Donges, Andreas
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Terahertz spin dynamics driven by a field-derivative torque
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.
Off-resonant magnetization dynamics in ferromagnetic thin films initiated by ultrastrong THz field
Summary form only given. The speed of magnetization switching is a key feature in next generation magnetic storage devices. The ongoing pursue towards faster magnetization control has triggered the development of laser sources at terahertz frequencies. In fact pulses in this spectral range are more suited than optical laser for coherent magnetization excitation by Zeeman precession [1]. The recent advent of THz pulses with field strength up to several Teslas [2] opens novel opportunities to drive ultrafast magnetization dynamics in the strong-field regime, which is different from the commonly used optical lasers where the magnetization control is mediated by heat deposition [3].Here we report on time-resolved measurements exploring the sub-cycle THz-induced magnetization dynamics in the ferromagnetic thin film samples Co, Fe and Ni [4]. We present the induced magneto-optical Kerr dynamics as function of the THz field strength up to extreme amplitudes of 7 T and 21 MV/cm, respectively. By increasing the THz pump fluence, we find a continuous transition from the regime of purely coherent Zeeman oscillations, to the incoherent regime, where spin oscillations are superimposed by thermal demagnetization. Our observations indicate that while the coherent response is driven only by the magnetic field, the incoherent dynamics are dominated by the associated THz electric field component. The observed magnetization evolution over sub-picosecond time scale is excellently reproduced by simulations based on ab-initio calculations for the Heisenberg spin Hamiltonian and the stochastic Landau-Lifshitz-Gilbert equation to describe the spin dynamics at finite temperature.
Thermal skyrmion diffusion used in a reshuffler device
Magnetic skyrmions in thin films can be efficiently displaced with high speed by using spin-transfer torques1,2 and spin–orbit torques3,4,5 at low current densities. Although this favourable combination of properties has raised expectations for using skyrmions in devices6,7, only a few publications have studied the thermal effects on the skyrmion dynamics8,9,10. However, thermally induced skyrmion dynamics can be used for applications11 such as unconventional computing approaches12, as they have been predicted to be useful for probabilistic computing devices13. In our work, we uncover thermal diffusive skyrmion dynamics by a combined experimental and numerical study. We probed the dynamics of magnetic skyrmions in a specially tailored low-pinning multilayer material. The observed thermally excited skyrmion motion dominates the dynamics. Analysing the diffusion as a function of temperature, we found an exponential dependence, which we confirmed by means of numerical simulations. The diffusion of skyrmions was further used in a signal reshuffling device as part of a skyrmion-based probabilistic computing architecture. Owing to its inherent two-dimensional texture, the observation of a diffusive motion of skyrmions in thin-film systems may also yield insights in soft-matter-like characteristics (for example, studies of fluctuation theorems, thermally induced roughening and so on), which thus makes it highly desirable to realize and study thermal effects in experimentally accessible skyrmion systems.
Magnetization compensation and spin reorientation transition in ferrimagnetic DyCo5 : Multiscale modeling and element-specific measurements
We use a multiscale approach linking ab initio calculations for the parametrization of an atomistic spin model with spin dynamics simulations based on the stochastic Landau-Lifshitz-Gilbert equation to investigate the thermal magnetic properties of the ferrimagnetic rare-earth transition-metal intermetallic DyCo5. Our theoretical findings are compared to elemental resolved measurements on DyCo5 thin films using the x-ray magnetic circular dichroism technique. With our model, we are able to accurately compute the complex temperature dependence of the magnetization. The simulations yield a Curie temperature of TC=1030K and a compensation point of Tcomp=164K, which is in a good agreement with our experimental result of Tcomp=120K. The spin reorientation transition is a consequence of competing elemental magnetocrystalline anisotropies in connection with different degrees of thermal demagnetization in the Dy and Co sublattices. Experimentally, we find this spin reorientation in a region from TSR1,2=320 to 360K, whereas in our simulations the Co anisotropy appears to be underestimated, shifting the spin reorientation to higher temperatures.
Coherent and incoherent ultrafast magnetization dynamics in 3d ferromagnets driven by extreme terahertz fields
Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective 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 ≈100mJ/cm2. 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 nonconducting materials for achieving coherent THz-magnetization reversal.