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Off-resonant magnetization dynamics phase-locked to an intense phase-stable terahertz transient


Controlling magnetization dynamics with a femtosecond laser is attracting interest both in fundamental science and in industry because of the potential to achieve magnetic switching at ever faster speeds. Here, we report a coherent, phase-locked coupling between a high-field single-cycle terahertz transient and the magnetization of ferromagnetic cobalt films. The visualized magnetization dynamics follow the temporal terahertz field oscillation, are tightly locked to the terahertz phase and are induced in the absence of resonant excitations and energy deposition. The magnetic response occurs on the timescale of the stimulus and is thus two orders of magnitude faster than the Larmor precession response. The experimental results are excellently reproduced by the Landau–Lifshift–Gilbert semi-empirical model, indicating its applicability to ultrafast magnetization dynamics and also demonstrating the marginal effect of the co-propagating terahertz electric field. This novel phenomenon of phase-locked control of magnetization with a strong terahertz field suggests new opportunities for ultrafast data storage.

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Figure 1: Time-resolved magneto-optical Kerr effect (MOKE) set-up that allows for the measurement of ultrafast moment dynamics on a room-temperature cobalt sample surface.
Figure 2: Measurements of the phase-locked out-of-plane magnetization dynamics initiated by the strong terahertz transient.
Figure 3: Magnetization response dependence on the terahertz input polarization direction.
Figure 4: Magnetic response as a function of terahertz field strength.

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This work was carried out at the Paul Scherrer Institute and was supported by the Swiss National Science Foundation (grant PP00P2_128493). The authors acknowledge A. Kleibert for helpful discussions and thank M. Paraliev for support. B.T. is supported by the ERASMUS Mundus program. J.L. acknowledges the DYNAVO project for financial support for upgrading the magnetron sputtering. C.P.H. acknowledges association with the National Center of Competence in Research on Molecular Ultrafast Science and Technology (NCCR-MUST).

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C.P.H. and J.L. conceived the experiment. C.P.H. coordinated the project and wrote the manuscript, together with P.M.D. and C.V. The experiment was performed by C.V., C.R., J.L., F.A. and C.P.H., and B.T. fabricated and characterized the samples. P.M.D. performed numerical investigations of the magnetization dynamics. C.P.H., J.L., C.V., C.R. and P.M.D. contributed to data analysis.

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Correspondence to C. P. Hauri.

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Vicario, C., Ruchert, C., Ardana-Lamas, F. et al. Off-resonant magnetization dynamics phase-locked to an intense phase-stable terahertz transient. Nature Photon 7, 720–723 (2013).

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