Searching Room Temperature Ferromagnetism in Wide Gap Semiconductors: Fe-doped Strontium Titanate and Zinc Oxide

Abstract

Scientific findings in the very beginning of the millennium are taking us a step further in the new paradigm of technology: spintronics. Upgrading charge-based electronics with the additional degree of freedom of the carriers spin-state, spintronics opens a path to the birth of a new generation of devices with the potential advantages of non-volatility and higher processing speed, integration densities and power efficiency. A decisive step towards this new age lies on the attribution of magnetic properties to semiconductors, the building block of today's electronics, that is, the realization of ferromagnetic semiconductors (FS) with critical temperatures above room temperature. Unfruitful search for intrinsic RT FS lead to the concept of Dilute(d) Magnetic Semiconductors (DMS): ordinary semiconductor materials where 3 d transition metals randomly substitute a few percent of the matrix cations and, by some long-range mechanism, order ferromagnetically. The times are of intense research activity and the last few years were astonishingly prolific. Room temperature ferromagnetism has already been reported for a number of such systems based on wide band gap semiconductors. Whereas low-temperature ferromagnetism, e.g. in GaMnAs, can be well described on the basis of carrier-mediated mean-field models, the nature of the high temperature ferromagnetism in wide band gap semiconductors is poorly understood. Indeed, such exotic new materials represent new unexplored grounds of condensed matter physics as dilute ferromagnetic semiconductors with Curie temperatures well above 300K and large ordered moments per magnetic atom challenge our understanding of magnetism in solids. This work addresses the main topic of search for new above room temperature and giant moment DMSs: Fe ion implanted ZnO and SrTiO$_{3}$. The giant moment FM state was observed in the whole investigated temperature ranges, up to 300K and 100 K, respectively, with strong indications of a much higher T$_{C}$ for both systems, along with evidence of the diluted configuration of the implanted Fe . Additionally, a simplified picture of defect-related magnetic ordering is forwarded. Special effort was devoted to SQUID measurements for magnetic characterization and to electron emission channeling (EC) experiments for lattice location of the implanted impurities. Furthermore, the lattice dynamics of Fe:SrTiO$_{3}$ was studied by means of Raman spectroscopy and some selected samples were analyzed by means of RBS/C and PIXE for complementary structural characterization. The demonstrated potential of ion implantation to produce FM layers in commercially available semiconductor materials takes semiconductor industry a step closer to the realization of above room temperature DMSs for a variety of new multifunctional phenomena in spintronic devices. Together with the forwarded insight over the mechanisms involved, it strongly motivates further thorough research