Spin and Orbital Moment Dynamics in Magnetic Materials
Andreas Scholl
Lawrence Berkeley National Laboratory.
Ultrafast magnetization dynamics using field and optical pulses has caught the interest of scientists because of the fascinating physics underlying the exchange of energy and angular momentum in a magnetic material and because of possible applications in data storage. However, experiments in ultrafast magnetism put harsh requirements onto the experimental technique, both in terms of temporal and of spatial resolution. The availability of pulsed x-ray sources, fast detectors, and high resolution microscopes have now opened the door to such studies using x rays as a probe.
First, I will introduce time-resolved imaging of magnetic patterns with 100 nm spatial resolution using the Photoemission Electron Microscope PEEM-2 at beamline 7.3.1.1 of the Advanced Light Source [1]. The bunch length of the storage ring sets the time resolution of 80 ps. It was observed that the chirality or handedness of the vortex, which is determined by the out-of-plane magnetization of the nanometer-size vortex core, governed the dynamics of the structure in response to fast field pulses.
On a faster time scale, magnetization dynamics experiments rely on direct laser excitation of the material. Experiments using the time-resolved magneto-optical Kerr effect (TR-MOKE) [2] demonstrated the possibility of manipulating magnetism using a femtosecond laser pulse.
The origin and the implications of the fsec-laser-driven demagnetization of Ni have been widely debated. X-ray magnetic dichroism is an ideal tool to probe such dynamics in complex materials because x-ray sum rules quantify spin moment, orbital moment and magnetic anisotropy of each element in a multi-element system. I will present measurements of the transient spin and orbital moment of Fe/Gd following a f-sec laser excitation. It was observed that the Fe orbital to spin moment ratio m_l /m_s stayed constant during the demagnetization, indicating a thermalized spin and orbital moment. Fe and Gd demagnetized at similar rates because of the strong exchange coupling between the layers. These experiments were conducted using an x-ray streak camera at BL 4.0.2 of the Advanced Light Source.
This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.
[1] S.B. Choe et al., Science 304, 420 (2004)
[2] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996)
