Research Programme
The central aim of the Collaborative Research Center 762 (CRC) is the fabrication and characterization of oxide heterostructures with components exhibiting ferroelectric, magnetic, semiconducting, and insulating properties, thus featuring additional degrees of freedom for the design of functional properties. The analysed materials and structures are multifunctional, i.e. in addition to magnetic and ferroelectric properties also electrical and/or optical properties are of general interest for the future application. The central foci of the CRC are functional properties driven by the oxide interfaces and the corresponding coupling via the oxide interfaces. The coupling is determined by the atomic structure as well as charge and spin order at the interface. The coupling can be influenced by external electric and magnetic fields, thus creating the respective functional effects. The coupling can be of different nature, e.g. electro-optical, electric, piezoelectric, magnetoeleastic or magnetoelectric.
Multiferroic interfaces are currently capturing a prominent position among these systems. Multiferroic materials per se are showing ferroelectric and magnetic order simultaneously. Ferroelectric properties are typically coupled to relative structural displacement of positive and negative ions against each other. These displacements are especially large if the d states of the systems are unoccupied (d0 occupation). On the other hand, ferromagnetism emerges preferably in systems with partially occupied d shells. Based on this consideration, ferroelectricity and ferromagnetism should actually be mutually exclusive. However, by the use of new methods of synthesis, single-phase multiferroics could be fabricated. The observed electric polarization and magnetization and particularly their coupling effects were frequently very small. A genuine breakthrough in this respect was achieved with multiferroic thin films, multiferroic heterostructures and nanostructured multiferroics. State-of-the-art layer deposition techniques allow for the synthesis of artificially grown layered structures and phases which cannot be fabricated with conventional concepts of synthesis. Furthermore, by the reduction of dimensionality an enhancement of the spontaneous ferroelectric polarization, the spontaneous magnetization and the coupling via the interface is expected. In this context, the magnetoelectric coupling via the oxide interface is particularly promising since it provides the possibility to control the spin order with electric fields and the charge with magnetic fields. This offers to switch the magnetic information by means of an external electric field and vice versa.
The design of ferroelectric and magnetic oxides with appropriate properties is one of the games of the CRC 762. Besides traditional magnetic materials priority is also given to magnetic oxides without d electrons. We are interested in solving the question whether defect-induced magnetism is stable and reproducible and would offer an alternative mechanism of magnetism which is particularly compatible with ferroelectric oxides.
The success of the CRC is based on the combined expertise of surface science, magnetism, semi-conductor physics, solid state chemistry, materials science, and theoretical physics in Halle and Leipzig. A whole variety of cutting-edge growth and characterisation techniques is available. The theoretical description is material-specific based on density functional theory. Besides ground state properties we study finite-temperature, transport, and time-dependent effects in the non-equilibrium state.
On a long-term scale the activities of the CRC 762 will offer new insight into the microscopic properties of oxide interfaces and will design and synthesize oxide heterostructures and prototypes of functional devices with potential applications in sensor and information technology.