Search for Dilute Magnetic Semiconductors - Results from Mössbauer Spectroscopy
by
Hilary Masenda(University of Witwatersrand)
→
Europe/Zurich
26-1-022 (CERN)
26-1-022
CERN
Description
ZnO and GaN doped with 3d metals have attracted much attention since the theoretical prediction [1] that these wide band-gap materials are potential dilute magnetic semiconductors with high Curie temperatures (Tc ≥ 300 K), resulting from carrier mediated magnetic interactions due to itinerant holes coupling with localized dopant spins. Several published reports [2,3] using different techniques reveal that these materials exhibit different forms of magnetism, the origin of which is still under debate. Most techniques measure the bulk magnetic properties, making it difficult to distinguish between magnetic behaviour stemming from precipitates or real dilute magnetism. The unique feature of Mössbauer spectroscopy is its ability to probe magnetic interactions on an atomic scale.
Using implantation of short lived radioactive isotopes produced at ISOLDE/CERN with fluences in the order of 3´1012 ions.cm-2, the precipitation problem can be avoided. 57Fe emission Mössbauer spectroscopy measurements following implantation of radioactive 57Mn+ ions were undertaken on nitrides (GaN, AlN, InN) and ZnO (as well as MgO, Al2O3). The samples were held at temperatures between 77-726 K in an implantation chamber and spectra were collected at gamma emission angles of 60o relative to the sample c-axis.
The Mössbauer spectra of GaN, AlN and the oxides, collected on a wide velocity scale of ±12 mm/s, reveal magnetic structure in the ‘wings’ which were analysed using a semi-empirical relaxation model [4] utilizing Blume-Tjon sextets. The observed magnetic effect may be explained by a slow spin-lattice relaxation due to paramagnetic Fe3+ weakly coupled to the lattice [5]. The central part of spectra was fitted with two quadrupole split doublets assigned to Fe on substitutional crystalline sites (FeC) and on interstitial sites (FeI) in the ZnO lattice. However, in nitrides three quadrupole split doublets were employed and assigned to Fe atoms on substitutional III sublattice (FeS), isolated amorphous zones (FeD) and the majority of Fe located on near substitutional sites associated with vacancy type defects.
Moreover, the spectra for InN did not reveal any presence of magnetic structure as observed in GaN and AlN which could be explained by the absence of high spin Fe3+. The spin–lattice relaxation rates of Fe3+, lattice site locations, annealing behaviour and variation of hyperfine parameters of fitted components in these materials will be presented.
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5. Gunnlaugsson, H. P., Mølholt, T. E., Mantovan, R. et al. Appl. Phys. Lett., 97 (2010), 142501.
