Spin-torque-induced dynamics at fine-split frequencies in nano-oscillators with two stacked vortices

Abstract

The gyrotropic rotation around the equilibrium position constitutes the fundamental excitation of magnetic vortices in nanostructures. The frequency of this mode varies with material and sample geometry, but is independent of the vortex handedness and its core direction. Here, we demonstrate that this degeneracy is lifted in a spin-torque oscillator containing two vortices stacked on top of each other. When driven by spin-polarized currents, such devices exhibit a set of dynamic modes with discretely split frequencies, each corresponding to a specific combination of vorticities and relative core polarities. The fine splitting occurs even in the absence of external fields, demonstrating that such devices can function as zero-field, multi-channel, nano-oscillators for communication technologies. It also facilitates the detection of the relative core polarization and allows for the eight non-degenerate configurations to be distinguished electrically, which may enable the design of multi-state memory devices based on double-vortex nanopillars.

Two-body problem of core-region coupled magnetic vortex stacks

“…We introduce an additional dimension to the collective gyrations of vortices known from spin-torque oscillators [22]. Stacking the vortices allows for a strongly increased packing density and has thus stimulated recent studies [23][24][25][26][27][28][29][30]. While for two-dimensional arrangements the minimization of the stray fields at the side surfaces creates the vortices and mediates their interaction, we observe a second coupling mechanism for three-dimensional stacks that has been investigated theoretically [26,31,32].…”

Magnetization dynamics of coupled ferromagnetic disks

“…They also provide new avenues to explore -with potentially interesting and useful applications -the dynamics of coupled magnetic nanostructures. The results presented here should also be useful in the context of designing and interpreting experiments exploring spin-transfer torque induced magnetization dynamics in nanopillars with diameters large enough to support vortex or meron structures 38,39 .…”

Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates

“…The complete layer stack was grown by molecular beam epitaxy on GaAs(001) wafers prior to the lithography process. Accordingly prepared nanopillars Microelectronic Engineering 140 (2015) 33-37 were successfully used for several studies of current-driven magnetization dynamics [7][8][9][10][11]. However, the fabrication process failed when it was applied to multilayers grown on MgO(001) substrates, which turned out to be more insulating than the GaAs(001) substrates and thus gave rise to severe charging effects.…”