Resonance fine-splitting in a spin-torque nano-oscillator containing two stacked vortices


Resonance fine-splitting in a spin-torque nano-oscillator containing two stacked vortices

Sluka, V.; Kákay, A.; Deac, A. M.; Bürgler, D. E.; Schneider, C. M.; Hertel, R.

Magnetic vortices have become subject of increasingly intense study in spintronics during the past years. These structures develop as flux-closure patterns in cylindrical ferromagnetic disks of dimensions between the single- and multi-domain regimes. The magnetization is basically in-plane and circulates the disk center while the sense of the circulation can be either clockwise or counter-clockwise, defining the chirality of the vortex. As a result of the curling magnetization, the exchange energy density increases when approaching the center region, leading to the formation of a tiny out-of-plane magnetized volume referred to as the vortex core [1]. This core has lateral dimensions in the order of the exchange length and can point either “up” or “down” with respect to the disk plane. Once dislocated from its equilibrium position, the vortex core performs a circular motion around the disk center which is referred to the gyrotropic mode [2,3]. Its frequency depends on the material parameters and the disk aspect ratio and is typically of the order of a few 100 MHz. The sense of gyration is thereby determined by the polarity of the vortex core.
It has been shown that the gyrotropic mode can be excited by spin-transfer torque [4], opening up the possibility of vortex-based spin-torque oscillators, which offer extremely narrow linewidths and operate at zero applied field. Most of the studies concerning spin-transfer torque-induced vortex motion involved systems consisting of a homogeneously magnetized polarizing layer and a vortex free layer.
Here, we study a nanopillar that comprises two magnetic vortices stacked on top of each other. The pillar consists of two Fe disks of 30 and 15 nm thickness, respectively, separated by a 6 nm Ag spacer (Fig. 1). The pillar diameter is about 150 nm. We investigate the magnetization dynamics in the double vortex system both experimentally, where the oscillator is excited by spin-transfer torque, and by means of micromagnetic simulations using the TetraMag [5] code. Specifically, we focus on configurations where the two vortices have opposed chiralities. We find that depending on the chirality combination and the relative core polarities, the resonances split into a fine-structure of four modes (Fig. 2). The four mode frequencies differ by hundreds of MHz, so that our spin-torque oscillator resembles a flute: Each combination of chiralities and relative core polarities results in a different “tone”, and all tones can be excited at zero field and equal currents. The frequencies obtained from micromagnetic simulations are in good agreement with the measured values, showing that the mode fine-splitting results from the magnetostatic interaction between the two ferromagnetic disks.
These results are not only interesting for designing new vortex-based spin-torque oscillators. They also show how to detect changes in vortex core polarity without resorting to high resolution imaging techniques and may thus open new perspectives on how to make use of vortex cores in, for example, core-based non-volatile memory applications.

[1] E. Feldtkeller and H. Thomas. Phys. Kondens. Mater. 4, 8 (1965).
[2] D. L. Hubert. Phys. Rev. B 26, 3758 (1982).
[3] B. van Wayenberge et al. Nature 444, 461 (2006).
[4] V.S. Pribiag et al. Nature Phys. 3, 498 (2007).
[5] A. Kákay, E. Westphal and R. Hertel. IEEE Trans. Magn. 46, 2303 (2010).

Keywords: magnetic vortex; spintronics; spin-torque oscillator; spin-valve; vortex-oscillator; magnetization dynamics; spin-transfer torque; vortex-core

  • Lecture (Conference)
    8th International Symposium on Metallic Multilayers (MML2013), 19.-24.05.2013, Kyoto, Japan

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Publ.-Id: 19920