Variable range hopping behavior in the magnetic semiconductor SnO$_2$:Co

To explore the origin of ferromagnetism in doped oxide systems we have grown SnO$_2$: Co thin films on R-plane Al$_2$O$_3$ substrates by laser ablation. We present detailed structural (x-ray diffraction and TEM), transport and magnetic measurements on films grown at different temperatures, growth rates and oxygen pressures. TEM studies show no evidence of cobalt clustering in any of the SnO$_2$:Co films investigated. Variations in growth conditions have been found to affect the physical properties of the material. For example, films with high crystallinity, grown at low growth rates, show transport consistent with Mott variable range hopping, with $\log(\rho)\propto T^{-1/4}$. This is in contrast to the $\log(\rho)\propto T^{-1/2}$ dependence found previously in TiO$_2$:Co containing metallic cobalt clusters$[1] $. We will discuss implications of these findings on the ferromagnetism observed in SnO$_2$:Co.\\ This work was supported by the DARPA SPINS program. The TEM work was supported by NSF DMR-0084173.\\ $[1]$ R.J. Kennedy, et al, Appl. Phys. Lett. 84 2832 (2004).