Low-temperature magnetic properties of submetallic phosphorous-doped silicon

Measurements of the electron-spin-resonance properties of Si:P with dopant concentration of $n=2.05\mathrm{}\times {10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$ (somewhat below the metal-insulating transition) at low temperatures (0.035 to 4.2 K) and low frequencies (11.5-58.2 MHz) are reported. They show a substantial deviation from Curie-law behavior for the susceptibility, in agreement with previous static experiments. An internal field is observed to develop as the temperature is lowered. At 35 mK, the internal field has a magnitude of 2.1 Oe and is directed opposite to the externally applied field. The buildup of the internal field is accompanied by an increase in the resonance linewidth. Both the linewidth and the internal field can be fitted with a power-law divergence that suggests critical behavior relative to a phase transition at zero temperature. At all temperatures investigated the relaxation time of the magnetization is observed to be less than 1 ms, and is interpreted as relaxation of the Zeeman reservoir to the exchange reservoir.