Enhanced multiferroic and magnetoelectric properties of Ho, Mn co-doped Bi5Ti3FeO15 films

Materials Letters 164 (2016) 618–622

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Enhanced multiferroic and magnetoelectric properties of Ho, Mn co-doped Bi5Ti3FeO15 films Yulong Bai, Jieyu Chen, Ruonan Tian, Shifeng Zhao n School of Physical Science and Technology, Inner Mongolia University, Hohhot 010021, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 27 October 2015 Received in revised form 10 November 2015 Accepted 18 November 2015 Available online 30 November 2015

Ho, Mn co-doped Bi5Ti3FeO15 films were prepared and the microstructures, ferroelectric properties, piezoelectric properties, magnetic properties as well as magnetoelectric effect were investigated. It shows that the ferroelectric, piezoelectric, ferromagnetic, magnetoelectric properties are enhanced obviously after doping with Ho and Mn. The origin of thus enhanced multiferroic and magnetoelectric properties was discussed detailedly. The present results suggest a candidate for magnetoelectric materials. & 2015 Elsevier B.V. All rights reserved.

Keywords: Ferroelectrics Piezoelectric materials Magnetic materials

1. Introduction

2. Experiment

The coupling between magnetic and electric ordering in multiferroics, namely, magnetoelectric effect, has attracted lots of attention due to their potential application in electronics, spintronics and transducers [1]. However, most single-phase multiferroics have many difficulties such as offering ferroelectric and spin order at low temperature, which limit their practical application. So worldwide investigations have been undertaken to develop high performance multiferroics to overcome above difficulties. Recently Aurivillius phases bismuth layered structured system Bi5Ti3FeO15 (BTF) with a four-layered perovskite unit of (Bi3Ti3FeO13)2
sandwiched by two (Bi2O2)2 þ layers along c-axis [2], has shown multiferroic properties, which combines ferroelectricity and ferromagnetism. However, lower magnetoelectric coefficients does not satisfy the actual demand on magnetoelectric performance. Especially, its magnetoelectric coupling and piezoelectric responses have few been concerned. Therefore, this work aims to investigate the effect of Ho, Mn co-coping on the multiferroic and magnetoelectric properties for BTF films. Enhanced multiferroic and magnetoelectric properties are was obtained after doping with Ho and Mn. The origins of thus enhanced multiferroic and magnetoelectric properties were discussed.

Bi5Ti3FeO15 (BTF), Bi4.75Ho0.25Ti3FeO15 (BHTF), Bi5Ti3Fe0.95Mn0.05O15 (BTFM), Bi4.75Ho0.25Ti3Fe0.95Mn0.05O15 (BHTFM) films were prepared using chemical solution deposition. Bi(NO3)3  5H2O, Fe(NO3)3  9H2O, Ho(NO3)3  9H2O, Mn(CH3COO)2 and butyl titanate [CH3(CH2)3O]4Ti were dissolved in glycol to form the precursor solutions, which were deposited on Pt(100)/Ti/SiO2/Si wafer by spin coating. Finally, the films were annealed by the rapid thermal processor (RTP-500) at 700 °C. The crystalline phases were characterized by X-ray diffraction (XRD, Panalytical Empyrean). The surface morphologies and piezoelectric coefficients d33 were investigated by an piezoresponse force microscopy (PFM, Asylum Research Cypher™). Ferroelectric hysteresis loops were studied by a multiferroic tester system (MultiFerroic100V, Radiant Technology). The magnetic measurement was performed using a physical property measurement system (PPMS, Quantum Design). For the magnetoelectric coefficient (αE) measurement, a magnetic bias field Hbias together with a small alternating magnetic field Hac ¼7.35 Oe and frequency f¼1 kHz was applied parallel to the film plane. The induced magnetoelectric voltage VME was recorded by a lock-in amplifier (SR830, SRS Inc.).

n

Corresponding author. E-mail address: [email protected] (S. Zhao).

http://dx.doi.org/10.1016/j.matlet.2015.11.083 0167-577X/& 2015 Elsevier B.V. All rights reserved.

