Higher-Order Crystal Field And Rare-Earth Magnetism In Rare-Earth-Co-5 Intermetallics

Crystal-field (CF) effects on the rare-earth (RE) ions in ferrimagnetic intermetallics NdCo5 and TbCo5 are evaluated using an ab initio density functional + dynamical mean-field theory approach in conjunction with a quasiatomic approximation for on-site electronic correlations on the localized 4f shell. The study reveals an important role of the high-order sectoral harmonic component of the CF in the magnetism of RECo5 intermetallics. An unexpectedly large value is computed in both systems for the corresponding crystal-field parameter (CFP) A(6)(6)< r(6)>, far beyond what one would expect from only electrostatic contributions. It allows solving the enigma of the nonsaturation of zero-temperature Nd magnetic moments in NdCo5 along its easy axis in the Co exchange field. This unsaturated state had been previously found out from magnetization distribution probed by polarised neutron elastic scattering but had so far remained theoretically unexplained. The easy plane magnetic anisotropy of Nd in NdCo5 is strongly enhanced by the large value of A(6)(6)< r(6)>. Counterintuitively, the polar dependence of anisotropy energy within the easy plane remains rather small. The easy plane magnetic anisotropy of Nd is reinforced up to high temperatures, which is explained through J-mixing effects. The calculated ab initio anisotropy constants of NdCo5 and their temperature dependence are in quantitative agreement with experiment. Unlike NdCo5, the A(6)(6)< r(6)> CFP has negligible effects on the Tb magnetism in TbCo5 suggesting that its impact on the RE magnetism is ion-specific across the RECo5 series. The origin of its large value is the hybridization of RE and Co states in a hexagonally coordinated local environment of the RE ion in RECo5 intermetallics.