Very High Frequency Electron Paramagnetic Resonance Studies on γ-BaCo₂(PO₄)₂

γ-BaCo₂(PO₄)₂ (γ-BCPO) is a quasi-two-dimensional frustrated honeycomb magnet with two separate short-range magnetic order – collinear and helical. Through magnetometry, specific heat and inelastic neutron studies, it was confirmed that the magnetism of γ-BCPO balances at the edge of the competing phases of the XXZ J₁-J₂-J₃ model. To better understand the local magnetism, very high frequency (120 – 614.4 GHz) electron paramagnetic resonance (VHF-EPR) measurements have been performed from 1.5 – 80 K. At any applied microwave frequency, we found no EPR signal from γ-BCPO above 20K. For T < 20 K, a broad signal is visible at low fields, with the spectral features better defined. As the frequency changed from 120 - 336 GHz, the broad signal split into several narrow modes, and for frequency 406.6 GHz and 614.4 GHz, a broad, but complete signal appeared with no splitting. The effective g-value shifts from g = 14.62 to 11.18 as the frequency increased from 406.6 to 614.4 GHz, in line with a magnetic gap of 300 GHz. The EPR signal narrows and shifts to lower fields as the temperature is lowered from 20 – 1.5 K. These features indicate that the resonances are likely related to antiferromagnetic modes because of large internal fields due to complex magnetic interactions in γ-BCPO.