Abstract
The development of spintronic technology with increasingly dense, high-speed, and complex devices will be accelerated by accessible microscopy techniques capable of probing magnetic phenomena on picosecond time scales and at deeply sub-micron length scales. A recently developed time-resolved magneto-thermal microscope provides a path towards this goal if it is augmented with a picosecond, nanoscale heat source. We theoretically study adiabatic nanofocusing and near-field heat induction using conical gold plasmonic antennas to generate sub-100 nm thermal gradients for time-resolved magneto-thermal imaging. Finite element calculations of antenna-sample interactions reveal focused electromagnetic loss profiles that are either peaked directly under the antenna or are annular, depending on the sample’s conductivity, the antenna’s apex radius, and the tip-sample separation. We find that the thermal gradient is confined to 40 nm to 60 nm full width at half maximum for realistic ranges of sample conductivity and apex radius. To mitigate this variation, which is undesirable for microscopy, we investigate the use of a platinum capping layer on top of the sample as a thermal transduction layer to produce heat uniformly across different sample materials. After determining the optimal capping layer thickness, we simulate the evolution of the thermal gradient in the underlying sample layer and find that the temporal width is below 10 ps. These results lay a theoretical foundation for nanoscale, time-resolved magneto-thermal imaging.
Highlights
Through finite element calculations of the tip-sample coupling, we reveal that in the range of realistic sample resistivities, apex radii, tip-sample separation, and film thickness, electromagnetic loss is either peaked directly under the apex with a full width at half maximum (FWHM) on the order of the apex radius, or it takes on an annular profile that peaks in a ring >10 nm from the center
The spatial distribution of the electromagnetic loss determines the thermal gradient in near-field heating
S 4 of the supplementary material, we show there is a very weak relationship between loss profile and sample permittivity
Summary
Spin-based electronics and high-density magnetic storage require precise control of local magnetic moments in devices,[1,2] often using either applied magnetic fields[3] or spin-transfer torques.[4,5,6,7] Development of these technologies will be aided by microscopy techniques enabling researchers to characterize dynamical, nanoscale magnetic phenomena[8,9] with relevant length scales that are typically 10 nm to 200 nm[10] and relevant time scales that are typically 5 ps to 50 ps.[11,12,13] One existing approach is x-ray magnetic circular dichroism-based microscopy, which offers the desired resolution with spot sizes down to 30 nm.[14] it requires a synchrotron facility[15] and it cannot be used in a normal laboratory setting Another approach is magneto-optical Kerr effect (MOKE) microscopy, which allows for table-top, stroboscopic imaging of spin dynamics with straightforward interpretation.[16] the visible to near-IR light that is typically used fundamentally limits the spatial resolution of MOKE to hundreds of nanometers, set by the diffraction-limited focal resolution of approximately half the wavelength.[17]. We find that centrally peaked thermal gradients are radially confined to below 100 nm, with temporal FWHM below 10 ps
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