Resonance Frequency Tuning of a 200 GHz Band Absorber by an External Magnetic Field

The effect of the external magnetic field on the millimeter‐wave absorption of rhodium‐substituted epsilon iron oxide, ε‐Rh 0.13 Fe 1.87 O 3 (1) and ε‐Rh 0.19 Fe 1.81 O 3 (2) nanomagnets, is investigated. Terahertz time‐domain spectroscopy (THz‐TDS) shows that 1 and 2 display a zero‐field ferromagnetic resonance (the so‐called natural resonance) at 201 GHz with a linewidth of 20 GHz and at 210 GHz with a linewidth of 30 GHz, respectively. The ferromagnetic resonance is measured under an external magnetic field ( H ex ). Applying +3.5 kOe parallel to the remnant magnetization and −3.5 kOe antiparallel to the remnant magnetization shifts the resonance frequency at 201 GHz in 1 by +6 and −4 GHz and that in 2 by +4 and −4 GHz, respectively. Simulations of the ferromagnetic resonance using the Landau−Lifshitz−Gilbert model reproduce the magnetic field‐induced shifts of the resonance frequencies. The 220 GHz band millimeter wave is the highest frequency with a high transparency for “atmospheric windows” and is expected to be a carrier frequency for sixth‐generation mobile communication systems (6 G) or 7 G. This study may help realize applications for millimeter‐wave isolators, circulators, or shutters.