Evidence for secondary ferromagnetic phases in the nuclear magnetic resonance spectra of GaMnAs epitaxial films and their nominal effect

Local fields in the diluted magnetic semiconductor GaMnAs were studied using zero-field nuclear magnetic resonance (NMR). After a careful search for ⁵⁵Mn and ⁷⁵As NMR signals over a wide frequency range from 20 MHz to 250 MHz, we detected a spectrum similar to α-MnAs. Its origins in the primary GaMnAs phase, however, cannot be fully ruled out. The temperature dependence of the NMR frequency is different from that of the material's macroscopic magnetic properties. After comparing the two, we conclude that the NMR spectrum originates in a secondary MnAs phase. Its effect on the overall macroscopic magnetic properties appears to be minimal; we find an upper bound of ̃0.1 % on the volume fraction of these complexes.