Spin-transfer excitations of permalloy nanopillars for large applied currents

Kiselev, S. I.; Sankey, J. C.; Krivorotov, I. N.; Emley, N. C.; Garcia, A. G. F.; Buhrman, R. A.; Ralph, Daniel

doi:10.1103/physrevb.72.064430

Cited by 67 publications

(60 citation statements)

References 38 publications

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“…The frequency f of the microwave peak increases with the current (blue shift), in contrast with the red shift generally observed in standard pillars with in-plane magnetization. Actually, with the standard angular dependence of the torque, the theoretical prediction is a succession of red and blue shift regimes at increasing current but, in experiments with in-plane applied fields, the crossover to a blue shift regime has been seldom observed [25]. In macrospin simulations, a blue shift in f is predicted for the regime of out-of-plane (OP) precessions and is also associated with a decrease of f with increasing in-plane field.…”

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Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

et al. 2007

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The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin transfer phenomenon. We report transport and microwave power measurements on specially designed nanopillars for which a non-standard angular dependence of the spin transfer torque (wavy variation) is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field, as this is also predicted by simulation with such a wavy angular dependence of the torque. This type of non-standard nanopillars can represent an interesting way for the implementation of spin transfer oscillators since they are able to generate microwave oscillations without applied magnetic field. We also emphasize the theoretical implications of our results on the angular dependence of the torque. 2The magnetization of a ferromagnetic body can be manipulated by transfer of spin angular momentum from a spin-polarized current. This is the concept of spin transfer introduced by Slonczewski [1] and Berger [2] in 1996. In most experiments, a spin-polarized current is injected from a spin polarizer into a "free" magnetic element, for example in pillar-shaped magnetic trilayers [3][4][5][6]. The phenomenon of spin transfer has a great potential for applications. It can be used either to switch a magnetic configuration (the configuration of a magnetic memory for example) [3][4][5] or to generate magnetic precessions and voltage oscillations in the microwave frequency range [6][7]. In the most usual situations, such oscillations are observed in the presence of a magnetic field.From a fundamental point of view, spin transfer effects raise two different types of problems [8]. First the spin transfer torque acting on a magnetic element is related to the transverse spin polarisation of the current (transverse meaning perpendicular to the magnetization axis of the element) and can be derived from spin-dependent transport equations [8][9][10][11][12][13][14][15][16][17]. On the other hand, the description of the magnetic excitations generated by the spin transfer torque raises problems of non-linear dynamics [8,[18][19][20]. For example, in the simple limit where the excitation is supposed to be a uniform precession of the magnetization (macrospin approximation), this precession can be determined by introducing the spin transfer torque into a Landau-LifshitzGilbert (LLG) equation for the motion of the magnetic moment. However, the determination of the spin transfer torque and the description of the magnetization dynamics cannot be regarded as independent problems. In standard trilayered structures with in-plane magnetizations and with the usual angular dependence, a switching regime is found at zero and low magnetic field and the precession regime with generation of voltage oscillations is mainly observed above some threshold field [8]. We will show that a new behavior, characterized by large a...

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Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

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“…The device is stable for a dc of up to 10 12 A / m 2 . 11 The allowed peak value for a pulsed current is higher. 13 We assume a spatially uniform magnetization of the free FM layer ͑"macrospin" approximation͒.…”

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“…Our model corresponds to a realistic 11,12 ferromagnetnormal metal-ferromagnet ͑FM-NM-FM͒ spin valve as sketched in Fig. 1, consisting of a thick FM Permalloy ͑Py͒ layer with fixed magnetization direction M f and a thin Py layer separated by a nonmagnetic ͑Cu͒ layer.…”

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Feedback control of noise in spin valves by the spin-transfer torque

