Ultrafast terahertz magnetometry

Abstract

A material's magnetic state and its dynamics are of great fundamental research interest and are also at the core of a wide plethora of modern technologies. However, reliable access to magnetization dynamics in materials and devices on the technologically relevant ultrafast timescale, and under realistic device-operation conditions, remains a challenge. Here, we demonstrate a method of ultrafast terahertz (THz) magnetometry, which gives direct access to the (sub-)picosecond magnetization dynamics even in encapsulated materials or devices in a contact-free fashion, in a fully calibrated manner, and under ambient conditions. As a showcase for this powerful method, we measure the ultrafast magnetization dynamics in a laser-excited encapsulated iron film. Our measurements reveal and disentangle distinct contributions originating from (i) incoherent hot-magnon-driven magnetization quenching and (ii) coherent acoustically-driven modulation of the exchange interaction in iron, paving the way to technologies utilizing ultrafast heat-free control of magnetism. High sensitivity and relative ease of experimental arrangement highlight the promise of ultrafast THz magnetometry for both fundamental studies and the technological applications of magnetism.

Fig. 1. Schematic of the experiment and the sample.

a Sketch of the THz emission experiment: the sample is placed in the static in-plane magnetic field of 63.4 mT, and excited with a 100 fs, 800 nm laser pulse at normal incidence. The THz emission from the sample in the forward-propagating direction is detected using free-space electro-optic sampling in a 1 mm ZnTe crystal, without any intermediate focusing or additional guiding. b Schematic of the sample: 10-nm-thick bcc Fe film, grown on a 0.5-mm-thick MgO substrate, and capped with 12-nm-thick MgO cap. The excitation gradient of the electrons in the Fe film in such a configuration is negligible, and hot-electron diffusion from the Fe film is blocked by MgO substrate and cap. This eliminates the possibility of any ISHE-type mechanism, which relies on the motion of spin-polarized electrons in the sample, in the laser-induced magnetization dynamics M(t). The only possible contributions to M(t) in the sample remain hot-electron-driven transient demagnetization causing incoherent hot-magnon excitation (primary dynamics), and modulation of exchange interaction in Fe due to generated coherent acoustic phonons S(t) propagating in the structure (secondary dynamics).

Fig. 2. Observable quantities: measured electro-optic signal of the THz emission, the reconstructed magnetization dynamics M(t), and its theoretical simulation.

a Measured FEOS signal of the THz emission from MgO/Fe/MgO sample, under the excitation of 0.51 mJ cm⁻². Inset: comparison of FEOS signal obtained from MgO/Fe/MgO sample via pure magnetic dipole emission due to M(t), red line; and MgO/Fe/Pd sample, dominated by the electric dipole emission via ISHE, green line. b Reconstructed magnetization from experimental EOS (solid line) and simulated magnetization dynamics (dashed line). Simulation conditions are identical to the experiment. μ₀M₀ = 2.15 T—equilibrium magnetization in Fe (see “Methods”). c Hot electrons excite hot magnons that drive primary demagnetization dynamics. d Dynamics of elongation of the Fe film due to the generated coherent optical phonon pulse. Complete magnetization dynamics M(t) comprises both incoherent hot-magnon and coherent acoustic phonon contributions.

Fig. 3. Comparison of measured and simulated magnetization dynamics M(t) in laser-excited MgO/Fe/MgO sample in this work.

a Ultrafast magnetization dynamics in the Fe film at various laser pump fluences in the range 0.25–1.02 mJ cm⁻² with errors shown by dashed lines, and b results of simulation corresponding to the experimental scenario in (a). The error bars in (a) were determined from the standard errors in the measured electro-optic signals (from 200 individual measurements). The experimental error was then propagated in the reconstruction protocol, leading to the intervals as indicated.