Confinement phenomena in chiral ferromagnetic films
11 January 2019
Campus Drie Eiken, Promotiezaal Q0.02 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Department of Physics
Milorad Milosevic, Bartel Van Waeyenberge
PhD defence Jeroen Mulkers - Faculty of Science, Department of Physics
The demand for data storage and computational power of ICT devices increases enormously. Moreover, these devices should have a low power consumption. The study of material properties is of utmost importance to meet this increasing demand. This is also true for research on new magnetic materials because they could lead to more efficient magnetic storage systems and logic devices. Over the last decade, chiral magnetic systems have been studied extensively. In this thesis, we contributed to this field of research by examining how lateral boundaries and material interfaces affect the magnetization configuration.
The ground state magnetization of thin ferromagnetic films with a perpendicular magnetic anisotropy is uniform at nanoscale; only at larger length scales, domains of ‘up’ and ‘down’ magnetization can appear. The magnetization in chiral ferromagnetic films is subjected to the Dzyaloshinskii-Moriya interaction (DMI). This chiral interaction introduces a chirality in the magnetization on a small length scale, and when strong enough, stabilizes magnetic configurations which consist out of chiral structures such as cycloids, helices, and skyrmions.
In this thesis, we showed that it is possible to confine magnetic structures such as magnetic domain walls and skyrmions in small magnetic strips and platelets. In heterochiral films — films which have a non-homogeneous DMI strength — we find a similar confinement of magnetic textures. In both cases, the existence of boundaries lead to multiple stable states. Furthermore, we derived, based on symmetry arguments, that the upper and bottom surface of the ferromagnetic film can lead to an additional DMI. This boundary-induced DMI causes a 3D deformation of the magnetization configuration.
Alongside the static magnetization configurations, we also studied the non-reciprocal behavior of spin waves in heterochiral magnets. A non-trivial refraction of spin waves occurs at interfaces between regions with a different DMI strength. By generalizing the well known Snell's law, we provide a theoretical description to describe this refraction.