QUANTUM-THEORY OF THE COERCIVE FORCE AND THE CAPPING EFFECT FOR MAGNETIC MULTILAYERS

A quantum theory of coercivity is established for magnetic systems. Th e concept of a metastable state is adopted and an order parameter that is actually the gap of the magnon excitations is used to tell the sta bility of the metastable state and to determine the coercive force of the system. Quantum theory is applied to study the capping effects in a magnetic double-film structure. A model Hamiltonian is presented for such a system, in which there are several magnetic layers with perpen dicular easy-axis anisotropy capped by a few layers with in-plane easy -axis anisotropy. The Hamiltonian is diagonalized under the harmonic a pproximation and the magnon excitation gap is determined. Numerical ca lculations are carried out and the experimental results about the coer cive force in the double-film structure are explained qualitatively. F inally, we shaw that there are: two types of phase transition in the c apping effect. The results show that the capping effect can be signifi cantly enhanced if one selects the anisotropy and the number of cappin g layers appropriately.