Pump-Probe Measurement Techniques for Determining Curie Temperature Distribution and Gilbert Damping
Creators: name: Zhengkun Dai
Publisher: name: Carnegie Mellon University
Year: 2020
Subjects: subject: 90699 Electrical and Electronic Engineering not elsewhere classified
subject: FOS: Electrical engineering, electronic engineering, information engineering
subject: FOS: Electrical engineering, electronic engineering, information engineering

description: This doctoral thesis presents two experimental studies utilizing the pump-probe techniques with pulsed laser systems to measure (1) Curie temperature distribution in granular FePt-L10 magnetic thin film for heat assisted magnetic recording (HAMR) applications and (2) Gilbert damping constants, with the ability to distinguish interfacial and bulk contributions in magnetic multilayer structures.<br>For the first study, the granular thin film sample consists of a two-dimensional closely packed FePt metallic grains with non-magnetic carbon grain boundaries. A laser pulse, of 1064 nm wavelength and 700 ps pulse duration, is used to heat a localized area uniformly in a controlled magnetic field. The magnetization of the grains after the heating is probed utilizing Kerr effect. By varying the laser pulse energy, the heating temperature of the sample can be varied with accurate precision in controlled manner. By heating/cooling the sample across the Curie temperature at different magnetic fields, Curie temperature distribution of the grains can be characterized with precision. This method was published and widely adopted by the recording industrial. In the thesis, all the detailed experimental setups and considerations, as well as experimental measurement procedures are clearly described. The idea of utilizing magnetic oxide grain boundary to reduce Curie temperature distribution is proposed and proved in experiment. The pump-probe method is extended to measure the temperature dependence of anisotropy field around Curie temperature.<br>For the second study, a different pulsed laser system was built using an ultrafast laser with pulse width of 80-100fs for examining the magnetization dynamics in multilayered magnetic thin films. In this scientific investigation, the pump pulse is used to excite the magnetic moment of the sample away from equilibrium and a probe pulse with varying time delay is used to measure the magnetization precession and damping motions that immediately follows excitation. Micromagnetic dynamic modeling is established to analyze the dynamic magnetization orientations to extract the ferromagnetic resonance frequency and the Gilbert damping constant. The developed method is capable of measuring magnetization dynamics in gigahertz to sub-terahertz range. Using this experimental setup, Co/Pt multilayered magnetic thin films with perpendicular anisotropy have been studied. It is found that in this system, there are two distinctive contributions to the overall Gilbert damping of the multilayers: One from interface and the other one from bulk. When the magnetic layers having thickness in nanometer scale, the interfacial damping dominates. Comparison between sandwiched magnetic thin films with Co as magnetic layer and Pt/Ir as nonmagnetic layers shows the nonmagnetic material and the interfacial coupling determine the overall damping together. <br>