Nanoscale Mapping of Magnetic Auto-Oscillations with a Single Spin Sensor

Abstract

Spin Hall nano-oscillators convert DC to magnetic auto-oscillations in the microwave regime. Current research on these devices is dedicated to creating next-generation energy-efficient hardware for communication technologies. Despite intensive research on magnetic auto-oscillations within the past decade, the nanoscale mapping of those dynamics remained a challenge. We image the distribution of free-running magnetic auto-oscillations by driving the electron spin resonance transition of a single spin quantum sensor, enabling fast acquisition (100 ms/pixel). With quantitative magnetometry, we experimentally demonstrate for the first time that the auto-oscillation spots are localized at magnetic field minima acting as local potential wells for confining spin-waves. By comparing the magnitudes of the magnetic stray field at these spots, we decipher the different frequencies of the auto-oscillation modes. The insights gained regarding the interaction between auto-oscillation modes and spin-wave potential wells enable advanced engineering of real devices.

Keywords: PL map; auto-oscillation; nano-oscillator; nitrogen-vancancy center; nonlinear oscillator; spin Hall effects.

Figure 1

Measurement overview and electrical characterization of the SHNO device. (a) Sample geometry and schematic representation of the electrical and optical measurements. (b) The simulated dc current density J_DC distribution reveals maximum values at the edges of the constriction. (c) A pure spin current generated via SHE in Pt generates a SOT in Ni₈₁F₁₉. This generates magnetic auto-oscillations which generate a microwave field interacting with the single spin sensor. (d) The SOT compensates the GT resulting in auto-oscillations. (e) Local magnetic field minima create spin wave potential wells which confine the auto-oscillations. (f) Top: Auto-oscillation spectra electrically measured as a function of dc currents exhibit the presence of two auto-oscillation modes with characteristic microwave frequencies. Bottom: Integrated auto-oscillation power.

Figure 2

Magnetic field distribution of the SHNO without dc current. (a) Measurement of the magnetic stray field component parallel to the NV axis depicting localized field minima. (b, c) High-resolution maps revealing characteristic magnetic field magnitudes at each constriction edge. (d) Simulation of the internal field distribution revealing local field minima at the edges of the constriction (circles). (e) Simulated magnetic stray field component along the NV axis at a height of 80 nm above the SHNO revealing localized field minima (circles) as fingerprint of the minima in (d).

Figure 3

NV-spin manipulation via SHNO microwave field. (a) The calculated NV resonance (black) at higher frequency overlaps with the SHNO frequency range (red) extracted from Figure 1(f). This auto-oscillation range is located below the calculated thin-film FMR range (blue) as expected for SHNOs in this measurement geometry. (b) NV photoluminescence (PL) drops by 24% when the SHNO is activated by a characteristic positive DC current of I_DC = 6.75 mA. The PL reduces only for positive currents for which the SHNO is active. The lowest PL is reached when the SHNO microwave oscillations are in resonance with the NV at 3.33 GHz. The inset shows the position of the NV-AFM tip during the dc sweep. (c) NV photoluminescence (PL) drops by 21% at the second constriction edge when the SHNO is activated by a characteristic positive dc current of I_DC = 7.95 mA.

Figure 4

Localization of the auto-oscillation modes (a), (b) PL map at I_DC = 6.75 mA (I_DC = 7.95 mA) when auto-oscillation Mode 2 (Mode 1) is in resonance with the NV at the bottom constriction edge (upper constriction edge). For both dc currents, only one of the modes is in resonance with the NV and the second one off-resonance, respectively. (c, d) Finer PL map at the interaction area of Mode 2 (Mode 1) revealing two lobes. (e) Micromagnetic simulation of the auto-oscillation power localized at the constriction edge. (f, g) Microwave field in-plane (out-of-plane) component being perpendicular to the NV axis. (h) Microwave field interacting with the NV revealing two lobes as seen in the experiment. (i) Total microwave field (microwave fields are calculated at a height of 80 nm above the SHNO).