3. Results and discussion Fig. 1(a) shows the XRD patterns of BFT, BHTF, BTFM, BHTFM films. It shows all films exhibit a perovskite structure with a four layered aurivillius structure. No impurity phases indicates substitution doping. Fig. 1(b) presents the enlarged version of the XRD

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Fig. 1. (a) XRD patterns of BFT, BHTF, BTFM, BHTFM films; (b) the enlarged version of the XRD patterns in the rang of 2θ from 29.5° to 34°.

patterns in the rang of 2θ from 29.5° to 34°. Comparing with BTF films, Ho doping or Ho–Mn co-doping lead to (200) diffraction peak slightly shift towards the higher 2θ direction, which indicates that grain size slightly decrease. This is attributed to the changing in the lattice parameters and the reduced average A-site radius derived from Ho3 þ ions with smaller radius replacing Bi3 þ ions. What's more, the separation of (119) peak indicates a structural transformation from the orthorhombic group F2mm to A21am after A-site Ho doping or Ho–Mn co-doping [3]. Thus structural transformations are put down to the lattice distortion. It is well known that the Goldschimidt tolerance factor τ determine the distortion degree of ABO3 perovskite and the stability [4]. τ reduces after doping with Ho and Mn due to the smaller ionic radius of Ho3 þ than that of Bi3 þ and the larger radius of Mn3 þ than that of Fe3 þ . The change indicates that Fe3 þ /Mn3 þ –O bonds and Bi3 þ /Ho3 þ –O bonds are under the compression or tension forces. In order to reduce the lattice stress, oxygen octahedron rotates cooperatively, which leads to the crystal structural transformation. However, comparing with BHTF and BHTFM films, both the separation of (119) peak and shifting of (200) peak are not obvious for BFFM films. It is because that the change of τ derived from Mn doping is much smaller than that of Ho doping. Fig. 2 presents AFM images of BTF, BHTF, BTFM, and BHTFM films. It shows BTF films are assembled with non-uniform rods, with length of  600 nm, and diameter of  270 nm. Some voids can be observed from the surface morphology. Compared with BTF films, the morphology of BTFM, BHTF and BHTFM films change greatly. BTFM becomes more uniform because rods are shortened with length of 450 nm and diameter of 220 nm. Thus decreased size leads to the decreased voids. It is worth noting that the films are assembled by the spheral particles with average diameter of 200 nm after doping with Ho. Thus particles assembling makes the films more smooth and density. What's more, for BHTFM films, the average diameter of the particles further decreases to 80 nm. Thus smaller particles make the films more smooth and density. Therefore, compared with BTF films, the more smooth and uniform films without voids were achieved after co-doping Ho and Mn. Similar results were reported by other studies [5–7]. Thus drastic changes of the morphology with doping is attributed to the fact that doping can effectively suppress the oxygen vacancy

concentration, which results in slower oxygen ion motion and consequently lowers the grain growth rate, finally forms smaller grain size and highly density films. Fig. 3(a) presents well-defined ferroelectric loops for all films. The saturation polarization Ps and the remnant polarization Pr is  46.54 mC/cm2 and 22.04 mC/cm2, respectively, at 450 kV/cm for BTF films. In comparison, Ps for BHTF, BTFM, and BHTFM films are 52.36 mC/cm2, 55.3 mC/cm2, and 69.4 mC/cm2, respectively. Correspondingly, Pr are improved to 24.4 μC/cm2, 27.3 μC/cm2 for BTFM, BHTF films, respectively. Especially, a larger Pr is obtained for BHTFM films, being 43.6 μC/cm2. Those show that the ferroelectric properties are improved after doping with Ho and Mn. Fig. 3(b)– (e) displays the applied voltage dependence of the piezoelectric response for BTF, BTFM BHTF, BHTFM films. They display complete butterfly shaped piezoresponse curves under the external electrical field. A voltage induced displacement was 1.23 nm at 15 V for BTF films with a deformation 1.23‰ ratio. The displacement is 2.51 nm with 2.51‰ ratio and 2.63 nm with 2.63‰ ratio for BTFM and BHTF films. However, for BHTFM films, the displacement is 4.15 nm with 4.15‰ deformation ratio. The piezoelectric hysteresis loops (d33–V) are calculated from the D–V curves based on the law of converse piezoelectric effect, which show that all films are switchable and ferroelectricity is retained. The piezoelectric coefficient d33 of BTF, BTFM, BHTF and BHTFM films is 66 pm/V, 188 pm/V, 204 pm/V and 269 pm/V, respectively. Those above show the ferroelectric and piezoelectric properties are greatly improved after doping with Ho, and Mn, especially co-doping Ho and Mn. Thus improved ferroelectric and piezoelectric properties are ascribed to four factors. Firstly, it comes from the structural transformation induced by Ho, Mn substitution doping. The orthorhombic structure lattice distortion changes the switching behavior of the polarization path to improve the ferroelectricity and piezoelectricity. Secondly, A-site rare earth doping can suppress the missing of bismuth and thus decrease chemical mismatch, so suppress domain pinning effect. As a result, improved ferroelectric and piezoelectric properties were obtained. Thirdly, the higher ferroelectric transition temperature (Tc) leads to the larger ferroelectric polarization and d33 [8]. And Tc of Aurivillius phase materials is inversely proportional to τ [4], which decreases after doping with Ho and Mn. Hence the improved ferroelectric and piezoelectric

