Optical and electric field control of magnetism
Ashima Arora · HZB Repository (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH (HZB)) · 2018
Future magnetic recording industry needs a high density data storage technology. However, switching the magnetization of small bits requires high magnetic elds that cause excessive heat dissipation. Therefore, controlling magnetism without applying external magnetic eld is an important research topic for potential applications in data storage devices with low power consumption. Among the di erent approaches being investigated, two of them stand out, namely i all optical helicity dependent switching AO HDS and ii ferroelectric control of magnetism. This thesis aims to contribute towards a better understanding of the physical processes behinds these e ects as well as reporting new and exciting possibility for the optical and or electric control of magnetic properties. Hence, the thesis contains two di erentiated chapters of results; the rst devoted to AO HDS on TbFe alloys and the second to the electric eld control of magnetism in an archetypal Fe BaTiO3 system. In the rst part, the scalability of the AO HDS to small laser spot sizes of few microns in the ferrimagnetic TbFe alloy is investigated by spatially resolving the magnetic contrast with photo emission electron microscopy PEEM and X ray magnetic circular dichroism XMCD . The results show that the AO HDS is a local e ect within the laser spot size that occurs in the ring shaped region in the vicinity of thermal demagnetization. Within the ring region, the helicity dependent switching occurs via thermally activated domain wall motion. Further, the thesis reports on a novel e ect of thickness dependent inversion of the switching orientation. It addresses some of the important questions like the role of laser heating and the microscopic mechanism driving AO HDS. The second part of the thesis focuses on the electric eld control of magnetism in an arti cial multiferroic heterostructure. The sample consists of an Fe wedge with thickness varying between 0 5 nm and 3 nm, deposited on top of a ferroelectric and ferroelastic BaTiO3 [001] oriented single crystal substrate. Here, the magnetic contrast is imaged via PEEM and XMCD as a function of out of plane voltage. The results show the evidence of the electric eld control of superparamagnetism mediated by a ferroelastic modi cation of the magnetic anisotropy. The changes in the magnetoelastic anisotropy drive the transition from the superparamagnetic to superferromagnetic state at localized sample positions