Ultrafast tuning of magnetization precession and magnetic anisotropy in thin iron films
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1230-MM04-10
Ultrafast tuning of magnetization precession and magnetic anisotropy in thin iron films E. Carpene1, E. Mancini1, D. Dazzi1, C. Dallera1, E. Puppin2 and S. De Silvestri1 1
ULTRAS, CNR-INFM, Dipartimento di Fisica, Politecnico di Milano, p.zza Leonardo da Vinci 32, 20133 Milano, Italy 2 CNISM, Dipartimento di Fisica, Politecnico di Milano, p.zza Leonardo da Vinci 32, 20133 Milano, Italy
ABSTRACT We have developed an experimental set-up based on time-resolved Magneto-Optical Kerr Effect (MOKE) that allows to retrieve the vectorial magnetization dynamics in thin films with sub-picosecond resolution. This method has been exploited to measure the variations of the magnetization (modulus and orientation) induced by an ultrashort laser pulse. The initial demagnetization is established at the electronic level within a few hundreds of femtoseconds through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 picosecond time-scale, around an effective, time-dependent field. Following the full dynamics of the magnetization, we have unambiguously determined the temporal evolution of the magneto-crystalline anisotropy, providing the clear experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective field. This method provides a simple and widely applicable way to study both magnetization and anisotropy in the sub-picosecond regime and therefore to unravel the mechanisms underlying the ultrafast evolution of the spin order in magnetic media.
INTRODUCTION A free magnetic moment misaligned with respect to a magnetic field undergoes a precessional motion known as Larmor precession. This motion is due to the torque the field exerts on M and it originates from its intrinsic angular momentum. The period of the precession depends on the intensity of the field and lies in the picosecond regime for magnetic fields of the order of a tesla. This time scale attracts technological interest on ferromagnetic materials, since the precession mechanism might provide a possible way to switch the magnetization, enhancing the speed of magnetic recording devices [1]. The precessional motion can be triggered by an ultrashort magnetic field pulse [2,3] or by an ultrashort and intense laser pulse acting as a heat source that modifies the magnetic anisotropy, thus leading to a rapid re-orientation of the magnetization [4,5]. We have set up a time-resolved magneto-optical Kerr effect (MOKE) experimental configuration that, without modifying neither the sample position nor the detection geometry, allows to retrieve quantitative sub-picosecond information about modulus and orientation of the magnetization in an epitaxial iron film, following an intense infrared laser pulse. Our quantitative approach allows to determine the heat-induced dynamics of the anisotropy field and provides the experimental evidence of the mechanism that launches the precession.
EXPERIMENTS The experiment was carried out at ro
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