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Fig. 2. Typical AFM images of BFT, BHTF, BTFM, BHTFM films.

properties are obtained. Lastly, the particle size decreases to form small domains, which are easy to switch. These substitutions can induce a chemical stress to the structure, change lattice constant, and even affect electronic structure through changing angle and length of Fe–O–Fe bonding [9]. So BTFM, BHTF and BHTFM films exhibit the enhanced ferroelectricity and peizoelectricity. However, the present films can’t applied a high electric filed due to the relatively higher leakage current, which showed the existence of defects, which is similar to the other report [6]. The magnetic hysteresis loops are shown in Fig. 4(a). The saturation magnetization Ms, remnant magnetization Mr, and coercity Hc are improved obviously after doping with Ho and Mn. Ms are 1.52 emu/cc, 3.04 emu/cc, 4.31 emu/cc and 4.83 emu/cc for BTFO, BHTFO, BTFMO and BHTFMO films, respectively. Correspondingly, Hc are 130 Oe, 150 Oe, 400 Oe, and 440 Oe, respectively. It shows BHTFM films exhibit the largest values of both the magnetization and coercity. Thus enhanced magnetic properties

are account for the lattice distortion. Different radii between Ho3 þ and Bi3 þ , as well as between Fe3 þ and Mn3 þ induce lattice distortion and tilting of FeO6 octahedral after doping Ho and Mn. Fe3 þ –O–Mn3 þ 180° super-exchange interaction could bring about ferromagnetic (FM) charter after doping with Mn, which destroyed Fe–O–Fe anti-ferromagnetic super-exchange interactions and further greatly enhance its magnetic properties [10]. Fig. 4(b) presents the magnetoelectric coefficient αE of these films with an applied magnetic bias field Hbias. A weak magnetoelectric coupling was observed for BTF films. With increasing Hbias, αE slowly increases, reaching the maximum of 4.4 mV/cm  Oe at Hbias ¼5.6 kOe, and then drops slowly. Similar change tendency is observed for BTFM films. However, an obvious improved αE ¼14.1 mV/cm Oe at Hbias ¼6.6 kOe. Different from BTF and BTFM films, αE rapidly increases with increasing Hbias for BHTF and BHTFM films, reaching 12 mV/cm Oe and 24.8 mV/cm Oe at at Hbias ¼3.6 kOe, which is 5 and 10 times of BTF films, respectively.

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Fig. 3. The polarization (P)-electric field (E) hysteresis loops, displacement curves and piezoelectric coefficients d33 of BFT, BHTF, BTFM, BHTFM films at room temperature.

Thus results show that the magnetoelectric effect is enhanced after doping with Ho and Mn, which are attributed to its enhanced electric and magnetic properties for BHTFM films. To be specific, the mechanisms of magnetoelectric effect is understood the interaction between electric and magnetic phases. A magnetic field applied to the materials will induce strain, passing along to the piezoelectric phase, which induces an electric polarization. The coupling nature depends on the mechanical coupling between (Bi2O2)2 þ and (Bi3Ti3FeO13)2
phases for the present films. If the transfer of stress from the (Bi3Ti3FeO13)2
magnetostrictive phase to the (Bi2O2)2 þ piezoelectric phase is strong enough, a high magnetoelectric coefficient will be obtained. The ferroelectric, piezoelectric and magnetic properties for BHTFM films are enhanced

greatly. Thus the largest αE is believable. Besides, the enhanced magnetoelectric properties are attributed to the lattice distortion accompanying with the structural transformation, which changes the rotation directions of the electric polarization and ferromagnetic moment. Modification of sublattice magnetization due to charge displacement within the unit cell, effective coupling between ferromagnetic and ferroelectric ordering, geometrically driven ferroelectricity with accompanying magnetic order, and modification of magnetic superexchange by lattice distortion, were identified as mechanisms [11]. Another is attributed to the fact that the energy barrier are decreased by the spin polarized charge after doping Mn3 þ , which make it more easy for the movement of domain walls to obtain the giant magnetoelectric effect.

Fig. 4. (a) Magnetic hysteresis loops for BFT, BHTF, BTFM, BHTFM films; (b) Magnetoelectric effect coefficient αE as a function of bias magnetic field.

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4. Conclusions

References

In summary, Ho–Mn co-doped BTF films were prepared by chemical solution deposition. The phase structure, topography, ferroelectric, piezoelectric and magnetizatic properties were investigated. A giant magnetoelectric coupling effect was achieved for Ho–Mn co-doped BTF films at room temperature. The present work suggests a potential mutiferroic material with giant magnetoelectric coupling effect.

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant nos. 11264026 and 11564028), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant no. 2014JQ01).