Pressure- and Field-induced Non-Fermi-liquid Behaviors in a Heavy-Fermion Antiferromagnet Ce₇Ni₃

We review recent investigations of the heavy-fermion antiferromagnet Ce₇Ni₃ with a geometrically frustrated structure under hydrostatic pressures and magnetic fields. Below T _N1=1.9 K, an incommensurate spin-density-wave (SDW) develops, and another antiferromagnetic transition occurs at T _N2 = 0.7 K. At a rather low critical pressure (P _c ), P _c  = 0.39 GPa, both T _N1 and T _N2 vanish, and the specific heat divided by temperature (C/T) exhibits –lnT dependence (i.e., non-Fermi liquid behavior). At P = 0.43 GPa > P _c , a T-independent behavior of C/T, i.e., Fermi-liquid behavior, recovers below 0.2 K. However, the magnetic susceptibility continues to increase down to 0.09 K at 0.43 GPa, which is not consistent with a conventional Fermi-liquid theory. On the other hand, upon applying magnetic fields B along the hexagonal c axis, T _N1 decreases and vanishes at 0.3 T. Magnetoresistance, specific-heat, and magnetization measurements reveal that a field-induced magnetic (FIM) phase appears in a B–T region for 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, the intensity of the magnetic reflection 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 non-Fermi-liquid behavior of Ce₇Ni₃.