dr.ir. E.P. Houwman (Evert)

Perovskites form a class of metal oxide materials ABO₃ with a diverse range of technologically important material properties. Here A and B are different metal ions. By (partially) substituting these ions with others, for example in AA’BO₃ or ABB’O₃, one can tailor the properties of the bulk material. These bulk properties are already fairly well known for many interesting perovskites.

For device applications, in particular the incorporation with Si technology, we develop processes to fabricate high quality –preferably epitaxial- thin films and multilayers of different types of perovskites, on different substrates (both –insulating perovskites and buffered Si). In that case the perovskite thin film is strained by the substrate, which in many cases has a significant influence on the film properties.

The influence of the strain on the crystallographic structure (change of lattice parameters and possibly polydomain growth) and the resulting effect on the material properties is studied experimentally and by detailed modeling.

In particular we have studied in detail the halfmetallic and ferromagnetic La_0.4Sr_0.6MnO₃compound under different strain conditions, concentrating on the magnetic and electric properties. A detailed model is developed from which the potential landscape of the magnetization vector in the film can be predicted from the imposed substrate strain. The model was confirmed experimentally for many different strain conditions. Further we were able to relate the unit cel deformations to oxygen-octahedra deformations, which explain the changes in the coupling between the Mn spins.

Presently we work on the ferroelectric and piezoelectric group of materials, especially the subgroupof PbZr_1-xTi_xO₃. These materials are important for many micromechanical devices, supercapacitors, energy scavengers. We seek to optimize the piezoelectric and ferroelectric properties for specific applications varying the composition x taking account for the strain by different substrates and deposition conditions, both experimentally and by modeling these systems.