Direct optically driven spin-charge dynamics govern the femtosecond response of ferromagnets

Ferromagnetic materials have strong electron correlations that drive quantum effects and make the physics that describes them extremely challenging. In particular, the electron, spin, and lattice degrees of freedom can interact in surprising ways when driven out of equilibrium by ultrafast laser excitation. In this thesis I uncover several previously unexpected connections between the electronic and spin systems in ferromagnets. Dynamics occur at unexpectedly fast timescales, driven using femtosecond laser excitation pulses. The tools that I use to observe the exceeding fast (10s of femtosecond) dynamics are bursts of extreme ultraviolet light resonant with the M-edge of transition metals and produced via high harmonic generation. We combine time-resolved transverse magneto-optical Kerr effect and time- and angle-resolved photoemission spectroscopies to show that the same critical behavior that governs the equilibrium magnetic phase transition in nickel also governs the ultrafast dynamics within 20 fs of laser excitation. When the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: the spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, whereas demagnetization and the collapse of the exchange splitting occur on much longer, fluence- independent time scales of 176 fs. This observation defines a new timescale in the field of ultrafast ferromagnetism. The next question is then whether or not a response at this speed or faster can be directly observed in more complex materials. To investigate this I perform experiments on the half-metallic heusler compound Co2MnGe. Here a single infrared femtosecond laser pulse drives ultrafast transfer of spin polarization from one elemental sublattice to another within its pulse duration. I simultaneously probe the magnetic response of cobalt and manganese to make a surprising finding: the magnetization of Co is transiently enhanced, while that of Mn rapidly quenches. This marks the first direct manipulation of electron spins via light, providing a path to spintronic logic devices such as switches and triggers that operate on few femtosecond or even faster timescales.
Year of Publication
Academic Department
Department of Physics
Number of Pages
Date Published
University of Colorado Boulder
Advisors - JILA Fellows