Pressure- and field-induced magnetic instabilities in a heavy-fermion antiferromagnet Ce₇Ni₃

We investigated ground state properties of a heavy-fermion antiferromagnet Ce₇Ni₃ under hydrostatic pressures and magnetic fields. This compound undergoes two antiferromagnetic phase transitions at T _N1 = 1.9 K and T_N2 = 0.7 K. Below T_N1, a spin-density-wave (SDW) develops. Upon applying rather weak pressure 0.39 GPa = P_c, both T_N1 and T_N2 vanish, and non-Fermi liquid behavior appears in the specific heat and magnetic susceptibility. The enhancement of residual resistivity along the a axis near P_c is attributed to the increased spin fluctuations along the a axis. By applying fields B along the c axis, T_N1 is suppressed and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements revealed another field-induced magnetic (FIM) phase in the region B || c > 0.7 T and T < 0.5 K. Neutron diffraction experiments indicate that the magnetic unit cell in the c-plane for the FIM phase is treble that of the chemical unit cell. Moreover, this magnetic reflection intensity remains even in the region between the FIM phase and SDW phase. This observation indicates the presence of large spin fluctuations in the c-plane associated with the magnetic frustration, which should be responsible for the magnetic instability of Ce ₇Ni₃.