What is the ultimate timescale for magnetization switching?
Prof. Theo Rasing, Dr. Alexey Kimel
Radboud University Nijmegen, Netherlands
Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for magnetic recording industry. Within a collaborative effort involving Radboud University Nijmegen (Netherlands), University of York (UK) and Nihon University (Japan) we have recently shown that it is indeed possible to manipulate the magnetic order on ultrashort time scales pertinent to exchange interaction. In particular, we have shown that upon fs laser excitation the ultrafast spin reversal of GdFeCo - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state . Such unexpected transient state occurs due to different dynamics of Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe.Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporary switch from an antiferromagnetic to ferromagnetic type of ordering. These observations supported by atomistic simulations , present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction.
What are the channels of ultrafast angular momentum transfer?
Prof. C. Boeglin/Dr. E. Beaurepaire/ Prof. J-Y. Bigot
Institute of Physics and Chemistry of Materials of Strasbourg (IPCMS), France
The essence of magnetization is angular momentum. Thus any change in magnetization will implicitly be accompanied by an angular momentum transfer from or to spin subsystem and eventually redistributed among the other degrees of freedom of the system. A very intriguing question is how and how fast such a transfer of angular momentum will take place upon, for instance, ultrafast demagnetization of a magnetic sample. Within a collaboration with Strasbourg University we have addressed this question for the case of CoPt alloy. Using XMCD as a probing tool we could disentangle and follow in real time the transient dynamics of the spin and orbital magnetic moments during demagnetization of the CoPt sample. The results conclusively show that orbital degree of freedom reacts first to the intense fs laser excitation and the spin follows with a certain time delay.
Ultrafast demagnetization of pure 4f elements and their alloys
Prof. Uwe Bovensiepen, Duisburg-Essen University
Prof. Martin Weinelt, Free University Berlin
So far, most of the studies on ultrafast magnetization dynamics were focused on 3d itinerant ferromagnetic systems. However, the dynamic investigations of the 4f elements, possessing large and strongly localized magnetic moments, remained sparse. Together with our colleagues from University Duisburg-Essen and from the Free University Berlin we have investigated the time-dependent magnetic moment of 4f electrons in the ferromagnets Gd and Tb. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to 50 times compared to equilibrium, as revealed by the fast femtosecond time scale. The slower picosecond time scale is determined by the equilibrium spin-lattice coupling following the 4f occupation.