Spin-Wave-Mode Coexistence on the Nanoscale: A Consequence of the Oersted-Field-Induced Asymmetric Energy Landscape

Abstract

It has been argued that if multiple spin wave modes are competing for the same centrally located energy source, as in a nanocontact spin torque oscillator, that only one mode should survive in the steady state. Here, the experimental conditions necessary for mode coexistence are explored. Mode coexistence is facilitated by the local field asymmetries induced by the spatially inhomogeneous Oersted field, which leads to a physical separation of the modes, and is further promoted by spin wave localization at reduced applied field angles. Finally, both simulation and experiment reveal a low frequency signal consistent with the intermodulation of two coexistent modes.

Mode-coupling mechanisms in nanocontact spin-torque oscillators

“…Each spectrum was averaged 10 times with a video bandwidth of 10 kHz in order to minimize the noise. In agreement with our circular NC-STOs (not shown) and previous results 8, 10,11,36 , the spectra show a transition between the propagating spin wave mode predicted by Slonczewski 5 and a modehopping regime. This transition occurs at a well-defined critical angle, θ c , corresponding to the bullet mode onset 10,36 .…”

“…The fabricated NC-STOs are based on a pseudo spin valve 11,35 with the layer structure Co (8 nm)/Cu (8 nm)/Py (4.5 nm), where the Co (fixed layer) acts as a relatively static current spinpolarizer whereas the Py (Ni 80 Fe 20 , free layer) is subject to STT induced magnetodynamics. Devices with both circular NCs and elliptical NCs with an aspect ratio r = 1.5 are defined via electron beam lithography.…”

“…1(c). Additionally, such a spatial localization was shown to promote mode coexistence in these devices 11 . In a recent multi-mode theory developed for STOs [25][26][27][28] , a coexistence regime was found to be favored by mode coupling.…”

“…Depending on the geometry and magnetic properties of the free layer, propagating spin waves (SWs) [5][6][7] , solitonic modes [8][9][10][11] , vortex gyration 12,13 , and magnetic dissipative droplets [14][15][16][17] have been observed. STOs can also be used to generate SWs in physically extended thin films, which is of particular interest for future magnonic applications 18,19 .…”

“…The effect of the currentinduced Oersted field, H Oe , has been recently shown to be critical in determining the SW propagation direction and mode volume 11,23 . Moreover, the energy landscape induced by the Oersted field was shown to determine the position of the localized modes 11,24 . Such localization can be observed from micromagnetic simulations by calculating the mode energy distribution, or the spatial representation of the power spectral density, shown in Fig.…”