Linewidths of Spin-Transfer-Driven Precession in Magnetic Nanopillars

In a magnetic multilayer device, spin-transfer torque from a spin-polarized DC current can drive magnetic layers into steady-state precessional modes [1,2]. We investigate how the linewidths of these modes depend on the angle and magnitude of an applied magnetic field. We find that the linewidths can change dramatically, decreasing by a factor of 50 in some devices, as the field is rotated away from the easy axis. We also find that the angular dependence of the linewidths depends on the device geometry.

People Involved

Kiran V. Thadani, Jack C. Sankey, Ilya N. Krivorotov, Ozhan Ozatay, and Patrick M. Braganca

Summary

In a magnetic multilayer, a spin-polarized current generated by one magnetic layer can deposit spin angular momentum into a thinner magnetic layer and generate a steady-state precessional excitation of the thinner layer. This effect is of technological interest because it may allow the production of high-quality nanoscale frequency-tunable microwave sources and resonators. It has been shown previously that the coherence time of the spin-transfer-driven precession is strongly temperature-dependent; it increases with decreased temperature [2]. From a practical standpoint, it is desirable to attain long coherence times even at room temperature. To this end, we have investigated the dependence of the coherence time on another parameter: the direction of the applied magnetic field.

Our devices consist of two magnetic layers [typically permalloy (Py), Co, or CoFe] separated by a Cu spacer. One of the magnetic layers, called the free layer, is very thin (about 5 nm), thereby making it susceptible to the spin-transfer torque. The other magnetic layer, called the fixed layer, remains stationary. This is accomplished by either; a) making it very thick (40 nm), b) exchange-biasing it with an antiferromagnet, such as IrMn (8 nm), or c) making it an extended film instead of a patterned ellipse. We call the three different kinds of devices "patterned," "exchange-biased patterned" and "unpatterned," respectively. The free layer in all three types of devices is etched using electron-beam lithography and ion milling to give a cross section that is approximately elliptical, with an aspect ratio of 2:1 or 3:1 and a minor axis of 50-70 nm. We use photolithography to pattern bottom leads and to make top contacts.

In our measurements, we vary the angle between the applied magnetic field and the energetically-favored magnetically easy axis of the free layer. As the applied field is rotated away from the easy axis, shape anisotropy changes the effective field on the moments of the fixed and free layers, thereby altering their orientation and current-induced precession dynamics. We use a spectrum analyzer to measure the resistance oscillations arising from spin-transfer-driven magnetic precession.

We find that the field direction studied most commonly, in-plane along the magnetic easy axis of the ellipse, generally gives the largest linewidths, corresponding to the least coherent precession. As the field is rotated away from the easy axis, the linewidths can change dramatically, decreasing by a factor of 50 in some devices. We have observed the largest angular dependence in exchange-biased devices and the smallest dependence in unpatterned devices. We are currently exploring several possible mechanisms to try to understand these dramatic variations.


Figure 1
Figure 1: The geometry of the devices.

Figure 2
Figure 2: Resistance versus magnetic field sweeps.

Figure 3

Figure 3: Power spectrum of DC-driven resistance oscillations for an applied field at 0 and 75 degrees from the easy axis. (inset): Linewidth versus field angle.

References

  1. S. I. Kiselev et al., Microwave oscillations of a nanomagnet driven by a spin-polarized current, Nature 425, 380 (2003).
  2. J. C. Sankey et al., Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by DC spin-polarized currents, Phys. Rev. B. 72, 224427 (2005).


Last updated: 11-July-2007

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