Current-Driven Magnetic Excitations in Permalloy-Based Multilayer Nanopillars

Urazhdin, Sergei; Birge, Norman O.; Pratt, W. P.; Bass, J.

doi:10.1103/physrevlett.91.146803

Cited by 300 publications

(308 citation statements)

References 26 publications

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“…2c and 2d, microwave signals can sometimes be observed not only at large H where dynamical modes have been postulated previously, 4,[6][7][8]12,[14][15][16] but also in the small-H regime of current-driven hysteretic switching. While sweeping to increasing currents at H = 500 Oe, for example, microwave peaks corresponding to small-angle precession exist for I within ~ 0.7 mA below the current for P to AP switching.…”

Section: † These Authors Contributed Equally To This Workmentioning

confidence: 94%

Microwave oscillations of a nanomagnet driven by a spin-polarized current

Kiselev

Sankey

Krivorotov

et al. 2003

Nature

1,976

1,595

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† These authors contributed equally to this workThe recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the intriguing possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. 1-16 However, a central question remains unresolved:What type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, 1-3 but existing measurement techniques have provided only indirect evidence for dynamical states. 4,6-8,12,14-16 The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a DC spin-polarised current. We show that in fact spin transfer can produce several different types of magnetic excitations. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a DC electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators. 2 We examine samples made by sputtering a multilayer of 80 nm Cu / 40 nm Co / 10 nm Cu / 3 nm Co / 2 nm Cu / 30 nm Pt onto an oxidized silicon wafer and then milling through part of the multilayer (Fig. 1a) to form a pillar with an elliptical cross section of lithographic dimensions 130 nm ¥ 70 nm. 17 Top contact is made with a Cu electrode.Transmission or reflection of electrons from the thicker "fixed" Co layer produces a spinpolarised current that can apply a torque to the thinner "free" Co layer. Subsequent oscillations of the free-layer magnetization relative to the fixed layer change the device resistance 18 so, under conditions of DC current bias, magnetic dynamics produce a timevarying voltage (with typical frequencies in the microwave range). If the oscillations were exactly symmetric relative to the direction to the fixed-layer moment, voltage signals would occur only at multiples of twice the fundamental oscillation frequency, f. To produce signal strength at f, we apply static magnetic fields (H) in the sample plane a few degrees away from the magnetically-easy axis of the free layer. All data are taken at room temperature, and by convention positive current I denotes electron flow from the free to the fixed layer.In characterization measurements done at frequencies < 1 kHz, the samples exhibit the same spin-transfer-driven changes in resistance reported in previous experiments 7,9 (Fig. 1b). For H smaller than the coercive field of the free layer (H c ~ 600 Oe), an applied current produces hysteretic switching of the magnetic layers between the low-resistance parallel (P) and high-resistance antiparallel (AP) states. Sweeping H can also drive switching between the P and AP states (Fig 1b, inset). For H larger ...

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Section: † These Authors Contributed Equally To This Workmentioning

confidence: 94%

Microwave oscillations of a nanomagnet driven by a spin-polarized current

Kiselev

Sankey

Krivorotov

et al. 2003

Nature

1,976

1,595

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“…By taking M = 7.8 × 10 5 A m −1 , the critical spin current density for the switching is calculated as being in the range of 1-3.5 × 10 10 A m −2 , in very good agreement with the estimate from our experimental data. On the other hand, in the previous CIMS experiments, the critical switching charge current density for Py/Cu/Py nanopillars 10,27,28 was reported in the range 1 × 10 11 -3 × 10 11 A m −2 . With a current polarization of ∼0.2, the critical local spin current density for the switching in the CIMS experiments is in the range 2-6 × 10 10 A m −2 .…”

mentioning

confidence: 99%

“…The generated pure spin current enables the magnetization reversal of a nanomagnet with the same efficiency as in the case of using charge currents. These results are important for further theoretical developments in multiterminal structures 2 , but also with a view towards realizing novel devices driven by pure spin currents.In a vertical spin-valve nanopillar consisting of a ferromagnet/non-magnet/ferromagnet trilayer, the magnetic state can be switched between the antiparallel and the parallel configurations by applying a charge current [1][2][3][4][5][6][7][8][9][10][11] . This charge-current-induced magnetization switching (CIMS) is the result of a direct transfer of spin angular momentum from the spin current carried along the charge current to the localized magnetic moment in the ferromagnet.…”

mentioning

confidence: 99%

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Giant spin-accumulation signal and pure spin-current-induced reversible magnetization switching

