Microwave Oscillations of a Nanomagnet Driven by a Spin-polarized Current
Kiselev, S.I. Sankey, J.C., Krivorotov, I.N., Emley, N.C., Schoelkopf, R.J., Buhrman, R.A. & Ralph, D.C., 2003. Microwave Oscillations of a Nanomagnet Driven by a Spin-polarized Current. Nature 425, 380
Essay about this article
The original experiments on spin torque discussed in Essay 15 focused on the threshold currents to switch the magnetization between two configurations. Indeed, phase diagrams showing the boundaries between different configurations were generated, in much the same way one has such diagrams for liquid-gas or liquid-solid phase boundaries. In several experiments it was noted that before reaching these boundaries there was a precursor. A.D. Kent’s group observed “In contrast to previous results ….., these results cannot be understood as small amplitude excitations of the magnetization. Micromagnetic modeling suggests that spin-transfer torques induce precessional states which evolve into a static state of antiparallel alignment of the layers. ” This was the beginning of an extended study of these current induced excitations or oscillations in which the magnetization of one thin layer is set into a precession by the spin-polarized current emerging from a fixed magnetic layer; this is now known as spin transfer oscillators.
This motion is due to the spin torque on the free layer and is understood from the solutions of the Landau-Lifshitz-Gilbert (LLG) equations of motion for the magnetization dynamics of a ferromagnetic layer, as modified by Slonczewski to include the spin torque acting on the magnetization, [see articles listed at the beginning of this section for references to these equations]. These precessions arise from a delicate balance of the forces acting on the magnetization, i.e., the magnetic anisotropy, external magnetic field, damping [which acts as viscous torque that dissipates motion], and the spin torque whose magnitude is controlled by the current. For propitious combinations of these parameters one can create various types of precessional motion along different axes of the free layer.
This current induced precession is reminiscent of the one induced to achieve ferromagnetic resonance (FMR), and indeed is described by similar LLG equations of motion. Albeit it is achieved not by spin torque but through a free rotation of the magnetization in the following way . In a resonance experiment, a magnetic field is applied to a ferromagnet that rotates the magnetization at the Larmor precession frequency; then a high frequency microwave signal is applied that is in the gigahertz frequency range. Either the magnetic field strength or the frequency of the microwave is adjusted so that the Larmor precession frequency of the magnetization is equal to that of the microwave signal; at this point the microwave signal creates a tiny magnetic field transverse to the axis of rotation so that in a rotating frame of reference, moving at the Larmor precession frequency, the magnetization only “sees” this transverse field and rotates around this axis. This is generically known as the resonance condition, which in a ferromagnet becomes FMR.
Spin transfer oscillators are a useful alternative to conventional FMR inasmuch as they do not require one to place the sample in a microwave cavity. While this cavity is used in some spin transfer FMR setups to determine when the free layer precesses, it is possible to monitor this rotation by observing changes in the electrical characteristics of the circuit containing the oscillator. That is, Albert Fert’s group at Thales has proposed stringing together in series a set of these oscillators and they have shown that the network can be locked not only in frequency but also in phase; their results show how the emitted microwave power of spin-transfer oscillators can be considerably enhanced by current-induced synchronization in an electrically connected network . One possible application envisaged for this network of spin transfer oscillators is for obtaining a better generator of microwaves.
 B. Özyilmaz and A. D. Kent, D. Monsma, J. Z. Sun, M. J. Rooks, and R. H. Koch, Phys. Rev. Lett. 91,067203 (2003).
 Charles P. Slichter, Principles of Magnetic Resonance (Harper, New York??, 1963).
 J. Grollier, V. Cros and A. Fert, Phys. Rev. B 73, 060409 (R) (2006).
S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, N. C. Emley, M. Rinkoski, C. Perez, R. A. Buhrman, and D. C. Ralph, Phys. Rev. Lett. 93, 036601 (2004) ;
W. H. Rippard, M. R. Pufall, S. Kaka, S. E. Russek, and T. J. Silva, Phys. Rev. Lett. 92, 027201 (2004);
W. H. Rippard, M. R. Pufall, S. Kaka, T. J. Silva, and S. E. Russek, Phys. Rev. B 70, 100406(R) (2004).
Why is it necessary to supply energy to sustain the precession of the magnetization in a ferromagnet? What mechanisms are responsible for the energy loss?
Select articles citing this paper
Zhao, E. and J. A. Sauls (2008). "Theory of nonequilibrium spin transport and spin-transfer torque in superconducting-ferromagnetic nanostructures." Physical Review B 78(17).
Florez, S. H., J. A. Katine, et al. (2008). "Effects of radio-frequency current on spin-transfer-torque-induced dynamics." Physical Review B 78(18).
Finocchio, G., O. Ozatay, et al. (2008). "Spin-torque-induced rotational dynamics of a magnetic vortex dipole." Physical Review B 78(17).
Tserkovnyak, Y., A. Brataas, et al. (2005). "Nonlocal magnetization dynamics in ferromagnetic heterostructures." Reviews of Modern Physics 77(4): 1375-1421.
Thiaville, A., Y. Nakatani, et al. (2005). "Micromagnetic understanding of current-driven domain wall motion in patterned nanowires." Europhysics Letters 69(6): 990-996.
Krivorotov, I. N., N. C. Emley, et al. (2005). "Time-domain measurements of nanomagnet dynamics driven by spin-transfer torques." Science 307(5707): 228-231.
Allwood, D. A., G. Xiong, et al. (2005). "Magnetic domain-wall logic." Science 309(5741): 1688-1692.
Zutic, I., J. Fabian, et al. (2004). "Spintronics: Fundamentals and applications." Reviews of Modern Physics 76(2): 323-410.
Koch, R. H., J. A. Katine, et al. (2004). "Time-resolved reversal of spin-transfer switching in a nanomagnet." Physical Review Letters 92(8).
Fuchs, G. D., N. C. Emley, et al. (2004). "Spin-transfer effects in nanoscale magnetic tunnel junctions." Applied Physics Letters 85(7): 1205-1207.