Room-temperature multiferroic properties of Bi4.15Nd0.85Ti3FeO15 thin films prepared by the metal–organic decomposition method

Solid State Communications 150 (2010) 1646–1649

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Room-temperature multiferroic properties of Bi4.15 Nd0.85 Ti3 FeO15 thin films prepared by the metal–organic decomposition method Fengzhen Huang, Xiaomei Lu ∗ , Cong Chen, Weiwei Lin, Xiaochun Chen, Junting Zhang, Yunfei Liu, Jinsong Zhu National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, Nanjing 210093, People’s Republic of China

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Article history: Received 4 February 2010 Received in revised form 19 March 2010 Accepted 19 June 2010 by T. Kimura Available online 26 June 2010 Keywords: A. Thin films B. Metal–organic decomposition D. Multiferroic property

abstract Aurivillius-structured Bi4.15 Nd0.85 Ti3 FeO15 multiferroic thin films with four perovskite slabs were deposited on Pt/Ti/SiO2 /Si substrates by the metal–organic decomposition method. The structural, dielectric and multiferroic properties of the films were investigated. Good ferroelectric behavior along with large dielectric constant and small loss factor were observed at room temperature. A weak ferromagnetic rather than an antiferromagnetic property was observed at room temperature by magnetic measurement. Moreover, the ferromagnetic property was enhanced when the temperature was below 13 K and a large saturation magnetization of about 5.4 emu/cm3 was obtained at 4 K. Possible reasons are put forward to discuss the complicated magnetic property. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Multiferroic materials simultaneously present both ferroelectric and spin orders, which enable them to have potential applications in both magnetic and ferroelectric devices [1–3]. Moreover, the coupling of electric and magnetic polarization in these materials provides an additional degree of freedom in device design [4]. Besides these potential applications, the fundamental physics of multiferroic materials is also fascinating from the viewpoint of science [5]. So far, the most frequently investigated multiferroic materials are BiMnO3 and BiFeO3 , offering ferroelectric order and spin order at relatively high temperature [6–9]. Unfortunately, BiMnO3 and BiFeO3 have some drawbacks, especially their low resistivity, which degenerate the electric and coupling properties of the materials. As alternatives, some single-phase magnetoelectric Bi-layered oxides, such as Bi5 Ti3 FeO15 (BTF), with high roomtemperature (RT) resistivity have been developed. BTF, composed of four perovskite layers sandwiched by two (Bi2 O2 )2+ layers, can be prepared by inserting the well-known multiferroic BiFeO3 unit into the typical ferroelectric compound Bi4 Ti3 O12 . The (Bi2 O2 )2+ layers play key roles as both space-charge compensation and insulation, which can effectively reduce the leakage current. However, polycrystalline Bi4 Ti3 O12 has a small polarization of about 10 µC/cm2 and poor fatigue endurance; thus almost all the results

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about BTF material also show poor ferroelectric properties with small polarization. It is known that La-modified or Nd-modified Bi4 Ti3 O12 material shows excellent fatigue resistance with larger remanent polarization Pr [10–12]. In particular, Bi3.15 Nd0.85 Ti3 O12 (BNT) films show large polarization and fatigue-free characteristics. Moreover, Nd dopant can also improve the multiferroic and electric transport properties of BiFeO3 [13]. Thus, in this paper, we report the structural, electric and magnetic properties of Bi4.15 Nd0.85 Ti3 FeO15 (BNTF) thin films prepared by the intergrowth of BiFeO3 and BNT. 2. Experiments The metal–organic decomposition (MOD) method was employed to fabricate the films because of its advantages in mass production. Bismuth nitrate, neodymium nitrate, titanium isopropoxide, and iron nitrate were used as the precursors for Bi, Nd, Ti, and Fe, respectively, and acetic acid was the solvent. A 10 mol% excess of bismuth nitrate was added to compensate for possible bismuth loss during the high-temperature process. The films were spin-coated on (111) Pt/Ti/SiO2 /Si substrates at 3000 rpm for 25 s, dried at 200 °C for 3 min and then prefired at 400 °C for 5 min in air. The above processes were repeated several times to get the desired film thickness. The coated films were crystallized at 750 °C for 50 min in O2 . To measure the electrical properties of the films, Pt dot electrodes were deposited by sputtering through a shadow mask. Finally, a post-annealing procedure at 500 °C for 10 min was applied to insure good contact between the

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Fig. 1. X-ray diffraction patterns of the BNTF film.

-400

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Fig. 3. RT ferroelectric hysteresis loops of the BNTF film measured at various applied electric fields.

Normalized Polarization

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Fig. 2. SEM surface morphology (a) and cross-sectional image (b) of the BNTF film.

