Current-Driven Excitation of Magnetic Multilayers
Slonczewski, J.C., 1996. Current-Driven Excitation of Magnetic Multilayers J. Magn. Magn. Mater. 159, L1
Essay about this article
Embedded in his 1989 paper Slonczewski had an out-of-equilibrium coupling between the electrodes of a Magnetic Tunnel Junction (MTJ) that amounted to a current induced torque created by the spin current. Independently, Luc Berger of Carnegie Mellon University in Pittsburgh was studying through the 1980’s and 1990’s the forces of a current on domain walls in ferromagnetic metals. By 1996 Slonczewski adapted his work on MTJ’s to a metallic trilayer, or spin-valve, and proposed that the torque exerted by one magnetic layer was such as to align the trilayers in parallel or antiparallel. Berger talked about the spin waves created by the current and the creation of a macroscopic number of these, and called this process a Spin-Wave Amplification by Stimulated Emission of Radiation (SWASER); which amounts to rotating the axis of spin quantization as if a torque acted on the magnetization.
In a sense spin torque is the converse of GMR, i.e., GMR arises from the effect of changes in the magnetic background on the resistivity, e.g., changing the alternate layers in a multilayer for parallel, whereas spin torque is the back action of the current on the magnetic background when it is non-collinear, i.e., when the magnetizations are not parallel or antiparallel. Whereas GMR is determined by changes in the charge current, the torque is controlled by the magnitude and polarization of the spin current; they are complementary effects arising from passing a current through a textured magnetic material. The prediction of both authors was that when a critical current was reached the non-collinear magnetic layers would align parallel or antiparallel to one another. The threshold for this current-induced switching, or magnetization reversal, for metallic multilayers was well within the reach of experimentalists to test, and they set out to test the idea. Depending on the strength of the forces that maintain the magnetization of a ferromagnetic layer in a specific direction, known as the magnetocrystalline or anisotropy energy, it was foreseen that once a threshold was reached, the magnetization could precess as well as switch.
Both of these effects have technologic applications; the precessional (e.g. gyrational) motion could be in the gigahertz range and would generate and amplify microwaves, whereas the switching effect could be used to reset magnetic memories which are the elements of magnetic random access memory (MRAM), an alternative to the dynamic RAM used in our computer’s active memory. MRAM is non-volatile, i.e., when one turns off a computer it retains its memory, so that when one turns on a computer there is no delay or boot-up time, as the active memory is immediately ready.
L. Berger, Phys. Rev. B 54, 9353 (1996); J. Appl. Phys. 89, 5521 (2001).
In light of more recent calculations of spin torque, what ingredients were not considered by Slonczewski in his calculations between 1996 and 1999?
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