  In this paper, we present results on magnetization properties of boron
nitride-carbon (BN-C) and boron carbide-carbon (B4C-C) granular mixtures. The
temperature-dependent magnetization for field-cooled during cooling and
field-cooled during warming shows a kind of thermal hysteresis that is always
seen around a metamagnetic phase transition from an antiferromagnetic
martensite to a ferromagnetic austenite phase. The low-temperature
magnetization has an upward turn that can be attributed to superparamagnetism,
diamagnetic shielding, and trapped flux characteristic to high-temperature
superconducting materials. After subtracting the diamagnetic background, the
field-dependent magnetization loops M(B) are ferromagnetic-like, more
significant for the BN-C than for the B4C-C mixture. In addition, the
magnetization loops show the kink feature characteristic to granular
superconductivity. The irreversibility temperature for a B4C-C mixture having
37.5 wt% B is Tc = 76 K. Combining our data with previous results on B-doped
diamond and Q-carbon, we find that Tc increases linearly with the B
concentration.
