Parallel Susceptibility of Thin Ferromagnetic Films within Many-Body Greens Function Theory

Parallel susceptibility of the quantum Blume-Capel model for ferromagnetic films is calculated within the Green function theory. The Hamiltonian includes a Heisenberg term with the different surface exchange coupling with respect to bulk, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the exchange anisotropy. The importance of collective excitations is demonstrated by comparing with result calculated within mean field approximation.