Stress-Induced Modification of Gyration Eigen-Frequencies in Stacked Double-Vortex Structures

The ground state of nanoscale circular magnetic disks of certain geometric aspect ratios is a spontaneously forming stable vortex configuration with circulating in-plane magnetization and a vortex core pointing out-of-plane. Resonantly exciting the VC via either an rf magnetic field or an rf spin-polarized current yields a gyrotropic motion around its equilibrium position, characterised by a specific eigen-frequency, which depends on the material parameters and the disk geometry [1]. Such oscillations, which can be read out via periodic magnetoresistance changes, generate rf signals with high quality factors (>10000) in the sub-GHz bandwidth [2,3]. While all these features make vortex-based nano-oscillators interesting as nanoscale rf sources, the major drawback remains their low frequency tunability associated with the linear characteristics of the gyrotropic mode.
Here, we investigate the role of magnetostriction in improving the tunability of vortex nano-oscillators. Specifically, we consider a double-disk structure comprising magnetostrictive (CoFe) and non-magnetostrictive (Py) layers separated vertically by a non-magnetic spacer. We show that, when the two vortices have different eigen-frequencies and the magnetostatic coupling between them is sufficiently strong, the stress-induced magnetoelastic anisotropy can lead to the synchronized gyration of the two vortex cores (Fig. 1). The stress-induced transition from double-frequency to single-frequency dynamics is mostly controlled by the polarization of the vortices and the magnetostatic coupling strength (i.e. spacer thickness). These findings offer a frequency tunability of vortex-based oscillators via mechanical stress, which can be generated and controlled electrically, for example, using piezoelectric substrates [4].

Funding from the EU Horizon 2020 project No. 737038 (TRANSPIRE) is acknowledged.

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[2] A. Dussaux, et al, Nat. Commun. 1, 1 (2010).
[3] N. Locatelli, et al, Appl. Phys. Lett. 98, 062501 (2011).
[4] M. Filianina, et al, Appl. Phys. Lett. 115, 062404 (2019).