Visible-light controlled ferroelectricity and magnetoelectric coupling in multiferroic BiCoO₃ nanoribbons

Abstract

Multiferroic materials hold promising potential for multifunctional applications because they simultaneously possess ferromagnetism and ferroelectricity. Moreover, the magnetoelectric coupling in multiferroic materials is especially important for basic research, and it has significant applications in nonvolatile multiple-state memory devices. In this work, for the first time, we report that visible light can control the ferroelectricity and magnetoelectric coupling in BiCoO₃ nanoribbons. The relative visible-light-induced improvement of the saturated ferroelectric polarization is more than 60%. The relative visible-light-induced enhancement of the magnetoelectric effect is about 10%, indicating that light offers additional degree of freedom in controlling the coupling between ferromagnetism and ferroelectricity.

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Introduction

It is known that ferroelectricity has long been viewed as a collective phenomenon, which is associated with a spontaneous macroscopic polarization resulting from the alignment of localized dipoles within a correlation volume.^1–6 Recently, ferroelectric materials are essential components in a wide spectrum of applications.^7–11 During the past few years, ferroelectric materials have been used in a number of commercial and wide spread products, including memory, microwave electronic components and microdevices with pyroelectric effect. In various recent applications, the application of nonvolatile ferroelectric random access memories (FRAMs) is most attractive,^12,13 which is based on the polarization reversal by an external applied electric field in metal–ferroelectric–metal capacitors,^14–16 where the logic “0” and “1” are represented by a nonvolatile storage of the negative and positive remanent polarization state, respectively. FRAMs show attractive advantages, such as write and read cycle times in the sub-100 ns range and low power consumption, which are in most cases superior to the performance of other nonvolatile technologies.

Moreover, magnetoelectric materials (also known as multiferroics) have attracted great scientific and technological interests due to their magnetoelectric properties with interesting device potentials, originating from the coupling between ferroelectric and magnetic order parameters.^17–28 The interplay between ferroelectricity and magnetism allows a magnetic control of ferroelectric properties and an electric control of magnetic properties.^29,30 Hence, they offer a wide opportunity for potential applications in information storage, magnetic recording media, spintronic devices and sensors.^31–33 Recently, several single-phase multiferroics combining large ferroelectric and ferromagnetic polarizations at room temperature are reported.^34–39 In a single-phase material, the magnetoelectric effect requires long-range ordering of atomic moments and electric dipoles.^40–45 Moreover, Jyoti Ranjan Sahu et al. have found rare earth multiferroic materials, indicating a new family of multiferroics.⁴⁶ In addition, some photoferroelectric materials have been also found,^47–50 which exhibit the light-induced change of ferroelectricity.

In addition, the perovskite BiCoO₃ with a band gap of 2.11 eV has received much attention not only in the application but also in basic research. In recent reports, the multiferroic BiCoO₃ was synthesized at high pressure and was not suitable as a parent compound for lead-free piezoceramics.^51–53 BiCoO₃ is isostructural with PbTiO₃, but the polar structural distortion is more pronounced, with a large displacement of the Co³⁺ ion from the center of the octahedron leading to a pyramidal rather than octahedral coordination.^51–53

Although there are a large number of reports on BiCoO₃ in previous works, to the best of our knowledge, the visible-light controlled ferroelectricity and magnetoelectric coupling in BiCoO₃ have not been investigated. Herein, we describe the visible-light controlled ferroelectricity and magnetoelectric coupling in single-crystalline BiCoO₃ nanoribbons.

Experimental

Materials preparation

In our work, BiCoO₃ nanoribbons were prepared by a hydrothermal process. All chemicals used in this work were of analytical reagent grade and commercially available, and used without further purification. Detailed experimental procedures are as follows: Firstly, Bi(NO₃)₃·5H₂O (0.025 mol) and CoCl₂·6H₂O (0.025 mol) were dissolved in 40 ml distilled water with stirring until completely dissolved. Secondly, 0.5 g polyvinylpyrrolidone (PVP) was added and stirred. After stirring continuously for 2 hour, 4.0 g of NaOH was added under vigorous stirring for 30 min. Then, the solution was transferred to a 50 ml sealed Teflon-lined steel autoclave. The Teflon-lined steel autoclave was heated at 120 °C for 180 hours. After the autoclave was cooled to room temperature, the obtained powder was washed with distilled water and ethanol and dried at 60 °C overnight.

Materials characterization

Crystal structure of BiCoO₃ nanoribbons was characterized by X-ray diffraction (XRD, Shimadzu XRD-7000 X-ray diffractometer) with Cu Kα radiation (λ = 1.5418 Å) at room temperature. Surface morphology of BiCoO₃ nanoribbons was characterized using a scanning electron microscope (SEM, JSM-6510) equipped with an INCA PentaFETx3 detector for energy dispersive X-ray spectroscopy (EDS). The size, selected area electron diffraction (SAED) and microstructure of the BiCoO₃ nanoribbons were observed by transmission electron microscopy (TEM, JEM-2100) at an acceleration voltage of 200 kV.

