We present a detailed study of the field (B) - temperature (T) phase diagram of the heavy-fermion antiferromagnet ${\mathrm{Ce}}_7{\mathrm{Ni}}_3$ that crystallizes in the hexagonal ${\mathrm{Th}}_7{\mathrm{Fe}}_3$-type structure with three nonequivalent Ce sites $(\mathrm{Ce}{}_{\mathrm{I}},$ $\mathrm{Ce}{}_{\mathrm{II}},$ and $\mathrm{Ce}{}_{\mathrm{III}}).\mathrm{}$ This compound undergoes two magnetic transitions at $T_{N1\mathrm{}}=1.9\mathrm{}\mathrm{K}$ and $T_{N2\mathrm{}}=0.7\mathrm{}\mathrm{K}$ in zero field. Below $T_{N1\mathrm{}},$ an incommensurate spin density wave (SDW) develops, and below $T_{N2\mathrm{}}$ a commensurate SDW appears independently. By applying fields along the c axis, both $T_{N1\mathrm{}}$ and $T_{N2\mathrm{}},$ are suppressed and vanish at 0.3 T. For $B\Vert c>\mathrm{}0.7\mathrm{}\mathrm{T},$ however, another magnetic phase appears below 0.5 K, which was found by magnetoresistance, specific heat, and magnetization measurements. The separation of the field induced phase from the SDW phase is attributed to large spin fluctuations of $\mathrm{Ce}{}_{\mathrm{III}},$ which originate from a geometrical frustration in the quasiregular tetrahedron made of $\mathrm{Ce}{}_{\mathrm{I}}$ and $\mathrm{Ce}{}_{\mathrm{III}}.$

• Received 14 November 2002

DOI:https://doi.org/10.1103/PhysRevB.67.144408