Driving degenerate infrared-active phonons with circularly polarized laser pulses is predicted to generate magnetization [1]. Estimates based on the circular motion of the mode’s Born effective charge typically yield values on the order of one nuclear magneton per unit cell. Recent work has revealed large effective magnetic fields from circularly driven phonons in materials showing large phonon Zeeman effect [2], and – more surprisingly – phonon magnetizations up to four orders of magnitude larger than predicted by calculations even in systems without magnetic ions [3,4]. In most cases, this induced magnetization has been probed optically, via the Faraday rotation in the photo-excited volume, making it challenging to unambiguously separate the Faraday signal from other contributions arising by nonlinear optical interactions [5,6]. A promising, artefact-free approach is to quantify the phonon-induced magnetization by the fringing field it generates. Recently, we developed an ultrafast magnetometry technique that measures magnetic fields with sub-picosecond time resolution and sub-microtesla sensitivity [7,8]. Here, we study 6H-SiC, driven with circularly polarized pulses resonant with a doubly degenerate E1 phonon. By combining measurements of the fringing field from phonon-induced magnetization with polarization rotation measurements within the photoexcited volume, we find that the induced magnetization is at least an order of magnitude smaller than inferred from Faraday rotation alone [3]. This approach yields a more accurate estimate of the induced magnetization and helps clarify the origin of the large magnetization values reported in literature [3, 4].
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