Element-Selective Ultrafast Magnetization Dynamics with a Tabletop Light Source

摘要

Next-generation hard-disk drives will require smaller magnetic bits and faster magnetization switching; hence, better understanding of nanoscale magnetic material is one of the key factors in developing of these devices. Here, I present the first ultrafast magnetization dynamics studies by use of extreme ultraviolet radiation from a tabletop high-harmonic generation source. This new probing technique offers three advantages over conventional ones: ultrafast time resolution, element selectivity, and the tabletop size.I report three experiments showing that high harmonics are a powerful tool for probing magnetization in magnetic materials. First, our group measures simultaneously the magnetizations of Ni and Fe in Permalloy using the transverse magneto-optical Kerr effect. Second, we study laser-induced demagnetization dynamics in two ferromagnetic alloys: Permalloy and Permalloy-Cu. Contrary to a common expectation that the dynamics in strong exchange-coupled alloys would be identical, we discover that the magnetization of Fe decays earlier than that of Ni during the first 60 fs. To explain this delay, we propose a simple model incorporating a finite exchange-time factor into the magnetization rate equations. Finally, to confirm the observed sequence of dynamics in alloys, we conduct the magnetization study of elemental Fe and Ni with identical experimental conditions. The results indicate that the order of demagnetizations in the elemental forms is the same as that in Permalloy: Fe demagnetizes faster than Ni does.