Tunable Reflective Structures Based on Weak Ferromagnetics and Their Application as Tunable Sub-Terahertz Resonators

Abstract

Electromagnetic eigenwaves in a weak ferromagnetic with anisotropy of the “easy plane” under magnetization in this plane are investigated. It is shown that at frequencies of anti-resonances the weak ferromagnetic can be considered as environment with characteristic impedance close to zero. The polarization of eigenwaves in such environment is analyzed on the example of high-temperature weak ferromagnetics—hematite and iron borate. It is proposed to consider the weak ferromagnet-metal structure as an element of the metasurface with a controlled phase shift of the reflected wave. A tunable Fabry-Perot resonator with a weak ferromagnet-metal structure (ferromagnet is of sub-wave thickness) is considered and the corresponding frequency-field dependencies in the sub-terahertz range are studied. It is shown that such resonator combines rather high Q-factor with a possibility of magnetic tuning of its resonance frequency. The maximum local slope of the field dependence of the resonator frequency reaches 0.3 MHz/Oe using a hematite layer and 0.6 MHz/Oe using an iron borate layer of 10 µm thickness. The resonator unloaded quality factor for the main mode varies within 300–1400 and 250–1000, respectively.