Poling-Written Ferroelectricity in Bulk Multiferroic Double-Perovskite

Jun 1, 2016 - *E-mail: [email protected]. Tel.: +39 0521 269220. ... compound BiFeO3. Under external electric poling, the system irreversibl...
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Poling-Written Ferroelectricity in Bulk Multiferroic Double-Perovskite BiFe0.5Mn0.5O3 Davide Delmonte,*,†,‡ Francesco Mezzadri,§ Edmondo Gilioli,† Massimo Solzi,‡ Gianluca Calestani,§ Fulvio Bolzoni,† and Riccardo Cabassi† †

IMEM-CNR, Parco Area delle Scienze 37/A, 43124 Parma, Italy Department of Physics and Earth Science, University of Parma, Parco Area delle Scienze 7/A, 43124 Parma, Italy § Department of Chemistry, University of Parma, Parco Area delle Scienze 17/A 43124 Parma, Italy ‡

S Supporting Information *

ABSTRACT: We present a comprehensive study of the electrical properties of bulk polycrystalline BiFe0.5Mn0.5O3, a double perovskite synthesized in highpressure and high-temperature conditions. BiFe0.5Mn0.5O3 shows an antiferromagnetic character with T N = 288 K overlapped with an intrinsic antiferroelectricity due to the Bi3+ stereochemical effect. Beyond this, the observation of a semiconductor−insulator transition at TP ≈ 140 K allows one to define three distinct temperature ranges with completely different electrical properties. For T > TN, electric transport follows an ordinary thermally activated Arrhenius behavior; the system behaves as a paramagnetic semiconductor. At intermediate temperatures (TP < T < TN), electric transport is best described by Mott’s variable range hopping model with lowered dimensionality D = 1, stabilized by the magnetic ordering process and driven by the inhomogeneity of the sample on the B site of the perovskite. Finally, for T < TP, the material becomes a dielectric insulator, showing very unusual poling-induced soft ferroelectricity with high saturation polarization, similar to the parent compound BiFeO3. Under external electric poling, the system irreversibly evolves from antiferroelectric to polar arrangement.

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INTRODUCTION The multiferroic properties of bismuth (Bi)-based perovskites have been a well-known and debated topic in material science since Wang et al.1 reported the room-temperature coexistence of ferroelectricity (TC ≈ 1100 K) and magnetism (TN ≈ 640 K) on an epitaxial film of BiFeO3. Since then, many other Bicontaining compounds have been studied and synthesized: in the form of films or 2D heterostructures, e.g., BiMnO3,2−4 LaxBi1−xMnO3,5 (Bi0.9La0.1)2NiMnO6,6 BiFe0.5Cr0.5O3,7 and Bi 2 CoMnO 6 , 8 or bulk polycrystals, e.g., Bi 2 NiReO 6 − Bi2MnReO6,9 BiMnO3,10,11 Bi2NiMnO6,12 Bi(Fe0.5Cr0.5)O3,13 and La1.2Bi0.8Mn2−x(Ni/Co)xO6±δ.14 The scientific community distinguishes between proper and improper ferroelectric magnetic perovskites referring to two different classes in which the ferroelectricity has an independent or a magnetostrictive origin, respectively. In proper ferroelectrics, the magnetic and electric ordering temperatures are usually different, so that the magnetoelectric effect is essentially absent, while the improper ferroelectrics have higher magnetoelectric coefficients but often quite low polarization intensity and very low ordering temperature of the multiferroic phase. The Bi-based double perovskites can simultaneously yield high ordering temperatures and a consistent overlap between the two order types, thanks to a deep structural modification induced by the ordering process itself. © XXXX American Chemical Society

In this work, we report a complete study of the electrical properties of bulk BiFe0.5Mn0.5O3, a double perovskite synthesized in high-pressure/high-temperature (HP/HT) conditions, that can be considered a solid solution of the two end members BiFeO3 and BiMnO3.15,16 Herein we show that bulk polycrystalline BiFe0.5Mn0.5O3, besides films and heterostructures,17−20 is definitely multiferroic; in addition to the already reported magnetic properties,21−24 a ferroelectric polarization at low temperature is clearly detected. Moreover, the observed correlation between the magnetic and electric degrees of freedom opens the route to further characterizations specifically focused on the study of possible magnetoelectric coupling.

