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We present a detailed study of the field ͑B͒ -temperature ͑T͒ phase diagram of the heavy-fermion antiferromagnet Ce 7 Ni 3 that crystallizes in the hexagonal Th 7 Fe 3 -type structure with three nonequivalent Ce sites (Ce I , Ce II , and Ce III ). This compound undergoes two magnetic transitions at T N1 ϭ1.9 K and T N2 ϭ0.7 K in zero field. Below T N1 , an incommensurate spin density wave ͑SDW͒ 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 BʈcϾ0.7 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 .

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