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arXiv · 2607.08519

A crystal-field route to THz-driven magnetization

T. Zalewski·M. S. Mrudul·Y. Lee·M. Weissenhofer·A. V. Boris·P. M. Oppeneer·A. Kirilyuk·C. S. Davies
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Abstract

Light carries angular momentum, but the microscopic pathways that transform it into magnetization remain elusive. Here we establish that crystal-field excitations, historically viewed primarily as equilibrium spectroscopic fingerprints of localized 4$f$ electrons, constitute an active microscopic route through which circularly-polarized terahertz (THz) light creates magnetic polarization. Using wavelength-selective ultrafast Faraday spectroscopy on the paramagnetic insulator CeF$_3$, we show that resonant excitation of localized 4$f$ crystal-field transitions generates a helicity-dependent magnetization that survives for up to about 100 ps. Most strikingly, while the optical helicity is held fixed, the THz-driven response reverses sign as the excitation wavelength is tuned across the crystal-field resonance. The resulting dispersive spectral response follows the crystal-field excitation spectrum rather than that of optical phonons, and is captured by resonant electronic theory of the inverse Faraday effect. Our results identify crystal-field excitations as a previously unrecognized dynamical reservoir for optical angular momentum and broaden the microscopic pathways through which THz light can create and manipulate magnetic states.

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T. Zalewski, M. S. Mrudul, Y. Lee, M. Weissenhofer, A. V. Boris, P. M. Oppeneer, A. Kirilyuk, C. S. Davies. 2026-07-09. A crystal-field route to THz-driven magnetization. https://arxiv.org/abs/2607.08519

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