β-Detected NMR of Li in Ga_1-xMn_xAs

The magnetic properties of a 180-nm-thick epitaxial film of the dilute magnetic semiconductor Ga_1-xMn_xAs with x=0.054 are investigated using beta-detected NMR of low-energy implanted 8Li⁺. There is a broad distribution of local magnetic fields in the Ga _1-xMn_xAs layer, reflecting the magnetic inhomogeneity of the system. The resonance (representing the local magnetic field distribution) is followed as a function of temperature through the ferromagnetic transition. The average hyperfine field at the 8Li nucleus is measured to be positive and on the order of 150 G at low temperature, implying a negative hyperfine coupling of the 8Li to the delocalized holes and suggesting that the holes are better described by an Mn-derived impurity band. The spin-lattice relaxation of 8Li shows a remarkably weak feature at the phase transition and no Korringa behavior despite metallic conductivity. No evidence is found of the microscopic magnetic phase separation that has been suggested by some low-energy muon spin-rotation measurements.