A platform for time-resolved scanning Kerr microscopy in the near-field

Abstract

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.

Nonequilibrium topological spin textures in momentum space

“…Therefore, our prediction is fully based on the leading order response in real systems. The ARPES light source typically bears a beam spot size 10-100µm upon the sample [1,5,35,38], which requests one to consider physical phenomena at the optical long-wavelength limit as the experimentally most relevant scenario, in contrast to the otherwise interesting space-resolved nano-ARPES or scanning Kerr magnetooptic microscopy study [39][40][41]. We thus introduce a spatially uniform Gaussian vector potential for the pump pulse vertically shone onto the xy-plane A(t) = A 0 exp(−t 2 /2t 2 0 ) [x cos Ωt + τ ŷ sin Ωt], where τ = 0, ±1 and t 0 the temporary width.…”

Magnetization dynamics of nanoscale magnetic materials: A perspective

“…The role of localized surface plasmons in enhancement of near-field MOKE signal will be significant here. Further development of near-field AFM probe, commercial supply of smaller aperture, polarization preserving plasmonic antenna, such as bow-tie and cross bow-tie geometry, would be required for routine measurements with sub-100 nm spatial resolution 39 . The experimental detection of intensity and phase maps of SWs for specific SW modes in nanoscale elements are still open challenges.…”

“…Time-resolved scanning Kerr microscopy (TRSKM) 38 facilitates the imaging of temporal evolution of spatial magnetization distribution by fixing the time delay and using scanning MOKE microscopy at various delays. However, the diffractionlimited spatial resolution is only few hundred nm, which can go down to sub-100 nm by using near-field MOKE 39 . In the frequency domain, ranging from MHz to tens of GHz, broadband FMR is a very useful technique to globally excite the sample by using absorption from the external source in the frequency spectrum.…”

Optically detected spin–orbit torque ferromagnetic resonance in an in-plane magnetized ellipse

“…Here time-resolved scanning Kerr microscopy 24 (TRSKM) has been used to perform opticallydetected, phase-resolved SOT induced ferromagnetic resonance (SOT-FMR) measurements in which the center localized magnetization dynamics of a microscale SOT device were directly probed with a spatial resolution of ~400 nm. Field swept SOT-FMR spectra were acquired from a 2×0.8 µm 2 inplane magnetized CoFeB(2 nm) ellipse deposited on a Pt(6 nm) underlayer as a function of the applied in-plane field angle.…”