Abstract
In this dissertation, we first investigate the in-plane magnetic anisotropy of Ga0.93Mn0.07As thin films grown on (001) and (311)A GaAs substrates. Temperature dependent magnetization (M□T) measurements reveal a less enhancement in Curie temperature (TC) of the (311)A Ga0.93Mn0.07As upon annealing compared to the (001) counterpart. However, in the studies of magnetic hysteresis loops, the (311)A Ga0.93Mn0.07As exhibits a simple in-plane magnetic anisotropy with easy-axis consistently set along the crystallographic orientations, while deviated easy-axis was observed in both the as-grown and annealed (001) Ga0.93Mn0.07As. We analyzed the in-plane azimuthal magnetostatic energy and proposed an equation for calculating the angular dependent uniaxial and biaxial anisotropic energy and locating the easy-axis exactly. Next, we present a different observation on the exchange biasing of Ga0.95Mn0.05As by ultra thin (6 □) MnOx. We have simultaneously observed exchange bias (HE) and vertical magnetization (M) shift in as-grown and field-annealed MnOx /Ga0.95Mn0.05As bilayers. HE initially decreases with increasing annealing time ta and then increases when ta > 30 min, while M shift remains almost unchanged with ta. X-ray photoelectron spectroscopy (XPS) analysis reveals that MnOx is composed of MnO and Mn3O4, and the volume amount ratio of Mn3O4 to MnO increases with increasing ta. A simple model of spin configurations based on a uniform MnO□Mn3O4 interface with constant “pinned” uncompensated interfacial spins during field annealing is proposed to account for the observed exchange-biased phenomena. The last of this study is to explore the influences of the Mn concentration, growth temperature, and growth time on the structure and magnetic properties of (In, Mn)As nanostructures under Stranski□Krastanov (S–K) growth mode. The (In, Mn)As nanodots and elongated dots (or nanowires), all have room-temperature ferromagnetism, can be grown at 320 oC ≦ Tg < 450 oC and Tg ≧ 450 oC, respectively. We particularly investigate the effects of the growth time and Mn concentration on the magnetic properties of the (In, Mn)As nanodots grown at 380 oC. Microstructure analysis reveals that the In1-xMnxAs nanodots with x < 0.44 present zinc-blende structure. For the In1-xMnxAs nanodots with x ≧ 0.44, a blocking temperature (TB) slight lower than TC can be located at the bifurcation between the zero-field-cooled and field cooled M□T curves. For these nanodot samples with ultrahigh Mn concentration, the spin of dots exhibits a long-range order state at T < TB and the magnetic spin configuration is confirmed as superferromagnetic ordering by memory effect test. By analyzing the M□T relations of the In0.56Mn0.44As nanodot samples with different growth times, a simple model is proposed to account for the spin configuration transition with quantum dots growth evolution. At last, it is found that high Mn concentration and high growth temperature are both necessary for the growth of well-ordered nanowires.