Event Details

Understanding new magnetic phenomena to enable emerging memory, logic, and oscillator technologies is aided by magnetic imaging techniques that possess simultaneous picosecond temporal resolution and 10 — 100 nm spatial resolution. Conventionally, this combination is available only at facility-based research centers using e.g., pulsed x-ray dichroism techniques. Likewise, many of the most exciting magnetic material systems, including ultrathin ferromagnetic or antiferromagnetic insulators coupled to layers that produce spin-orbit interactions, are difficult to image with any method. To address these challenges in an accessible way, we have developed a table-top spatiotemporal magnetic microscope based on nanoscale, picosecond thermal pulses. Our method takes advantage of magneto-thermal interactions that couple heat flow to spin transport, including the anomalous Nernst effect  [1] and the longitudinal spin Seebeck effect  [2]. Using focused light as a picosecond heating source, we demonstrate that these imaging modalities have time resolution on the order of 10  ps and sensitivities to magnetization angle of 0.1—0.3 °/ for ferromagnetic metals and insulators. In combination with phase-sensitive microwave current imaging, phase-sensitive ferromagnetic resonance imaging [3] enables direct imaging of the gigahertz-frequency magnetic driving torque vector, which is valuable for understanding spin-orbit interactions [4]. We also demonstrate magneto-thermal imaging of Neel order in FeRh [5] (an antiferromagnetic metal) and NiO [6] (an antiferromagnetic insulator), offering an accessible method to study spin-orbit torque switching of antiferromagnetic devices. Finally, I will describe how the resolution of time-resolved magnetic imaging with heat can be improved to greatly exceed the optical diffraction limit, both theoretically  [6] and experimentally. We demonstrate scanning a sharp gold tip illuminated by picosecond laser pulses as the basis of a nanoscale spatiotemporal magnetic microscope.

[1] J. M. Bartell, D. H. Ngai, Z. Leng, and G. D. Fuchs, Nat. Commun. 6, 8460 (2015).
[2] J. M. Bartell, C. L. Jermain, S. V. Aradhya, J. T. Brangham, F. Yang, D. C. Ralph, and G. D. Fuchs, Phys. Rev. Appl. 7, 044004 (2017).
[3] F. Guo, J. M. Bartell, D. H. Ngai, and G. D. Fuchs, Phys. Rev. Appl. 4, 044004 (2015).
[4] F. Guo, J. M. Bartell, and G. D. Fuchs, Phys. Rev. B 93, 144415 (2016).
[5] I. Gray, G. M. Stiehl, A. B. Mei, D. Schlom, J. T. Heron, D. C. Ralph, and G. D. Fuchs, in prep. (2019).
[6] I. Gray, T. Moriyama, N. Sivadas, G. M. Stiehl, J. T. Heron, R. Need, B. J. Kirby, D. H. Low, K. C. Nowack, D. G. Schlom, D. C. Ralph, T. Ono, and G. D. Fuchs, arXiv:1810.03997 (2018).
[7] J. C. Karsch, J. M. Bartell, and G. D. Fuchs, APL Photonics 2, 086103 (2017).