Ferromagnetism inSnO2-based multilayers: Clustering of defects induced by doping

Abstract

Several series of $\text{A}/{\text{SnO}}_{2}$ and Mn/B multilayers, where A and B are thin layers (0.4--10.1 nm) of ${\text{SnO}}_{2}$, ${\text{SiO}}_{2}$, Si, Al, Mn, or ${\text{MnO}}_{x}$, have been investigated magnetic, electronic, and structurally. The study demonstrates that the detected ferromagnetism is related to regions of high density of defects in ${\text{SnO}}_{2}$ induced by doping under particular conditions. The observed room-temperature ferromagnetic (RTFM) signal does not scale to Mn content but increases with the number of interfaces and their roughness. The presence of Mn in $3+$ oxidation state in the ${\text{SnO}}_{2}$ lattice is a necessary condition but is not enough to promote ferromagnetism which also requires the presence of ${\text{Sn}}^{2+}$. The high oxygen deficiency induced by Mn doping in the tin-oxide layers is mostly compensated by the formation of the stable SnO phase. Moreover, the RTFM signal decreases upon annealing either in ${\text{O}}_{2}$ rich or in vacuum atmospheres. The combination of Mn with ${\text{SiO}}_{2}$, Si, or Al produces paramagnetic signals but no ferromagnetism; consequently ${\text{SnO}}_{2}$ is a crucial ingredient of this RTFM. The observed ferromagnetism may be explained by short-range ferromagnetic correlations between Mn probably mediated by induced holes at oxygen sites of ${\text{SnO}}_{2}$ in the vicinity of trivalent ${\text{Mn}}^{3+}$ doping ions. The inhomogeneous Mn distribution inside tin oxide at the multilayer interfaces may produce large enough regions with high defect concentration to allow long-range ferromagnetic order. All undoped ${\text{SnO}}_{2}$ films, grown in a wide set of different conditions, show paramagnetic signals with high-$J$ values but no ferromagnetism is detected probably because of the high density of defects required to establish a ferromagnetic order based in a short-range mechanism.