Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic N é el temperature of the multiferroic compound BiFeO 3 is suppressed to around room temperature by heteroepitaxial misfi t strain. Remarkably, the ferroelectric state undergoes a fi rst-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our fi ndings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.

The phase separation of multiple competing structural/ferroelectric phases has attracted particular attention owing to its excellent electromechanical properties. Little is known, however, about the strain-gradient-induced electronic phenomena at the interface of competing structural phases. Here, we investigate the polymorphic phase interface of bismuth ferrites using spatially resolved photocurrent measurements, present the observation of a large enhancement of the anisotropic interfacial photocurrent by two orders of magnitude, and discuss the possible mechanism on the basis of the flexoelectric effect. Nanoscale characterizations of the photosensitive area through position-sensitive angle-resolved piezoresponse force microscopy and electron holography techniques, in conjunction with phase field simulation, reveal that regularly ordered dipole-charged domain walls emerge. These findings offer practical implications for complex oxide optoelectronics.

Topological defects in matter behave collectively to form highly non-trivial structures called topological textures that are characterised by conserved quantities such as the winding number. Here we show that an epitaxial ferroelectric square nanoplate of bismuth ferrite subjected to a large strain gradient (as much as 105 m−1) associated with misfit strain relaxation enables five discrete levels for the ferroelectric topological invariant of the entire system because of its peculiar radial quadrant domain texture and its inherent domain wall chirality. The total winding number of the topological texture can be configured from − 1 to 3 by selective non-local electric switching of the quadrant domains. By using angle-resolved piezoresponse force microscopy in conjunction with local winding number analysis, we directly identify the existence of vortices and anti-vortices, observe pair creation and annihilation and manipulate the net number of vortices. Our findings offer a useful concept for multi-level topological defect memory.

Leaky ferroelectric oxides can serve as optoelectronic circuit elements in which potential gradients due to the spontaneous polarization yield asymmetric and nonlinear photocarrier dynamics. Ferroelectric domains and domain walls should each influence photocarrier generation, separation, and transport differently, but the microscopic mechanisms are unknown. Here, we present scanning photocurrent images of epitaxial BiFeO 3 thin films that reveal how the photoresponse depends on dynamic domain configurations. Locally, photocurrent direction is determined by local domain orientation, whereas the photocurrent magnitude is spectrally centered around charged domain walls associated with oxygen vacancy migration. Our observations demonstrate that photodetection can be electrically controlled by manipulating domains, suggesting non-volatile optoelectronic memory applications.In non-centrosymmetric media, nonequilibrium photocarriers can be directed along polar directions. This yields a unidirectional photocurrent (I ph ) parallel to the spontaneous polarization (P S ), and thus high photovoltages (V ph ) [1]−[6] that are linearly proportional to channel length. [4] Photovoltaic effects in ferroelectric oxides were previously investigated back in the 1960s, [1,2,7] and a high V ph accompanied by nonlinear and unidirectional I ph have been recently observed in a small bandgap ferroelectric BiFeO 3 (BFO). [3,4] This material is the only roomtemperature multiferroic, [8][9][10] and represents an interesting platform for investigating photon interactions with internal ferroic order. [11][12][13][14] For example, bulk photo-diode effects [3,15] and photo-induced mechanical striction [16] were recently observed in BFO monodomains with a switchable ferroelectric polarization. Meanwhile, in polydomain samples, it was also found that the sign and the magnitude of I ph and V ph are predominantly determined by domain walls (DWs), particularly when multiple DWs are regularly spaced. [4] Polarization switching in rhombohedral BFO is closely associated with magnetoelectric coupling, and is manifested in ferroelastic switching associated with 71° or 109° DWs. The spatial extent of ferroic order induces changes in polarization, and must thus impose a local variation in the electrical potential and bandgap that govern the photoresponses. Nevertheless, the interplay between these internal states and the local photoresponse has not been fully addressed to date. Here we report a vectorial mapping of the local I ph , scanned over the static domain configuration of BFO thin films, manipulated by an applied E-field. By the spatiallyand spectrally-resolved photoresponses of (001)-oriented BFO films, it was revealed that photodetection is determined by the local P S . Specifically, we captured direct images of the evolution of the local I ph on switching domains via 'charged' domain-wall motion, [17][18][19] with the magnitude of I ph being enhanced across a reduced band gap.The device structure of our BFO photodetector is shown in Figure 1, w...

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼10 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (E). Here, we present a promising FPV based on hexagonal YbFeO (h-YbFO) thin-film heterostructure by exploiting its narrow E. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/AlO device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces E substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.