Abstract
Seminconductor-base spintronics has attracted intense attentions over last decade for its potential applications. In this work, we have focused on the exploration of developing new spintronics materials, and the corresponging distinict physical properties, such as microstructure, Curie temperature (TC), and magnetic anisotropy. Here, three main topics have been involved in details in this dissertation. In the first topic, we have demonstrated the high Curie temperature (TC >350 K) ferromagnetic semiconductor (Zn,Co)O fabricated by standard solid-state reaction method. In addition, the different quenching conditions and co-doping with the additional element Cu show remarkable influence on ferromagnetic behavior of polycrystalline (Zn,Co)O samples. The microstructure of these polycrystalline samples were identified by XRD and HRTEM, in which, the lattice constant of ZnO wurtzite structure reveal slight increase and decrease with small amount of additional Co and Cu doping, respectively. Furthermore, the chemical environments analysis by ESCA, and the composition distributions characterized by nano-beam EDS spectra mapping confirm that Co element was uniform and homogeneously dispersed in the ZnO matix, and exhibit a single-phase solid solution property instead of magnetic secondary phase of Co clusters. The enhancement effect of ferromagnetic behavior by various queching temperature or small amounts of additional Cu doping seems to be ascribed to the reduction of electron carriers. In the second topic, we have developed the planar Hall effect measurement to examine the in-plane magnetic anisotropy of epitaxial (Ga,Mn)As layer and Mn delta (δ)-doped GaAs-based heterostructures. The planar Hall resistance in (Ga,Mn)As layer and Mn δ-doped GaAs heterostructures measured at liquid helium temperature (2.6 K) reveal very large RPH jumps of ~90 ohm and ~300 ohm, respectively, that is more than four order lager than those of metallic ferromagnets. This observed ”giant” Hall effect enables a very sensitive measurement of the field, and it is also robust to determine the magnetic anisotropy and associated magnetization switching behaviors. In particular, a specific in-plane <100> biaxial magnetic anisotropy was observed in (Ga,Mn)As epilayer. However, Mn δ-doped GaAs heterostructures shows the distinct in-plan uniaxial magnetic anisotropy along [110], which is not expected on the basis of the zinc-blend structure of GaAs. It has been suggested that the locally high Mn and holes concentrations may have led to the distinct in-plan uniaxial magnetic anisotropy along the [110] direction in the Mn δ□-doped atomic plane. Finally, in the last topic, we have demonstrated a new formation method for obtaining nano-thickness ferromagnetic Mn(Ga)As layer by post field-annealing treatment of IrMn/(Ga,Mn)As hetero-structure. The Mn(Ga)As reaction layer can be self-organized between DMS (Ga,Mn)As and antiferromagnetic IrMn layers even though the annealing temperature is as low as 100 oC. In particular, TEM analyses clearly indicated the monotonical increase in thickness of Mn(Ga)As layer with increasing annealing temperature and high quality interface distribution through the whole sample. Furthermore, we have demonstrated the exchange bias effect in IrMn/Mn(Ga)As/(Ga,Mn)As hetero-structure at low temperature. The coercive field HC and exchanged field HE are estimated to be 260 ± 10 G and 90 ± 10 G at 10 K, respectively. These implication are reasonably attributed by the effective exchange coupling interaction between antiferromagnetic IrMn layer and ferromagnetic Mn(Ga)As layer.