Magnetic Constitution of PbTe:Cr - Experimental Aspects

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One of the most promising ways of improvement of thermoelectric energy conversion leads via band gap engineering towards the enlargement of the density of states at the Fermi level, e.g. via a co-doping to induce a resonant level in the electric transport relevant band [1]. Cr in PbTe is a particularly interesting example since Cr atoms form mixed valence Cr2+/3+ donor centers resonant with the PbTe conduction band [2].We report on detailed temperature and magnetic field dependence of magnetization of PbTe doped with transition metal Cr2+/3+. The material is studied by a SQUID magnetometer in order to quantitatively determine the contribution of single substitutional Cr2+/3+ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about 1019 cm-3 paramagnetic Cr ions, of which 2/3 are the Cr3+ ions formed via self-ionization of Cr2+ resonant donors. The magnetic finding agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr2+/3+ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr2Te3 and Cr5Te8 are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibit (blocked) superparamagnetic properties. Also HR-TEM confirmed the presence of numerous single nanometer size precipitates of Cr-Te compounds. The magnetic susceptibility of both n- and p-type PbTe in the temperature range 100 < T < 400 K has been established. These magnitudes are essential to properly account for the high temperature magnetic susceptibility of PbTe:Cr [3].This study has been supported by the National Science Centre (Poland) through project OPUS (UMO - 2017/27/B/ST3/02470) and by the National Science Centre for Development (Poland) through grant TERMOD No TECHMATSTRATEG2/408569/5/NCBR/2019 and by the Foundation of Polish Science through the IRA Programme co-financed by EU within SG OP.