Electric-field control of magnetic domain wall motion and local magnetization reversal in multiferroic heterostructures
Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. In this project, new approaches to electric-field control of local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, are explored. The methods are based on strain transfer from ferroelastic 90 degree stripe domains in ferroelectric substrates to continuous magnetostrictive films. We have demonstrated that the dominance of magnetoelastic anisotropy in these multiferroic heterostructures enables full imprinting of ferroelectric domain patterns into ferromagnetic films, an effect that critically scales with ferroelectric domain size. Moreover, strong pinning of magnetic domain walls onto narrow ferroelastic boundaries leads to flexible tuning of the spin rotation and width of magnetic domain walls in an external magnetic field. Finally, using optical polarization microscopy to image both the ferroelectric and ferromagnetic domain structures, we have shown that domain correlations and strong ferroic domain wall coupling are maintained during temperature cycling across ferroelectric phase transitions and polarization reversal in an applied electric field. The latter result demonstrates unprecedented electric-field control over the formation of ferromagnetic domains and the lateral motion of magnetic domain walls, an accomplishment that opens the way to electric-field driven spintronics.
Imprinting of ferroelectric domain patterns from a BaTiO3 substrate (FE) into a ferromagnetic CoFe film (FM). The schematic illustrations and images in (a,b) and (c,d) indicate the domain structure after CoFe thin film growth and after the application of an out-of-plane electric field of 10 kV/cm, respectively.
Micromagnetic simulation of the ferromagnetic domain wall structure on top of a ferroelectric domain boundary. Depending on the direction of the applied magnetic field, narrow uncharged (a) or broad charged (b) ferromagnetic walls form.
Active tunnel barriers for solid-state devices (ACTIVE-BAR)
Quantum mechanical tunneling of electrons through thin insulating barriers forms the basis of many solid-state nanoelectronic devices including resonant tunneling diodes (RTDs), resonant tunneling transistors (RTTs), superconducting quantum interference devices (SQUIDs), and magnetic tunnel junctions (MTJs). The functionality of these multilayer structures relies on the quantization of energy levels in thin films, the quantum phase difference and critical current or the spin-dependence of the electron tunneling probability, which are determined by the physical properties of the electrodes. In RTDs and RTTs the electrodes are III/V semiconductors, SQUIDs use low- or high-temperature superconductors, and the electrodes of MTJs are ferromagnetic. Tunnel barrier materials that are used include wide band III/V semiconductors and insulating nonpolar oxides like Al2O3. The predominant role of the barriers is to quantize electron energy levels between two closely spaced barriers or to provide direct tunneling between electronic states in the electrodes. The barrier materials are thus mainly selected because of their insulating nature and since it is impossible to alter their properties during device operation they can be considered as passive elements.
In the ACTIVE-BAR project, we explore new concepts of electron tunneling across barriers whose properties can be tuned by external actuation. The tunnel junctions that we fabricate consist of conducting oxide electrodes that are separated by a thin insulating barrier exhibiting either (1) ferrimagnetic, (2) ferroelectric, or (3) multiferroic order. The application of a magnetic field, an electric field, or a combination of both alters the physical properties of the tunnel barrier and thereby the electron transport and spin filtering properties of the entire junction. These active tunnel barriers offer unique functionalities to nanoelectronic tunneling devices in future logic, memory, and sensor technology. Moreover, this experimental study provides valuable information on the physics behind electronic transport, finite-size effects of ferroic order parameters, and multiferroics at ferromagnetic-ferroelectric interfaces.
In this project, we are studying the interaction between magneto-optical and plasmonic properties of ferromagnetic nanostructures. In particular, we are focusing on the tailoring of magneto-optical responses under plasmonic resonance conditions in circular and elliptical Ni nanodots on glass substrates. Recently, we have demonstrated that excitations of local plasmon resonances transverse to the polarization of the incident light determine the magneto-optical spectral features of ferromagnetic nanostructures. The sensitivity of the magneto-optical signal on the local dielectric environment can be used for refractive index sensing applications and single molecule detection.
Ferroelectricity and magnetic ordering effects at the nanoscale
Manipulation of ferroelectric material properties in thin-film epitaxial heterostructures are studied in this project. For example, anomalous polarization switching and very large dielectric nonlinearities have been demonstrated for Pb0.5Sr0.5TiO3 films with epitaxial nanocolumns.
Also, a new mechanism for bandgap reduction in ferroelectric nanofilms has been identified. Normally, the bandgap of bulk crystals widens in the presence of ferroelectric polarization, an effect that has been confirmed by experiments and first-principles calculations. By studying epitaxial ferroelectric nanofilms of KTaO3 (KTO), KNbO3 (KNO), and NaNbO3 (NNO) on SrTiO3 (STO) substrates, we have found anomalous bandgap narrowing in the presence of robust ferroelectric order. This effect, which is pronounced for KNO and NNO films with a thickness of 10 – 30 unit cells, is ascribed to the formation of higher symmetry structural phases with suppressed oxygen octahedral tilts. The desirable red shift of the absorption spectrum opens up new prospects for the design of nanoscale photonic and optoelectronic devices.
Finally, we are studying magnetic ordering phenomena at epitaxial perovskite interfaces. Charge imbalance at epitaxial d0 STO/KTO, STO/KNO, and STO/NNO interfaces can lead to the formation of 2D hole or electron gases in the STO layer (depending on interface termination). Ab initio analysis and experiments suggest that ferromagnetic ordering and half-metallicity occur in hole-doped d0 perovskites, whereas partial magnetization and complex magnetic ordering patterns are possible under electron localization. The influence of lattice strain and ferroelectricity on magnetic ordering at perovskite interfaces are currently under investigation.