On the photovoltaic effect asymmetry in ferroelectrics.

Despite symmetrical polarization, the magnitude of a light-induced voltage is known to be asymmetric with respect to poling sign in many photovoltaic (PV) ferroelectrics (FEs). This asymmetry remains unclear and is often attributed to extrinsic effects. We show here for the first time that such an asymmetry can be intrinsic, steaming from the superposition of asymmetries of internal FE bias and electro-piezo-strictive deformation. This hypothesis is confirmed by the observed decrease of PV asymmetry for smaller FE bias. Moreover, the both PV effect and remanent polarization are found to increase under vacuum-induced expansion and to decrease for gas-induced compression, with tens percents tunability. The change in cations positions under pressure is analysed through the first-principle density functional theory calculations. The reported properties provide key insight for FE-based solar elements optimization.

Tuning a sign of magnetoelectric coupling in paramagnetic NH2(CH3)2Al1-xCrx(SO4)2 × 6H2O crystals by metal ion substitution.

Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (ME) interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric NH2(CH3)2Al1-x Cr x (SO4)2 × 6H2O. Doing so we are able to induce and even tune a sign of the ME interactions, in the paramagnetic ferroelectric (FE) state. Both studied samples with x = 0.065 and x = 0.2 become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature (T c ) increases nonlinearly, with the shift being larger for the 6.5% of Cr. A magnetic field applied along the polar c axis increases ferroelectricity for the x = 0.065 sample and shifts T c to higher values, while inverse effects are observed for x = 0.2. The ME coupling coefficient αME = 1.7 ns/m found for a crystal with Cr content of x = 0.2 is among the highest reported up to now. The observed sign change of αME with a small change in Cr content paves the way for ME coupling engineering.

Optical Writing of Magnetic Properties by Remanent Photostriction.

We present an optically induced remanent photostriction in BiFeO_{3}, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO_{3}/Ni structure. The 75% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO_{3}. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.

Multistate nonvolatile straintronics controlled by a lateral electric field.

We present a multifunctional and multistate permanent memory device based on lateral electric field control of a strained surface. Sub-coercive electrical writing of a remnant strain of a PZT substrate imprints stable and rewritable resistance changes on a CoFe overlayer. A proof-of-principle device, with the simplest resistance strain gage design, is shown as a memory cell exhibiting 17-memory states of high reproducibility and reliability for nonvolatile operations. Magnetoresistance of the film also depends on the cell state, and indicates a rewritable change of magnetic properties persisting in the remnant strain of the substrate. This makes it possible to combine strain, magnetic and resistive functionalities in a single memory element, and suggests that sub-coercive stress studies are of interest for straintronics applications.

Light-induced size changes in BiFeO3 crystals.

Multifunctional oxides are promising materials because of their fundamental physical properties as well as their potential in applications. Among these materials, multiferroics exhibiting ferroelectricity and magnetism are good candidates for spin electronic applications using the magnetoelectric effect, which couples magnetism and ferroelectricity. Furthermore, because ferroelectrics are insulators with a reasonable bandgap, photons can efficiently interact with electrons leading to photoconduction or photovoltaic effects. However, until now, coupling of light with mechanical degrees of freedom has been elusive, although ferroelasticity is a well-known property of these materials. Here, we report on the observation, for the first time, of a substantial visible-light-induced change in the dimensions of BiFeO(3) crystals at room temperature. The relative light-induced photostrictive effect is of the order of 10(-5) with response times below 0.1 s. It depends on the polarization of incident light as well as applied magnetic fields. This opens the perspective of combining mechanical, magnetic, electric and optical functionalities in future generations of remote switchable devices.

Polar properties of Eu(0.6)Y(0.4)MnO(3) ceramics and their magnetic field dependence.

Eu(1-x)Y(x)MnO(3), compared against other magnetoelectric systems, exhibits very distinctive features. Its magnetoelectric properties are driven by the magnetic spin of the Mn(3+) ion, but they can be drastically changed by varying the content of Y(3+), which does not carry any magnetic moment. Although the x = 0.40 composition has been studied extensively, some basic areas still remain to be thoroughly understood. Thus, this work is aimed at studying some of its polar properties and their magnetic field dependence as well. The experimental results reported here show that this material is very easily polarizable under external electric fields, and so, whenever the polarization is obtained from time integration of the displacement currents, an induced polarization is superposed on the spontaneous one, eventually masking the occurrence of ferroelectricity. We have found clear evidence for the influence of a magnetic field in the polar properties of Eu(0.6)Y(0.4)MnO(3). The study of electric polarization of Eu(0.6)Y(0.4)MnO(3) under an external magnetic field yields a value with the same order of magnitude of the remanent polarization as was determined from polarization reversal experiments. The comparison of the magnetically induced changes in the polarization obtained for polycrystalline samples and single crystals confirms the threshold magnetic field value for the polarization rotation from the a-direction to the c-direction, and provides evidence of the importance of the granular nature of the samples in the polar response to the magnetic field.