Haoliang Huang

Low magnetic field response single-phase multiferroics under high temperature

A single-phase material where ferroelectricity and ferromagnetism coexist at room temperature (RT) is hardly available at present, and it is even more rare for such a material to further have an intrinsic and low magnetic field response magnetoelectric (ME) coupling at temperatures higher than RT. In this communication, a new single-phase Aurivillius compound, SrBi5Fe0.5Co0.5Ti4O18 has been discovered that exhibits a plausible intrinsic ME coupling. Remarkably, this property appears at a high temperature of 100 °C, surpassing all single-phase multiferroic materials currently under investigation. With a magnetocapacitance effect detectable at 100 °C and under a low response magnetic field, a RT functioning device was demonstrated to convert an external magnetic field variation directly into an electric voltage output. The availability of such a single-phase material with an intrinsic and low magnetic field response that is multiferroic at high temperature is important to the fundamental understanding of physics and to potential applications in sensing, memory devices, quantum control, etc.

Bipolar loop-like non-volatile strain in the (001)-oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals.

Strain has been widely used to manipulate the properties of various kinds of materials, such as ferroelectrics, semiconductors, superconductors, magnetic materials, and “strain engineering” has become a very active field. For strain-based information storage, the non-volatile strain is very useful and highly desired. However, in most cases, the strain induced by converse piezoelectric effect is volatile. In this work, we report a non-volatile strain in the (001)-oriented Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals and demonstrate an approach to measure the non-volatile strain. A bipolar loop-like S-E curve is revealed and a mechanism involving 109° ferroelastic domain switching is proposed. The non-volatile high and low strain states should be significant for applications in information storage.

Large anisotropic remnant magnetization tunability in (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 multiferroic epitaxial heterostructures

A large anisotropic remnant magnetization tunability was observed in multiferroic (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN-0.3PT) epitaxial heterostructures. The remnant magnetization along [100] direction was suppressed by an electric field applied to the substrate while the remnant magnetization along [011¯] was enhanced. The tunabilities of the remnant magnetization along the [100] and [011¯] directions are about −17.9% and +157% under electric field of +7.27u2009kV/cm, respectively. This large anisotropic remnant magnetization tunability may find potential applications in the electrically written and magnetically read memories.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Angular Dependence of Exchange Bias and Magnetization Reversal Controlled by Electric‐Field‐Induced Competing Anisotropies

The combination of exchange-biased systems and ferroelectric materials offers a simple and effective way to investigate the angular dependence of exchange bias using one sample with electric-field-induced competing anisotropies. A reversible electric-field-controlled magnetization reversal at zero magnetic field is also realized through optimizing the anisotropy configuration, holding promising applications for ultralow power magnetoelectric devices.

Piezo-strain induced non-volatile resistance states in (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 epitaxial heterostructures

The non-volatile resistance states induced by converse piezoelectric effect are observed in ferromagnetic/ferroelectric epitaxial heterostructures of (011)-La2/3Sr1/3MnO3/0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 (LSMO/PMN0.7PT0.3). Three stable remnant strain states and the corresponding resistance states are achieved by properly reversing the electric field from the depolarized direction in ferroelectric PMN0.7PT0.3 substrate. The non-volatile resistance states of the LSMO film can be manipulated by applied electric-field pulse sequence as a result of the large coupling between the electronic states of LSMO film and the strain transferred from the ferroelectric substrate. The electrically tunable, non-volatile resistance states observed exhibit potential for applications in low-power-consumption electronic devices.

Electric-field-control of resistance and magnetization switching in multiferroic Zn0.4Fe2.6O4/0.7Pb(Mg2/3Nb1/3)O3―0.3PbTiO3 epitaxial heterostructures

Multiferroic (001)–Zn0.4Fe2.6O4/0.7Pb(Mg2/3Nb1/3)O3–0.3PbTiO3 (ZFO/PMN–PT) epitaxial heterostructures have been investigated to demonstrate the electric-field-controlled resistance and magnetization switching. The tunabilitiy of resistance of the ZFO film is about −0.1% under the in-plane strain −0.02% at 296 K and 0.2% for the electric field 1.0 kV/cm at 80 K, respectively, and the tunabilitiy of magnetization is about 1.1% under the in-plane strain −0.11% at 296 K, which is attributed to the controllable strain transferred into the ZFO film from the piezoelectric PMN–PT substrate. A possible microscopic mechanism of the manipulation of resistance and magnetization is the enhancement of hopping amplitude of electrons between mixed-valent Fe2+ and Fe3+ ions under the electric-field-induced in-plane compressive strain.

Multifunctional Single-Phase Photocatalysts: Extended Near Infrared Photoactivity and Reliable Magnetic Recyclability.

A practical photocatalyst should be able to integrate together various functions including the extended solar conversion, a feasible and economic recyclability, and above the room temperature operation potential, et al., in order to fulfill the spreading application needs in nowadays. In this report, a multifunctional single-phase photocatalyst which possesses a high photoactivity extended into the near infrared region, an easy magnetic recyclability and the high temperature stability was developed by doping Co into a new layer-structured Bi7Fe3Ti3O21 material. Light absorption and photocatalytic activity of the resulted Bi7Fe3-xCoxTi3O21 photocatalyst were extended to the long wavelength as far as 800u2009nm. Its strong ferromagnetism above the room temperature enables the nanopowders fully recyclable in viscous solutions simply with a magnet bar in an experimental demonstration. Furthermore, such photoactivity and magnetic recyclability were heavily tested under high-temperature and high-viscosity conditions, which was intended to simulate the actual industrial environments. This work brings the bright light to a full availability of a new multifunctional photocatalyst, via integrating the much enhanced ferromagnetic, ferroelectric, optoelectronic properties, most importantly, into a single-phase structure.

Interface engineering in epitaxial growth of layered oxides via a conducting layer insertion

There is a long-standing challenge in the fabrication of layered oxide epitaxial films due to their thermodynamic phase-instability and the large stacking layer number. Recently, the demand for high-quality thin films is strongly pushed by their promising room-temperature multiferroic properties. Here, we find that by inserting a conducting and lattice matched LaNiO3 buffer layer, high quality mu2009=u20095 Bi6FeCoTi3O18 epitaxial films can be fabricated using the laser molecular beam epitaxy, in which the atomic-scale sharp interface between the film and the metallic buffer layer explains the enhanced quality. The magnetic and ferroelectric properties of the high quality Bi6FeCoTi3O18 films are studied. This study demonstrates that insertion of the conducting layer is a powerful method in achieving high quality layered oxide thin films, which opens the door to further understand the underline physics and to develop new devices.

Growth of single-crystalline Bi6FeCoTi3O18 thin films and their magnetic–ferroelectric properties

High-quality single-crystalline thin films of n = 5 (n denotes the period of the Aurivillius structure) multiferroic Aurivillius oxides were obtained by determining and applying an optimal growth temperature of 605 °C and oxygen pressure of 20 Pa using pulsed laser deposition. The optimal growth ranges were narrower than 30 °C for the temperature and 10 Pa for the pressure. The optimized thin films exhibited obvious ferroelectric polarization switching in the out-of-plane direction as well as weak ferromagnetism with a saturation magnetization of less than 10 emu/cm3.