Controlling the gyrotropic critical current by resonant excitation of the higher order spin wave modes in vortex-based magnetic tunnel junctions

Abstract

Vortex-based magnetic tunnel junctions (MTJs) are a rich dynamic system with a variety of different resonant modes, spanning a wide frequency spectrum from hundreds of MHz to tens of GHz [1, 2]. The diverse spectral properties of vortex-based MTJs makes them potential candidates for high frequency signal detectors [3, 4], with a dc rectification ‘spin-diode’ effect resulting when one of the resonant modes are excited with an rf current passing across the MTJ. In this study, we investigate the spin-wave modes present in a vortex-based MTJ, by analysis of the spin-diode rectification effect and by comparison with Fourier analysis of micromagnetic simulations. We demonstrate that the nature of the spin wave mode is strongly related to the position of the vortex core, varying from a pure azimuthal mode when the core is located at the centre of the MTJ, to a more standing wave edge mode when the core is strongly displaced to the edge. Furthermore, we demonstrate that the dominant excitation mechanism also changes depending upon core position, with the azimuthal mode being more sensitive to the field-like spin torque effect, and with the edge mode being more susceptible to the radial Oersted field. In addition to analysing the nature and the excitation scheme of these modes, we also show how their resonant excitation can lead to the modification of the critical current of the steady state excitation of the gyrotropic mode. The ability to control the gyrotropic mode via excitation of the higher order mode gives rise to a frequency down converter which can demodulate a signal with a carrier signal at 7.7 GHz with a modulation rate of 1 MHz. By harnessing the higher order spin wave modes present in vortex-based MTJs, new radio-frequency technologies such as signal detection and demodulation, which target GHz frequency bands can be envisaged. VC and PB acknowledge the ANR project “SPINNET” ANR-18-CE24-0012.