Date on Master's Thesis/Doctoral Dissertation

12-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Smadici, Serban

Committee Member

Mendes, Sergio

Committee Member

Yu, Ming

Committee Member

Liu, Jinjun

Author's Keywords

Ultrafast demagnetization dynamics; magnetic entropy dynamics; MOKE technique; time-resolved pump-probe experiment

Abstract

It has been observed that ultrathin films, multilayers, or magnetic nanostructures indicate novel magnetic phenomena that differ profoundly from the respective bulk properties. Besides, because of the broad applications of these magnetic materials in the industry, they are an exciting research area. Hence, investigating the low-dimensional magnetic systems is one of the most active fields in experimental condensed matter physics. Magnetization dynamics can occur over a wide range of time scales (from seconds to femtoseconds). Some of these processes even occur on time scales as short as a few picoseconds (10-12s) or femtoseconds (10-15s). Measurement of these fast processes is essential to fully understand the dynamics of various excitations in the magnetic thin films and multilayers, which has motivated significant advancements in the accuracy of time-resolved measurements in the past several decades. Development of ultrafast femtosecond pulsed laser sources and pump-probe techniques enabled measurements of the dynamics of these fast processes, specifically ultrafast spin dynamics in magnetic materials. Entropy is vital in various fields in thermodynamics and statistical mechanics. Investigating and measuring the nonequilibrium magnetic entropy dynamics of a ferromagnetic material, in addition to the magnetization dynamics, is of great interest because entropy is crucial for fully characterizing the nonequilibrium magnetic state. The first part of this work is dedicated to the ultrafast time-resolved measurement using a pump-probe technique to measure transient features on a 35 nm Co/Au sample induced by an intense pump beam with 190 fs pulses. A new method is developed to modulate the delay between the pump and the probe pulses. A delay modulator, made of glass plates rotating in a pump beam, is applied to measure time resolved rates of transient processes in a pump-probe experimental setup. Glass plates modulate both delay and beam power. Delay modulation results obtained with this procedure are consistent with the derivative in the delay of the usual power modulation and can show a more significant signal-to-noise ratio. Next, the focus was on magnetization and magnetic entropy dynamics measurements of ferromagnets simultaneously. We are the first group to report it. A double probe beam polarization and pump power modulation configuration were applied to measure the magnetization and magnetic entropy dynamics in ultrafast demagnetization of ferromagnets. Two magnetic properties, obtained by subtracting measurements at the opposite fields, have the same onset at small delays and different relaxations at large delays. The different dynamics suggest that spin relaxation in ultrafast demagnetization is a combination of two processes. Measurements of ultrafast magnetization and magnetic entropy relaxation rates were also made on a 100 nm Co/Ag at different equilibrium temperatures obtained with heat accumulation. The relaxation rate of a state in internal equilibrium was determined by the curvature of its thermodynamic potential surface at the equilibrium point. This model explains the observed magnetization and magnetic entropy relaxation rates different dependence on equilibrium temperature.

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