Magnetic excitations in diluted magnetic semiconductors and their superlattices.

摘要

Diluted magnetic semiconductors present a wide variety of phenomena of which some are here investigated. A study of the properties of transition metal ions in II-VI semiconductors and in thiogallates is presented. These ions have an incomplete 3d shell, consequently exhibiting a magnetic moment. Their properties when embedded in the semiconductor are different from those of the free ion. In the crystal, the magnetic ion under study is surrounded by other ions forming a polyhedron. The electrostatic potential arising from these is called the crystalline potential (or crystal field). The crystal field reflects the symmetry of the site of the impurity, i.e., T{dollar}sb{lcub}rm d{rcub}{dollar} symmetry in a zinc-blende semiconductor and C{dollar}sb{lcub}rm 3v{rcub}{dollar} in a wurtzite host. The energy level structure of the ground term of the iron group ions is investigated, assuming that the crystal potential is stronger than the spin-orbit coupling. The effect of a magnetic field is introduced as an additional perturbation, thus permitting an evaluation of the g-factors. This provides the theoretical basis for the interpretation of experimental studies of optical absorption and of spin-flip Raman scattering. In this study the ions are regarded as non-interacting. If the concentration of iron group ions increases, collective phenomena appear. The spin wave excitations are investigated in superlattices formed by a series of alternating layers of diluted magnetic semiconductors in an antiferromagnetic phase separated by paramagnetic or nonmagnetic gaps.; The theory predicts the existence of three kinds of magnetic modes in superlattices: (i) "pure-bulk" modes for which the magnetization is a trigonometric function of the component of the wavevector along the superlattice axis, (ii) "pure-interface" modes where the magnetization decays in both types of layers and (iii) "bulk-interface" modes where the spin wave propagates in one medium and decays in the other. Such "bulk-interface" waves can be regarded as confined to the layer in which they propagate, in close analogy to the situation occurring in the propagation of optical phonons in superlattices.