BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Multiferroics: progress and prospects in thin films.

https://chemistry.beloit.edu/classes/nanotech/ZnO/mattoday10_05_40.pdf

ZnO - nanostructures, defects, and devices

Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

Electric-field control of local ferromagnetism using a magnetoelectric multiferroic

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.

Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature.

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents Doping of 2at% Ti4+ ions increased the dc resistivity by more than three orders of magnitude In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Large Electric Polarization and Exchange Bias in Multiferroic BiFeO3

The recently discovered1 superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are ‘pinned’ in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research2, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the ‘persistent’ supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

Vortex dynamics in superconducting MgB2 and prospects for applications.

High-resolution x-ray diffraction and transmission electron microscopy (TEM) have been used to study BiFeO3 thin films grown on the bare and SrRuO3 buffered (001) SrTiO3 substrates. Reciprocal space mapping (RSM) around (002) and (103) reflections revealed that BFO films with a thickness of about 200 nm were almost fully relaxed and had a rhombohedral structure. Cross-sectional, high-resolution TEM showed that the films started to relax at a very early stage of growth, which was consistent with the RSM results. A thin intermediate layer of about 2 nm was observed at the interface, which had a smaller lattice than the overgrown film. Twist distortions about the c axis to release the shear strain introduced by the growth of rhombic (001) BiFeO3 on cubic (001) SrTiO3 were also observed. The results indicate that a strained, coherent BiFeO3 film on (001) SrTiO3 is very difficult to maintain and (111) STO substrates are preferable.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

ZnO nano-rods were grown on polycrystalline Zn foil by cathodic electrodeposition in an aqueous zinc chloride/calcium chloride solution at 80?C. Variations in the solution concentration and substrate surface preparation were explored to shed light on the nucleation of the nano-rods. It was found that the nano-rod diameter increased with increasing solution concentration. Rolling striations and native ZnO on the surface of the Zn appeared to enhance nucleation and allowed more highly aligned, dense structures to be grown. By using low solution concentrations (5.0 ? 10?4?M?ZnCl 2) and non-electropolished Zn substrates, well faceted, hexagonal nano-rod structures of dimension ~80?nm diameter and }1\ntilde \micmu {\mathrm {m}}$>>1??m length were obtained. X-ray studies showed the samples to be highly aligned but containing a Zn-oxychloride impurity phase. Annealing caused the impurity phase to disappear and resulted in the films having a sharp photoluminescence double peak at 380/396 nm.

https://iopscience.iop.org/article/10.1088/0957-4484/14/9/306/pdf

Xiaoding Qi

Papers

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Large Electric Polarization and Exchange Bias in Multiferroic BiFeO3

Vortex dynamics in superconducting MgB2 and prospects for applications.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

Electrochemical growth of ZnO nano-rods on polycrystalline Zn foil