Ferromagnetic resonance in coupled magnetic nanostructured arrays

Ivan Nekrashevich,Dmitri Litvinov

doi:10.1063/1.5038093

Abstract

The modulated field sweep ferromagnetic resonance (FMR) spectroscopy was used to study the magnetization dynamics in large arrays of interacting cubic nanomagnets. A 60nm thick permalloy (Ni80Fe20) thin films were patterned using a lift-off process into several large arrays of 60 x 60 nm2 nanostructures where the spacing between the magnetic nanocubes was varied to control the strength of dipolar coupling. Electron beam lithography was used for device patterning using lift-off. The permalloy films were deposited using magnetron sputtering. DC magnetic properties were evaluated using alternating gradient force magnetometer (AGFM). In the FMR measurements, the orientation of DC bias magnetic field was varied from the in-plane to out-of-plane with respect to the 2D plane of the arrays. The FMR peak splitting and multiple FMR modes were observed in the evolution of the FMR spectra as the function of the bias field orientation, and were strongly influenced by the nanomagnet geometry and the spacing between the nanomagnets. Two resonance modes, shape and lattice, were particularly well pronounced in the observed FMR spectra. These modes are characterized by effective demagnetizing factors representing different symmetries of the system of interacting nanomagnets. Micromagnetic modeling suggest that micromagnetic texture within the nanomagnets modulated by the stray fields from the neighbors correlates with the FMR spectra.

Highlights

Following the pattern quality verification using Phillips XL 30S scanning electron microscopy (SEM) and Digital Instruments NanoScope IIIa atomic force miscropcopy (AFM), a 100nm thick layer of PMMA was spun over the samples surface to avoid sample degradation and damage during handling and magnetic measurements
All three samples in this study show the in-plane anisotropy with the coercivity Hc ranging from 6.8 to 134.1 Oe when the bias field applied in-plane and parallel to the sides of cubic nanomagnets, FIG. 3
The in-plane and out-of-plane hysteresis loops of the arrays shown on the Fig. 3 along with the top-down SEM images of the arrays with various duty cycles

Summary

Introduction

The ever-growing operational speeds combined with ever-increasing device packing densities in current and future magnetics-based technologies, including magnetic random-access memory, magnetic data storage, reprogrammable magnetic system, etc., brings about the critical need for a deeper understanding of high-frequency properties of underlying magnetic nanostructured materials and devices.[1,2,3,4,5,6,7,8,9] Significant progress has been made in both theoretical and experimental studies of spin wave generation and propagation in arrays of nanomagnets with or without magnetic coupling.[10,11,12,13,14] Edge modes, spin wave quantization and confinement were studied theoretically[15,16,17] and characterized experimentally.[11,18,19,20,21,22] Studies indicate considerable influence of the lattice symmetry on the magnonic band structure of closely packed ferromagnetic nanostructuress magnetized in-plane at various azimuthal angles.[23,24] Theoretical formalism was developed for magnetic excitations in arrays of coupled macrospins.[25] Dependence of the FMR spectrum and magnetization of arrays of isolated nanomagnets on the angle of applied external field was studied in nanoellipses with relatively low aspect ratio.[26]

Methods

Results

Conclusion