Magnetic structure of films: Dependence on anisotropy and atomic morphology

Abstract

In this review a number of magnetic properties of different thin film systems are investigated as functions of the temperature and the atomic morphology. Special attention is paid to determine the influence of the collective magnetic excitations and the noncollinear magnetic structure. At finite temperatures these problems are studied within a Heisenberg model by application of a mean field approximation as well as by a many-body Green’s function theory. First, the magnetization profiles and the magnetic ordering (Curie-) temperatures are calculated for different magnetic systems. In particular, single ferromagnetic (FM) films, coupled magnetic bilayers with two FM films, and trilayers consisting of two FM films separated by a nonmagnetic spacer layer are studied. Here the role of the magnetic fluctuations are highlighted, which are particularly important for these low-dimensional magnets. For the different systems under consideration we show that the strongly varying magnetic properties caused already by weak interlayer couplings can be explained only by taking into account the collective magnetic excitations. Hence, the effect of these excitations for two-dimensional magnets can be studied explicitly. The calculated results are partly compared with measurements. Moreover, the thin-film magnetic structure of materials with a helical bulk magnetization is investigated. We show that due to the breaking of nearest- and next-nearest-neighbor bonds in the surface region the helical magnetic structure of, e.g., a thin Ho film becomes significantly disturbed. Even a FM phase may result in a decreasing film thickness or an increasing temperature. The possibility of a paramagnetic layer within an ordered magnetic structure is pointed out. In addition, the spin reorientation transition of thin FM films is studied as a function of the temperature, the film thickness, and an external magnetic field. This phenomenon results from competing effective anisotropy contributions with a different dependence on the temperature, for example. Special attention is paid to investigate the influence of magnetic noncollinearities caused by an atomic roughness or a variation of the film thickness. We show that these noncollinearities result in a much broader magnetic reorientation as compared to the one of a smooth film. This feature can be considered by effective higher-order anisotropies for an otherwise collinear thin film magnetization. Approximate expressions for these quantities are presented. We show that for a strongly inhomogeneous system the magnetization profile during SRT exhibits a smooth behavior, hence is not characterized by magnetic domains separated by comparatively thin domain walls. The magnetic reversal as induced by a transversal magnetic field is studied for the simple cases of FM and antiferromagnetic (AFM) monolayers. For the latter case we show that with increasing strength of the transversal field the magnetization component perpendicular to that field exhibits a maximum. This unexpected property is explained by the presence of quantum fluctuations which are particularly important for antiferromagnets. Finally, the magnetic structure of coupled FM–AFM layers is studied. The interlayer coupling induces a net magnetic binding energy and thus results in an effective interface anisotropy of the FM layer. A noncollinear magnetization is induced in the AFM layers and possibly also in the FM layers close to the interface. We show that the magnetic structure of the interface layers and the resulting magnetic ordering temperature(s) depend sensitively on the lattice symmetry. The importance of the collective magnetic excitations for these coupled FM–AFM systems is also pointed out. The resulting interface anisotropy may enhance the total anisotropy of the FM subsystem. Alternatively, if the interface anisotropy competes with the intrinsic FM anisotropy, a magnetic reorientation of the FM film can take place with an increasing temperature and varying FM film thickness.