Few-nm tracking of current-driven magnetic vortex orbits using ultrafast Lorentz microscopy

Abstract

Transmission electron microscopy is one of the most powerful techniques to characterize nanoscale magnetic structures. In light of the importance of fast control schemes of magnetic states, time-resolved microscopy techniques are highly sought after in fundamental and applied research. Here, we implement time-resolved Lorentz imaging in combination with synchronous radio-frequency excitation using an ultrafast transmission electron microscope. As a model system, we examine the current-driven gyration of a vortex core in a 2 µm-sized magnetic nanoisland. We record the trajectory of the vortex core for continuous-wave excitation, achieving a localization precision of ±2 nm with few-minute integration times. Furthermore, by tracking the core position after rapidly switching off the current, we find a temporal hardening of the free oscillation frequency and an increasing orbital decay rate attributed to local disorder in the vortex potential. arXiv:1907.04608v1 [cond-mat.mes-hall]

Ultrafast electron microscopy for probing magnetic dynamics

“…Specifically, this tool is wellsuited to help answer a number of open questions in magnetism related to microscopic magnetic energy landscapes, 127,128 angular momentum transfer processes, 134 ultrafast demagnetization at interfaces, skyrmion dynamics, and coherent ultrafast magnetism. The growth of other ultrafast electron techniques, including time-resolved SEMPA 67-71 and potentially SPLEEM, opens the door to complementary surface-sensitive imaging of dynamics.…”

“…Ultrafast Lorentz microscopy has been employed to study several types of dynamics, including domain wall motion under an oscillating magnetic field 109 or laserinduced thermal gradients, 110,124 laser-induced demagnetization, 100,[124][125][126] and current-driven vortex oscillations. 127 In two recent examples, Möller et al investigated the response of a magnetic vortex to an applied highfrequency current. 127,128 The interplay between the current-induced force on the vortex, consisting both of Oersted-field and spintransfer torque contributions, and the confinement potential within a magnetic nanostructure results in resonant gyration dynamics of the vortex.…”

“…127 In two recent examples, Möller et al investigated the response of a magnetic vortex to an applied highfrequency current. 127,128 The interplay between the current-induced force on the vortex, consisting both of Oersted-field and spintransfer torque contributions, and the confinement potential within a magnetic nanostructure results in resonant gyration dynamics of the vortex. Driving the vortex gyration by applying 101.5 MHz radiofrequency currents across a 2.1 µm permalloy square, Möller et al were able to track the vortex position with a 2 nm precision, 127 as shown in Figure 4, and observed the frequency dependence of the response in real space.…”

Femtosecond laser driven precessing magnetic gratings

“…21 The recent developments within Lorentz ultrafast TEM (LUEM), through integration of pump-probe into the system, [22][23][24] have demonstrated its ability for studies of ultrafast magnetization dynamics on systems such as skyrmions, vortices, and domain walls. [25][26][27][28][29] Direct imaging of magnetization dynamics at combined nanoscale and picosecond resolutions may uncover new details central to the understanding of spintronic processes including ultrafast magnetic switching, formation and pertur-bation of topological magnetic structures, and spin wave propagation.…”

Pinning and gyration dynamics of magnetic vortices revealed by correlative Lorentz and bright-field imaging

“…Excellent spatial resolution for polycrystalline samples with higher surface roughness is possible using electron microscopy techniques [33] and was used to characterize core pinning via one-dimensional differential-phasecontrast line scans [21]. Moreover, recent advances in time-resolved Lorentz microscopy enable imaging magnetic dynamics at simultaneous high spatial and temporal resolution [34][35][36][37][38][39]. To date, however, the full capabilities of electron microscopy in both magnetic and structural imaging have yet to be leveraged in correlated studies.…”