Current-induced magnetization dynamics in a spin-valve comprising two spin-transfer torque-coupled ferromagnetic layers


Current-induced magnetization dynamics in a spin-valve comprising two spin-transfer torque-coupled ferromagnetic layers

Sluka, V.; Fowley, C.; Bernert, K.; Gan, H.; Fassbender, J.; Deac, A. M.

Typical all-metallic spin-torque oscillators are nanopillars comprising two easy-plane ferromagnetic elements separated by a nonmagnetic metal spacer. One of the layer magnetizations is fixed and serves as the polarizer for the current while the second layer is susceptible to spin-transfer torque. Subjected to d.c. currents and external magnetic fields, these devices exhibit wellknown types of magnetization dynamics such as small-/ large-angle or clamshell precession, generating microwave signals at several Gigahertz [1]. Here, we investigate a system that differs from the above in two ways. First, our nanopillar combines an uniaxial in-plane anisotropy-ferromagnet (FM1) with a second layer (FM2) exhibiting a perpendicular anisotropy (PMA). Thus our system resembles the one studied in [2], however, in our case none of the layers is fixed, meaning that both layer magnetizations are coupled by the current and can move freely under the action of the spin-torque. Using the generalization of Slonczewski’s model [3] to asymmetric spin-valves [4,5], the d.c. current-induced magnetization dynamics in perpendicular applied field is studied numerically within the framework of the macrospin model. The resulting current-field phase diagram displays regimes of single and combined layer motion. Within fields below the effective PMA and currents smaller than -2 mA or larger than 4 mA (for the parameters chosen), the magnetization of FM1 basically precesses in-plane for the external field exceeding a threshold value. The latter increases with the current magnitude for both current polarities, climbing up to ±176 mT at +25 mA. The simultaneous excitation of FM1 and FM2 however exhibits a more asymmetric current dependence (Fig. 1). Within the simultaneously excited regime, the complexity of the trajectories is found to vary strongly with applied currents and fields. Fig. 2 shows example spectra taken at the locations indicated in Fig. 1. Viewed from a commoving frame, some trajectories correspond to largeangle precession, while others behave entirely differently from what is found in common fixed-free layer spin-valves. This is reflected in the obtained power spectra which range from near-harmonic to broad noise-like.

[1] S. I. Kiselev et al., Nature 425, 380 (2003).
[2] W. H. Rippard et al., Phys. Rev. B 81, 014426 (2010).
[3] J. C. Slonczewski, J. Magn. Magn. Mater. 247, 324 (2002).
[4] J. Xiao et al., Phys. Rev. B 70, 172405 (2004).
[5] J. Xiao et al., Phys. Rev. B 72, 014446 (2005).

Keywords: spin-transfer torque; magnetization dynamics; spin-valve; spin-torque oscillator

  • Poster
    12th Joint MMM/Intermag Conference, 14.-18.01.2013, Chicago, Illinois, USA

Permalink: https://www.hzdr.de/publications/Publ-19899
Publ.-Id: 19899