Novel topological states in multiferroics

Materials with strong electric and magnetic properties are normally different from each other. Surprisingly, in some systems these two features can exist simultaneously. Materials where electricity and magnetism not only coexist, but strongly interact with each other, are called multiferroics. Multiferroics reveal a series of novel physical phenomena, and they are also interesting for such applications like memory devices, controlling of magnetic properties by electric voltage, or of electric polarization by magnetic field. Switching of most physical properties, e.g. charge, magnetization or polarization is normally achieved via two states, normally referred to as 0 and 1. In praxis this is realized by either switching on and off of, e.g. electric voltage, or by a reversal of magnetization or electric polarization. In recent experiment on multiferroic materials based on gadolinium and manganese oxide an unusual new situation could be detected: instead of two, these materials exhibit a series of four distinct magnetoelectric states, each with its own unique polarization and magnetic properties. Detailed investigations suggest that each of the four states corresponds to a 90-degree rotation of magnetic moments, thus completing a full 360-degree rotation in a cycle. This rotation is observed by ramping the external magnetic field back and forth, thus resembling a mechanical crankshaft. In the present project, the new four-state cycle should be investigated in a broader class of materials. We will look for the possibilities to control and possibly modify this cycle and to understand the underlying microscopic mechanisms including the unusual 360-degree rotation. In the course of investigations, we will extend the classical combination of experimental stimuli like electric and magnetic fields. As a new parameter, rotation of external field acting on ordered magnetic structure will be explored. These experiments will be supported by measurements of magnetic and magnetoelectric spectra in the sub-terahertz frequency range, giving valuable information on the dynamic of the investigated materials. This project is led by Andrei Pimenov, with Janek Wettstein as a co-author, both affiliated with the Vienna University of Technology.