All-optical switching in ferromagnetic superlattices.

The unexpected observation of ultrafast demagnetization (UDM) and subsequent time-resolved studies of laser-induced magnetization switching opened a new door to both fundamental physics and technological applications of magnetic materials. All-optical switching (AOS) can be initiated faster than the precession limit, hinting to its potential in increasing the writing speed and data storage density. However, notwithstanding considerable research interest, the mechanism of AOS in ferromagnetic materials remained unclear. Ferromagnetic superlattices were deposited on glass substrates with e-beam evaporation and sputtering. Magnetization curves were measured in magneto-optical and vibrating sample magnetometer experiments. A femtosecond Ti:S laser was utilized in a writing setup to induce AOS in Co/Pd ferromagnetic superlattices at different fluences and beam polarizations. Magnetic force and polarizing microscopy were applied to image the magnetic structure and identify optimal AOS parameters. High-repetition rate pulses of the Ti:S laser resulted in heat accumulation in the samples. For a better understanding of the relation between temperature and domain wall motion, we solved the heat diffusion equation numerically. The solution for our sample displayed a tilted thermal wave front, consistent with the tilted magnetic domains observed. This supports our model of AOS, in which thermal forces acting on domain walls leads to their expansion and magnetization switching (chapter 3). Polarizing microscopy images also revealed a complementary pattern of magnetic domains after laser writing, suggesting that demagnetizing fields are not negligible. Furthermore, comparison of pump-probe UDM measurements with AOS writing measurements pointed to a demagnetized state before AOS emergence. This motivated us to apply micromagnetic simulations to investigate the time evolution of a demagnetized state and in particular the role of demagnetizing fields in the development of different final states. We show using this method that demagnetizing fields can nucleate and, together with thermally-induced forces, develop a switched state (chapter 4). Using the pump-probe setup, we measured the frequency dependence of laser-induced temperature modulations in Co/Au, Co/Ag and Co/Pd superlattices on glass substrates. Green\u27s function solutions of the heat diffusion equation show that a glass layer with properties distinct from the glass substrate is present near the metallic superlattices (chapter 5)

Anharmonic multiphonon origin of the valence plasmon in SrTi1-xNbxO3

Doped SrTi1-xNbxO3 exhibits superconductivity and a mid-infrared optical response reminiscent of copper-oxide superconductors. Strangely, its plasma frequency, omega_p, increases by a factor of ~3 when cooling from 300 K to 20 K, without any accepted explanation. Here, we present momentum-resolved electron energy loss spectroscopy (M-EELS) measurements of SrTi1-xNbxO3 at nonzero momentum, q. We find that the infrared feature previously identified as a plasmon is present at large q in insulating SrTiO3, where it exhibits the same temperature dependence and may be identified as an anharmonic, multiphonon background. Doping with Nb increases its peak energy and total spectral weight, drawing this background to lower q where it becomes visible in IR optics experiments. We conclude that the "plasmon" in doped SrTi1-xNbxO3 is not a free-carrier mode, but a composite excitation that inherits its unusual properties from the lattice anharmonicity of the insulator.Comment: 5 pages, 4 figure