Distinguishing the laser-induced spin precession excitation mechanism in Fe/MgO(001) through field orientation dependent measurements

Abstract

Rotational field dependence of laser-induced magnetization precession in a single-crystal Fe/MgO(001) sample was studied by the time resolved magneto-optical Kerr effect. Polar and longitudinal magnetization components were separated by measuring precession dynamics under opposite fields. When the applied field is weaker than the anisotropy field of an Fe film, the precession amplitude is small for the field direction near the easy axis and becomes larger as the field rotates towards the hard axis, showing a four-fold symmetry in agreement with the in-plane magnetic anisotropy; whereas at higher fields, the amplitude displays a drop near the hard axis. Such precession behavior can be well reproduced using an excitation model with rapidly modified but slowly recovered magnetic anisotropy and considering the elliptical precession trajectory. Our results indicate that the dominant mechanism for triggering Fe spin precession is the anisotropy modulation correlating with the lattice thermalization, rather than the transient anisotropy modulation due to the high electron temperature within 1 ps.

Ultrafast changes of magnetic anisotropy driven by laser-generated coherent and noncoherent phonons in metallic films

“…temperature rise) is dominant in agreement with previous observations [7][8][9][10][11]. The thermal tilt of Beff keeps it in the film plane, thus 0 should be close to zero at any B, which is observed experimentally in Fig.…”

“…In accordance with the model proposed by van Kampen et al [18] and followed by a number of works [7][8][9][10][11]19], we assume that the femtosecond laser pulse launches the magnetization precession by modifying the magnetic anisotropy and, thus, tilting the effective magnetic field Beff.…”

“…Due to electron-phonon interaction this process takes place on subpicosecond time scale and often may be considered as an instantaneous rise of the lattice temperature T. The increase of T affects the values of the MCA parameters and Beff tilts from its equilibrium position. Such a scenario has been considered in a number of previous works and may be attributed to the modulation of MCA by non-coherent phonons generated as a result of optical excitation [7][8][9][10][11].…”

“…Contrary to earlier works [7][8][9][10][11]13,18,19], where the optically induced precession was studied, we use a ferromagnetic film with low symmetry orientation. It is important that we use the experimental geometry, which allows us to exclude the ultrafast demagnetization as a stimulus for the excitation of magnetization precession [7,18,19] leaving only the changes of MCA as the main mechanisms for launching the precession.…”

“…[14,15] and Ref. [7][8][9][10][11] respectively, but there is no way to compare quantitatively the contributions of these mechanisms to changes of the MCA as a result of direct optical excitation.…”