Laser-induced magnetization dynamics in a cobalt/garnet heterostructure

We report on magnetization dynamics triggered in a Co/garnet heterostructure by femtosecond laser pulses. Although laser excitation of a bare Co-doped yttrium iron garnet leads to magnetization precession, the phase of which strongly depends on the linear polarization of the light pulses, the deposition of an ultrathin Co layer on a top of a garnet film results in substantial changes of the laser-induced dynamics. The precession in the garnet is shown to lose its sensitivity to the polarization. Instead, light triggers polarization insensitive precession in both the magnetostatically coupled Co layer and the garnet film at two distinct frequencies typical for Co and garnet layers. Copyright c © EPLA, 2014 Introduction. – Manipulation of magnetization with the help of femtosecond laser pulses is a hot topic in fundamental science [1–4]. Understanding optical control of the magnetism in magnetic heterostructures is a particularly important issue for further development of faster magnetic information storage/processing and spintronic nanodevices. Optical control of spins in Co/SmFeO3 heterostructures has been recently demonstrated using X-ray photoemission electron microscopy, revealing that the dynamics of the spins in the metallic Co and the dielectric SmFeO3 are correlated [5]. However, since the time resolution of these experiments was limited by the duration of the X-ray pulse (about 70 ps), these findings have raised the question whether the correlations between metallic and dielectric layers in such heterostructures are also present at the sub-100 ps time scale. To investigate this issue we studied a similar system consisting of a dielectric yttrium iron garnet (YIG) film and an ultrathin Co layer. It is known that in such a heterostructure one can observe a strong influence of a 2 nm Co layer on both the domain structure and magnetization (a)E-mail: [email protected] reversal processes in the garnet, due to magnetostatic coupling [6]. A priori, one can anticipate several mechanisms which can be responsible for possible correlations between the magnetization dynamics in the Co and garnet layers of such a heterostructure. For instance, it is known for a metal/dielectric heterostructure that spin-orbital interaction may initiate a transfer of angular momentum between the layers, and thus cause correlations in the magnetization dynamics [7]. In addition, it can occur as a consequence of exchange coupling, such as has been found in a Py/YIG bilayer [8]. The goal of the research reported in this paper is to understand whether the ultrathin Co film also affects laser-induced magnetization dynamics in the garnet film. Here we compare laser-induced magnetization dynamics in a cobalt/garnet heterostructure with that observed for the bare garnet thin film. Although optical excitation of a bare garnet leads to magnetization precession, the phase of which strongly depends on the polarization of the pump light, the deposition of an ultrathin Co layer on top of the garnet film is shown to lead to substantial changes of the laser-induced dynamics. The precession becomes insensitive to the polarization of the laser pulse and occurs