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EXPERIMENTAL SECTION

Sample Preparation. High-purity (>95%) polycrystalline BiFe0.5Mn0.5O3 was synthesized via a solid-state reaction carried out in HP/HT, using a Walker-type multianvil press, at 6 GPa (isotropic pressure) and 1100 °C for 1.5 h of reaction time. The phase was frozen by quenching to room temperature before pressure release. The starting powder binary oxides (Bi2O3, Fe2O3, and Mn2O3, provided by Alfa Aesar; purity >99.9%) were ground together in stoichiometric amounts and encapsulated into gold foils. Received: April 20, 2016

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DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Characterization. Synchrotron powder X-ray diffraction (SPXRD) data were collected at the ID09A beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The ground sample was inserted into a 0.2 mm boron−glass capillary, and data were measured in transmission using a flat-panel MAR555 detector. The X-ray wavelength was set to 0.50899 Å. From the as-grown cylindrical pellets of polycrystalline bulk BiFe0.5Mn0.5O3, disk-shaped specimens with an area of 4 mm2 and a thickness of 0.50 mm were prepared for electric characterization. When required (pyroelectric, resistive, and ferroelectric measurements), the samples have been metalized by sputtering on both the sides with a 200 nm platinum layer at an argon pressure of 1.2 × 10−2 mbar. Platinum has been selected to optimize the ohmic behavior and uniformity of the junction. The dielectric constant has been measured on nonmetalized samples to avoid contact effects25,26 using an Agilent 4284 capacitive bridge. Pyroelectric and resistive currents were measured using a Keithley 6517-B electrometer, while electric poling was applied with a Keithley 2400 sourcemeter. All of these characterizations have been integrated through homemade probes into a SQUID magnetometer, used as a cryostat. The ferroelectric characterization was performed by means of an AIXACCT TF-2000E analyzer equipped with a Trek 2200 high-voltage amplifier. Resistive measurements were performed also on a single crystal of approximate dimensions of 10 × 10 × 30 μm3, extracted from an HP/ HT synthesis batch. The crystal was glued on a copper plate with a thin film of conductive silver paste and then completely covered with a drop of epoxy resin in order to avoid short circuits and leakage currents. After hardening, the top of the epoxy was mechanically removed until one face of the crystal was exposed, and then a copper wire was applied on the top contact using silver paste. Data Analysis. Structural data were integrated with the Fit2D software,27 taking into account polarization and geometrical correction. Rietveld refinement was carried out by using the GSAS package28 with the EXPGUI interface.29 Ferroelectric measurements were collected and preelaborated with the AiXplorer software. The presented electric characterization measurements were entirely computed by means of the Matlab platform.

Figure 1. Rietveld plot of SPXRD data collected with λ = 0.50899 Å. Black crosses indicate the experimental data. The red line is the computed pattern, while the blue line at the bottom is the experimental−calculated intensity difference. Top purple thick marks: BiFe0.5Mn0.5O3. Bottom green thick marks: Bi2CO5. In the inset, a high 2θ portion of the pattern is shown, indicating the absence of relevant structural distortions.

represented in Figure 2, which are mutually compensated for (in agreement with the centrosymmetric character of the structure) so that the overall resultant is zero.

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RESULTS Structural Characterization. SPXRD data were refined in the space group, in agreement with the single-crystal information published in ref 23 (see Tables S1 and S2). No significant deviation from this symmetry is observed, reaching excellent agreement factors of the Rietveld refinement R(F2) = 7.55% and Rwp = 4.39% for 2644 observed reflections and 46 variables (Figure 1). The refined lattice parameters are a = 5.553(2) Å, b = 11.1755(4) Å, c = 15.6897(4) Å, and V = 973.77(11) Å3. A small amount ( 10 GΩ·cm). Therefore, TP is possibly the thermal threshold of an insulator-to-semiconductor transition. At higher temperatures, the dc resistivity trend measured on a bulk sample reproduces an exponential T dependence characterized by a mean value of the activation energy EA,bulk = 350 meV, in good agreement with respect to the single crystal, EA,cryst = 335 meV. The data were analyzed by Arrhenius linearization, using the three main models usually adopted in similar systems:30−33 the semiconducting thermal-activated (TA) transport,34−36 the adiabatic nearest-neighbors hopping of small polarons or a Holstein polaron (ANHSP) model,37 and Mott’s variable range hopping (M-VRH) with different dimensionalities (3D and 1D) 38,39 (see the complete description in the Supporting Information). As shown in Figure 4b, the Arrhenius plot shows the presence of an anomaly near TN = 288 K, although discernible for low applied voltages only, matching the magnetic TN. As a consequence, the modeling of electric transport was carried out separately for the two noninsulating regimes: TP < T < TN and T > TN. For TP < T < TN, the data fittings show excellent convergence to R2 = 1

