Ultrafast Lorentz Microscopy using High-Coherence Electron Pulses

by Nara Rubiano da Silva
Doctoral thesis
Date of Examination:2019-03-29
Date of issue:2019-05-08
Advisor:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Claus Ropers
Referee:Prof. Dr. Stefan Mathias
Persistent Address: http://dx.doi.org/10.53846/goediss-7437

 

 

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Abstract

English

Information processing and storage applications demand increasing miniaturization and operation speeds, where optical manipulation of magnetic order may play a very important role. It is hence essential to study how light stimulates magnetization reordering at very short spatial and temporal scales. The development of Ultrafast Transmission Electron Microscopy (UTEM), a laser-pump/electron-probe setup, provides a unique means for that. However, magnetic information can only be visualized in UTEM provided a spatially coherent pulsed electron source is available, since phase-contrast techniques such as Lorentz microscopy are required. In this work, highly coherent electron pulses are achieved by implementing a laser-driven Schottky field emitter in UTEM. We employ the electron pulses to map the ultrafast demagnetization of a single magnetic structure with very high spatial and temporal resolution. We show that the transient magnetization can be quantitatively tracked from the image contrast, and that the magnetic order remains rigid with decreased strength on short timescales following the optical excitation. This work builds a solid framework for the acquisition, and quantitative evaluation and interpretation of magnetic contrast images of transient magnetic states. Moreover, it establishes magnetic imaging with high spatiotemporal resolution, and constitutes a step forward in ultrafast phase-contrast imaging on the nanoscale.
Keywords: Ultrafast transmission electron microscopy; UTEM; Coherent ultrashort electron pulses; Nanoscale magnetization dynamics; Lorentz microscopy; Magnetic imaging; Ultrafast dynamics; Ultrafast solid-state physics
 
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