Magnetic susceptibility study of boron carbides

Abstract

We have measured the magnetic susceptibility of several boron carbides, including ${\mathrm{B}}_4$C, ${\mathrm{B}}_{15}$ ${\mathrm{C}}_2$, and ${\mathrm{B}}_9$C, over the temperature range 2–400 K with magnetic fields between 1 and 50 kOe. In general, the susceptibility follows a Curie law with a temperature-independent diamagnetic contribution. There is some ferromagnetic contribution to the susceptibility. However, the density of ferromagnetic impurities is considerably less in our samples than in commercial samples. The paramagnetic spin concentration is compared with the carrier concentration determined by transport measurements (conductivity, Hall effect, and Seebeck effect) on the same samples. The spin concentration is about 2 orders of magnitude smaller than the carrier concentration. We attribute the observed spins to defects and impurities. Thus, our results support the view that the charge carriers are spinless, small, singlet bipolarons.