Performance test

The measurement of ferroelectric hysteresis loops (P–E) was performed by a Precision Premier Workstation ferroelectric test system (Radiant Technology, USA), where Ag served as the two electrodes. The Ag electrodes were prepared by vacuum deposition, and the preparation process of the Ag electrodes is as follows. Firstly, the small amounts of BiCoO₃ nanoribbons were dispersed in ethanol. Then, the dispersion was dropped onto the surface of single-crystalline insulator silicon. After the insulator silicon completely dried, the mask was covered on it. Second, we put placed insulator silicon with a mask into the vacuum sputtering system to grow Ag electrodes. In this experiment, an ordinary filament lamp with various power densities as light source was used, and the light power density was measured by an irradiatometer. The wavelength range of visible light is 400–760 nm. Magnetic properties were characterized by a vibrating sample magnetometer (VSM) from ADE Corporation in USA.

Results and discussion

Fig. 1(a) shows the X-ray powder diffraction (XRD) patterns of BiCoO₃ nanoribbons at room temperature. The XRD profile matches very well with that in the reported work.^45,46 The samples can be indexed in a tetragonal system, corresponding to space group P4mm.^54,55 The XRD pattern also indicates that there are no impurity phases in the as-synthesized samples. In addition, the low background and sharper peaks suggest that the nanoribbons retain their crystallinity. The BiCoO₃ nanoribbons were further confirmed by elemental analysis carried out from energy-dispersive X-ray spectra (EDS). The EDS data in Fig. 1(b) confirms that the elements of nanoribbons are Bi, Co and O without any other impurities.

Fig. 1 (a) The X-ray powder diffraction (XRD) patterns of BiCoO₃ nanoribbons at room temperature. (b) The energy-dispersive X-ray spectra (EDS) of BiCoO₃ nanoribbons.

Fig. 2(a) and (b) present the scanning electron microscopy (SEM) images of BiCoO₃ nanoribbons. The as-prepared sample consists of uniform nanoribbons (Fig. 2(a)). The high resolution SEM image is shown in Fig. 2(b). Fig. 2(c) exhibits the transmission electron microscope (TEM) image of BiCoO₃ nanoribbons, which reveals the BiCoO₃ nanoribbons are about a dozen microns long and 2 μm wide, and thickness is in nanoscale. The inset to Fig. 2(c) displays the corresponding selected area electron diffraction (SAED) pattern, which shows that the BiCoO₃ nanoribbons are single-crystalline structures. Fig. 2(d) presents the high resolution TEM (HRTEM) image of BiCoO₃ nanoribbons, where the fringes with spacing of 0.42 nm and 0.47 nm correspond to (010) and (100) planes of BiCoO₃.

Fig. 2 (a) The low-magnification and (b) high-magnification scanning electron microscopy (SEM) images of as-prepared BiCoO₃ nanoribbons. (c) The transmission electron microscopy (TEM) image of individual BiCoO₃ nanoribbons, the inset is the corresponding selected area electron diffraction (SAED) pattern of BiCoO₃ nanoribbons. (d) The high resolution TEM (HRTEM) image of BiCoO₃ nanoribbons.

Fig. 3(a) exhibits the schematic diagram of the circuit used for the P–E hysteresis loop measurements, where the BiCoO₃ nanoribbon lays on a single-crystalline insulated silicon substrate and Ag acts as the two electrodes. The light source is vertically positioned above the sample during the measurement, and the distance between the sample and the lamp is always more than 30 cm. Homogeneous illumination between the two electrodes can be achieved because the distance of the two Ag electrodes is a few microns. Fig. 3(b) demonstrates the P–E ferroelectricity hysteresis loops of the single-crystalline BiCoO₃ nanoribbon in the dark and under illumination with various power densities at room temperature. We first measured the P–E hysteresis loop in the dark, and then we gradually increased the light power density and measured the P–E hysteresis loops under visible-light irradiation. The P–E hysteresis loops of the single-crystalline BiCoO₃ nanoribbon greatly changes when subjected to light irradiation. In addition, the hysteresis loops are relatively stable under various conditions, as shown in Fig. 4(a) and (b). However, when we removed the light source, the enhanced polarization cannot be retained (Fig. 4(c)).

Fig. 3 (a) Schematic diagram of the circuit used for the P–E hysteresis measurements, where the BiCoO₃ nanoribbons lay on an insulated silicon substrate and Ag acts as the two electrodes. (b) The P–E hysteresis loops were obtained at 100 Hz illuminated under an ordinary filament lamp with various power densities at room temperature. (c) The visible-light-induced changes ΔP of the saturated ferroelectric polarization are nearly linear to the light power density when H = 0 T and 2.5 T respectively. (d) The M–H hysteresis loop for BiCoO₃ nanoribbons measured at room-temperature.

Fig. 4 (a) The P–E hysteresis loops with 5 cycles in the dark. (b) The P–E hysteresis loops with 5 cycles under visible-light illumination with power density 40 mW cm⁻². (c) The P–E hysteresis loop was firstly obtained under visible-light illumination with power density 40 mW cm⁻², and then the P–E hysteresis loop was obtained after removing the light. (d) The current–voltage curves of the BiCoO₃ nanoribbon in the dark and under visible-light illumination with power density 40 mW cm⁻² at room temperature.

