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Transient Grating Spectroscopy of Magnetic Thin Films

Jansma, T. (2016) Transient Grating Spectroscopy of Magnetic Thin Films. Master's Thesis / Essay, Applied Physics.

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Materials that exhibit coupling between elastic and magnetic degrees of freedom are of both fundamental and technological interest. Such `magnetoelastic' materials possess great potential for tuneable multifunctional devices and show opportunities for application in novel types of magnetic data storage, in which the generation of elastic waves lead to precessional switching of the magnetic orientation. In this report we use an all-optical approach to investigate the magnetoelastic properties of several ferromagnetic thin films. The `transient grating' (TG) geometry is employed, in which the interference of two intense, spatially and temporally overlapped pump pulses are used to excite a `transient' grating on the sample surface. This leads to the excitation of frequency tunable (surface) acoustic waves and, using a weaker probe pulse, the setup allows us to witness simultaneously both the acoustic waves and their coupling to the sample magnetization in real-time at sub-picosecond resolution. The sensitivity of the magnetization detection scheme in the TG setup is closely linked to temperature-induced partial demagnetization of the sample. The primary goal of this work is to understand the details of the temperature dynamics after optical excitation and how they affect the magnetization dynamics. In order to fully reconstruct the magnetization in time, it is therefore necessary to carefully consider the temperature dynamics inside the sample after excitation. Numerical simulations are performed to calculate the temperature dynamics with picosecond time resolution. The result of the simulations are used to correct the amplitudes of the magnetization precession of experimental data sets appropriately, revealing the `real' magnetization dynamics as a function of time. As a secondary goal, we began a new experimental activity to study the magnetoelastic effects in the strongly magnetostrictive material `Terfenol'. The results show multiple fundamental departures from previous measurements on nickel films. Whereas the response in nickel showed the resonant elastic driving of magnetization precession, in Terfenol, the most striking effect is the apparent complete, field-periodic, suppression of elastic wave propagation. Understanding of the remarkable results for Terfenol is still limited, and additional experimental work is needed as well as a clear theoretical framework. Nonetheless, these effects are remarkable in their ability to control thermal expansion and strain generation using magnetic fields.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Applied Physics
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 08:30
Last Modified: 15 Feb 2018 08:30

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