Session Y6: Ultrafast Magnetization Dynamics: Where Are We Today?

The x-ray few of femtosecond spin-orbit excitations in ferromagnets

Polarized soft x-rays have been used over the past 20 years to obtain fascinating new insights into nanoscale magnetism. The separation of spin and orbital magnetic moments, for instance, enabled detailed insights into the interplay of exchange and spin-orbit interactions at the atomic level. The now available polarized soft x-ray pulses with only 100 fs duration allow us to observe the magnetic interactions at work in real time. The ultimate goal of such studies is to understand how spins may be manipulated by ultrashort magnetic field, spin polarized current or light pulses. In this talk I will focus on fs laser induced spin-orbit dynamics in 3d transition metals. Using fs x-ray pulses from the BESSY II femtoslicing facility I will show how fs excitation of the electronic system modifies the spin-orbit interaction enabling ultrafast angular momentum transfer between spin, orbital and lattice degrees of freedom.   

Possibility of Nanoscale Imaging of Ultrafast Magnetization Dynamics

Understanding the microscopic mechanisms driving the magnetization dynamics on the fs time scale is of essential importance for manipulating and controlling the macroscopic state in magnetic storage devices. The demagnetization in ferromagnetic films by an ultrashort laser excitation on a time scale of a few hundred fs raised controversies about the effective path to dissipate angular momentum to the lattice, see e.g. [1]. Even more intriguing is the demonstration of all-optical magnetization reversal in ferrimagnetic compounds using circularly polarized, fs laser pulses [2]. Until only recently, the field of ``Femto-magnetism'' has naturally been driven by all-optical pump-probe techniques. Femtosecond time-resolved X-ray magnetic circular dichroism spectroscopy has been utilized to unambiguously determine the ultrafast quenching of spin and orbital moments after ultrashort laser excitation [3]. While all-optical pump-probe techniques allow ultrafast excitations (pump) and the study of their evolution (probe) on the macroscopic scale by use of the magneto-optical Kerr or Faraday effect, little is known about the microscopic processes on nano- and sub-nanometer length scales because of the lack of real or momentum space resolution of optical techniques. By combining resonant coherent resonant magnetic scattering with the unique high peak-brightness, short pulse structure, and fully transverse coherence of the new x-ray free-electron lasers, the dynamics of magnetic fluctuations and magnetization relaxation processes can be studied on the nanometer scale with sub-picosecond time resolution. We demonstrate the possibility of nondestructive single shot imaging of the magnetization in Co/Pd multilayers at LCLS. \\[4pt] [1] Koopmans, B, et al., Nature materials 9, 259 (2010). \\[0pt] [2] Stanciu, C.D., et al., Phys. Rev. Lett. 99, 047601 (2007). \\[0pt] [3] Stamm, C., et al., Nature materials 6, 740 (2007).   

Imaging Magnetization Dynamics on the Nanoscale Using X-ray Microscopy

We aim at time- and spatially resolved imaging of excitations in ferromagnetic materials such as spin waves, the motion of domain walls and the gyration of magnetic vortices and antivortices. Special emphasize is given to the interaction of electrical currents with magnetic inhomogeneities like domains walls and vortices. The spin-polarized current can give rise to a spin torque on spatially inhomogeneous magnetization configurations. With magnetic transmission X-ray microscopy we observe a current-driven oscillation of an individual domain wall on its genuine time scale. In the framework of an analytical model insight into the domain-wall motion and its characteristic damping time is gained by examination of different phase spaces [1]. Current-induced depinning of a domain wall from a pinning site depends on the temporal shape of the current pulse. Apart from resonant excitation of the wall this effect arises from an additional force on the wall due to a fast changing current. Efficient depinning is achieved for rise times smaller than the damping time of the domain wall [2]. Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of vortices and antivortices. They behave as two-dimensional oscillators with a gyrotropic eigenmode which can be resonantly excited by spin currents and magnetic fields [3]. It is shown that the two excitation types couple in an opposing sense of rotation in case of resonant antivortex excitation with circular-rotational currents [4]. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. Financial support by the DFG via SFB 668 and via GK 1286 as well as by the City of Hamburg via the Landesexzellenzcluster Nano-Spintronics is gratefully acknowledged. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy. \\[4pt] [1] L. Bocklage et al., Phys. Rev. B \textbf{81}, 054404 (2010) \\[0pt] [2] L. Bocklage et al. Phys. Rev. Lett. \textbf{103}, 197204 (2009) \\[0pt] [3] A. Drews et al., Phys. Rev. B \textbf{77}, 094413 (2008); M. Bolte et al., Phys. Rev. Lett. \textbf{100}, 176601 (2008). \\[0pt] [4] T. Kamionka et al., Phys. Rev. Lett. \textbf{105}, 137204 (2010).   