Fig. 3(c) shows that the visible-light-induced changes ΔP of the saturated ferroelectric polarization are nearly linear to the light power densities, where ΔP is defined as (P_light − P_dark)/P_dark. Here, P_dark is the saturated ferroelectric polarization in the dark and P_light is the saturated ferroelectric polarization under visible-light irradiation. The saturated ferroelectric polarization increases quickly with the light power densities increasing. The saturated ferroelectric polarization increases about 55% with light power density 40 mW cm⁻² when H = 0 T. However, the relative change of the saturated ferroelectric polarization ΔP reaches to 60% with light power density 40 mW cm⁻² when H = 2.5 T. In previous reports, laser can change the ferroelectric hysteresis loop, and the relative change of the saturated ferroelectric polarization and the remanent ferroelectric polarization were less than 20% at room temperature.^56,57 Therefore, the great change of the saturated ferroelectric polarization induced by visible light in the single-crystalline BiCoO₃ nanoribbons at room temperature is remarkable, which reveals a potential application for nonvolatile multistate memory devices. In previous reports, the light-induced reduction of ferroelectric polarization in ferroelectric compounds was explained by trapping of photogenerated charge at domain boundaries to minimize internal depolarizing fields.^58,59

The possible mechanisms of the light-induced ferroelectricity change include increased conductivity (namely increased current density), photovoltaic effect and ferroelectric domain mobility under visible-light illumination. The current–voltage curves of the BiCoO₃ nanoribbons in the dark and under visible-light illumination at room temperature show that the current under visible-light illumination is 10% higher than that in the dark (Fig. 4(d)). Therefore, increased conductivity is impossibly the dominant origination for the light-induced ferroelectricity change because the saturated ferroelectric polarization and the remanent ferroelectric polarization increase about 60% under visible-light illumination. Generally, the photovoltaic effect is unidirectional, and therefore not the dominant origination because the change of light-induced ferroelectricity is symmetrical. Here, the observed remarkable improvement in the ferroelectric polarization of the BiCoO₃ nanoribbons should be attributed to trapping of photogenerated charges at domain boundaries, which enhance ferroelectric domain mobility.

Fig. 3(d)shows the magnetic hysteresis loop (M–H) of the single-crystalline BiCoO₃ nanoribbons at room temperature (300 K). It is obvious that the single-crystalline BiCoO₃ nanoribbons exhibit weak ferromagnetism with coercivity of about 30 Oe at room temperature.

To confirm that the single-crystalline BiCoO₃ nanoribbons are also a magnetoelectric material, we tested the magnetoelectric coupling effect at 10 K. Fig. 5 presents the P–E hysteresis loops under visible-light irradiation with power density of 40 mW cm⁻² without magnetic field and under applied magnetic field H = 2.5 T at 10 K. Obvious magnetoelectric effect is observed in the BiCoO₃ nanoribbons.

Fig. 5 The P–E hysteresis loops is obtained at 100 Hz illuminated under ordinary filament lamp with power density of 40 mW cm⁻² when H = 0 T and 2.5 T, respectively, at 10 K.

Fig. 6(a) exhibits the magnetoelectric effects of the single-crystalline BiCoO₃ nanoribbons in the dark and under visible-light irradiation with various light power densities at 10 K, where the magnetoelectric effects ΔP_S is defined as (P_S-H − P_S-0)/P_S-0, and P_S-0 is the saturated ferroelectric polarization without magnetic field and P_S-H is the saturated ferroelectric polarization under applied magnetic field. We can see that the magnetoelectric effect can be controlled by visible-light irradiation. Fig. 6(b) presents the light-power-density-dependent ΔP_S of the single-crystalline BiCoO₃ nanoribbons at 10 K, where ΔP_S is achieved as the magnetic field increases from zero to 2.5 T. The magnetoelectric effects ΔP_S is greatly enhanced by visible-light. ΔP_S increases five times from 2% to 10% when the light power densities increase from 0 to 40 mW cm⁻². Therefore, BiCoO₃ is a favourable optical magnetoelectric material; i.e. the magnetoelectric coupling of BiCoO₃ is closely controlled by illumination.

Fig. 6 (a) The visible-light-induced change of the magnetoelectric coupling of the BiCoO₃ nanoribbons at 10 K. ΔP_S is defined as (P_S-H − P_S-0)/P_S-0, where P_S-0 is the saturated ferroelectric polarization without magnetic field and P_S-H is the saturated ferroelectric polarization under applied magnetic field. (b) The light-power-density-dependent ΔP_S of the single-crystalline BiCoO₃ nanoribbons when H = 2.5 T at 10 K.

Conclusions

In conclusion, we report that visible light can control the ferroelectricity and magnetoelectric coupling of the BiCoO₃ nanoribbons. Thus, the single-crystalline BiCoO₃ nanoribbons are definitely an interesting material, which opens the perspective of combining optical, ferroelectric and ferromagnetic functionalities.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (Grant no. 51372209).

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