Dr. Rodolfo Gallardo,
Universidad Técnica Federico Santa María.
Avenida España 1680, Casilla 110V,
Non-reciprocity of wave-phenomena describes the situation that the wave dispersion depends on the sign of the wave vector, i.e. counter-propagating waves exhibit different wavelengths for the same frequency and vice versa. As broken time reversal symmetry creates non-reciprocity connected with the wave propagation, it imposes itself in the field of signal processing. In this context, data processing by using spin waves – the elementary excitations in magnetic systems with coupled electron spins – is considered as promising route towards low-power electronics, as their propagation does not involve electric charges. The non-reciprocity induced by dynamic interactions of dipolar nature is studied in a coupled magnetic bilayer and in magnetization-graded films. The advantage of such systems lies in their simplicity.
Although similar systems have been studied intensively, a clear theoretical formulation and discussion with focus on non-reciprocity is missing so far. The study is carried out by means of analytical calculations that are complemented by micromagnetic simulations in order to identify limitations of the model. By analyzing the dynamic interlayer stray field interactions, the spin-wave dispersion is obtained and optimal parameters for maximizing the non-reciprocal properties are established by providing easy to use equations for the limiting case of two identical ferromagnetic layers. It is found that the frequency shift of two counter-propagating spin waves is notoriously enhanced when a graduation is included in the thickness of a ferromagnetic film; while in the bilayer system this frequency shift is enhanced when
an antiparallel equilibrium state is produced in the magnetizations. These results show that the above systems can be used to channel and control spin waves, thus promoting different kinds of applications for magnon-based devices.