Magnetocrystalline anisotropy of YCo5 and related RECo(5) compounds

The magnetocrystalline anisotropy energy of YCo5 has been calculated from first principles using the local-spin-density approximation. The easy magnetization axis is predicted correctly and the anisotropy energy is only 20% smaller than the experimental value if a recently proposed orbital polarization correction is included; otherwise it is about a factor of 7 too small. Our analysis indicates that magnetic materials with substantially larger anisotropy energies than the best available nowadays should be possible. A large orbital moment is found to contribute to the magnetization bringing the calculated moment, 8.0 mu(B) per YCo5 unit cell, into good agreement with the experimental value of 8.3 mu(B). A large anisotropy in the magnetization is calculated which is nearly completely due to an anisotropic orbital moment associated with the Co atoms. The magnetocrystalline anisotropy energy is shown to be strongly correlated with the anisotropy in the total orbital moment. There is a large reduction in the hyperfine fields compared to the value in bulk hcp Co, not only due to large orbital contributions, but also to different values of the valence contact term. The contribution of the rare-earth (RE) ions to the anisotropy energy of the related RECo(5) compounds may be understood in terms of the crystal-held and exchange interactions felt by the localized RE(4f) electrons. The RE(4f)-Co exchange interactions and the Hartree contribution to the crystal field have been calculated under the assumption that these interactions may be treated as small perturbations. The electric-field gradient V-zz and the A(2)(0) crystal-field parameter at the RE site are a factor of 2 and 3 larger than the experimental values, respectively. The order of magnitude of the calculated exchange field agrees with the values derived from experiment.