Abstract-Ultrafast terahertz magnetometry

Wentao Zhang, Pablo Maldonado, Zuanming Jin, Tom S. Seifert, Jacek Arabski, Guy Schmerber, Eric Beaurepaire, Mischa Bonn, Tobias Kampfrath, Peter M. Oppeneer, Dmitry Turchinovich

https://www.nature.com/articles/s41467-020-17935-6

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.

Abstract-Terahertz spin dynamics driven by a field-derivative torque

Ritwik Mondal, Andreas Donges, Ulrike Ritzmann, Peter M. Oppeneer, and Ulrich Nowak

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.060409

Efficient manipulation of magnetization at ultrashort timescales is of particular interest for future technology. Here, we numerically investigate the influence of the so-called field-derivative torque, which was derived earlier based on relativistic Dirac theory [R. Mondal et al., Phys. Rev. B 94, 144419 (2016)], on the spin dynamics triggered by ultrashort laser pulses. We find that only considering the THz Zeeman field can underestimate the spin excitation in antiferromagnetic oxide systems such as, e.g., NiO and CoO. However, accounting for both the THz Zeeman torque and the field-derivative torque, the amplitude of the spin excitation increases significantly. Studying the damping dependence of the field-derivative torque we observe larger effects for materials having larger damping constants.

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Abstract-Coherent and incoherent ultrafast magnetization dynamics in 3 d ferromagnets driven by extreme terahertz fields

Mostafa Shalaby, Andreas Donges, Karel Carva, Rolf Allenspach, Peter M. Oppeneer, Ulrich Nowak, and Christoph P. Hauri

https://journals.aps.org/prb/accepted/6a074Y81Tf81f278f2a37c59cec82beaa7ac0c3d7

Ultrafast spin dynamics in magnetic materials is generally associated to ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and non-selective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase-locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of $\approx \text{SI}100\text{milli}\text{joule}\text{per}\square \text{centi}\text{meter}$. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting non-conducting materials for achieving coherent THz-magnetization reversal.