Incommensurate magnetic structure of the heavy fermion antiferromagnet Ce7Ni3

Magnetic structures of the heavy fermion antiferromagnet Ce7Ni3 have been studied by neutron diffraction using a single crystal. It undergoes the two successive phase transitions at TN1 = 1.8 K and TN2 = 0.7 K in zero field. At TN1 ordering of a single-k type magnetic structure with an incommensurate modulation vector kIC ≅ 0.22c* takes place. This structure is approximately a sinusoidally modulated structure of c-axis moments, which belongs to the Γ2 and Γ3 irreducible representations of the space group P63mc. Root mean squares of the moments at T = 1.4 K are 0.46±0.07, 0.70±0.03 and 0.10±0.05 μB for the Ce1, Ce2 and Ce3 sites, respectively. Below TN2 the incommensurate modulation vector becomes temperature independent, and a commensurate structure with kC = 1/4c* appears in addition to the incommensurate structure. The metamagnetic transition under the magnetic field of Hsub(parallel)c = 0.16 T at T = 1.5 K is a first order transition to a ferromagnetic state. (author)