Theory of Spin Transfer Torque

The working of countless electronic devices involves electric and magnetic effects interacting within nanostructured materials. In the phenomenon known as spin transfer torque, a current can give a jolt to thin magnetic layers sandwiched between nonmagnetic materials. The effect can potentially disturb the operation of the read-heads of standard computer hard drives but may also make possible a novel kind of magnetic memory as well as other electronic devices. Controlling the deleterious effects and harnessing the useful potential of spin transfer torques requires both quantitative measurements of their effects and a refined theoretical understanding of these measurements, the goal of this project.

A ferromagnetic material such as iron takes on permanent magnetization when the magnetic properties of its atoms all line up in the same way. Because individual electrons also have an intrinsic magnetic alignment, defined by their so-called spin direction, they can interact with ferromagnets in some unusual ways. For example, an electric current flows more easily through a ferromagnet if electron spins line up with, rather than opposite to, the magnetization.

The device that reads data from a hard disk makes use of this effect. In the read-head are two ferromagnetic layers separated by a nonmagnetic spacer. Data bits encoded magnetically in the surface of a hard disk realign the magnetization of the head's outermost layer, producing a measurable change in electrical resistance. 

However, there's also an effect by which the current doesn't merely respond to the magnetization of the layer but actually disturbs it. When an electron with misaligned spin passes into a magnetized material, the mismatch gives rise to a small twisting force -- a torque -- between the electron and the magnet. A large current can in principle generate a force big enough to shift the magnetization direction of the material it is passing through. In current hard disk read-head designs, this "spin transfer torque" is below the magnitude at which it could disrupt the reading of data, but as designs evolve it's likely to become more of a problem.

Our theoretical investigation of spin transfer torques breaks down the intricate phenomenon into a series of steps. The first step is to use elementary quantum physics to understand precisely how a single electron interacts with a magnetized material. This information then feeds into models that describe the action of a current consisting of many electrons with different alignments. The final ingredient is to include the response of the magnetization to the influence of changing currents. The theoretical picture built up in this way can help engineers improve read-head designs and work out solutions to problems before entering into large-scale production.

Spin transfer torques may also be put to good use. A blossoming technology known as magnetic random access memory (MRAM) records data bits by making the magnetization of tiny ferromagnetic layers either the same or opposite to that of an adjacent, permanently magnetized base. A small current passing through the layers reads data by indicating high or low resistance, but a much larger current can exert a spin transfer torque sufficient to flip the upper layer's magnetization. In that case, the applied current provides a means of writing and rewriting MRAM data.

In another possible application, theory shows that in certain situations the spin transfer torque can cause the magnetization of a ferromagnetic layer to spin around, or precess, at a frequency controlled by the current. This technique could make possible gigahertz oscillators with a wide range of electronic uses.

Researchers within the CNST and in other NIST laboratories are developing methods to accurately quantify spin transfer torques. The goal of our studies of spin transfer torque is to advance theory to the point that it can reliably interpret these measurements, and enable rigorous and reliable predictions of how specific systems will perform in a range of possible device applications.

• First-principles calculation of the nonadiabatic spin transfer torque in Ni and Fe, K. Gilmore, I. Garate, A. H. MacDonald, and M. D. Stiles, Physical Review B 84, 224412 (2011).
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• Spin transfer torques, D. C. Ralph and M. D. Stiles, Journal of Magnetism and Magnetic Materials 320, 1190-1216 (2008).
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• Spin-transfer torque and dynamics, M. Stiles and J. Miltat, in Spin Dynamics in Confined Magnetic Structures III (Springer Berlin/Heidelberg, 2006), p. 225-308.
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