Paramagnetism in ion-implanted oxides

This thesis describes the investigation on para-magnetism in dilute ion-implanted single-crystal oxide samples studied by on- and off-line $^{57}$Fe emission Mössbauer spectroscopy. The ion-implantation of the radioactive isotopes ( $^{57}$Mn and $^{57}$Co) was performed at the ISOLDE facility at CERN in Geneva, Switzerland. The off-line measurements were performed at Aarhus University, Denmark. Mössbauer spectroscopy is a unique method, giving simultaneously local information on valence/spin state of the $^{57}$Fe probe atoms, site symmetry and magnetic properties on an atomic scale. The utilisation of emission Mössbauer spectroscopy opens up many new possibilities compared with traditional transmission Mössbauer spectroscopy. Among them is the possibility of working with a low concentration below 10$^{-4}$ –10$^{-3}$ at.%, where the implanted Mössbauer $^{57}$Fe probes are truly dilute impurities exclusively interacting with their nearest neighbours and therefore the possibility of creating precipitations or clusters are avoided. The main results were obtained on the wide-band gap semiconductor ZnO. Two other oxides, MgO and $\alpha$-Al$_2$O$_3$, were studied as comparative model systems. The ZnO system is of great interest in terms of theoretically predicted dilute ordered magnetism at room temperature. In all the three oxides Fe$^{3+}$ was found to exhibit slow paramagnetic spin relaxation and there is no evidence of Fe atoms taking part in a dilute magnetic order. A model was developed to extract the temperature dependent relaxation rate of paramagnetic Fe$^{3+}$. The spin-lattice relaxation rates of Fe$^{3+}$ in MgO and $\alpha$-Al$_2$O$_3$ was found to be consistent with theory and available experimental data in the literature, but ZnO was found to exhibit a ~ T$^9$ temperature dependence above room temperature, which cannot be explained by the expected Raman processes. In ZnO a fluence dependency (<10$^{12}$ cm$^{-2}$ ) of the charge state of Fe$^{3+}$ /Fe$^{2+}$ was found in the temperature range of 300–390K. This behaviour was explained as caused by a change of the Fermi level upon ion-implantation. In MgO and $\alpha$-Al$_2$O$_3$ interstitial Fe was suggested to exhibit a novel type of cage motion. In addition it was proposed that the interstitial Fe in MgO exhibits long-range diffusion, with diffusivity orders of magnitude higher than that expected for vacancy assisted diffusion of Fe in MgO.