Competing interactions in multiferroics and low-dimensional systems

Promotie: dhr. M.A. van der Vegte, 16.15 uur, Academiegebouw, Broerstraat 5, Groningen
Proefschrift: Competing interactions in multiferroics and low-dimensional systems
Promotor(s): prof.dr. J. Knoester
Faculteit: Wiskunde en Natuurwetenschappen
Contact: Michiel van der Vegte, tel. 06 149 639 30, e-mail: m.a.van.der.vegte@rug.nl

Competing interactions in multiferroics and low-dimensional systems

Magnetic and electronic materials permeate every aspect of modern technology. Magnetic materiales are for example used in data storage devices like hard disks and credit cards. The sensor industrie makes use of a related class of materials called ferroelectrics. They have a spontaneous electric polarization that can be switched by an applied electric field.

Materials which combine two or more “ferroic” properties in the same phase are known as multiferroics. Trends toward device miniaturization have led to increased interest in combining electronic and magnetic properties into multifunctional materials, so that a single device component can perform more than one taks. Ferroelectric magnets are particularly appealing not only because they have the properties of both parent compounds, but also because interactions between the magnetic and electric polarizations lead to additional functionalities. An example of such is the magnetoelectric effect: the induction of a magnetization by an electric field, or of a polarization by a magnetic field.

De magnetoelectric coupling gives rise to a new type of excitations, called electromagnons. They are excitations of the magnetic structure which can be excited by the electric field of light. Theoretical studies in this thesis describe the microscopic origin of electromagnons. In experiments they are only observed in the multiferroic fase, characterized by a non-collinear spin structure. In an external magnetic field the multiferroic fase decays and the electro-magnon peak disappear out of the spectrum. In other words, a small magnetic field can switch the optical properties. One of the main conclusions of this thesis is that electro-magnons can be present in the optical spectrum of magnets with non-collinear spin orders.