First-Principles Study of Magnetism-Dependent Phonons Governed by Exchange Ligand Field

We present first-principles phonon calculations for ferromagnetic and paramagnetic (disordered local moment) states in B2-type (FeCo, MnNi) and L1(0)-type (CoPt, FePt, FePd) materials. The calculations show a clear relationship between the degree of the phonon softening due to magnetic disordering and crystal symmetry. We discuss the mechanism of the phonon softening by introducing a concept, exchange ligand field. Randomness in the exchange ligand field induces the broadening of density of states in paramagnetic states, as a result; a significant difference in electronic structures between both magnetic states in high symmetric materials causes the phonon softening.