Multiferroic Rashba Semiconductors based on GeTe

Multiferroics, materials in which ferromagnetism and ferroelectricity coexist, have attracted tremendous interest in the last few years due to their extraordinary physical properties as well as high potential for device application such as sensors, programmable logic and data storage. Magnetically doped GeTe using transition metal ions such as Mn, Fe, Cr or V is a highly attractive material system for manifestation of multiferroic behavior. This is due to the fact that the Ge2+ ions induce the driving force for ferroelectricity while the Mn2+ or Cr2+ ions provide the magnetic moments that can couple to each other for ferromagnetic ordering. As a result, these materials represent one of the simplest conceivable multiferroic systems, but up to now the mechanisms and consequences of the coexistence and coupling between ferromagnetism and ferroelectricity has remained largely explored. The objective of this research project is to systematically map out the multiferroic properties of Ge1- xXxTe magnetically doped with X = Mn, Cr and V. Thereby we will establish a versatile platform for this novel class of multiferroic materials in which the magnetic anisotropy, magnetoelectric coupling strength as well as the Curie temperatures can be tuned independently from each other. Moreover, we seek to unravel the consequences of multiferroicity on the electronic band structure and spin- texture due to the giant Rashba-Zeeman effect and demonstrate how this can be controlled by external electric and magnetic fields. A multilevel approach will be employed for realization and investigation of our tunable multiferroic systems, combining a wide range of advanced techniques and expertise provided by the University of Linz and European collaboration partners. This will allow to cover all aspects from material developments, epitaxial growth, magnetic doping, structure analysis, scanning tunneling microscopy and spectroscopy, magnetic characterization, spin-resolved photoemission studies, scattering experiments at synchrotrons as well as first principles ab initio calculations. Through this project we seek to achieve (a) independent control of ferroelectricity and ferromagnetism with enhanced Curie temperatures and establish the multiferroic phase diagrams of GeTe materials, (b) control of magnetic anisotropy and ferroelectric domains by strain, thickness and substrate orientation, (c) demonstrate ferroelectric and ferromagnetic coupling and their switching using electric and magnetic fields, and (d) reveal the entanglement of the spin texture through the Rasbha Zeeman effect and how it can be controlled by external means. We expect that our results will not only unravel novel physics and a profound understanding of the fundamental mechanisms but will also open new avenues for multiferroic device applications.

The interplay between electronic eigenstates, spin and orbital degrees of freedom is one of the most exciting fields of research. It forms the basis for giant magnetoresistance, spin-torque manipulation, and giant Rashba effects and leads to new quantum phases such as topological insulators and Weyl semimetals. The combination of several of these effects in multiferroic materials is of outstanding fundamental interest but also provides vast potentials for device applications. In this project, we have focused on doped GeTe as a prototypical multiferroic system where ferroelectricity (FE) is due to the relative displacement between the Ge and Te atoms, whilst ferromagnetism (FM) is induced by transition metal doping. Thin film heterostructures were produced by molecular beam epitaxy and multiferroic materials in which the magnetic anisotropy and magnetoelectric coupling can be tuned independently from each other were developed. A multilevel approach encompassing a wide range of techniques was employed to cover all aspects from material development, growth, magnetic doping, x-ray scattering and nanobeam diffraction, scanning tunneling microscopy, magnetic dichroism as well as spin- and angle resolved photo-emission spectroscopy performed at European synchrotrons complemented by ab initio calculations. Switching of FE domain by applying external fields through gate electrodes was demonstrated and the dynamics of carrier excitation and Rashba spin splitting revealed by time resolved photoemission experiments. The results opens up new pathways for ultra-fast spin manipulation, which is of key importance for future spintronic device applications.