† 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 ...

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

Abstract:Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the dynamics of nanomagnets. A peculiar consequence of this spin-torque, the ability to induce persistent oscillations of a nanomagnet by applying a dc current, has previously been reported only for spatially uniform nanomagnets. Here we demonstrate that a quintessentially nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized dc current. Comparison to micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz, making these highly compact spin-torque vortex oscillator devices potential candidates for microwave signalprocessing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures. Pribiag et al.1A spin-polarized electron current can apply a torque on the local magnetization of a ferromagnet. This spin-transfer effect 1,2 provides a new method for manipulating magnetic systems at the nanoscale without the application of magnetic fields and is expected to lead to future data storage and information processing applications 3 . Experiments have demonstrated that spin-torque can be used to induce current-controlled hysteretic switching, as well as to drive persistent microwave dynamics in spin-valve devices 3,4,5,6,7,8,9,10,11,12 . While it is known that spin-torque switching of a magnetic element can sometimes occur via non-uniform magnetic states 13 , a central remaining question is whether spin-torque can be used to efficiently excite steady-state magnetization oscillations in strongly non-uniform magnetic configurations in a manner suitable for fundamental investigations of nanomagnetic dynamics and improved device performance. A relatively simple type of non-uniform magnetic structure is a magnetic vortex, the lowest-energy configuration of magnetic structures just above the singledomain length scale 14 . Previous studies, typically performed on single-layer permalloy (Py) structures, focused on the transient or resonant response of a magnetic vortex to an applied magnetic field and identified the lowest excitation mode of a vortex as a gyrotropic precession of the core 15,16,17,18 . It has also been demonstrated that the vortex core polarization can be efficiently switched by short radio-frequency magnetic field pulses 19 . Recently, the spin-transfer effect has been used to drive a magnetic vortex into resonant precession by means of an alternating current incident on a single Py dot 20 . Here we report by means of direct frequency-domain measurements that a dc spinpolarized current can drive highly coherent gigahertz-frequency steady-state oscillations of the magnetic vortex in a nanoscale magnetic device. The high sensitivity of ou...

We present time-resolved measurements of gigahertz-scale magnetic dynamics caused by torque from a spin-polarized current. By working in the time domain, we determined the motion of the magnetic moment throughout the process of spin-transfer-driven switching, and we measured turn-on times of steady-state precessional modes. Time-resolved studies of magnetic relaxation allow for the direct measurement of magnetic damping in a nanomagnet and prove that this damping can be controlled electrically using spin-polarized currents.Spin-polarized electrons traversing a ferromagnet can transfer spin-angular momentum to the local magnetization, thereby applying a torque that may produce magnetic reversal or steady-state precession (1, 2). This spintransfer mechanism allows nanomagnets to be manipulated without magnetic fields, and it is the subject of extensive research for applications in nonvolatile memory, programmable logic, and microwave oscillators (3-11). However, the gigahertz-scale magnetic dynamics that can be driven by spin transfer have previously been measured using only frequency-domain techniques (12-16). Here we report direct time-resolved studies of dynamics excited by spin-transfer torques. By working in the time domain, we are able to characterize the full time-dependent magnetic response to pulses of spin-polarized currents, including transient dynamics. These measurements allow a direct view of the process of spin-transfer-driven magnetic reversal, and they determine the possible operating speeds for practical spin-transfer devices. The results provide rigorous tests of theoretical models for spin transfer (1, 9, 17-19) and strongly support the spin-torque model (1, 18) over competing theories that invoke magnetic heating (9, 20).We studied nanopillar-shaped samples consisting of two 4-nm-thick permalloy (Py K Ni 80 Fe 20 ) ferromagnetic layers separated by an 8-nm-thick Cu spacer layer (Fig. 1A, inset). Both Py layers and the Cu spacer were etched to have an elliptical area of approximately 130 Â 60 nm. Current flows perpendicular to the layers through Cu electrodes. The relative angle between the magnetic moments of the Py layers was detected by changes in the sample resistance due to the giant magnetoresistance effect. For time-resolved measurements on subnanosecond scales, signal-to-noise considerations require averaging over multiple signal traces. If the signal is oscillatory, the phase of the oscillations has to be the same in each trace or else the signal will be lost in averaging (21). This requires that the samples be engineered so that the initial (equilibrium) angle between the magnetic moments of the two layers, q 0 , is different from zero and is well controlled. Our devices were specially designed to provide this control. The equilibrium orientation of our top free-layer Py moment was governed primarily by the shape anisotropy of the elliptical device. We exchange-biased the bottom layer at an angle of 45-to the top layer easy axis using an 8-nm-thick antiferromagnetic Ir 20 Mn 80 underl...

We demonstrate a technique that enables ferromagnetic resonance measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by more than a factor of 50 compared to individual ferromagnetic samples measured by other resonance techniques. Studies of the resonance frequencies, amplitudes, linewidths, and line shapes as a function of microwave power, dc current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.