Ultrafast studies on the coupling of magnetic and structural dynamics in solid-state ferromagnets

Abstract

The thesis describes primarily two experiments which investigate the coupling of structural and magnetic properties in solid-state ferromagnets on an ultrafast ($\approx$100~fs) timescale. To achieve this goal, both studies use ultrafast optical and x-ray pump-probe methods in combination, integrating information about magnetic and structural dynamics to form a more complete picture of the underlying physics. In the first experiment, the well-known shape-memory ferromagnetic Heusler alloy $\text{Ni}_2\text{MnGa}$ is studied. This compound is a multiferroic material stabilised by the correlations between electronic, magnetic, and structural order. The shape-memory effect is based on a structural phase transition between a low-temperature martensite phase and a high-temperature austenite phase. In both phases, the material is ferromagnetic. It has been observed for different stoichiometries that the martensite phase exhibits slightly incommensurate structural modulations whenever the shape-memory effect is present, but the exact connection between the two is unclear. We performed ultrafast optical measurements of the magnetisation of $\text{Ni}_2\text{MnGa}$, using 100~fs, 800~nm pulses for both the pump and the magneto-optical Kerr effect probe. We found a rapid demagnetisation with a timescale of $320\pm50$~fs, followed by a very slow recovery. At the FEMTO slicing source, we performed ultrafast x-ray diffraction on the weak (20201) satellite reflection of the (202) Bragg peak, which is a measure of the structural modulation. At high fluences of the 800~nm near-IR pump, we were able to observe a rapid and almost complete suppression of the structural modulation on a timescale of $300\pm40$~fs, very similar to the behaviour of the magnetisation. However, the longer timescale dynamics are markedly different from the one observed in the magnetisation: we observed an intermediate recovery of the structural modulation, before a second drop was caused by the thermal phase transition to the high-temperature austenite structure, which does not support the structural modulation. Our conclusion is that magnetisation and structural modulation are not strongly coupled in $\text{Ni}_2\text{MnGa}$, as the structural modulation was able to recover even while the material was demagnetised. The second experiment studied the movement of the lattice during ultrafast demagnetisation of a thin single-crystal iron film. While ultrafast demagnetisation is a very well-established effect, its microscopic mechanism remains unclear, especially with respect to the flow of angular momentum. Our goal was therefore to find a signature of the ultrafast Einstein--De Haas effect, i.e. the flow of spin angular momentum from the magnetisation to the mechanical angular momentum of the lattice. We identified that transverse shear waves would be launched from the surfaces of a rapidly demagnetising crystal, with an initial deflection whose direction depends on the magnetisation. A double-grazing x-ray diffraction geometry to measure the crystal truncation rod (CTR) of an in-plane and in-surface Bragg peak of the iron was chosen for the experiment. This choice maximises the sensitivity to the transverse strain wave and simultaneously minimises sensitivity to longitudinal strain waves caused by laser heating. The ultrafast pump-probe experiment was performed at the XPP endstation of the LCLS free-electron-laser, using a 6.9~keV x-ray probe beam and a 800~nm near-IR pump. The intensity along the (2~2~L) CTR was recorded simultaneously for each shot by using an imaging detector, with L in the range 0.02--0.09. Measurements were made for two opposite directions of magnetisation by selecting one of the two polarities of the sample electromagnet for each shot. By taking the difference of the time-resolved diffraction intensities for the two magnetisation directions along the CTR, we were able to positively identify a transverse mechanical strain wave in the data. Comparison with a detailed lattice dynamics and diffraction model allowed us to extract the timescale of the appearance of angular momentum in the lattice as around 200~fs, and the magnitude of angular momentum as corresponding to 8\% of the saturation magnetisation of iron. Using magneto-optical data for calibration, where we observed a 10\% demagnetisation under the same conditions, we conclude that a majority of about 80\% of the angular momentum lost from the spin system appear as mechanical angular momentum in the lattice; a complete transfer is also plausibe with the given experimental uncertainties. This shows that the interaction with the lattice plays an essential role in the ultrafast demagnetisation of iron.