Electron spin resonance study of hot‐pressed boron carbide

Electron spin resonance (ESR) spectra are reported for four hot‐pressed boron‐rich boron carbide samples B1−xCx in the composition range 0.1≤x≤0.2. At 2 K the spectra for all four compositions are characterized by a single Lorentzian absorption with a relatively narrow linewidth and free‐electron g‐value. The B4C ESR linewidth remains narrow between 2 and 295 K, while the linewidth of the other three samples containing less carbon increases by a factor of 20 over this temperature range. Above 20 K the ESR signal in the latter three samples has a lineshape that cannot be described by a single Lorentzian, but can be explained as the sum of two Lorentzians with different linewidths, suggesting the presence of two distinct paramagnetic centers. The integrated intensities of the ESR signals in all samples follow a Curie law below 100 K, and the Currie constant corresponds to spin concentrations of 2 to 3×1019 per gram. While these values are in agreement with static magnetic susceptibility data for samples cut...