Broadband characterization and time-resolved nanosecond magneto-optics of thin magnetic films

In the field of magnetic devices as in all the fields related to information technology and electronics the demand for a higher clock rate and faster device operations is continuously increasing. In data storage, the effort to reach the highest information density is leading to the usage of patterned films having micrometer and sub-micrometer features. Both data storage and telecommunication fields are focusing on the magnetization switching behavior at the highest attainable frequency. For data storage it is necessary to obtain a high read/write speed, while for telecommunications a magnetization switching in the sub-nanosecond range is important due to the equivalence with wave propagation frequencies in the GHz range, where current technologies operate. Most investigations in these fields have been performed up to now in the frequency domain, where important results have been obtained starting from the identification of the ferromagnetic resonance and spinwave modes. Ultra fast magnetization dynamics on the nano- and picosecond time scale thus plays and will continue to play an important role for a large variety of present and future applications of magnetic nanostructures. At this time scale, magnetization dynamics is dominated by the damped precessional motion of the magnetization about the acting internal and external fields. It is usually described by the Landau-Lifshitz-Gilbert equation, the two main parameters involved in it being the ferromagnetic resonance frequency and the phenomenological Gilbert damping parameter . Very different requirements on the FMR frequency and the Gilbert damping parameter have to be met for different technological applications: microwave absorbers based on ferromagnetic nanoparticles require large damping, whereas in general all applications related to memory storage require a very low Gilbert damping . For fast sensors or spin transfer torque memory cells the dynamical properties of the magnetic devices must be precisely matched to the specific application. In ferromagnetic resonance experiments, forced precession oscillations of the magnetization are excited by means of a high-frequency magnetic field, which is provided either by a Vector Network Analyzer (VNA), which performs a frequency sweep at a fixed external magnetic eld and returns scattering parameters from which the complex magnetic permeability component can be obtained, or by means of a microwave oscillator, in a setup called field-sweep FMR where the same information about complex permeability are obtained through a varying magnetic field, the oscillator providing a fixed frequency signal. If, instead of the ac field, a dc step pulse field is applied, free precession oscillations are excited, which then decay under the damping action of eddy currents and spin relaxation mechanisms. This is what is referred to as Pulsed Inductive Microwave Magnetometer (PIMM), i.e. to a setup where the waveguide is used both as a source of fast pulsed magnetic fields and as an inductive flux sensor. The pulses are provided by a pulse generator, and a 20 GHz digital sampling oscilloscope is used to acquire the fast pulse data. The system provides direct information on the dynamical behavior of the magnetization as a function of several variables, including applied magnetic bias field, magnetic pulsed field amplitude and width, and sample orientation. This thesis work is devoted to the experimental broadband investigation of several types of thin plates, films and multilayer samples in a wide frequency range, from DC up to several tens of GHz, and to the physical interpretation of the material behavior upon such a wide frequency range and in particular to the development of experimental setups for the aforementioned characterizations.