Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.

The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.

Ferromagnetic cobalt-doped indium oxide, (In${}_{1-x}$Co${}_x$)${}_2$O${}_3$, thin films with $x$ between 1.6% and 8.1% have been studied by x-ray, magnetic, and optical methods. Evidence gathered from x-ray diffraction and x-ray absorption fine-structure studies suggest that the Co atoms are substitutional within the In${}_2$O${}_3$ matrix. The magnetization of the films were found to consist of two components: a paramagnetic term that agrees with what is expected for paramagnetic cobalt ions and a temperature-dependent ferromagnetic hysteresis loop. The ferromagnetic component was too large to have been generated by the free carriers, implying that it originates from electrons bound in defect states associated with oxygen vacancies. This is confirmed by optical absorption and magneto-optical studies because the magnetic circular dichroism scales with concentration of cobalt and temperature in the same way as the measured magnetization.

A theoretical investigation of the structure, magnetism and thermal conductivity of LaMn_1−xGa_xO₃ has been made. A local, cluster model is introduced. It is proposed that gallium substitution may cause neighbouring Mn³⁺ (3x² − r²)/(3y² − r²) orbitals to transform into the 3z² − r² state. This unique idea is the basis of a successful description of the structure and magnetism of LaMn_1−xGa_xO₃. The evolution of the crystal lattice parameters has been simulated; agreement with experimental data is excellent; changes in orthorhombic strain and cell volume are also well predicted. The behaviour of the magnetization has been successfully described by a spin-flipping model: over the entire range of doping, good qualitative consistency between simulation predictions and experimental data is observed. Ultimately, the aforementioned results indicate that lattice distortions alone may be responsible for the observed magnetic order in these manganites. Additionally, the thermal conductivity of LaMn_1−xGa_xO₃ has been analysed. The results are fitted with a kinetic model. It appears that thermal currents in LaMn_1−xGa_xO₃ may be limited by scattering of phonons from the structural changes near the Ga ions.

Magnetization is produced in samples of ZnO by implanting krypton ions. Krypton is chemically inert; hence, the magnetization is the consequence of the radiation damage that occurred due to the implantation process. We have found magnetization as large as 185 emu cm⁻³ at room temperature which corresponds to a moment of ∼40 Bohr magnetons per implanted krypton ion. The results are compared for implantation into different ZnO samples, O- and Zn-polar thin films and thin ZnO crystal wafers. The structure of the films has been investigated using RHEED, AFM, XRD and EXAFS. The O-polar film grew in a layer-by-layer mode and after implantation, its lattice spacing increased and the grain size decreased which indicated that the Kr atoms had been incorporated into the lattice. In contrast the Zn-polar film grew in a Stranski–Krastanov mode and its lattice spacing and grain size were almost unchanged by the implantation indicating that the Kr atoms were in the grain boundaries. Raman scattering is used to characterize the radiation damage and to identify scattering from amorphous regions in the grain boundaries and the scattering from point defects. The largest magnetization is found in the O-polar ZnO films where there is a marked grain fragmentation, leading to an increase in the volume of the sample occupied by grain boundaries. The increase in the number of point defects, which was identified by Raman scattering for films of both polarity, was less effective at producing a magnetic signal than the grain boundaries.

Carbon-doped ZnO films were prepared on c-cut sapphire substrates by conventional PLD method. All films exhibit RT ferromagnetism. However, the samples deposited at low temperature and low pressure with low C concentration show large M _s. The substitution of carbon for O in ZnO causes the electron transfer of 3d orbital of Zn ion, which results in the generation of a net spin with one half in the d-orbit of Zn, and accordingly makes ferromagnetism possible. XPS and Auger results indicate the formation of donor defects such as V_O or/and Zn_i in the film, which makes the net spin align, accordingly induces magnetic ordering of the films. Both a certain number of donor defects and the net spin of Zn ions caused by the substitution of O by C are two key factors in inducing magnetic ordering in C-doped ZnO films. Therefore, defect-induced ferromagnetism is reasonable to explain the observed magnetism in our experiment.