Magnetic Constitution of Topologically Trivial Thermoelectric PbTe:Cr

Abstract

In this paper we report on detailed temperature and magnetic field dependence of m agnetization of IV-VI semiconductor PbTe doped with mixed valence transition metal Cr${}^{2+/3+}$. The material is studied solely by an integral superconducting quantum interference device magnetometer in order to quantitatively determine the contribution of single substitutional Cr${}^{3+}$ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about $10^{19}$~cm${}^{-3}$ paramagnetic Cr${}^{3+}$ ions formed via self-ionization of Cr${}^{2+}$ resonant donors. These are known to improve the thermoelectric figure of merit parameter zT of this semiconductor. The magnetic finding excellently agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr${}^{2+/3+}$ 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: Cr${}_2$Te${}_3$ and Cr${}_5$Te${}_8$ are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibits (blocked) superparamagnetic properties. 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 in proper accounting for the high temperature magnetic susceptibility of PbTe:Cr.

19 pages, 5 figures

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