A single-cycle terahertz pulse applied to single-crystal holmium iron garnet ${\mathrm{Ho}}_3{\text{Fe}}_5{\mathrm{O}}_{12}$ strongly perturbs the magneto-optical Faraday effect—an effect conventionally employed for optical readout of the magnetic state. This ultrafast perturbation sets in when cooling below the magnetization compensation temperature, where the unit cell distorts and the spins of ${\mathrm{Ho}}^{3+}$ form a noncollinear “double-umbrella” structure. The experimental dependencies reveal that the findings can be described as the ultrafast terahertz-electric-field-induced magneto-optical Faraday effect caused by the resonant excitation of crystal-field-split $f\text{−}f$ electronic transitions of ${\mathrm{Ho}}^{3+}$ in a noncentrosymmetric environment of oxygen ions. We demonstrate that the effect is not limited to ${\mathrm{Ho}}_3{\text{Fe}}_5{\mathrm{O}}_{12}$ but is also present in other rare-earth iron garnets, such as ${\mathrm{Tb}}_3{\text{Fe}}_5{\mathrm{O}}_{12}$. The effect can facilitate ultrafast optical readout of magnetic bits at a sub-100 nm spatial scale.

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• Electric polarization
• Faraday effect
• Magnetoelectric effect
• Ferrimagnets
• Rare-earth magnetic materials
• Coherent control
• Terahertz spectroscopy
• Ultrafast pump-probe spectroscopy