Spin-Torque Excitations in Magnetic Nanopillars with an Exchange-Biased Fixed Layer

We have fabricated and are studying IrMn / permalloy (Py) / Cu / Py spin-valve nanopillars. We use exchange-bias coupling between IrMn layer and Py to strongly pin the magnetic moment of the lower Py layer and keep its orientation approximately 40 degrees from the easy axis of the other permalloy layer. We have used these samples to measure the bias dependence of the spin transfer torque vector using spin-transfer-driven ferromagnetic resonance (ST-FMR), and found that the torque stays in the plane defined by the two magnetic moments in the bias range |I|<2mA. We are also studying resonant magnetic switching using the spin-transfer torque from microwave-frequency pulses.

People Involved

Yong-Tao Cui, Kiran Thadani, Jack Sankey, Zhi-Pan Li

Summary

When current passes perpendicularly through the layers of a spin-valve structure (Ferromagnet / Metal / Ferromagnet), electrons polarized by one magnetic layer (the "fixed layer") can transfer their spin angular momenta to the other magnetic layer (the "free layer") and hence exert a torque on the free layer [1]. This effect provides a new way to manipulate small magnets by electrical current rather than magnetic field, and has potential applications in developing non-volatile magnetic memories. For many types of experiments it is convenient to fabricate samples with an offset angle between the magnetic moments of the two magnetic layers in the absence of any magnetic field. This can be achieved by using an antiferromagnetic layer to exchange-bias the fixed layer, and pin its moment in the desired direction.[2]

We have fabricated exchange-biased spin valve nanopillars of the structure (in nm): Py 4 / Cu 80 / IrMn 8 / Py 4 / Cu 8 / Py 4 / Cu 2 / Pt 30. The multi-layers are first deposited in a sputtering system and then annealed in a magnetic field to induce an exchange bias between the IrMn and Py layers. Then electron-beam lithography and ion milling are used to define pillars with elliptical cross sections having an aspect ratio ~3:1 and a short axis diameter between 30 nm and 100 nm. The long axis of the ellipse is oriented so that the easy magnetic axis of the free layer is offset approximately 40 degrees from the exchange-bias direction. The contact leads are made by photolithography, and SiO2 is deposited to provide electrical insulation between the top and bottom leads.

The samples are initially characterized by measuring the differential resistance, dV/dI, as a function of magnetic field at different field angles. By fitting these curves to a macrospin Stoner-Wolfarth model, we can measure that the exchange bias strength to be typically 400 G at room temperature, oriented approximately 40 degrees from the easy axis of the magnetic free layer, in agreement with the sample design. We then used the samples to perform the recently-developed technique of spin-transfer-driven ferromagnetic resonance (ST-FMR), which requires an initial offset angle between the moments in the two magnetic layers. Based on the magnitude and peak shape of the resonance signals, ST-FMR enables measurements of the bias dependences of the strength and direction of the spin-transfer torque on the free layer due to the spin-polarized current. We found that the FMR resonance peaks were symmetric in the bias range of ||I|<2mA, which indicates that the spin-transfer torque is confined strictly in the plane defined by the magnetic moments in the two layers. This is in contrast to the recent observation of an out-of-plane component of torque in MgO magnetic tunnel junctions under finite DC bias.3

These same samples can also be used in RF-enhanced switching experiment in which an applied microwave signal may resonantly excite the free layer and reduce the critical current for switching. Experiments to explore this effect are underway.


Figure 1
Figure 1: SEM image: cross section of the nanopillar device.

Figure 2
Figure 2: dV/dI vs. applied magnetic field, with a field along direction 45 degrees from the easy axis.

Figure 3

Figure 3: Bias dependence of the frequency-symmetric and antisymmetric components of ST-FMR resonance peak shapes.

References

  1. Slonczewski, J. C., Current-Driven Excitation of Magnetic Multilayers J. Magn. Magn. Mater. 159, L1 (1996).
  2. Krivorotov, I. N. et al., Time-Domain Measurements of Nanomagnet Dynamics Driven by Spin-Transfer Torques, Science 307, 228 (2005).
  3. Sankey, J. C. et al., Measurement of the Spin-Transfer-Torque Vector in Magnetic Tunnel Junctions, arXiv:0705.4207.


Last updated: 11-July-2007

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