Abstract
Cobalt-doped ZnO nanoparticles (NPs) with different Co concentrations are investigated by means of - and -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 ions' ESR powder spectra. Depending on cobalt concentration, common additional weak ESR lines are detected which are here demonstrated to arise from some 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 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 value. For resonance frequency in the microwave range, four cobalt pairs can satisfy the condition 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.
- Received 9 September 2021
- Revised 8 November 2021
- Accepted 6 January 2022
DOI:https://doi.org/10.1103/PhysRevB.105.035424
©2022 American Physical Society