Static and Dynamic Magnetic Properties of Exchange-coupled Thin Film Systems

by Bin Zhang

Institution: Freie Universität Berlin
Department: FB Physik
Degree: PhD
Year: 2014
Record ID: 1099251
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000097825


This thesis focuses on the coupling and dynamics of magnetic films. Epitaxial FM/Mn/FM [FM (ferromagnetic): Ni, Co] trilayers with a varied thickness (wedged) Mn layer were grown on Cu(001) and studied by magneto-optical Kerr effect measurements. The bottom FM film as well as the Mn film exhibit a layer-by-layer growth mode, which allows to modify both interface roughnesses on the atomic scale by tuning the thicknesses of the films to achieve a certain filling of their topmost atomic layers. For the Co/Mn/Co systems, the onset of antiferromagnetic order in the Mn layer at room temperature was found at a higher thickness (tAFM) for a filled compared to a half-filled topmost atomic layer of the bottom Co layer. Magnetization loops with only one step were found for half-filled topmost atomic layer of the bottom Co film, while loops with two separate steps have been observed in trilayers with an integer number of atomic layers in the bottom Co film. The small coercivity of the top Co film shows an oscillation with 1 ML period as a function of the Mn thickness above 10 ML, which is interpreted as the influence of the atomic-scale control of the interface roughness on the interface exchange coupling between the antiferromagnetic Mn and the top ferromagnetic Co layer. The strength of the magnetic interlayer coupling between the top and bottom Co layers through the Mn layer for an integer number of atomic layers in the bottom Co layer exhibits an oscillation with a period of 2 ML Mn thickness, indicative of direct exchange coupling through the antiferromagnetic Mn layer. A long-period interlayer coupling of the two FM layers with antiparallel coupling is observed and attributed to indirect exchange coupling of the Rudermann-Kittel-Kasuya-Yosida type. After deposition of Ni on Mn wedge/atomically filled Co film, the coercivity is enhanced compared to Mn/Co or Co/Mn/Co samples. In the Ni/Mn/Ni trilayer, the top Ni layer also shows a higher coercivity than the bottom layer above 8 ML Mn thickness. Comparing the medium energy electron diffraction oscillations during growth of Mn on Ni and Ni on Cu(001) shows that the surface roughness is reduced by the Mn layer. This indicates that the coercivity of Ni/Mn depends on the interface roughness, i.e., a smoother interface shows a higher coercivity. The magnetization dynamics of Co wedge/Ni bilayers induced by fs laser pulses close to the spin-reorientation transition (SRT) between out-of-plane and in-plane magnetization has been investigated by photoelectron emission microscopy with pump-probe measurements. The response of the magnetization of the Co layer close to the SRT shows a slower recovery after demagnetization comparing to the region far away from SRT, probably due to a lower Curie temperature. Diese Arbeit befasst sich mit der Kopplung und der Dynamik von magnetischen Schichtsystemen. FM/Mn/FM [FM (Ferromagnet): Ni, Co] Dreilagensysteme mit einer Mn-Lage variable Dicke (Keilform) wurden epitaktisch auf Cu(001) aufgewachsen und mit Hilfe des magneto-optischen…