High-frequency electron spin resonance (HFESR) spectroscopy is a powerful tool to get microscopic insights into the magnetic structure of magnetic materials. In HFESR measurements, solid-state multipliers and backward traveling-wave oscillators are often used as a light source, but they only cover a limited frequency range so it is necessary to combine multiple light sources to cover broad frequency range. On the other hand, terahertz (THz) photomixer is a continuously frequency-tunable source/detector that covers a broad frequency range beyond 1 THz. This remarkable feature of THz photomixer allows broadband and seamless THz spectroscopy [1].
We developed a novel broadband HFESR spectroscopy technique using THz photomixers. The source and detector could be compactly assembled so that the whole system was accommodated in a magnet bore. ESR signals were detected as transmitted THz-wave intensity through a sample placed between the source and the detector. Our systems can be operated in both field- and frequency-swept configurations [2, 3]. Our technique allowed HFESR spectroscopy in a broad frequency range between 0.05-1.1 THz and in magnetic fields of up to 10 T.
This technique was applied to antiferromagnetic resonance (AFMR) spectroscopy of single-crystalline nickel oxide (NiO) [4]. NiO is known as a typical easy-plane antiferromagnet with alternating stacks of ferromagnetic nickel layer along the <111> direction. However, its detailed magnetic structure has not been clarified yet. Indeed, the experimental results are not consistent with the standard two-sub lattice model. We systematically carried out frequency-swept AFMR measurements of NiO at room temperature, and found that the field dependence of the AFMR mode at 1 THz differs remarkably upon field orientations. The results are analyzed, taking the inherent magnetic domain structure of NiO into account.