Magnetization Reversal And Spin Dynamics In Magnetic Films

Spin dynamics of magnetic nanostructures is one of the intense research fields in spintronics,whose magnetic relaxation governs the magnetization switching time and the critical current density in spintronic devices.In the recent two decades,there have been tremendous interests in the pure spin current-related spintronic devices due to no Joule heating created by charge transport.The devices necessitate a deeper understanding of the pure spin current-related magnetic relaxation.In this thesis,multilayer films composed of nonmagnetic and magnetic layers were prepared by molecular beam epitaxy?MBE?.By means of microwave and pulsed laser triggering spin pumping of the multilayer films,we have studied the orientation dependence of the pure spin current-related relaxation,and the relaxation of collective precession.The main researches are as follow:1.Epitaxial Pd/Fe bilayers were grown on MgO?001?substrates via MBE.Vector Network Analyzer ferromagnetic resonance?VNA-FMR?measurements were performed to characterize the magnetic relaxation of the Pd/Fe bilayers.The non-local Gilbert damping induced by spin pumping is demonstrated by the Gilbert damping dependence on Pd thickness.We verified the emergence of the magnetic dragging effect except magnetic field along hard or easy axis based on numerical calculations.The magnetic dragging effect can result in the anisotropic linewidth and the nonlinear linewidth versus frequency relations.Taking both magnetic dragging and extrinsic contributions into account,we unambiguously demonstrated that magnetization orientation has a negligible contribution to the non-local Gilbert damping based on both theoretical and experimental results,revealing that the spin transport across the interface of Pd?100?[110]/Fe?001?[100]is independent of the Fe magnetization orientation.We proposed to explain the experimental observations in view of the Kambersky s theory and the Elliott-Yafet mechanism.2.Another Fe layer was deposited on the epitaxial Pd/Fe bilayers via MBE.We measured the FMR spectra of the pseudo-spin values Fe/Pd/Fe with various Pd thicknesses and reported the modification of the Gilbert damping related to the exchange interaction in reference to Pd/Fe bilayers.The exchange coupling between two Fe layers hinges on the Pd insertion layer thickness.The Gilbert damping parameters between interlayer coupling and interlayer decoupling have a significant difference.We observed a damping enhancement of the optical mode and a nearly constant damping of the acoustic model.The two Fe layers magnetically decouple by the thick Pd spacer layers,and only dynamically couple via spin currents emitted by the spin pumping.One of Fe layer is excited by microwave,while the other Fe layer?off resonant?and Pd layer adopt roles as spin sinks.3.Time resolved magneto-optical Kerr effect?TRMOKE?has been utilized to investigate optically induced magnetization precession and magnetic relaxation.We explicitly formulate the correlation between the Gilbert damping and the effective damping in the optical approach.The non-local Gilbert damping induced by spin pumping is demonstrated by the Gilbert damping dependence on Pd and Fe thickness.Furthermore,the spin pumping-related parameters,i.e.the interfacial spin mixing conductance g^??=?2.7±0.4?×10¹⁹m^-2 and spin diffusion length of Pd ?_SD=?7.4± 3.0?nm,are self-consistently determinated.Apart from the researches of magnetic relaxation,we also investigated the magnetization reversal processes of GdFeCo film and Pd/wedged Co/Pd film nearby the spin reorientation transition.The researches include the following parts:1.An amorphous GdFeCo film has a temperature-driven spin reorientation transition?SRT?.Its magnetization reversal processes have been characterized by the hysteresis loops and first-order reversal curve?FORC?in the vicinity of the SRT temperature.Along with the SRT from out-of-plane to in-plane,the total irreversible components decrease with increasing temperature and the reversal process gradually changes from the pinning-type dominated reversal to the nucleation type reversal,and then to the uniform rotation.2.A Pd suffer layer was evaporated on the annealed MgO?111?substrate using MBE,and then the wedged Co layers were deposited via a motor driven shutter.Finally,the sample was covered by Pd as capping layers.We observed the Co thickness driven SRT from out-of-plane to in-plane.By means of the magneto-optical Kerr effect and FORC,we analyzed the different types of magnetization reversal in the continuous SRT process,which are related to the formation of different magnetic compositions.