The intrinsic nature of $Zn_{1-x}TM_xO$ (TM: Co, Mn) as a relatively new DMSs(Diluted Magnetic Semiconductors) are not well characterized until now. $Zn_{1-x}TM_xO$ (TM : Co, Mn) ceramics and thin films are fabricated by conventional solid state method and pulsed laser deposition(PLD) method, respectively.
The solubility limit of Co in ZnO ceramic is 20 mol%. Using PLD method, the optimum thin film deposition condition on (0001) sapphire substrate was found to be at the substrate temperature 600 ℃ in an oxygen pressure of $1\times10^{-5}$ Torr. 40 mol% Co-doped ZnO thin film could be obtained due to non-equilibrium growth characteristics of PLD in the optimum growth condition. The epitaxial relationship between film and substrate was confirmed by x-ray φ scan, pole figure, and TEM.
Temperature dependence of resistivity of $Zn_{1-x}Co_xO$: Al thin films showed almost metallic behavior for x = 0.06 ~ 0.15, while semiconducting behavior for x≥0.2 was observed. The temperature of maximum MR (magnetoresistance) increased with magnetic field in the ρ (H, T)/(0, T) vs T curve up to x = 0.15, but was almost constant for x≥0.2.
The isothermal MR was measured for $Zn_{1-x}Co_xO$: Al thin films (x = 0 ~ 0.25). The $Zn_{1-x}Co_xO$: Al (x ≤0.02) thin films showed only negative MR. Very interesting MR behavior was observed for x = 0.06, 0.1. Negative MR exists below $H ≤H_{kink}$. Positive MR is observed in the field between $H_{kink}$ and $H_{max}$. Above $H_{max}$, positive MR appears again. The peculiar phenomena were discussed in relation to weak localization, the increase of Thomas-Fermi radius, s-d exchange interaction, and the spin alignment with the magnetic field.
The nearest neighbor exchange interaction $J_1/k_B$ is calculated to be -27 K, which means antiferromagnetic interaction between Co ions. The spin freezing temperature of x = 0.2 and 0.25 was 8 K and 12 K, respectively from FC-ZFC experiment.
The transmittance and absorption measurement illustrated the d-d electron transition inside Co ion and increase of band gap with Co content in $Zn_{1-x}Co_xO$ thin films.
The maximum solubility limit of Mn was 20 mol% irrespective of Mn raw material such as MnO, $Mn_3O_4$, and $MnO_2$. XPS results showed that Mn was incorporated into ZnO as Mn2+ in all specimens in which MnO, $Mn_3O_4$, and $MnO_2$ are used as raw Mn-oxide powder. The lattice parameter increased with increasing Mn content. Electrical resistivity increased with the incorporating Mn which acts as deep donor in ZnO.
$Zn_{1-x}Mn_xO$: Al (x = 0.05 ~ 0.15) thin films showed semiconductor behavior. Isothermal MR curve showed positive MR below $H_{max}$ and negative MR above $H_{max}$. The value of $J_1/k_B$ was -14.5 K, showing that the minus sign is antiferromagnetic exchange interaction. The spin glass transition was observed for x = 0.2 and 0.25 in which spin freezing temperature is 6 K and 9 K, respectively. The absorption spectra of $Zn_{1-x}Mn_xO$ revealed linear increase of band gap with x.
The origin of room temperature ferromagnetism was ascribed to the Co clusters by x-ray diffraction, electrical resistivity, XPS, Auger depth profile, and HRTEM.
