Second order perturbed Heisenberg Hamiltonian of Fe3O4 ultra-thin films
  Due to the wide range of applications, theoretical models of Fe3O4 films are found to be important. Ultra thin Fe3O4 films with ferrite structure have been theoretically investigated using second order perturbed modified Heisenberg Hamiltonian. Matrices for ultra thin films with two and three spin layers are presented in this manuscript. Total magnetic energy was expressed in terms of spin exchange interaction, magnetic dipole interaction, second order magnetic anisotropy and stress induced magnetic anisotropy. Magnetic properties were observed for films with two spin layers and variant second order magnetic anisotropy. For the film with three spin layers, second order anisotropy constant was fixed to avoid tedious derivations. Magnetic easy axis rotates toward the in plane direction as the number of spin layers is increased from two to three because the stress induced anisotropy energy dominates at higher number of spin layers. According to some other experimental data, the magnetic easy axis of thin films rotates toward the in plane direction as the thickness is increased. For ferrite film with two spin layers, magnetic easy and hard directions can be observed at 0.75 and 1.2 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 3.9. For ferrite film with three spin layers, magnetic easy and hard directions can be observed at 2.4 and 2.3 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 4.2. 
P. Samarasekara
cond-mat.mtrl-sci