https://ralphgroup.lassp.cornell.edu/papers/JMMM-320-1190-2008.pdf

Spin transfer torques

The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

/pdf/ultrafast-optical-manipulation-of-magnetic-order-2ms2t5k1zo.pdf

Ultrafast optical manipulation of magnetic order

An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. However, the electrons within an insulator possess spin as well as charge, so it is possible for them to transmit a signal in the form of a spin wave. Kajiwara et al. have now developed a hybrid metal–insulator–metal structure in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer. This wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The observation of voltage transmission in an insulator raises the prospect of insulator-based spintronics and other novel forms of signal delivery. An insulator does not conduct electricity, and so cannot in general be used to transmit an electrical signal. But an insulator's electrons possess spin in addition to charge, and so can transmit a signal in the form of a spin wave. Here a hybrid metal–insulator–metal structure is reported, in which an electrical signal in one metal layer is directly converted to a spin wave in the insulating layer; this wave is then transmitted to the second metal layer, where the signal can be directly recovered as an electrical voltage. The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction1. But in addition to charge, an electron also has spin2, and the collective motion of spin can propagate—and so transfer a signal—in some insulators3. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y3Fe5O12 and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y3Fe5O12 via spin-Hall effects4,5,6,7,8,9,10,11. Second, making use of the transfer in a Pt/Y3Fe5O12/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect7,8,10,11 in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y3Fe5O12 layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping3 in Y3Fe5O12 is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices.

Transmission of electrical signals by spin-wave interconversion in a magnetic insulator

TOPICAL REVIEW: Nanomagnetics

This paper formulates a general analytic approach to the theory of microwave generation in magnetic nano-structures driven by spin-polarized current and reviews analytic results obtained in this theory. The proposed approach is based on the universal model of an auto-oscillator with negative damping and nonlinear frequency shift. It is demonstrated that this universal model, when applied to the case of a spin-torque oscillator (STO) based on a current-driven magnetic nano-pillar or nano-contact, gives adequate description of most of the experimentally observed properties of STO. In particular, the model describes the power and frequency of the generated microwave signal as functions of the bias current and magnetic field, predicts the magnitude and properties of the generation linewidth, and explains the STO behavior under the influence of periodic and stochastic external signals: frequency modulation, phase-locking to external signals, mutual phase-locking in an array of STO, broadening of the generation linewidth near the generation threshold, etc. The proposed nonlinear auto-oscillator theory is rather general and can be used not only for the development of practical nano-sized STO, but, also, for the description of nonlinear auto-oscillating systems of any physical nature.

Nonlinear Auto-Oscillator Theory of Microwave Generation by Spin-Polarized Current

We show experimentally and by model calculations that in finite, nonellipsoidal, micrometer size magnetic thin film elements the dynamic magnetic eigenexcitations (spin waves) may exhibit strong spatial localization. This localization is due to the formation of a potential well for spin waves in the highly inhomogeneous internal magnetic field within the element.

/pdf/spin-wave-wells-in-nonellipsoidal-micrometer-size-magnetic-1bxf3p5nae.pdf

Spin wave wells in nonellipsoidal micrometer size magnetic elements.

A Brillouin light scattering study and theoretical interpretation of spin-wave modes in arrays of in-plane magnetized micron-size rectangular ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ elements are reported. It is shown that two-dimensional spin-wave eigenmodes of these elements can be approximately described as products of one-dimensional spin-wave eigenmodes of longitudinally and transversely magnetized long finite-width permalloy stripes. The lowest eigenmodes of rectangular elements are of dipole-exchange nature and are localized near the element edges, while the higher eigenmodes are of a mostly dipolar nature and are weakly localized near the element center. The frequency spectra and spatial profiles of these eigenmodes are calculated both analytically and numerically, and are compared with the results of the Brillouin light scattering experiment.

Spin-wave excitations in finite rectangular elements of Ni 80 Fe 20

We present spectral measurements of spin-wave excitations driven by direct spin-polarized current in a free layer of nanoscale ${\mathrm{Ir}}_{20}{\mathrm{Mn}}_{80}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕\mathrm{Cu}∕{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ spin valves The measurements reveal that large-amplitude coherent spin-wave modes are excited over a wide range of bias current The frequency of these excitations exhibits a series of jumps as a function of current due to transitions between different localized nonlinear spin-wave modes of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanomagnet We find that micromagnetic simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented by the Slonczewski spin-torque term (LLGS) accurately describe the frequency of the current-driven excitations including the mode transition behavior However, LLGS simulations give qualitatively incorrect predictions for the amplitude of excited spin waves as a function of current

https://arxiv.org/pdf/cond-mat/0703458.pdf

Large-amplitude coherent spin waves excited by spin-polarized current in nanoscale spin valves

We demonstrate (using full-scale micromagnetic simulations) that the spin-injection driven steady-state precession of a thin magnetic nanoelement exhibits a complicated transition from a quasimacrospin to chaotic behavior with increasing element size. For nanoelement parameters and magnetic fields typical of those used experimentally we show that the macrospin approximation is invalid already for very small nanoelement sizes $(\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$.

/pdf/transition-from-the-macrospin-to-chaotic-behavior-by-a-spin-5efc1900w1.pdf

Transition from the macrospin to chaotic behavior by a spin-torque driven magnetization precession of a square nanoelement

In this paper a detailed numerical study in frames of the Slonczewski formalism of magnetization oscillations driven by a spin-polarized current through a thin elliptical nanoelement is presented. We show that a sophisticated micromagnetic model, where a polycrystalline structure of a nanoelement is taken into account, can explain qualitatively all most important features of the magnetization oscillation spectra recently observed experimentally S. I. Kiselev et al., Nature 425, 380 2003, namely, existence of several equidistant spectral bands, sharp onset and abrupt disappearance of magnetization oscillations with increasing current, absence of the out-of-plane regime predicted by a macrospin model, and the relation between frequencies of so-called small-angle and quasichaotic oscillations. However, a quantitative agreement with experimental results especially concerning the frequency of quasichaotic oscillations could not be achieved in the region of reasonable parameter values, indicating that further model refinement is necessary for a complete understanding of the spin-driven magnetization precession even in this relatively simple experimental situation.

/pdf/magnetization-precession-due-to-a-spin-polarized-current-in-r129mesgyw.pdf

N. L. Gorn

Papers

Spin wave wells in nonellipsoidal micrometer size magnetic elements.

Spin-wave excitations in finite rectangular elements of Ni 80 Fe 20

Large-amplitude coherent spin waves excited by spin-polarized current in nanoscale spin valves

Transition from the macrospin to chaotic behavior by a spin-torque driven magnetization precession of a square nanoelement

Magnetization precession due to a spin-polarized current in a thin nanoelement: Numerical simulation study