Magnetic Anisotropy and Jahn-Teller Distortion of Itinerant Ferromagnets with Cubic Structures

An itinerant ferromagnet with a cubic structure such as α-Fe has no uniaxial magnetism, while uranium monosulfide US has the strongest-known uniaxial magnetism despite its cubic structure. We probed the cause for this difference by density functional theory (DFT) calculations for α-Fe, FeNi, and US to determine their magnetocrystalline anisotropy energies (MAEs), partial density of states (PDOS) plots, and pair polarization indexes (i.e., the strengths of the pair polarizations), Δp. By comparing these results with those already reported for MnAl and SmCo₅, we find that the MAEs of these itinerant ferromagnets increase in the order, α-Fe < FeNi < MnAl < SmCo₅ < US, and so do their Δp values, revealing that US and α-Fe differ in their uniaxial magnetism due to the large difference in their pair polarizations. We also investigated the possibility for pair polarization to induce Jahn-Teller instability by analyzing the weak cubic-to-rhombohedral distortion of US that accompanies its ferromagnetic transition at 177 K to show that this is indeed a weak Jahn-Teller distortion. The absence of such a Jahn-Teller instability in other permanent magnets was ascribed to the weak covalent character of their interatomic bonds.