Master of Science – Research
School of Physics
Hargreaves, Rhys, Ultrafast demagnetisation as a Terahertz source, Master of Science – Research thesis, School of Physics, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4274
Ultrafast demagnetisation of ferromagnets by ultrafast laser pulses provides a unique window for the investigation of spin dynamics in ferromagnets. Ultrafast demagnetisation has previously been observed via the generation of Terahertz emissions (among other methods) using amplified lasers, and by magneto-optical methods using non-amplified lasers. Using a fast (short pulse length), low power laser, with lower pulse energies, but comparable peak pulse power density, models of ultrafast demagnetisation were compared.
The models compared were the Two Temperature model proposed by Vaterlaus et al., the Three Temperature Model by Beaurepaire et al., the Non- Thermal-Electron model by Ju et al., and the Microscopic Three Temperature Model by Koopmans et al.. A quasi-null result of THz spectral power density < 10−22 W · THz−1, from Nickel samples, was inconsistent with predictions of the Two Temperature Model. More accurate measurements of Terahertz emission were not possible due to cumulative noise effects from the high pulse repetition frequency, and low pulse energy used.
In addition, theoretical investigations into the Microscopic Three Temperature Model were undertaken. These predicted that the peak Terahertz emission frequency is inversely proportional to the ultrafast pulse length. It was also predicted that (for the laser modelled) peak power output is achieved by initiating demagnetisation from ≈80% of the Curie temperature of the material, and that this result is approximately independent of the material.