Experimental Evidence of Self-Localized and Propagating Spin Wave Modes in Obliquely Magnetized Current-Driven Nanocontacts

Abstract

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

Mode-coupling mechanisms in nanocontact spin-torque oscillators

“…Each spectrum was averaged 10 times with a video bandwidth of 10 kHz in order to minimize the noise. In agreement with our circular NC-STOs (not shown) and previous results 8, 10,11,36 , the spectra show a transition between the propagating spin wave mode predicted by Slonczewski 5 and a modehopping regime. This transition occurs at a well-defined critical angle, θ c , corresponding to the bullet mode onset 10,36 .…”

“…In agreement with our circular NC-STOs (not shown) and previous results 8, 10,11,36 , the spectra show a transition between the propagating spin wave mode predicted by Slonczewski 5 and a modehopping regime. This transition occurs at a well-defined critical angle, θ c , corresponding to the bullet mode onset 10,36 . Additionally, we observe a second critical angle, θ L , where both the bullet mode and a high frequency mode are observed, maintaining an approximately constant frequency difference 11 .…”

“…1(a-b), where black indicates zero power and light yellow indicates the (scaled) maximum power. In particular, a low frequency mode, identified as a spin wave bullet 8,10 , is primarily located at the local field minimum (B) while a high frequency mode is primarily located at the local field maximum (HF). This situation can also be schematically represented by the frequency landscape along X = 0, Fig.…”

“…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 .…”

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

“…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] .…”