Magnetic vortex oscillator driven by d.c. spin-polarized current

Abstract

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...

Mode-coupling mechanisms in nanocontact spin-torque oscillators

“…Depending on the geometry and magnetic properties of the free layer, propagating spin waves (SWs) [5][6][7] , solitonic modes [8][9][10][11] , vortex gyration 12,13 , and magnetic dissipative droplets [14][15][16][17] have been observed. STOs can also be used to generate SWs in physically extended thin films, which is of particular interest for future magnonic applications 18,19 .…”

Chirality control via double vortices in asymmetric Co dots

“…Magnetic vortices in sub-micron sized dots have gained considerable interests in recent years due to their unique reversal mechanisms, fascinating topological properties, and potential applications in information storage, [1][2][3][4][5][6] spin-torque oscillators, 7,8 magnetic memory and logic devices, 9 and targeted cancer-cell destruction strategies. 10 Vortices are one type of topological defects characterized by an in-plane magnetization with a clockwise (CW) or counter-clockwise (CCW) chirality and a central core with an out-of-plane magnetization (up or down polarity).…”

Superconducting mesa-structures with a spin filtering manganite interlayer

“…The precession of magnetization excited by a spin-polarized current was theoretically considered in [5][6][7]. Experimentally, the excitation of precession of the magnetization with a sufficiently high current density (above 10 6 A/cm 2 ) was observed in multilayer structures made of ferromagnets and metals [8][9][10] and in the tunnel-type structure [11]. At the same time, the dynamics of the spin-polarized current in superconducting structures with a ferromagnetic interlayer remains less studied.…”