High-field effects in a metallic ferromagnet on the femtosecond timescale [electronic resource]
Keywords: Applied Physics

The dynamics initiated when a magnetic system is excited out of its equilibrium position are influenced by the strength of the fields driving the excitation and the timescales over which those fields are applied. This thesis explores dynamics induced in relatively simple magnetic systems by extremely strong fields applied on ultrafast timescales. Here, "strong" describes magnetic fields of tens of tesla and electric fields of gigavolts per meter, and "ultrafast" describes timescales faster than 100 picoseconds. Specifically, we look at the process of magnetic switching initiated by strong, true half-cycle field pulses with temporal durations of either a few picoseconds or a few hundred femtoseconds. The experiment utilizes a single shot technique to initiate the switching which relies on the electromagnetic fields created by relativistic electron bunches at the SLAC National Accelerator Laboratory. The femtosecond duration pulses have peak magnetic and electric fields of 60 T and 20 GV/m respectively and coherent frequency spectra which extend into the terahertz. These intense pulses are unique to SLAC and thus provide us with a novel set of conditions with which to study ultrafast magnetization dynamics. While traditional magnetic switching experiments rely on magnetic fields alone to initiate the switching, the first part of the thesis will discuss a new type of switching mechanism which utilizes a combination of magnetic and electric fields. We will show that the 20 GV/m electric field of the femtosecond duration electron bunch acts to create a new, transient magnetic anisotropy axis through a distortion of valence electron states and that this new anisotropy axis dramatically affects the magnetization switching dynamics induced by the accompanying magnetic field. We will show that this hybrid switching mechanism triggers a deterministic reversal of the magnetization on the timescale of 100's of femtoseconds, and that the experiment acts as a proof of principle demonstration for an all electric field induced magnetic switching technique. While electric fields have been used to manipulate magnetization before, there has never been a clear demonstration of a technique which would actually use an electric field to switch. The second part of the thesis will address the rather surprising observation that passage of these ultrafast high-field pulses through our thin film sample leaves it remarkably damage free, in striking contrast to lower field, longer pulses which leave signatures of heating. This latter result is particularly encouraging for potential future applications. In short, this thesis includes three contributions to the fields of ultrafast magnetization dynamics and high-field physics: 1. We observe a new type of transient, all electric field induced magnetic anisotropy caused by a pure electronic distortion of the valence electron states in a thin film metallic ferromagnet. This magnetoelectronic anisotropy saturates at a value larger than any other magnetic anisotropy previously observed in a magnetic metal. 2. We demonstrate the first clear path for an all electric field induced magnetization switching technique viable for normal thin film metallic ferromagnets at room temperature. 3. We show that it is possible to deposit the energy necessary to induce such effects in a thin film and dissipate it before it can cause appreciable heating of the sample. We propose coherent transition radiation as the possible dissipation channel.