Figure 5. Dielectric constant of BiFe0.5Mn0.5O3 as a function of the temperature for different frequencies of the applied electric field.

decrease of the dielectric constant is detected as the system is cooled, dropping from ∼50 in the high-frequency/hightemperature limit to ∼38 in the low-temperature limit. This phenomenon appears at TN = 288 K, ending between 140 and 200 K for the high- and low-frequency measurements, respectively, below which the system behaves as a dielectric, highlighting good linearity as a function of the temperature. In agreement with the transport characterization, below TP = 140 K, the dielectric measurements indicate the formation of the insulating phase, independently from the frequency. In addition, the dielectric constant is observed to be inversely dependent on the frequency of the applied alternating-current electric field, uncommon behavior for standard dielectrics. This

Figure 4. (a) Bulk electrical resistivity as a function of the temperature measured for different applied voltages (10, 25, 50, and 100 V) and plotted in a logarithmic scale. (b) Arrhenius plot in the high-temperature range showing a less pronounced transition at TN. (c) Arrhenius plot showing a clear transition at TP ≈ 140 K in the transport mechanism. C

DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry is by no means surprising if the presence of a partially delocalized electron is taken into account within the whole investigated thermal spectrum. Consequently, the higher the field frequency, the more localized the electrons appear to be and so the larger the integrated charge (from which the dielectric constant is proportionally derived). It is worth noting that the dynamics of lowering and stabilization of the electrical permittivity roughly overlaps the same temperature range necessary to completely settle the magnetic long-range order of the system.21,23 Pyroelectric and Ferroelectric Characterization. Pyrocurrent measurements were performed during a timecontrolled heating ramp of the sample from 5 to 320 K (Figure 6). Before that, a dc electric poling of about 1.4 kV/cm

Figure 7. Pyrocurrent responses during thermal heating after an electric field poling of E = +1.4 kV/cm (blue squares) and E = −1.4 kV/cm (green diamonds) compared to the measurement of Figure 6 (red circles).

current peak measured in the thermal region between TP = 140 and 90 K characterized by two different maxima with different intensities. It is worth noting that this peak precisely occurs when the system evolves from the dielectric phase to the thermal region in which the M-VRH mechanism sets in, specifically in correspondence of activation of the M-VRH polaronic state. The same pyroelectric measurements cannot be performed on the tiny single crystals available. This is by no means surprising because a rough estimate, based on the current intensities of Figure 6 and on the geometrical size of the two samples, allows one to estimate a pyroelectric current far below the instrumental sensitivity for the single crystal. The ferroelectric hysteresis loop of bulk BiFe0.5Mn0.5O3, externally written in the structure through several cycles of dc electric field poling at EP ≈ 5.7 kV/cm, is shown in Figure 8 (together with the corresponding magnetic hysteresis loop23). The induced polarity was tested by means of a dynamic hysteresis measurement (DHM). The obtained saturation polarization PS ≈ 31 μC/cm2 is characteristic of a material with very significant polarization, comparable to the one reported for single crystals of the parent compound BiFeO340,41 even though slightly affected by a leakage current contribution. In parallel, a very interesting soft character is detected, with a coercive electric field EC ≈ 2.1 kV/cm, which is 10 or 100 times less than those of commercial ferroelectrics. We tentatively checked the room temperature polarization versus electric field response, but in this regime, the pure conductive contribution (as evidenced by all of the previous characterizations and by Figure S5) becomes predominant with respect to the dielectric one, so that we were not able to observe the persistence of this induced polar character at higher temperature. However, after the poling, the trend of the P versus E curve at low temperature significantly changes. The observation of a ferroelectric loop in this system, whose structure is found to be centrosymmetric, deserves some consideration. The system seems to respond to sufficiently high electric field poling cycles by irreversibly moving from an antiferroelectric (centrosymmetric) arrangement to a polar (noncentrosymmetric) arrangement. The DHM performed at 77 K on the sample, before stressing it with electric poling cycles, highlights the presence of a simple

Figure 6. Pyrocurrent response during thermal heating after a dc electric field cooling of 1.4 kV/cm.

