Spin resonance investigations of (formula presented) in wurtzite GaN and AlN films

High quality Mn-doped GaN and AlN films grown by molecular beam epitaxy have been investigated with X-band electron spin resonance (ESR). The observed resonance patterns are well described by the spin Hamiltonian for isolated (formula presented) centers with electronic spin (formula presented) and nuclear spin (formula presented) Isotropic g factors (formula presented) and hyperfine parameters (formula presented) are observed both in GaN and AlN, while the fine-structure parameters vary from (formula presented) for strongly strained GaN films to (formula presented) for almost relaxed GaN films, and to (formula presented) for relaxed AlN films. At intermediate orientations of the crystalline c axis with respect to the magnetic field, intermixing occurs between the nuclear spin eigenstates due to off-diagonal elements in the spin Hamiltonian, which strongly enhances the transition probabilities of usually forbidden ESR transitions with (formula presented) This is confirmed experimentally as well as by numerical simulations. It is concluded that (formula presented) impurities are present as isolated, paramagnetic centers at the investigated doping concentration of (formula presented) without any evidence for electrostatic or magnetic coupling to extended valence band states, which is a prerequisite of ferromagnetic exchange required for spintronic devices.