Bandopadhyay

Brataas

Bauer

2011

Applied Physics Letters

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The miniaturization of magnetic read heads and random access memory elements makes them vulnerable to thermal fluctuations. We demonstrate how current-induced spin-transfer torques can be used to suppress the effects of thermal fluctuations. This enhances the fidelity of perpendicular magnetic spin valves. The simplest realization is a dc to stabilize the free magnetic layers. The power can be significantly reduced without losing fidelity by simple control schemes, in which the stabilizing current-induced spin-transfer torque is controlled by the instantaneous resistance. © 2011 American Institute of Physics. ͓doi:10.1063/1.3556270͔Magnetic spin valves with metal or insulator spacers are used for hard disk read heads as well as for magnetic random access memory elements. The physical principle of operation is the giant magnetoresistance ͑GMR͒ or tunnel magnetoresistance ͑TMR͒, which varies with the relative magnetization between a layer with fixed magnetization ͑e.g., by exchange biasing͒ and a free layer with a magnetization that is allowed to ͑re͒orient the magnetization direction. Further miniaturization is thwarted by the increased thermal fluctuations associated with smaller magnets, however, because the magnetic anisotropy energies scale with the total magnetic moment. 1 GMR read heads in the form of nanometer-scale pillars are currently considered as possible replacements for TMR read heads.2 Their higher sensitivity comes at the cost of noise which is enhanced by the spin-transfer-torque-induced coupling of electronic and magnetic fluctuations.3,4 Here, we demonstrate that the spin-transfer torque can also be used to suppress the noise in spin valves. The necessary power to achieve a given fidelity can significantly be reduced by feedback control of the electric current-induced torque by the instantaneous resistance.An electric current passed through a spin valve interacts with the magnetic order parameter when the magnetizations are noncollinear. This is caused by the spin current component polarized normal to the magnetization of the free layer, which is absorbed at the interface. The loss of angular momentum in the spin current is transferred to the magnetization as a spin-transfer torque, 5-10 which effectively leads to magnetic damping or antidamping of the magnetization dynamics, depending on the current direction. Low frequency noise can be suppressed by increasing the magnetic anisotropy of the free magnetic layer at the cost of reduced sensitivity. Covington 3 proposed to reduce noise by applying a spin-transfer torque which opposes the intrinsic damping so that the damping seems to be reduced. According to the fluctuation-dissipation theorem the rms random field that perturbs the magnetization at thermal equilibrium is proportional to the square root of the Gilbert damping constant ͓as Eq. ͑3͒ below͔. The average spin-transfer torque dynamically changes the energy of the ferromagnet ͑FM͒ but does not change the Brownian fluctuating fields the ferromagnet experiences, except at very low temperat...

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“…A more likely mechanism for the decrease in TMR with increasing bias is the excitation of spin-waves or magnons by the spin-polarized tunneling current [21][22][23] . In our devices, electron spin injection orthogonal to the magnetization of the island can exert a torque acting like a transverse magnetic field which creates magnons.…”

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Large magnetoresistance in Co∕Ni∕Co ferromagnetic single electron transistors

Liu

Pettersson

Michalak³

et al. 2007

Applied Physics Letters

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We report on magnetotransport investigations of nano-scaled ferromagnetic Co/Ni/Co single electron transistors. As a result of reduced size, the devices exhibit single electron transistor characteristics at 4.2K. Magnetotransport measurements carried out at 1.8K reveal tunneling magnetoresistance (TMR) traces with negative coercive fields, which we interpret in terms of a switching mechanism driven by the shape anisotropy of the central wire-like Ni island. A large TMR of about 18% is observed within a finite source-drain bias regime. The TMR decreases rapidly with increasing bias, which we tentatively attribute to excitation of magnons in the central island.

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Spin-transfer excitations of permalloy nanopillars for large applied currents

Cited by 67 publications

References 38 publications

Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

Shaped angular dependence of the spin-transfer torque and microwave generation without magnetic field

Feedback control of noise in spin valves by the spin-transfer torque

Large magnetoresistance in Co∕Ni∕Co ferromagnetic single electron transistors

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