Tunable Optical Mode Ferromagnetic Resonance in FeCoB/Ru/FeCoB Synthetic Antiferromagnetic Trilayers under Uniaxial Magnetic Anisotropy

• Published in: Advanced functional materials Vol. 26; no. 21; pp. 3738 - 3744
• Main Authors: Li, Shandong, Li, Qiang, Xu, Jie, Yan, Shishen, Miao, Guo-Xing, Kang, Shishou, Dai, Youyong, Jiao, Jiqing, Lü, Yueguang
• Format: Journal Article
• Language: English
• Published: Blackwell Publishing Ltd 07.06.2016
• Subjects: composition gradient sputtering; Coupling; electric field tunability; Exchange; Ferromagnetism; Interlayers; Inverse; Magnetic anisotropy; Magnetic permeability; magnetoelectric coupling; microwave ferromagnetic films; multiferroic; Switches
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201600122

Ferromagnetic resonance (FMR) is one of the most important characteristics of soft magnetic materials, which practically sets the maximum operation speed of these materials. There are two FMR modes in exchange coupled ferromagnet/nonmagnet/ferromagnet sandwich films. The acoustic mode has relatively lower frequency and is widely used in radio‐frequency/microwave devices, while the optical mode is largely neglected due to its tiny permeability even though it supports much higher frequency. Here, a realistic method is reported to enhance the permeability in the optical mode to an applicable level. FeCoB/Ru/FeCoB trilayers are carefully engineered with both uniaxial magnetic anisotropy and antiferromagnetic interlayer exchange coupling. This special magnetic structure exhibits a high optical mode frequency up to 11.28 GHz and a maximum permeability of 200 at resonance. An abnormally low inverse switch field (<200 Oe, less than 1/5 of the single layer) is observed which can effectively switch the system from optical mode with higher frequency into acoustic mode with lower frequency. The optical mode frequency and inverse switch field can be controlled by tailoring the interlayer coupling strengths and the uniaxial anisotropy fields, respectively. The tunable optical mode resonance thus can increase operation frequency while reduce operation field overhead in FMR based devices. In a ferromagnet/nonmagnet/ferromagnet exchange coupled trilayer system, the ferromagnetic resonance can be tuned with the intrinsic magnetic anisotropy and interlayer coupling strength. The lower frequency acoustic mode can be eliminated while the higher frequency optical mode is enhanced. A low inverse switch field allows one to efficiently toggle between these two modes.