Date on Master's Thesis/Doctoral Dissertation

12-2017

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Physics and Astronomy

Degree Program

Physics, PhD

Committee Chair

Smadici, Serban

Committee Member

Jayanthi, Chakram

Committee Member

Sumanasekera, Gamini

Committee Member

Cohn, Robert

Author's Keywords

all-optical switching; magnetization; magneto-optical effects; ultrafast demagnetization; pump-probe experiment; time-resolved study

Abstract

The unexpected observation of ultrafast demagnetization (UDM) and subsequent time-resolved studies of laser-induced magnetization switching opened a new door to both fundamental physics and technological applications of magnetic materials. All-optical switching (AOS) can be initiated faster than the precession limit, hinting to its potential in increasing the writing speed and data storage density. However, notwithstanding considerable research interest, the mechanism of AOS in ferromagnetic materials remained unclear.

Ferromagnetic superlattices were deposited on glass substrates with e-beam evaporation and sputtering. Magnetization curves were measured in magneto-optical and vibrating sample magnetometer experiments. A femtosecond Ti:S laser was utilized in a writing setup to induce AOS in Co/Pd ferromagnetic superlattices at different fluences and beam polarizations. Magnetic force and polarizing microscopy were applied to image the magnetic structure and identify optimal AOS parameters.

High-repetition rate pulses of the Ti:S laser resulted in heat accumulation in the samples. For a better understanding of the relation between temperature and domain wall motion, we solved the heat diffusion equation numerically. The solution for our sample displayed a tilted thermal wave front, consistent with the tilted magnetic domains observed. This supports our model of AOS, in which thermal forces acting on domain walls leads to their expansion and magnetization switching (chapter 3).

Polarizing microscopy images also revealed a complementary pattern of magnetic domains after laser writing, suggesting that demagnetizing fields are not negligible. Furthermore, comparison of pump-probe UDM measurements with AOS writing measurements pointed to a demagnetized state before AOS emergence. This motivated us to apply micromagnetic simulations to investigate the time evolution of a demagnetized state and in particular the role of demagnetizing fields in the development of different final states. We show using this method that demagnetizing fields can nucleate and, together with thermally-induced forces, develop a switched state (chapter 4).

Using the pump-probe setup, we measured the frequency dependence of laser-induced temperature modulations in Co/Au, Co/Ag and Co/Pd superlattices on glass substrates. Green's function solutions of the heat diffusion equation show that a glass layer with properties distinct from the glass substrate is present near the metallic superlattices (chapter 5).

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