(50 V) was applied at 290 K for 30 min, while the system was cooled to 5 K. The overall behavior of the pyrocurrent signal is quite complex, constituted by three principal regimes with opposite sign of the current flow. The high-temperature negative-sign region is characterized by conductive losses through the sample, as can be argued from the activation energy EA,pyro = 360 meV obtained by the Arrhenius plot of Figure S3 in the range T > TN: in fact, such a value is very close to the values of EA,bulk and EA,cryst. The discontinuity at T ≈ 345 K, which has never been observed in any other measurement, does not affect the value of EA,pyro and could be due to possible contact effects. This is confirmed by the fit performed in the adjacent range (between 340 and 295 K), characterized by almost the same activation energy (Figure S3). Below TN, a continuous reduction of the curve slope confirms the onset of a different transport regime (possibly the M-VRH) guided by stabilization of the antiferromagnetic ordering, which likely forces partial localization of the electrons on the Mn3+ site. At lower temperatures, the pyrocurrent is positive and originates from the sample thermal depolarization. This is coherent with the space group centrosymmetry, related to an antiferroelectric arrangement of the Bi ions that are significantly shifted from the neutral position, generating local electric dipoles that are compensated for by symmetry constraints. Looking at the red circles curve of Figure 7, the dc electric poling of 1.4 kV/cm seems to induce a sort of polar relaxation as the system is cooled, identified by the presence of a broad D

DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. (a) Ferroelectric loop obtained with a DHM, exploiting fast triangular electric pulses with a frequency of 2000 Hz. The measurement was performed at 77 K. (b) Magnetic hysteresis of BiFe0.5Mn0.5O3 at 100 K generated by weak ferromagnetism (see refs 21 and 23).

fluctuations (differently from the TA and ANHSP models, which both assume the crystallographic order of the structure). The very good quality of the data fittings suggests that the electric potential fluctuations may be reasonably associated with BiFe0.5Mn0.5O3 intrinsic disorder and inhomogeneity, which determine the presence of mesoscopic clusters with different concentrations of manganese and iron. Once the system is cooled, the incoming long-range magnetic order, guided by the magnetic iron-rich cluster regions (which get antiferromagnetically ordered at 420 K), forces gradual electron localization at the Jahn−Teller active Mn3+ ion site.21,23 Consequently, a variable-range-hopping polaronic state may be activated by the induced local distortion, while M-VRH transport would be disfavored along the iron-rich clusters from d5-electronic shell symmetry considerations. In this framework, the array constituted by Mn3+-rich clusters would represent a low resistive path for the current because the effective potential barrier is lowered by the negligible number of Fe3+ ions; therefore, polarons may preferably jump along macroscopic percolative arrays made by Mn3+ clusters (in close analogy to what happens for the so-called Efros−Shklovskii periodical granular arrays,42,43 verified experimentally in different compounds and morphologies44−46), so determining the observed low-dimensional transport mechanism, correctly described by the 1D M-VRH model (eq S4). On the contrary, when the system is heated in the paramagnetic phase (T > TN), the electrons on the manganese atoms are mainly involved in the delocalization process so the M-VRH polaronic state decades. In this context, it is not possible anymore for the delocalized electrons to distinguish manganese ions from iron ions at least in terms of the electrical potential. Hence, it is reasonable to figure the whole system as a pure disordered system and to state the space invariance of the electrical potential (TA mechanism; see complete treatise of this interpretation in the paragraph “transport analyses for bulk sample” provided in the Supporting Information). These considerations configure a strong correlation between the magnetic ordering process and evolution of the electrical properties of BiFe0.5Mn0.5O3, highlighting how magnetism is able to directly change the electronic structure at the cationic sites and allow the formation of an insulating phase (or vice versa). Such a fascinating coupling preludes the possible existence of macroscopic magnetoelectric or magnetoresistive effects in BiFe0.5Mn0.5O3.

dielectric response without hysteresis (Figure S4), indicating the induced nature of the ferroelectric loop. The pyrocurrent has been retested on the sample used for DHM, applying the same poling field (E = 1.4 kV/cm) of the first pyroelectric measurement, in order to check possible modifications induced in the sample. The results are reported in Figure 7. The symmetry with respect to the I = 0 axis for the curves corresponding to opposite poling fields [+E (blue squares) and −E (green diamonds)] states the polar nature of the observed current peaks. The pyrocurrent curves of the poled sample have the same shape as those of the unpoled sample (red circles of Figure 7); namely, they appear to result from convolution of major and minor peaks separated by about 15 K from each other. However, the poling procedure has reduced the intensity of the major peak and shifted its position to lower temperature, while the minor peak is unaffected by poling. An increase of the baseline current at temperatures above TP also can be noticed. The integrated charge increases with respect to the first measurement on the sample: this can be explained by assuming that the sample was effectively brought into a new polar state with an irreversibly higher residual polarization by means of the high field poling treatment applied during the ferroelectric cycle.

