Field-induced magnetic transition in the heavy-fermion antiferromagnet Ce7Ni3 -: art. no. 144408

We present a detailed study of the field (B) - temperature (T) phase diagram of the heavy-fermion antiferromagnet Ce7Ni3 that crystallizes in the hexagonal Th7Fe3-type structure with three nonequivalent Ce sites (Ce (I), Ce (II), and Ce (III)). This compound undergoes two magnetic transitions at T-N1=1.9 K and T-N2=0.7 K in zero field. Below T-N1, an incommensurate spin density wave (SDW) develops, and below T-N2 a commensurate SDW appears independently. By applying fields along the c axis, both T-N1 and T-N2, are suppressed and vanish at 0.3 T. For Bparallel toc>0.7 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 Ce (III), which originate from a geometrical frustration in the quasiregular tetrahedron made of Ce (I) and Ce (III).