High-Frequency Properties of Permalloy Nanowire Arrays for RF Devices

Abstract

We present our simulation and experimental results on high-frequency properties of permalloy (Py) nanowire arrays. The planar Py nanowires are 10 μm long, 100 nm thick with widths of ~ 240 nm, and ~ 500 nm (layout dimension). The distance between adjacent Py nanowires is ~ 100 nm. OOMMF simulations indicate that single domain structures exist in these nanowires with ferromagnetic resonance frequencies (FMR) around 10 GHz. Coupling effects are negligible. The Oersted field of an external DC current reduces the FMR frequencies. The obtained Py properties can be used to simulate RF inductors to evaluate the application potentials of Py nanowire arrays in RF devices. I. BACKGROUNDIn the pursuit of on-chip high-performance radio-frequency (RF) devices, such as inductors [1-3], filters [4], and circulators [5], ferromagnetic materials have attracted much attention. The high-frequency permeability of the obtained ferromagnetic materials determines the effectiveness of performance boosting. For patterned ferromagnetic thin films, ferromagnetic resonance (FMR, frequency f r ) is often the main loss mechanism and application limiting factor. Therefore, boosting f r is of great interest.Exploiting shape anisotropy [4, 6, 7] of metallic ferromagnetic thin films is one of the promising approaches to boost f r . Among the materials that have been tested, permalloy (Py, Ni 80 Fe 20 ) films have attracted much attention. Nevertheless, only limited f r improvements have been demonstrated even though large aspect ratios were used [6][7][8][9][10]. No efforts have been made to control domain structures despite the fact that domain-wall motion and magnetization rotation are distinctly different. On the other hand, it was shown theoretically that Py structures can have 20 GHz or above FMR frequency [11]. It was also shown that Ni-Fe nanowires with small cross section areas have natural FMR frequencies around 20 GHz [12,13]. However, the fabrication processes are not compatible with CMOS process. Therefore, it is interesting to investigate the high frequency properties of planar submicron Py structures with controllable magnetization distributions.In a recent brief communication [14], we reported patterned submicron Py films with 8-10 GHz FMR frequencies. In this work, we present additional experimental results with simulation analyses of magnetization distributions, coupling effects, and Oersted field induced FMR frequency shift of permalloy nanowire arrays. With the obtained high-frequency permeability data, we also evaluate the potential of Py nanowire arrays for integrated inductor applications.