New magnetic structure study of TbNi₅: Evidence of incommensurate structure

We have investigated the magnetic structure of TbNi₅ using neutron diffraction from 2.2 to 30 K to find new magnetic satellites below T_C = 23 K, which have never been reported before. These satellites can be indexed as (0, 0, 1 ± τ), (1, 0, 1 ± τ), (0, 0, 2 ± τ), and (1, 0, 2 ± τ), where τ is equal to 0.0194 r.l.u. (reciprocal lattice units). We have observed these new peaks in addition to magnetic peaks seen in previous measurements. We can understand this long-period incommensurate structure in terms of two propagation vectors: k₁ = (0, 0, 0) and k₂ = (0, 0, τ). From the temperature dependence of the intensity of the commensurate and incommensurate peaks as well as our heat capacity data, we suggest that TbNi₅ should have a modulated magnetic structure over the whole temperature range from 23 to 2 K, in contradiction to the long held view that TbNi₅ is a collinear ferromagnet below T_C = 23 K.