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Disputation Johan Julius Frederick Heitz

12.01.2022 | 18:45
Thema der Dissertation:
Spintronic Operations Driven by Terahertz Electromagnetic Pulses
Abstract: Spintronic devices, supplementing and surpassing charge-based electronics by including the electron spin, have recently begun to reach the market. So far, it is unclear whether fundamental spintronic effects such as spin accumulation or spin-orbit torque can be transferred to THz frequencies. Additionally, the THz frequency range coincides with many fundamental excitations, for instance phonons, magnons, and the relaxation of electronic currents. Strong THz electromagnetic pulses can be used to study such fundamental excitations, making use of both the electric and magnetic fields of the electromagnetic pulse.
In this thesis, strong THz electromagnetic pulses are applied to spintronic thin-film stacks to drive charge and spin currents, apply torque and manipulate magnetic order. A short optical probe pulse or an electrical resistance probe interrogate the transient magnetic response. First, a measurement strategy based on magnetic circular and linear birefringence (MCB, MLB) is developed to simultaneously detect all components of the vector magnetization of thin film magnets in optical transmission probe experiments at normal incidence, requiring only a variation in the initial probe polarization. Thereafter, using this detection scheme, we study the THz frequency operation of spintronic effects in ferromagnetic(FM)/non-magnetic (NM) heavy metal stacks. We find signatures of THz spin accumulation at the FM/NM interface. Third, an effective method to modulate the relative THz electric and magnetic field amplitudes in thin film samples is presented and experimentally verified, enabling one to disentangle effects driven by the electric or the magnetic component of the THz electromagnetic pulse. Finally, we utilize the electric-field suppression effect close to metals to optically gate the THz electric field driven resistance modulation of an antiferromagnet (AFM) grown on a semiconducting substrate. We show that an optically induced transient substrate conductance depletes the THz electric field in the AFM layer, while not perturbing the AFM magnetic order directly.
In conclusion, this thesis is an important contribution to push fundamental spintronic effects such as spin accumulation and spin-orbit torque to the THz range. The developed methodologies are helpful to advance nonlinear THz spectroscopy of magnetic materials.

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12.01.2022 | 18:45