Showing papers by "Michel Viret published in 2012"
TL;DR: In this article, it was shown that under intense illumination and electric field, oxygen vacancies can be controllably generated in BiFeO3 to dramatically increase the conductance of single crystals to a controllable value.
Abstract: Energy harvesting from sunlight is essential in order to save fossil fuels, which are found in limited amount in the earth's crust. Photovoltaic devices converting light into electrical energy are presently made of semiconducting materials, but ferroelectrics are also natural candidates because of their internal built-in electric field. Although they are clearly uncompetitive for mainstream applications, the possibility to output high photovoltages is making these materials reconsidered for targeted applications. However, their intrinsic properties regarding electronic transport and the origin of their internal field are poorly known. Here, it is demonstrated that under intense illumination and electric field, oxygen vacancies can be controllably generated in BiFeO3 to dramatically increase the conductance of BiFeO3 single crystals to a controllable value spanning 6 orders of magnitude while at the same time triggering light sensitivity in the form of photoconductivity, diode, and photovoltaic effects. Properties of the bulk and the Schottky interfaces with gold contacts are disentangled and it is shown that bulk effects are time dependent. The photocurrent has a direction that can be set by an applied field without changing the ferroelectric polarization direction. The self-doping procedure is found to be essential in both the generation of electron hole pairs and the establishment of the internal field that separates them.
88 citations
TL;DR: In this paper, optical measurements have been carried out on high-quality BiFeO3 single crystals in order to show the presence of electronic defect states and calculate the band gap.
Abstract: Optical measurements have been carried out on high-quality BiFeO3 single crystals in order to show the presence of electronic defect states and calculate the band gap. The photoluminescence spectra show an intense electronic transition peak at a wavelength of 410 nm. This peak is large and asymmetric, indicating the existence of defects inside the gap, which we attribute to oxygen vacancies. These defects are likely to originate from the slow heating rate and long sintering times necessary to synthesize BiFeO3 single crystals. The optical band gap is measured to be 3 eV, a larger value than those previously reported.
56 citations
TL;DR: In this article, the authors report on the mechanical deformation of BiFeO${}_{3}$ and its response time to discrete wavelengths of incident light ranging from 365 to 940 nm.
Abstract: Optomechanical effects in polar solids result from the combination of two main processes, electric field-induced strain and photon-induced voltages. Whereas the former depends on the electrostrictive ability of the sample to convert electric energy into mechanical energy, the latter is caused by the capacity of photons with appropriate energy to generate charges and, therefore, can depend on wavelength. We report here on mechanical deformation of BiFeO${}_{3}$ and its response time to discrete wavelengths of incident light ranging from 365 to 940 nm. The mechanical response of BiFeO${}_{3}$ is found to have two maxima in near-UV and green spectral wavelength regions.
39 citations
TL;DR: In this paper, it was shown that β-NaFeO 2 is also a multiferroic material at room-temperature but with the most interesting extra property of showing weak ferromagnetism.
33 citations
TL;DR: In this paper, a photostrictive BiFeO3 (BFO) substrate was used to measure the magnitude of resistance and magnetoresistance in a piezomagnetic CoFe thin film.
Abstract: We present a magnetoresistive—photoresistive device based on the interaction of a piezomagnetic CoFe thin film with a photostrictive BiFeO3 (BFO) substrate that undergoes light-induced strain. The magnitude of the resistance and magnetoresistance in the CoFe film can be controlled by the wavelength of the incident light on the BiFeO3. Moreover, a light-induced decrease in anisotropic magnetoresistance is detected due to an additional magnetoelastic contribution to magnetic anisotropy of the CoFe film. This effect may find applications in photo-sensing systems, wavelength detectors and can possibly open a research development in light-controlled magnetic switching properties for next generation magnetoresistive memory devices.
22 citations
TL;DR: In this paper, a light-induced decrease in anisotropic magnetoresistance is detected due to an additional magnetoelastic contribution to magnetic anisotropy of the CoFe film.
