Autonomous and forced dynamics in a spin-transfer nano-oscillator: Quantitative magnetic-resonance force microscopy

Abstract

Using a magnetic resonance force microscope (MRFM), the power emitted by a spin transfer nano-oscillator consisting of a normally magnetized Py Cu Py circular nanopillar is measured both in the autonomous and forced regimes. From the power behavior in the subcritical region of the autonomous dynamics, one obtains a quantitative measurement of the threshold current and of the noise level. Their field dependence directly yields both the spin torque efficiency acting on the thin layer and the nature of the mode which first auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. From the MRFM behavior in the forced dynamics, it is then demonstrated that in order to phase-lock this auto-oscillating mode, the external source must have the same spatial symmetry as the mode profile, i.e., a uniform microwave field must be used rather than a microwave current flowing through the nanopillar.

Nutation Spectroscopy of a Nanomagnet Driven into Deeply Nonlinear Ferromagnetic Resonance

“…When pumping fields excite SWs well above their thermal amplitudes, a rich variety of phenomena emerges, such as the formation of dynamical solitons [4,5], SW turbulences and chaos [4,[6][7][8], and Bose-Einstein condensation of magnons [9], the quanta of SWs.Recent developments in magnetic nanotechnologies have also demonstrated that FMR and SW dynamics can be excited either by microwave magnetic fields or by spin transfer torques, with the promise of innovative magnonic and spintronic devices for information and communication technologies [10]. In this area, spin torque nano-oscillators (STNOs) [11][12][13][14][15], which exhibit strong nonlinear properties [16], have even been successfully implemented to perform neuromorphic tasks [17,18].The complexity of magnetization dynamics when strongly nonlinear regimes set in is usually detrimental to the reliable control of nanomagnetic devices, such as oscillators, memories, and logic gates. In this respect, it is important to establish how far from equilibrium magnetic nanostructures can be driven before the coherent magnetization dynamics becomes highly perturbed by the onset of SW instabilities [19].…”

Mechanical magnetometry of Cobalt nanospheres deposited by focused electron beam at the tip of ultra-soft cantilevers

“…A SEM image of it is presented in the inset of Figure 3a. It is a nanosphere of diameter 700 nm made of an amorphous FeSi alloy with 3% in mass of silicon, which was already employed in several MRFM experiments [6,[23][24][25]. In these studies, the magnetic moment of this MRFM probe, m = (2.5 ± 0.5) · 10 -10 emu, was inferred from its stray field, that can be calculated by assuming a punctual magnetic moment at the center of the sphere.…”

Single-Particle Phenomena in Magnetic Nanostructures

“…The MRFM technique lends itself to the study of many nanometric magnetic systems, such as coupled nanodisks, exchange-coupled magnetic layers, nanopillars, as well as spin torque oscillators in which a spin current is passed through a multilayered nanopillar [275][276][277]. The sensitivity of the MRFM technique was shown in the extreme in the magnetic resonance study of a single-electron spin [267], as illustrated in Fig.…”