Abstract text (incl. figure legends and references)

1. Introduction In situ transmission electron microscopy (TEM) offers a unique approach to observe dynamic physical and chemical processes in real time and with atomic-scale resolution. Due to several advances in electron microscopy including aberration corrected optics, specimen environment control, custom stages, fast and sensitive data acquisition, the electron microscope has become an increasingly popular tool to synthesize nanostructures of all three dimensions [1, 2]. Furthermore, the diverse radiation damage mechanisms that are a hurdle to high resolution electron microscopy [3] are put to the experimenter"s advantage when the microscope is repurposed for nanoengineering processes. We focus on the influence of an experimental aspect of beam-induced fragmentation which can offer direct insights into the physical nature of this process, namely, the influence of the magnetic field originating from the pole pieces in the TEM. Our findings show that the presence or absence of the magnetic field affects the fragmentation of the particles in ways that are consistent with an explanation based on beam-induced charging. 2. Materials and Methods We deposited Au microparticles onto the amorphous carbon (a-C) film of a commercial TEM grid by direct dry transfer. We then irradiated the particles under two irradiation conditions: first, in bright field (BF) mode, where the magnetic field around the microparticles is of around 2 T, and second, in Lorenz mode of TEM, where the field is turned off. The irradiation protocol is the same for both irradiation conditions and it is schematically represented in Figure 1 a) and b). Initially, the Au microparticle is stable as long as the irradiating current density is below a critical threshold value JT (Fig. 1 a) and c) ). Above JT , the particle begins to shrink as its outermost layers are expelled and form Au nanoparticles deposited on the a-C substrate (Fig. 1 b) and d) ). 3. Results The irradiation procedures with and without magnetic field allow us to investigate if and how magnetoresistive effects on the Au/a-C interface modulate JT at which fragmentation occurs. We found that the presence or absence of the field significantly affects the fragmentation of the particles: Au microparticles subjected to convergent irradiation in Lorentz mode (B = 0 T) fragment or "shrink" at average lower current densities than particles subjected to the same protocol in BF mode (B = 2 T). This can be observed in Figure 2 where JT at which the shrinking of the particles begins is plotted as a function of the particle"s mass. The results are consistent with the beam-induced charging paradigm. 4. Conclusion The role of the magnetoresistive mechanism as inferred from the field-dependent variation of the critical current density to initiate the fragmentation of the Au particles strongly4. Conclusion The role of the magnetoresistive mechanism as inferred from the field-dependent variation of the critical current density to initiate the fragmentation of the Au particles strongly indicates that the fragmentation phenomenon is electrostatically driven instead of thermally triggered. Furthermore, the magnetoresistive effect seems to be negative, which is consistent with current knowledge about the electrical properties of a-C films [4]. This leads us to conclude that beam-induced fragmentation is a phenomenon that should be conceptualized as an instance of Coulomb explosion.