anomaly is considered to be intrinsic to the magnetic order of the H2LLAF system, which seems to be triggered by the modulation of the staggered moment due to local defects. Even below the magnetic ordering temperature, the paramagnetic spins coexit with the magnetic order of the H2LLAF system. These pressure effects are similar to the impurity effects in another typical $S=1/2$ H2LLAF ${\mathrm{SrCu}}_{2}{\mathrm{O}}_{3}$ doped with nonmagnetic ${\mathrm{Zn}}^{2+}$ ions.