Local field of spin-spin interactions in the nuclear spin system of n -GaAs

At low lattice temperatures the nuclear spins in a solid form a closed thermodynamic system that is well isolated from the lattice. Thermodynamic properties of the nuclear spin system are characterized by the local field of spin-spin interactions, which determines its heat capacity and the minimal achievable nuclear spin temperature in demagnetization experiments. We report the results of measurement of the local field for the nuclear spin system in GaAs, which is a model material for semiconductor spintronics. The choice of the structure, a weakly doped GaAs epitaxial layer with weak residual deformations, and of the measurement method, the adiabatic demagnetization of optically cooled nuclear spins, allowed us to refine the value of the nuclear spin-spin local field, which turned out to be two times less than one previously obtained. Our experimental results are supported by calculations, which take into account dipole-dipole and indirect (pseudodipolar and ``exchange'') nuclear spin interactions as well as quadrupole splitting of nuclear spins in the vicinity of charged impurity centers.