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DISCUSSION The electric characterizations carried out on BiFe0.5Mn0.5O3 reveal a phase diagram characterized by three distinct thermal regimes: T > TN = 288 K, TP ≈ 140 < T < TN, and T < TP, where TN is the Neél temperature and TP is the insulator− semiconductor transition temperature. 1. T > TN. This regime corresponds to the paramagnetic region of the material; here the system behaves as a band-gap semiconductor, and transport is described by a TA mechanism, with an activation energy of about 350 meV (confirmed in three different cases: by measurement of the material resistivity on both the bulk and single-crystal samples as well as by pyrocurrent measurements). In this regime, BiFe0.5Mn0.5O3 cannot be electrically polarized. 2. TP < T < TN. At the long-range antiferromagnetic transition, all of the performed characterizations detect a change of the transport properties, well described by M-VRH, usually employed to describe the resistivity thermal behavior of compositionally disordered phases,31,32 in which the transport mechanism is led by carriers localized by random potential E

DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX

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3. T < TP. In the lower temperature regime, below TP, the system undergoes an electric transition detected by different characterization techniques (dielectric, transport, and pyrocurrent measurements) and rapidly collapses into a dielectric insulating state by electron localization. In this regime, it is possible to polarize the system with an external electric field and to induce an irreversible antiferroelectric−ferroelectric transition, as observed by direct ferroelectric loop measurements and verified by pyroelectric measurements. In these conditions, at least at 77 K and below, the coexistence of ferromagnetic and ferroelectric hysteresis makes BiFe0.5Mn0.5O3 a multiferroic material, revealing stabilization of a noncentrosymmetric state induced by poling. The ferroelectric hysteresis, measured on a bulk phase for the first time, shows a soft nature, with a relatively low coercivity but a very high saturation; this is a remarkable result from the applicative point of view because it was obtained on a polycrystalline sample.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39 0521 269220. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The corresponding author acknowledges Fondazione Cariparma Credit Agricole for its financial support. Our colleagues at IMEM-CNR, Dr. Marco Calicchio and Dr. Roberto Mosca, are sincerely thanked for the sputtering deposition. aixACCT Systems GmbH in Aachen, Germany, is also thanked. The ESRF is acknowledged for beamtime allocation.

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REFERENCES

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CONCLUSIONS BiFe0.5Mn0.5O3, a metastable double perovskite synthesized at HP/HT conditions, shows a distorted, centrosymmetric orthorhombic cell (Pnam space group) induced by the Bi3+ stereochemical effect. As a consequence, the spatial symmetry reveals a global antiferroelectric character. The compositional inhomogeneity at the B site determines the presence of mesoscopic regions (clusters) with different concentrations of Mn3+ and Fe3+ ions, responsible for a large number of interesting properties. The antiferromagnetic long-range transition (TN = 288 K) arises from the magnetic Fe3+-rich clusters (TC = 420 K), which drives the ordering of the surrounding manganese paramagnetic ions. This process is accompanied by a gradual localization of εg electrons on the Jahn−Teller active Mn3+ site; the related local polar distortion leads to the activation of a polaron-mediated 1D M-VRH conductivity, confined along the arrays of clusters highly rich in manganese. This phenomenon vanishes below TP = 140 K, where the system crosses a semiconductor-to-insulator transition. Within this framework, complete stabilization of the antiferromagnetic order directly yields stabilization of a pure dielectric phase. In the low-temperature magnetic regime, the intrinsic antiferroelectricity is removed by the application of dc electric bias cycles: a polar phase is formed showing a ferroelectric hysteresis loop characterized by an extraordinarily soft character and a very high saturation polarization of 31 μC/ cm2. Surprisingly, the system retains the new written ferroelectric character also when heated to room temperature, transforming BiFe0.5Mn0.5O3 into a new multiferroic system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00961. Supporting structural refinement data, complete treatise and proposed interpretation of the electric transport process by means of comprehensive data analysis, general description of the ferroelectric measurement technique, and additional electric (ferroelectric) characterization in different thermal ranges and under different conditions (PDF) F

DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00961 Inorg. Chem. XXXX, XXX, XXX−XXX