Crystal Structure, Magnetic Properties And Mixing Effects Of Light And Heavy Rare Earths In The Metamagnetic Compounds RCo₂

Crystal structure, magnetic properties and mixing effects of light and heavy rare earths in the metamagnetic compounds RCo2 have been systematically investigated by x-ray powder diffraction, neutron powder diffraction and magnetization measurements. The main results are summarized as follows:In Nd1-xTbxCo2, the compounds with x=0.4~1.0 exhibit a rhombohedral distortion with space group R3?m. Near the critical concentration xc≈0.33(where the minimum of MS is reached), the compounds show many complex magnetic behaviors including anomaly of saturated magnetization MS, minimum of magnetocrystalline anisotropy constant, compensation point of magnetization, filed-induced paramagnetic to ferromagnetic transition and filed-induced collinear to noncollinear magnetic structural transition. Around x0≈0.19 (where the Nd moment equals to the Tb moment), a double peak was observed on the curve of AC susceptibility, but it does not indicate an occurrence of a first-order phase transition at TC. The theory of Landau phase transition shows that the transition is still a second-order one. A possible magnetic phase diagram is constructed. In Nd1-xGdxCo2, MS varies linearly in the whole range of Gd concentration and a relatively simple magnetic behavior is observed. The salient difference between these two systems can be understood by considering the structural distortion of cubic cell and the difference of magnetocrystalline anisotropy between Tb and Gd.The results of powder neutron diffraction show that as the temperature decreases, NdCo2 undergoes a second-order transition from the cubic to a tetragonal structure (space group I41/amd) at TC and a first-order transition from the tetragonal to an orthorhombic structure (space group Fddd) at T≈42 K. The lattice constant, atomic moment and the anisotropic magnetostriction exhibit abrupt changes at about 42 K, which is connected with the first-order structural transition and indicative of a strong coupling between magnetism and crystal structure. TbCo2 exhibits a transition from the cubic to a rhombohedral structure (space group R3?m) at TC. The moments of Co1 (3b) and Co2 (9e) are almost equal. The temperature dependence of the Tb moment follows very well the Brillouin...