2008

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A number of proposed next-generation electronic devices, including novel memory elements 1 and versatile transistor circuits 2 , rely on spin currents, that is, the flow of electron angular momentum. A spin current may interact with a magnetic nanostructure and give rise to spin-dependent transport phenomena, or excite magnetization dynamics 1-11 . In contrast to a spin-polarized charge current, a pure spin current does not produce any charge-related spurious effects 12,13 . One way to produce a pure spin current is non-local electrical-spin injection 12-18 , but this approach has suffered so far from low injection efficiency. Here, we demonstrate a significant enhancement of the non-local injection efficiency in a lateral spin valve prepared with an entirely in situ fabrication process. Improvements to the interface quality and the device structure lead to an increase of the spin-signal amplitude by an order of magnitude. The generated pure spin current enables the magnetization reversal of a nanomagnet with the same efficiency as in the case of using charge currents. These results are important for further theoretical developments in multiterminal structures 2 , but also with a view towards realizing novel devices driven by pure spin currents.In a vertical spin-valve nanopillar consisting of a ferromagnet/non-magnet/ferromagnet trilayer, the magnetic state can be switched between the antiparallel and the parallel configurations by applying a charge current [1][2][3][4][5][6][7][8][9][10][11] . This charge-current-induced magnetization switching (CIMS) is the result of a direct transfer of spin angular momentum from the spin current carried along the charge current to the localized magnetic moment in the ferromagnet. Separation of the charge and spin components raises the possibility of chargeless pure spin-current-induced magnetization switching (pure spin CIMS).The pure spin current transfers only spin angular momentum, and thus provides an attractive means to manipulate the magnetic state in magnetic nanostructures as well as a quiet electrical background for experimental studies. The pure spin current I S can be generated by the diffusion of the accumulated spins 12-20 in a metallic lateral spin-valve (LSV) structure with non-local electrical spin injection, as shown in Fig. 1a. When the spin accumulation at the interface between the permalloy (Py) and Cu wires on the detector side is non-collinear to the Py magnetization, the transverse component of the pure spin CuAu Py

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“…Telegraph noise, where there is random switching between two stable states 10 , originates from such diverse phenomena as thermal activation of an unstable impurity 11-13 , non-equilibrium activation of a bistable system 14-17 , switching of magnetic domain orientation [18][19][20] , or a reversible chemical reaction in a biological ion channel 21 .…”

mentioning

confidence: 99%

Conditional statistics of electron transport in interacting nanoscale conductors

Sukhorukov

Jordan

Gustavsson³

et al. 2007

Nature Phys

103

137

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There is an intimate connection between the acquisition of information and how this information changes the remaining uncertainty in the system. This trade-off between information and uncertainty plays a central role in the context of detection. Recent advances in the ability to make accurate, on-chip measurements of individual-electron current through a quantum dot 1-8 (QD) have been enabled by exploiting the sensitivity of a second current, passing through a nearby quantum point contact (QPC), to the fluctuating charge on the QD 4-8 . An important characteristic of QPC detectors is their minimal influence on the systems they probe. Here we show that even the operation of an effectively non-invasive QPC detector can statistically alter the system's behaviour. By observing a particular QPC current, the statistical distribution of the QD conditional current undergoes a substantial change in comparison to that expected for unconditional shot noise 9 . These results are in almost perfect agreement with a theoretical model we develop to predict the joint current probability distribution and conditional transport statistics of interacting nanoscale systems.Noise is generally due to randomness, which can be classical or quantum in nature. Telegraph noise, where there is random switching between two stable states 10 , originates from such diverse phenomena as thermal activation of an unstable impurity 11-13 , non-equilibrium activation of a bistable system 14-17 , switching of magnetic domain orientation [18][19][20] , or a reversible chemical reaction in a biological ion channel 21 .In nanoscale conductors, where charge motion is quantum coherent over distances comparable to the system size, shot noise and telegraph noise have recently been shown to be two sides of the same coin 6,7,22,23 . A quantum dot (QD) is sufficiently small that it is effectively zero dimensional, and behaves as an artificial atom, holding a small number of electrons. Figure 1a shows the sample used in the experiment reported here. The QD is marked by the dotted circle 24 . An extra electron can tunnel into the QD from the source lead (S), stay in the QD for a random amount of time and then tunnel out into the drain lead (D) if the applied voltage bias exceeds the temperature. This singleelectron transport produces a fluctuating electrical current. In order to detect the statistical properties of this current, a sensitive electrometer with a bandwidth much higher than the tunnelling rates is required. The electrometer is a nearby quantum point contact (QPC) that is capacitively coupled to the QD via the Coulomb interaction. The voltage-biased QPC detector transports many electrons through a narrow constriction in the surrounding two-dimensional electron gas (represented with an arrow). The resistance of the QPC is susceptible to changes in the surrounding electrostatic environment, and can therefore be used to sense the presence (or absence) of an extra electron on the QD 4 . When the extra electron tunnels into or out of the QD, the current I ...

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Current-Driven Magnetic Excitations in Permalloy-Based Multilayer Nanopillars

Cited by 300 publications

References 26 publications

Microwave oscillations of a nanomagnet driven by a spin-polarized current

Microwave oscillations of a nanomagnet driven by a spin-polarized current

Giant spin-accumulation signal and pure spin-current-induced reversible magnetization switching

Conditional statistics of electron transport in interacting nanoscale conductors

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