Dynamical Mode Coupling and Coherence in a Spin Hall Nano-Oscillator with Perpendicular Magnetic Anisotropy

Abstract

We experimentally study the dynamical modes excited by spin current in Spin Hall nano-oscillators based on the Pt/[Co/Ni] multilayers with perpendicular magnetic anisotropy. Both propagating spin wave and localized solitonic modes of the oscillation are achieved and controlled by varying the applied magnetic field and current. At room temperature, the generation linewidth broadening associated with mode hopping was observed at currents close to the transition between different modes and in the mode coexistence regimes. The mode hopping was suppressed at cryogenic temperatures, confirming that the coupling between modes is mediated by thermal magnons. We also demonstrate that coherent single-mode oscillations with linewidth of 5 MHz can be achieved without applying external magnetic field. Our results provide insight into the mechanisms controlling the dynamical coherence in nanomagnetic oscillators, and guidance for optimizing their applications in spin wave-based electronics.

Magnetic Droplet Mode in a Vertical Nanocontact-Based Spin Hall Nano-Oscillator at Oblique Fields

“…Prior experimental and theoretical studies have shown that the bullet mode emerges from the linear spectrum of spin-waves above a critical current determined by damping, without any energy barrier for its formation [13,14,19]. In contrast, the droplet mode involves an inverted static core, whose nucleation is associated with a certain energy barrier.…”

“…In contrast to the droplet, the bullet mode is characterized by large-amplitude precession of the central region, with the spatial characteristics determined by a combination of exchange stiffness, nonlinearity, and spin torque [27]. This mode is commonly observed in SHNOs and the conventional multilayer STNOs with in-plane magnetic anisotropy, or PMA free layer at sufficiently large inplane fields [13,15,19,36]. The magnetic droplet mode is generally expected to exhibit larger spectral intensities than the bullet mode, due to the large precession angle of magnetization in the dynamical domain wall.…”

“…In contrast, the droplet mode involves an inverted static core, whose nucleation is associated with a certain energy barrier. This barrier is likely too small to affect the dynamics at room temperature, but its effects can become pronounced at lower temperatures [19,37,38]. To investigate these effects, we have performed additional spectroscopic measurements at cryogenic temperatures.…”

“…This geometry is commonly referred to as nano-gap SHNO. However, studies have shown that the spin injection geometry in the planar nano-gap SHNO favors simultaneous excitation of two dynamical modes [14,18], resulting in significant degradation of oscillation characteristics at ambient temperatures, due to the thermal magnon-mediated mode hopping [19]. In in-plane magnetized nano-gap SHNOs, the secondary mode is stabilized mostly by the dipolar field of the primary bullet mode, which creates two effective potential wells in the regions localized near the edges of the center bullet mode along the axis defined by the direction of magnetization [18,20].…”

Width dependent auto-oscillating properties of constriction based spin Hall nano-oscillators

“…Spin Hall nano-oscillators (SHNOs) are a class of miniaturized, ultra-broadband, microwave signal generators [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] where the spin Hall effect [21][22][23] converts a direct charge current into a transverse pure spin current, transferring angular momentum [24][25][26] into a nearby ferromagnetic layer, resulting in current and magnetic field tunable spin wave (SW) autooscillations (AOs); a microwave voltage is then generated via the anisotropic magnetoresistance of the magnetic layer. Thanks to their simple bilayer structure, nano-constriction SHNOs offer a number of advantages such as straightforward fabrication processes, flexibility in material choices, significant freedom in layout, and direct optical access to the active auto-oscillating region, which makes them attractive for fundamental studies of highly nonlinear and interacting SWs.…”