SPIN WAVES AND PHASE TRANSITIONS IN FERROMAGNETIC SUPERLATTICES

Abstract

With the advance in experimental techniques such as sputtering and epitaxy, spin excitations in magnetic superlattices have been the subject of increasing interest in recent years. Much of this work is motivated by a desire to achieve technically useful characteristics, such as man-tailored superlattice materials, surface and interface condition probe etc. In this dissertation, the spin waves in both magnetostatic region and exchange region, as well as phase transitions in Ising Model for ferromagnetic superlattices are studied. A new method of recurrence relations is used for the first time to study the spin waves or Curie temperatures in ferromagnetic superlattices. Compared with the previous method, such as transfer matrix method, this recurrence relation method has its unique advantages. Using this method, we can relate the potentials or spin constants of any two layers for a periodic superlattice in an analytical expression, and extend our results to an infinite limit easily. In the magnetostatic region, a macroscopic approach based on Maxwell's equations is used and the following superlattice structures are studied: I. Ferromagnetic / Nonmagnetic Superlattices With a Modified Surface or Inner Layer: It is found that for superlattices with modified magnetization MI at the surface layer, in addition to the spin waves present originally, there is a new surface superlattice spin wave if the wave vector is greater than a critical kc for a given M1. However, for defect superlattices, the localized mode always exists for M1 ? M. 2. Ferromagnetic / Nonmagnetic Superlattices With Different Layer Magnetization or Thickness: It is found that in an N layer superlattice with uniformly increased magnetization from left to right, for sufficient large k, there are N localized modes at the N left interfaces of the N layers (for positive q). 3. Three Models of Ferromagnetic / Ferromagnetic Superlattices: In the two-component superlattices, two surface modes are found to be characteristic of the ferromagnetic / ferromagnetic interface. In addition, a new kind of surface mode is found. In the exchange region, a Heisenberg model is used and an (m, n) two-component superlattice is investigated. The (m,n) two-component superlattice consists of two alternating magnetic materials of m and n atomic layers, respectively. It is found that the maximum number of surface modes in a semi-infinite surface is equal to the atomic layer number (m+n) of a superlattice unit. The range of J, when there are (m+n) surface modes, and when all the surface modes disappear, is also obtained. In the study of the phase transitions, an Ising model with mean field approximation is used and the (m, n) two-component superlattice is considered. The effect of the surface constants on the phase transitions is investigated. Numerical results are obtained for the dependence of the Curie temperature on the bulk as well as the surface exchanges for typical superlattices. The critical curves for the onset of surface transition are also obtained. In conclusion, Ill this dissertation, a new method of recurrence relations is developed and used to predict new magnetostatic modes and exchange modes, as well as the phase transition behavior in the ferromagnetic superlattices. Although some of them have not been observed, we expect more fabrication of superlattices studied here and Brillouin scattering measurements.