Change in the magnetic structure of (Bi,Sm)FeO3 thin films at the morphotropic phase boundary probed by neutron diffraction

We report on the evolution of the magnetic structure of BiFeO3 thin films grown on SrTiO3 substrates as a function of Sm doping. We determined the magnetic structure using neutron diffraction. We found that as Sm increases, the magnetic structure evolves from a cycloid to a G-type antiferromagnet at the morphotropic phase boundary, where there is a large piezoelectric response due to an electric-field induced structural transition. The occurrence of the magnetic structural transition at the morphotropic phase boundary offers another route towards room temperature multiferroic devices

Nanoscale domain wall dynamics in ferroelectric thin films: effects of electro-mechanical field interactions

Ferroelectric oxide thin films are currently used in ultra high density non-volatile memories (FeRAM) and Nano/Micro-Electro-Mechanical Systems (NEMS/MEMS). In all these application, the functional property is determined by the ferroelectric and ferroelastic domain wall movement. Moreover, commercially developed systems are based on polycrystalline (or textured) ferroelectric thin films and therefore their performance rely heavily on the microstructural features such as orientation, grain size, grain boundary contribution etc. Furthermore, as the thin films are patterned onto the substrate for any device fabrication, the film-substrate interface defects such as lattice misfit, dislocations etc., highly influence the domain wall behavior.This dissertation, investigates the nanoscale domain switching behavior in polycrystalline perovskite lead zirconate titanate, Pb(ZrxTi(l-x)03 (PZT) ferroelectric thin films. Systematic Piezoresponse Force Microscopy (PFM) studies provide direct visual evidence of the complex interplay between electrical and mechanical fields in a polycrystalline system, which causes effects such as correlated switching between grains, ferroelastic domain switching, inhomogeneous piezostrain profiles and domain pinning on very minute length scales. Furthermore, the grain to grain long range interaction and ensuing collective dynamics in the domain switching behavior have been investigated using the time resolved PFM and Switching Spectroscopy PFM (SSPFM). Finite element method (FEM) has been employed to quantify the local ferroelectric interaction and assess the several possible switching mechanisms. The experiments find that of the three possible switching mechanisms, namely, direct electromechanical coupling, local built-in electric field and strain, and grain boundary electrostatic charges, the last one is the dominant mechanism.Having studied in detail the nanoscale domain wall behavior, we are now able to control and engineer the domain behavior in the PZT ferroelectric thin film materials. In the special case of thin film ferroelectrics that have a large ferroelastic self-strain associated with their phase transformation, a key aspect is the interaction of this self-strain with the boundary conditions of the film. In this thesis, by depositing a strongly tetragonal ferroelectric thin film on a soft rhombohedral bottom layer, we show that these elastic interactions result in a ferroelastic domain structure in the tetragonal film that is susceptible to external perturbation. High-resolution piezoresponse force microscopy images demonstrate gross movement (nm scale) of the ferroelastic domains under local bias and shows enhanced electromechanical response through this movement. Band excitation piezoforce spectroscopy investigation presents visual evidence for the local mechanism that underpins the ferroelastic domain wall movement and reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. We find that while reversible switching is essentially a linear motion of the ferroelastic domains, irreversible switching takes place via domain-wall twists. Critically, real-time images of in-situ domain dynamics under an external bias reveal that the reversible component leads to reduced coercive voltages. Finally, we show that junctions representing three-domain architecture represent facile interfaces for ferroelastic domain switching. The results presented here thus provide (hitherto missing) fundamental insight into the correlations between the physical mechanisms that govern ferroelastic domain behavior and the observed functional response in domain-engineered thin film ferroelectric devices

Electric-Field Induced Reversible Switching of the Magnetic Easy Axis in Co/BiFeO₃ on SrTiO₃

Electric-field (E-field) control of magnetism enabled by multiferroic materials has the potential to revolutionize the landscape of present memory devices plagued with high energy dissipation. To date, this <i>E</i>-field controlled multiferroic scheme has only been demonstrated at room temperature using BiFeO₃ films grown on DyScO₃, a unique and expensive substrate, which gives rise to a particular ferroelectric domain pattern in BiFeO₃. Here, we demonstrate reversible electric-field-induced switching of the magnetic state of the Co layer in Co/BiFeO₃ (BFO) (001) thin film heterostructures fabricated on (001) SrTiO₃ (STO) substrates. The angular dependence of the coercivity and the remanent magnetization of the Co layer indicates that its easy axis reversibly switches back and forth 45° between the (100) and the (110) crystallographic directions of STO as a result of alternating application of positive and negative voltage pulses between the patterned top Co electrode layer and the (001) SrRuO₃ (SRO) layer on which the ferroelectric BFO is epitaxially grown. The coercivity (H_C) of the Co layer exhibits a hysteretic behavior between two states as a function of voltage. A mechanism based on the intrinsic magnetoelectric coupling in multiferroic BFO involving projection of antiferromagnetic G-type domains is used to explain the observation. We have also measured the exact canting angle of the G-type domain in strained BFO films for the first time using neutron diffraction. These results suggest a pathway to integrating BFO-based devices on Si wafers for implementing low power consumption and nonvolatile magnetoelectronic devices

Microstructure and texture development in single layered and heterolayered PZT thin films

10.1007/s10853-010-4712-0Journal of Materials Science45226187-6199JMTS

Synthesis of Epitaxial Metal Oxide Nanocrystals via

Perovskite phase instability of BiMnO3 has been exploited to synthesize epitaxial metal oxide magnetic nanocrystals. Thin film processing conditions are tuned to promote the breakdown of the perovskite precursor into Bi2O3 matrix and magnetic manganese oxide islands. Subsequent cooling in vacuum ensures complete volatization of the Bi2O3, thus leaving behind an array of self-assembled magnetic Mn3O4 nanostructures. Both shape and size can be systematically controlled by the ambient oxygen environments and deposition time. As such, this approach can be extended to any other Bi-based complex ternary oxide system as it primarily hinges on the breakdown of parent Bi-based precursor and subsequent Bi2O3 volatization.<br/