Time-resolved scanning Kerr microscopy has been used to directly image the magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators (STVOs) when phase-locked to an injected microwave (RF) current. The Kerr images reveal free layer magnetization dynamics that extend outside the NC footprint, where they cannot be detected electrically, but which are crucial to phase-lock STVOs that share common magnetic layers. For a single NC, dynamics were observed not only when the STVO frequency was fully locked to that of the RF current, but also for a partially locked state characterized by periodic changes in the core trajectory at the RF frequency. For a pair of NCs, images reveal the spatial character of dynamics that electrical measurements show to have enhanced amplitude and reduced linewidth. Insight gained from these images may improve understanding of the conditions required for mutual phase-locking of multiple STVOs, and hence enhanced microwave power emission.

Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a Al2O3/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the centre of a Py/Pt mesa of 4 µm diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current, and exploiting the phase-sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device centre that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilise the position of the selflocalised bullet mode excited by the DC current, and inhibit injection locking. The present study demonstrates the need to characterise both DC and RF current distributions carefully.

Time-resolved scanning Kerr microscopy (TRSKM) is a powerful technique for the investigation of picosecond magnetization dynamics at sub-micron length scales by means of the magneto-optical Kerr effect (MOKE). The spatial resolution of conventional (focused) Kerr microscopy using a microscope objective lens is determined by the optical diffraction limit so that the nanoscale character of the magnetization dynamics is lost. Here we present a platform to overcome this limitation by means of a near-field TRSKM that incorporates an atomic force microscope (AFM) with optical access to a metallic AFM probe with a nanoscale aperture at its tip. We demonstrate the near-field capability of the instrument through the comparison of time-resolved polar Kerr images of magnetization dynamics within a microscale NiFe rectangle acquired using both near-field and focused TRSKM techniques at a wavelength of 800 nm. The flux-closure domain state of the in-plane equilibrium magnetization provided the maximum possible dynamic polar Kerr contrast across the central domain wall and enabled an assessment of the magneto-optical spatial resolution of each technique. Line profiles extracted from the Kerr images demonstrate that the near-field spatial resolution was enhanced with respect to that of the focused Kerr images. Furthermore, the near-field polar Kerr signal (∼1 mdeg) was more than half that of the focused Kerr signal, despite the potential loss of probe light due to internal reflections within the AFM tip. We have confirmed the near-field operation by exploring the influence of the tip-sample separation and have determined the spatial resolution to be ∼550 nm for an aperture with a sub-wavelength diameter of 400 nm. The spatial resolution of the near-field TRSKM was in good agreement with finite element modeling of the aperture. Large amplitude electric field along regions of the modeled aperture that lie perpendicular to the incident polarization indicate that the aperture can support plasmonic excitations. The comparable near-field and focused polar Kerr signals suggest that such plasmonic excitations may lead to an enhanced near-field MOKE. This work demonstrates that near-field TRSKM can be performed without significant diminution of the polar Kerr signal in relatively large, sub-wavelength diameter apertures, while development of a near-field AFM probe utilizing plasmonic antennas specifically designed for measurements deeper into the nanoscale is discussed.

We employ an extended finite-element model as a design tool capable of incorporating the interaction between plasmonic antennas and magneto-optical effects, specifically the magneto-optical Kerr effect (MOKE). We first test our model in the absence of an antenna and show that for a semi-infinite thin-film, good agreement is obtained between our finite-element model and analytical calculations. The addition of a plasmonic antenna is shown to yield a wavelength dependent enhancement of the MOKE. The antenna geometry and its separation from the magnetic material are found to impact the strength of the observed MOKE signal, as well as the antenna's resonance wavelength. Through optimization of these parameters we achieved a MOKE enhancement of more than 100 when compared to a magnetic film alone. These initial results show that our modeling methodology offers a tool to guide the future fabrication of hybrid plasmonic magneto-optical devices and plasmonic antennas for magneto-optical sensing.

Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to the TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear 'bullet' mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.Within a spin torque oscillator (STO), magnetic autooscillations, with MHz to GHz frequencies, are driven by the spin transfer torque (STT) associated with injection of DC spin current. Their frequency and amplitude can be tuned via either the DC electrical bias current or an applied magnetic field, while the magnetoresistance of the constituent materials leads to the generation of voltage oscillations. STOs have strong potential for magnetic sensing, signal processing, and neurmorphic computing applications 1,2 . The ability to lock the frequency and phase of the STO to an injected RF signal is an important property within applications, while arrays of STOs promise increased output power through mutual synchronization. However it is first necessary to understand the character of the underlying magnetization dynamics. More specifically, the dynamics excited by both DC and RF currents must be determined if the conditions required for phase-locking are to be fully understood.Within a spin Hall nano-oscillator (SHNO) the Spin Hall effect (SHE) 3,4 drives a pure spin current from a heavy metal with large spin-orbit interaction into a ferromagnet layer 5-7 . The de-coupling of charge and spin currents opens up new device geometries, for example enabling exploitation of magnetic insulators 8 , and in the present study, allows optical access to the active region of the device.The generation of magnetic auto-oscillations requires a critical spin current density to be exceeded. Within t...