The perovskite antiferromagnetic ($T_{rm N}$ $sim$ 220 K) insulator EuNiO$_3$ undergoes at ambient pressure a metal-to-insulator transition at $T_{rm MI}$ = 460 K which is associated with a simultaneous orthorhombic-to-monoclinic distortion, leading
to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO$_3$ with pressure (up to $sim$ 20 GPa) across its quantum critical point (QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and $^{151}$Eu nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure we find that after a small increase of $T_{rm N}$ ($p$ $leq$ 2 GPa) and the induced magnetic hyperfine field $B_{rm hf}$ at the $^{151}$Eu nucleus ($p$ $leq$ 9.7 GPa), both $T_{rm N}$ and $B_{rm hf}$ are strongly reduced and finally disappear at $p_{rm c}$ $cong$ 10.5 GPa, indicating a magnetic QCP at $p_{rm c}$. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressure-induced insulator-to-metal transition occurs at $p_{rm IM}$ $cong$ 6 GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high pressure electrical resistance data on EuNiO$_3$ at low-temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as $T^{rm n}$, with n=1.6, whereas it becomes a normal Fermi-liquid, n = 2, for pressures above $sim$15 GPa. On the basis of the obtained data a magnetic phase diagram in the ($p$, $T$) space is suggested.
We have performed magnetization measurements at high magnetic fields of up to 53 T on single crystals of a uranium heavy-fermion compound U$_2$Zn$_{17}$ grown by the Bridgman method. In the antiferromagnetic state below the N{e}el temperature $T_{rm
N}$ = 9.7 K, a metamagnetic transition is found at $H_c$ $simeq$ 32 T for the field along the [11$bar{2}$0] direction ($a$-axis). The magnetic phase diagram for the field along the [11$bar{2}$0] direction is given. The magnetization curve shows a nonlinear increase at $H_m$ $simeq$ 35 T in the paramagnetic state above $T_{rm N}$ up to a characteristic temperature $T_{{chi}{rm max}}$ where the magnetic susceptibility or electrical resistivity shows a maximum value. This metamagnetic behavior of the magnetization at $H_m$ is discussed in comparison with the metamagnetic magnetism of the heavy-fermion superconductors UPt$_3$, URu$_2$Si$_2$, and UPd$_2$Al$_3$. We have also carried out high-pressure resistivity measurement on U$_2$Zn$_{17}$ using a diamond anvil cell up to 8.7 GPa. Noble gas argon was used as a pressure-transmitting medium to ensure a good hydrostatic environment. The N{e}el temperature $T_{rm N}$ is almost pressure-independent up to 4.7 GPa and starts to increase in the higher-pressure region. The pressure dependences of the coefficient of the $T^2$ term in the electrical resistivity $A$, the antiferromagnetic gap $Delta$, and the characteristic temperature $T_{{rho}{rm max}}$ are discussed. It is found that the effect of pressure on the electronic states in U$_2$Zn$_{17}$ is weak compared with those in the other heavy fermion compounds.