Ultrafast magnetization dynamics in lanthanide ferromagnets: From bulk to surfaces

The intense research on femtosecond laser-induced magnetization dynamics resulted in rich ultrafast phenomena [1]. A microscopic description of the underlying elementary processes, however, remains a challenge. Most efforts focus on the $3d$ transition metal ferromagnets and related compounds. This talk discusses recent work on the lanthanide ferromagnets Gd and Tb. Their magnetic moment is dominated by $4f$ electrons which are localized at the ion core. Their spin-lattice coupling is determined by the angular momentum of the $4f$ electrons. Using femtosecond x-ray magnetic circular dichroism at the femtosecond slicing facility at the BESSY II storage ring in Berlin, Germany, we measure the ultrafast change in the magnetic moment, which occurs on two specific timescales [2]. The faster one is 0.75~ps. It is driven by hot electrons and is identical for both lanthanides. The slower one is different for Gd (40~ps) and Tb (8~ps) due to the stronger spin-lattice coupling in Tb. The talk also discusses time-resolved non-linear optical studies on Gd(0001) and Tb(0001) surfaces [3]. We find a coherent surface phonon which is strongly coupled with the ultrafast magnetic response and pronounced differences compared to the bulk dynamics which are attributed to spin-polarized transport effects. \\[4pt] [1] A. Kirilyuk, A. V. Kimel, Th. Rasing, Rev. Mod. Phys. \textbf{82}, 2731(2010).\\[0pt] [2] M. Wietstruk {\it et~al.}, http://arxiv.org/abs/1010.1374.\\[0pt] [3] A. Melnikov and U. Bovensiepen, in {\it Dynamics at solid state surfaces and interfaces} Vol. 1, edited by U. Bovensiepen, H. Petek, M. Wolf (Wiley-VCH, Weinheim, 2010); A. Melnikov {\it et~al.}, J. Phys. D: Appl. Phys. \textbf{41}, 164004 (2008).   

Ultrafast magnetization dynamics in a system with tunable angular momentum

Many peculiarities of the magnetization dynamics are related to the fact that a certain amount of angular momentum is associated with magnetic moment. Here the dynamics of angular momentum is studied in ferrimagnetic rare-earth -- transition metal alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. Depending on their composition, such ferrimagnets can exhibit a magnetization compensation temperature TM where the magnetizations of the sublattices cancel each other and similarly, an angular momentum compensation temperature TA where the net angular momentum vanishes. At the latter point, the frequency of the homogeneous spin precession diverges. As a consequence, ultrafast heating of a ferrimagnet across its compensation points may result in a subpicosecond magnetization reversal [1]. Moreover, we have experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field [2,3]. This optically induced ultrafast magnetization reversal is the combined result of laser heating of the magnetic system and circularly polarized light acting as a magnetic field with amplitudes of up to several Teslas. The direction of this opto-magnetic switching is determined only by the helicity, i.e. angular momentum, of light. This novel reversal pathway (see [4]) is shown to crucially depend on the net angular momentum reflecting the balance of the two opposite sublattices. \\[4pt] [1] C.D. Stanciu et al., Phys. Rev. B 73, 220402 (2006); Phys. Rev. Lett. 99, 217204 (2007). \\[0pt] [2] C.D. Stanciu et al, Phys. Rev. Lett. 99, 047601 (2007). \\[0pt] [3] A. Kirilyuk, A.V. Kimel, and Th. Rasing, Rev. Mod. Phys. 82, 2731 (2010). \\[0pt] [4] K. Vahaplar et al., Phys. Rev. Lett. 103, 117201 (2009).