Abstract: We present a magnetoresistive-photoresistive device based on the interaction of a piezomagnetic CoFe thin film with a photostrictive BiFeO3 substrate that undergoes light-induced strain. The magnitude of the resistance and magnetoresistance in the CoFe film can be controlled by the wavelength of the incident light on the BiFeO3. Moreover, a light-induced decrease in anisotropic magnetoresistance is detected due to an additional magnetoelastic contribution to magnetic anisotropy of the CoFe film. This effect may find applications in photo-sensing systems, wavelength detectors and can possibly open a research development in light-controlled magnetic switching properties for next generation magnetoresistive memory devices.
21 citations
TL;DR: In this article, the magnetic and ferroelectric configurations of BiFeO3 single crystals were studied using X-ray (magnetic) linear dichroism in a photoemission electron microscope (X-PEEM).
Abstract: In this work, we propose to study the magnetic and ferroelectric configurations in ferroelectric multidomain BiFeO3 single crystals. Using x-ray (magnetic) linear dichroism in a photoemission electron microscope (X-PEEM), we are able to directly image both the antiferromagnetic and ferroelectric domains. We find that inside one single ferroelectric domain several antiferromagnetic domains coexist. This is different from what was observed on epitaxial thin films, where the ferroelectric domains perfectly match the antiferromagnetic ones, but also from previous neutron measurements on ferroelectric monodomain single-crystals for which one single antiferromagnetic domain was identified. This underlines the fundamental differences between thin films, bulk samples, and single versus ferroelectric multidomain samples.
17 citations
TL;DR: In this article, the inverse spin-Hall effect was used to detect ferromagnetic resonance in a permalloy nanostructure using a platinum layer in contact, and the spin currents driven out of the precessing magnetization of a micron square sized structure generated an electrical voltage in the platinum layer.
Abstract: We report here on the detection of ferromagnetic resonance in a permalloy nanostructure using the inverse spin-Hall effect in a platinum layer in contact. The tiny spin currents driven out of the precessing magnetization of a micron square sized structure generate an electrical voltage in the platinum layer because of spin-orbit scattering. We have achieved isolating this signal from other resistive contributions and show that it dominates in certain field geometries. This detection technique can therefore be applied in ferromagnetic nanostructured materials under certain experimental precautions. We also have been able to modify the damping of our Py nanostructures by injecting spin polarized currents using the spin-Hall effect in Pt.
14 citations
TL;DR: It is found that the atoms on the {111} planes were the easiest to be removed by electron irradiation and fluctuations between low-energy and high-energy facets were observed.
Abstract: Direct chemical and structural characterization of transient iron−nickel alloy nanowires was performed at subnanometer spatial resolution using probe spherical aberration-corrected scanning transmission electron micros- copy and electron energy-loss spectroscopy. Nanowires with diameter less than 2 nm retaining their nominal bulk alloy composition were observed. In some cases, the nanowires were oxidized. Before rupture, a nanojunction as thin as three atoms in width could be imaged. The time-dependent structural analyses revealed the nanowire rupture mechanisms. It is found that the atoms on the {111} planes were the easiest to be removed by electron irradiation and fluctuations between low-energy and high-energy facets were observed. The hitherto unknown rich variety of structural and chemical behavior in alloyed magnetic nanojunctions should be considered for understanding their physical properties.
10 citations
TL;DR: It is argued that the RF fields induce an internal distortion of the magnetization profile that depends on the shape of the domain wall, which leads to an enhanced resistive response as compared to saturated domains.
Abstract: Changes in domain wall resistance under radio-frequency (RF) irradiation are experimentally studied for transverse walls. An original experimental technique is applied to the measurement in a permalloy nano-stripe with a notch, where the walls are found to provide a largely enhanced resistive response as compared to saturated domains. Their susceptibility is found to be an order of magnitude larger than that of the domains in a frequency range between 5 and 20 GHz. We argue that the RF fields induce an internal distortion of the magnetization profile that depends on the shape of the domain wall.
3 citations