Quantum Model for Ferromagnetic Thin Films with an Alternating Crystal Field

Abstract

Within the framework of many-body Green's function theory there are studied the properties of the quantum Blume-Capel model for ferromagnetic films with an alternating single-ion anisotropy on the odd atomic layers and on the even ones. We analyse various possible phase diagrams for the surface exchange couplings and the single-ion anisotropy parameters. Model and methodThe ferromagnetic Blume-Capel-Ising (BCI) model has been studied within the mean field approximation [1], the effective field theory [2], the two-spin cluster approximation in the cluster expansion method [3,4], Monte Carlo simulations [5], a thermodynamically self-consistent theory based on an Ornstein-Zernike approximation [6], the exact solution based on the Bethe lattice by means of the exact recursion relations [7]. Most of the studies mentioned above displays also the existence of a tricritical point at which the phase transition changes from second order to first order when the value of K 2 becomes negative. Our work represents the first atempt to consider a quantum version of BCI model. Within quantum Blume-Capel (QBC) model we will study the influence of the enhancement of the surface exchange coupling and the alternative single-ion anisotropy K 2 (1) on the odd atomic layers and K 2 (2) on the even ones on the critical behaviour of thin ferromagnetic films.The Hamiltonian of the considered system consists a Heisenberg exchange interaction with strength J ij > 0 between nearest neighbour lattice sites, an exchange anisotropy with strength D > 0, and a second-order single-ion anisotropy with strength K 2 > 0: (197)

Parallel Susceptibility of Thin Ferromagnetic Films within Many-Body Green's Function Theory

“…Using achieved parameters quantitative agreement between theory and experiment was obtained. In recently appeared work [5] there have been studied phase diagrams of the quantum Blume-Capel model. In the present paper we modify this work on the calculation within GFT and mean field approximation (MFA) the parallel susceptibility χ zz with respect to the easy z-axis.…”