Whispering Gallery mode ESR spectroscopy and parameters measurement in single crystal SrLaAlO₄ at millikelvin...

A cylindrical single crystal SrLaAlO₄ Whispering Gallery mode dielectric resonator was cooled to millikelvin temperature using a dilution refrigerator. By controlling a DC-magnetic field, impurity ions' spins were coupled to a variety of modes allowing the measurement of hybrid spin-photon systems. This Electron Spin Resonance mapping technique allowed us to detect Cu²⁺,Fe³⁺ and Mn⁴⁺ impurity ions (at the level of parts per million (ppm) to parts per billion (ppb)), verified by the measurement of the spin parameters along with their site symmetry. Whispering Gallery modes exhibited Q-factors ⩾10⁵ at a temperature less than 20mK, allowing sensitive spectroscopy with high precision. Measured hyperfine line constants of the Cu²⁺ ion shows different parallel g-factors, g_‖Cu, of 2.526,2.375,2.246 and 2.142. The spin-orbit coupling constant of the Cu²⁺ ion was determined to be λ≃-635cm^-1. The low-spin state Fe³⁺ ion's measured parallel g-factor, g_‖Fe, of 2.028 reveals tetragonal anisotropy. The Mn⁴⁺ ion is identified in the lattice, producing hyperfine structure with high-valued g-factors,g_‖Mn, of 7.789,7.745,7.688,7.613,7.5304 and 7.446. The hyperfine structures of the Cu²⁺ and Mn⁴⁺ ions show broadening of about 79G between 9.072GHz and 10.631GHz, and 24.5G broadening between 9.072GHz and 14.871GHz, respectively.