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B11: Multiferroic dynamics at oxide interfaces

The aim of this project is the understanding of coupled dynamics in multiferroic epitaxial heterostructures under the influence of external fields. The main goal is the detection and quantification of the dynamic coupling between the two multiferroic parameters, i.e. polarization and magnetization. Systems under investigation are primarily ferroelectric thin films with ferromagnetic electrodes. Practically the samples will be fabricated by patterning epitaxial heterostructures consisting of a conductive base electrode, an epitaxial ferroelectric layer and a ferromagnetic top electrode using photo-lithography or electron-beam patterning. The dynamics of multiferroic coupling between polarization and magnetization is investigated by exciting of magnetization / polarization and measuring the complementary parameters (polarization / magnetization).

In the first instance, the influence of the magnetization on the ferroelectric polarization is quantified. The magnetization will be switched using a waveguide through which short current pulses will generate magnetic field pulses. The magnetoelectric coupling is detected by a conventional dynamic measurement of the ferroelectric polarization (PNUD positive-negative-up-down measurement).

In a later stage the inverse magnetoelectric coupling i.e. the influence of the polarization on the magnetization will be detected by time-resolved magneto-optical Kerr effect (TR-MOKE). The polarization of the ferroelectric is switched with short electrical pulses while the magnetization of the top electrode is determined shortly after this switching process with MOKE. The above studies are supported by ferromagnetic resonance (FMR) with different approaches. The spin pumping through a ferroelectric intermediate layer will be analyzed using frequency-domain measurements. In addition, in ferroelectric / ferromagnet bilayers the change in the anisotropy of the ferromagnet upon applying a voltage to the ferroelectric will be studied. In a later stage time/domain measurements will be performed in which pulse-induced FMR (PIM) will be synchronized with short electrical voltage pulses applied to the ferroelectric. Here, the pulses are shifted in time relative to one another so that it is possible to switch the ferroelectric polarization while stimulating the precession of the magnetization.

The experimental studies are closely supported by theory. In our approach we describe, in the absence of the multiferroic coupling, the ferromagnetic dynamics on the level of the Landau-Lifshits-Gilbert equation and the polarization dynamics by means of the Landau-Ginzburg theory. Turning on the multiferroic coupling we can then study the influence of an electric (magnetic) field on the magnetization (polarization) of the sample. The numerical implementation on a grid allows us to resolve in time and space the multiferroic dynamics under the influence of external homogeneous and non-homogenous fields. Furthermore, we will calculate the FMR signals for the experiments involving temperature effects. Special attention is paid to the influence of the multiferroic coupling and spatial structure of the order parameters on the measured FMR signals.