Magnetic nanoparticles have attracted much research interest in the past decades due to their potential applications in microwave devices. Here, we adopted a novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation. We observed that the cut-off frequency can be enhanced from 5.3 GHz for Fe3O4 to 6.9 GHz forFe3O4@SiO2 core-shell structure superparamagnetic nanoparticles, which are much higher than the natural resonance frequency of 1.3 GHz for Fe3O4 bulk material. This finding not only provides us a new approach to enhance the resonance frequency beyond the Snoek's limit, but also extend the application for superparamagnetic nanoparticles to microwave devices.

High-frequency properties of oil-phase-synthesized ZnO nanoparticles * Ding Hao-Feng(丁浩峰) a)b) , Yang Hai-Tao(杨海涛) b) † , Liu Li-Ping(柳丽平) b) , Ren Xiao(任 肖) b) , Song Ning-Ning(宋宁宁) b) , Shen Jun(沈 俊) c) , Zhang Xiang-Qun(张向群) b) , Cheng Zhao-Hua(成昭华) b) , and Zhao Guo-Ping(赵国平) a)b) ‡