Zero-field magnetic resonance of cobalt ion pairs in ZnO nanocrystals

Abstract

Cobalt-doped ZnO nanoparticles (NPs) with different Co concentrations are investigated by means of X-and Q-band electron spin resonance (ESR) near liquid-helium temperature in both parallel and perpendicular modes. The high crystal quality of the NPs allows for the hyperfine-structure resolution within the single Co2+ ions' ESR powder spectra. Depending on cobalt concentration, common additional weak ESR lines are detected which are here demonstrated to arise from some Co2+ high-spin pairs with a distance of about 4-6 Å. ESR simulations show that these 3/2 spin pairs are weakly coupled by an isotropic Heisenberg Hamiltonian with either ferromagnetic or antiferromagnetic J coupling constants, almost identical to those previously detected in bulk and microwire ZnO:Co. The presence of substantial (axial) single-ion anisotropy in ZnO:Co makes the different pairs' resonance positions strongly depending on the J value. For resonance frequency ν in the microwave range, four cobalt pairs can satisfy the condition |J|∼hν/3 to resonate at almost zero magnetic field. Such near-zero-field transitions notably resonate in the parallel ESR mode, which is the signature of the gapped nonlinear Zeeman effect, which is of particular interest for highly stable atomic-clock transitions.