Field-Angle-Dependent Multi-Frequency Electron Spin Resonance Spectroscopy In Submillimeter Wave Range Based On Thermal Detection

We report the thermally detected electron spin resonance (ESR) spectroscopy in the frequency range of millimeter and submillimeter waves. Under high vacuum conditions, a cantilever-shaped device detects ESR absorption of a mounted sample as a temperature difference in its beam direction. Despite the simple experimental setup, the spin sensitivity of the order of 10(12) spins/G was achieved at 10 K. The developed sample stage is small enough to be used in a 10 T split-pair superconducting magnet with a bore of 25 mm, enabling precise field-angle-dependent ESR measurements at multi-frequencies above 500 GHz. We demonstrate its usefulness by studying the field-angle dependence of the excitation energy of the dimer triplet state in the Shastry-Sutherland magnet SrCu2(BO3)(2).