Non-equilibrium States of Matter: Ultrafast Dynamics and Phase Transitions

Licentiatavhandling, 2025

Although increasingly accessible by experiments and present in nanotechnology, non-equilibrium dynamics still pose challenges for theoretical physics. That includes the description of ultrafast demagnetization processes, magnetic phase transitions, and induced magnetization. This thesis is a contribution to the advancement of coarse-grained mesoscopic descriptions of non-equilibrium dynamics of magnetization.

Thermodynamics provides a framework to include fluctuations and coarse-grained dynamics. We extend it to non-equilibrium collective phenomena on subpicosecond timescales, although it was originally developed for slower timescales, and introduce the concept of ultrafast thermodynamics. We use three examples to demonstrate its potential.

First, the entropy production in ultrafast magnetization is considered using the Landau-Lifshitz-Gilbert equation, and the path-integral formalism is applied to it. This yields the entropy production that strongly depends on the nutation of the magnetization.

Second, we use entropy production as a measure of Markovianity in magnetization dynamics. Using three different Landau-Lifshitz-Gilbert equations (without nutation, with nutation, with nutation and higher orders), we numerically compute their entropy production rates. Negative entropy production rates indicate non-Markovian dynamics, making them a good measure of the Markovianity.

Third, we investigate the phase transition into quadrupole ordering, observed slightly above the magnetic transition. Using a field-theoretical approach, we find the transition temperatures and coupling to strain, explain why quadrupole order is visible only in anisotropic materials, and derive an analytical expression for the transition temperatures.

Finally, this thesis includes a review of recent theories of the phono-magnetic effect, a non-equilibrium magnetization phenomenon subject to significant debate. Altogether, the work broadens the theoretical foundations for understanding ultrafast non-equilibrium magnetization dynamics.