材料期刊网

NdCo(12-x)V(x) (2.2 <= x <= 2.6) crystallizes in the ThMn(12)-type structure and exhibits an unusual jump in the magnetization at a critical field B(C) that decreases with increasing V content. It was intriguing that the magnetization of the bulk samples below B(C) was very close to that of YCo(12-x)V(x) with the same V content. To investigate the nature of the magnetic order in these materials we have carried out powder neutron diffraction measurements as a function of temperature and magnetic field on the NdCo(9.5)V(2.5) composition. In zero field we find that both the Nd and Co ions have substantial ordered moments (similar to 2.9 and 0.4 mu(B), respectively) at low temperature (4 K), with the moments coupled ferromagnetically (T(C)=180 K) and aligned along the c axis. On a loose powder an applied field of 4 to 7 T rotates the particles so the c axis aligns with the field, indicating a strong uniaxial anisotropy that renders the system Ising-like. However, we find the same in-field magnetic structure and essentially the same values for the saturated magnetic moments as those in zero field. Magnetization data on magnetically prealigned samples reveal that for fields applied parallel to the (easy) c axis the magnetization saturates below 0.5 T at a magnitude that is in very good agreement with the moments determined from neutron diffraction. For fields applied perpendicular to the c axis, on the other hand, the magnetization data show a two-plateau behavior, explaining the original magnetization data on randomly oriented powders. The strong dependence of the magnetization on the direction of the applied magnetic fields indicates an occurrence of the first-order magnetization process due to the competing magnetocrystalline anisotropies of the Nd and the Co sublattices and of the higher-order terms of the anisotropy energy.