Direct observation of a propagating spin wave induced by spin-transfer torque

Abstract

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Mutually synchronized bottom-up multi-nanocontact spin–torque oscillators

“…In a second four-nanocontact STO (not shown) and the five-nanocontact STO, the individual nanocontacts show a larger spread in threshold currents, which may reflect some variation in nanocontact size. The frequency generally blue shifts with increasing I dc , with the same current density dependence in all devices, which is consistent with the generation of a propagating SW mode with similar characteristics, regardless of the nanocontact number 22,24,32 . In the single nanocontact STO, a maximum total power of 95 pW is reached at B28 mA, where the device also exhibits its narrowest linewidth of about 10 MHz (Fig.…”

“…S pin-torque oscillators (STOs) [1][2][3][4][5][6][7] represent a novel class of nanoscopic microwave signal generators combining ultra-broadband operation [8][9][10] , ultrafast modulation [11][12][13][14][15][16] , an extremely small footprint, straightforward CMOS integration using commercial MRAM processes 17,18 , and potential for magnonic devices [19][20][21][22][23][24] . Their main drawbacks are their limited output power and high phase noise caused by tiny mode volumes and a strong frequency-power nonlinearity [25][26][27][28] .…”

“…Each nanocontact can sustain several tens of mA current, which gets spin polarized and transfers angular momentum between the two magnetic layers via the so-called spin-transfer torque (STT) effect [35][36][37] . At sufficiently high current densities (B10 8 A cm À 2 ), STT can sustain continuous precession of the local magnetization underneath the nanocontact and also inject high amplitudes of either localized or propagating spin waves (SWs) into the free layer 20,22,23,[38][39][40][41][42][43] .…”

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 .…”

Quantum Spin Torque

“…Spin transfer [1][2][3]-the transfer of angular momentum from spin-polarized electrical current to magnetic materials-has been extensively researched as an efficient mechanism for the electronic manipulation of nanomagnetic systems, advancing our understanding of nanomagnetism and electronic transport, and enabling the development of magnetoelectronic nanodevices [3][4][5][6][7][8][9][10][11][12][13][14][15]. This effect can be understood based on the argument of angular momentum conservation for spin-polarized electrons, scattered by a ferromagnet whose magnetization ⃗ M is not aligned with their polarization.…”