Condensed Matter/Materials & Biological Physics Seminar with Mikhail Cherkasskii on Ferromagnets

The inertial dynamics of magnetization in ferromagnets is investigated theoretically. The analytically derived dynamic response upon microwave excitation shows two peaks: ferromagnetic and nutation resonances. We demonstrate that the frequency of ferromagnetic resonance is shifted down by inertia of magnetization and the shift is dependent on magnetic anisotropy. The nutation resonance yielded by inertia exhibits field dependence, which is similar to the ferromagnetic resonance. We derive an equation allowing one to calculate resonances based on free energy density including the energy of magnetocrystalline anisotropy and demonstrate that previously obtained anisotropy constants and g-factors of ferromagnets should be revised.

Inertia effects in magnetization dynamics are shown to result in a new type of spin waves, i.e. nutation surface spin waves, which propagate at terahertz frequencies in in-plane magnetized ferromagnetic thin films. Considering the magnetostatic limit, i.e. neglecting exchange coupling, we calculate dispersion relation and group velocity, which we find to be slower than the velocity of conventional (precession) spin waves. In addition, we find that the nutation surface spin waves are backward spin waves. Furthermore, we show that inertia causes a decrease of the frequency of the precession spin waves, namely magnetostatic surface spin waves and backward volume magnetostatic spin waves. The magnitude of the decrease depends on the magnetic properties of the film and its geometry.