Study on gilbert damping in ferromagnetic ultrathin films by all optical method

Abstract

Perpendicular magnetic anisotropy (PMA) architectures are desired to improve memory density and to overcome the thermal stability problems in spin-transfer torque magnetic random access memory. The PMA architecture typically has a smaller switching current density Jc because it is free from demagnetization fields. However, PMA materials typically have larger Gilbert damping constants than typical in-plane ferromagnetic materials, leading to larger values of Jc because Jc is proportional to the Gilbert damping constant α. Therefore, the determination of the Gilbert damping constant of PMA materials is important. Furthermore, it is widely accepted that α is closely related to the spin-orbit interaction, which is also a main source of PMA. Therefore, the relationship between the Gilbert damping and other magnetic properties has been widely investigated. In this thesis, we have been studied the relationship between the Gilbert damping and other magnetic properties, such as stack number, magneto-crystalline anisotropy, and exchange bias field, at FePt, Co/Ni multilayer, and IrMn/Cu/CoFe thin films. We have investigated the Gilbert damping constant in ferromagnetic films using the time-resolved magneto-optical Kerr effect (TR-MOKE) system. From the spin dynamics in the L10-ordered FePt thin film is investigated by using an all-optical method. We get the resonance frequency f = 15.6 GHz and the effective Gilbert damping constant αeff = 0.156 which are 3 times and 10 times larger than the value of Permalloy. We systematically studied the intrinsic Gilbert damping constant in perpendicular magnetic anisotropy Co/Ni multilayer system by means of an all-optical method. We find that the intrinsic Gilbert damping constant does not depend on the stack number and the perpendicular magnetic anisotropy when the magnetic field is high enough. In contrast, the extrinsic Gilbert damping is strongly correlated with the inhomogeneous anisotropy distribution in the low-field regime...