Magnetic anisotropy of rare-earth—transition-metal compounds

The exchange coupling between rare-earth ($R$) and transition-metal ($T$) moments in $R_x{T}_y$ compounds is not so large as to hold the $R$ and $T$ moments rigidly parallel (for light rare-earth elements) or antiparallel (for heavy rare-earth elements); ionic magnetocrystalline anisotropy energy can be comparable to this intersublattice exchange. Anisotropy torques and/or an external field can induce canting between the sublattice moments. We show that this canting, even when small, can reduce the macroscopic effective anisotropy by a very large amount—more than an order of magnitude—from the intrinsic sublattice anisotropies. Consideration of the effects of canting reconcile hitherto conflicting reports, by spin-wave scattering and moment measurements, of the magnetic anisotropies of ${\mathrm{Ho}}_2$ ${\mathrm{Co}}_{17}$ and ${\mathrm{Ho}}_2$ ${\mathrm{Fe}}_{17}$. One conclusion of our analysis is that since a huge degeneration of macroscopic magnetic anisotropy, and hence of the coercivity, comes from sublattice flexing, it seems to follow that to increase coercivity one should be trying to increase $R-T$ exchange; the ionic magnetic anisotropy is already greater than its effective value.