74 research outputs found
Unusual spin-wave population in nickel after femtosecond laser pulse excitation
The spin-wave relaxation mechanisms after intense laser excitation in
ferromagnetic nickel films are investigated with all-optical pump-probe
experiments. Uniform precession (Kittel mode), Damon-Eshbach surface modes and
perpendicular standing spin waves can be identified by their dispersion f(H).
However, different to other ferromagnets f(H) deviates from the expected
behavior. Namely, a mode discontinuity is observed, that can be attributed to a
non-linear process. Above a critical field the power spectrum reveals a
redistribution of the energy within the spin-wave spectrum populated.Comment: 7 pages, 6 figure
Analytical expression of the magneto-optical Kerr effect and Brillouin light scattering intensity arising from dynamic magnetization
Time-resolved magneto-optical Kerr effect (MOKE) and Brillouin light
scattering (BLS) spectroscopy are important techniques for the investigation of
magnetization dynamics. Within this article, we calculate analytically the MOKE
and BLS signals from prototypical spin-wave modes in the ferromagnetic layer.
The reliability of the analytical expressions is confirmed by optically exact
numerical calculations. Finally, we discuss the dependence of the MOKE and BLS
signals on the ferromagnetic layer thickness
Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions
Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B
magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia,
and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance
ratios we found were 73% for alumina and 323% for magnesia-based tunnel
junctions. Additionally, tunnel junctions with a unified layer stack were
prepared for the three different barriers. In these systems, the tunnel
magnetoresistance ratios at optimum annealing temperatures were found to be 65%
for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The
similar tunnel magnetoresistance ratios of the tunnel junctions containing
alumina provide evidence that coherent tunneling is suppressed by the alumina
layer in the composite tunnel barrier.Comment: 3 pages,4 figures, 1 tabl
Ultrafast photocurrents at the surface of the three-dimensional topological insulator
Topological insulators constitute a new and fascinating class of matter with
insulating bulk yet metallic surfaces that host highly mobile charge carriers
with spin-momentum locking. Remarkably, the direction and magnitude of surface
currents can be controlled with tailored light beams, but the underlying
mechanisms are not yet well understood. To directly resolve the "birth" of such
photocurrents we need to boost the time resolution to the scale of elementary
scattering events ( 10 fs). Here, we excite and measure photocurrents in
the three-dimensional model topological insulator
with a time resolution as short as 20 fs by sampling the concomitantly emitted
broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast
surface current response is dominated by a charge transfer along the Se-Bi
bonds. In contrast, photon-helicity-dependent photocurrents are found to have
orders of magnitude smaller magnitude than expected from generation scenarios
based on asymmetric depopulation of the Dirac cone. Our findings are also of
direct relevance for optoelectronic devices based on topological-insulator
surface currents
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