Pt electrodes and the films. The microstructures of the films were analyzed by X-ray diffraction (XRD, D/Max-RB) with Cu Kα radiation and scanning electron microscopy (SEM 1530YP, Leo Co., Germany). The ferroelectric and magnetic properties were measured by using an RT66A standard ferroelectric test unit and a superconducting quantum interference device (SQUID, Quantum Design, XL-7) magnetometer, respectively. The dielectric properties were evaluated using an HP4194 impedance/phase analyzer. 3. Results and discussions Fig. 1 shows the X-ray diffraction patterns of the BNTF films. The peaks are indexed on the basis of an orthorhombic cell [14]. It is seen that the BNTF films exhibit single-phase perovskite structure, and no other impurity diffraction peaks are observed, which indicates that an iron–oxygen octahedron is successfully inserted into the BNT films and an Aurivillius structure with four perovskite slabs is achieved. Fig. 2 presents the SEM micrographs of the BNTF films. The surface micrograph (Fig. 2(a)) indicates a well-grown polycrystalline film with the grain size varying from 100 to 120 nm, while the cross-sectional picture (Fig. 2(b)) enables us directly obtain the film thickness of about 370 nm. The P–E hysteresis loops of the BNTF films were measured at RT. As shown in Fig. 3, the films exhibit well-saturated hysteresis loops with a large remanent polarization (2Pr ) of about 43 µC/cm2 and a large coercive field (2Ec ) of about 350 kV/cm at a maximum applied electric field of 470 kV/cm. This 2Pr value is found to be greatly enhanced in comparison with that of BTF materials [15–18]. The remanent polarization of ferroelectric materials has been reported to be affected by various factors, such as the displacement of polar ions [19], domain pinning by defects [20], the orientation

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Fig. 4. Fatigue behavior of the BNTF film.

of grains [10], etc. In this case, similar to Nd-doped Bi4 Ti3 O12 materials, the improved ferroelectricity probably comes from the tilted titanium–oxygen and/or iron–oxygen octahedra induced by Nd doping. Moreover, the decrease of amount of chemical defects such as bismuth vacancies and the accompanied oxygen vacancies may be another contribution to the enhanced ferroelectric property. Fig. 4 presents the result of the fatigue test performed at RT using 286 kV/cm and a 50 kHz bipolar square wave. The normalized polarization decreases to 75% after almost 109 switching cycles. Though the fatigue endurance behavior of BNTF film is improved compared with pure BTF material [21], it is still not as good as that of BNT films [10]. It is widely accepted that oxygen vacancies [22] and internal stress [23] should be mainly responsible for the fatigue behavior. Similar to BiFeO3 material, during annealing at high temperature, Fe2+ ions may be present in BNTF films, and they will be accompanied by oxygen vacancies so as to reach charge balance. As a result, the content of oxygen vacancies in BNTF films could be higher than that in BNT films. Moreover, the difference in chemical valence, ion radius and binding energy between Fe ions and Ti ions should give rise to more internal stress in BNTF films. These two factors may combine and thus result in the fatigue behavior of BNTF films. The dielectric characteristics of materials are also important and interesting for their electrical applications. Fig. 5 shows the RT

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dielectric constant ε and loss factor tan δ of BNTF films as a function of frequency. It is can be seen that ε decreases while the loss factor increases slightly with increasing frequency, and no sudden changes are observed in both ε and tan δ curves. Compared with the results in BTF films [15], a larger dielectric constant but a lower loss factor (189 for ε and 0.023 for tan δ at the measuring frequency 10 kHz) are obtained for our BNTF films in the whole measuring frequency range, which may be induced by Nd substitution. Since BNTF can be viewed as an atomic stacking of the threelayered ferroelectric compound BNT and the well-known multiferroic BiFeO3 , its multiferroic behavior becomes immediately attractive. It was reported that BTF materials exhibit an antiferromagnetic state at RT [24,25]. In order to illustrate the magnetic ordering in our BNTF films, the temperature (T ) dependence of magnetization with zero-field cooling (ZFC) and 0.2 T field cooling (FC) were measured, and the results are presented in Fig. 6(a). It is shown that the FC and ZFC curves separate at around 150 K and the difference between them (MFC − MZFC ) increases as the temperature decreases. However, the FC and ZFC curves merge at about 13 K and then M abruptly increases with further decreasing temperature. This temperature-dependent behavior of magnetization is generally indicative of antiferromagnetic ordering [26], and is probably translated to the ferromagnetic state when the temperature is below 13 K. More interestingly, a magnetic hysteresis loop with small saturation magnetization Ms of about 0.31 emu/cm3 is observed at RT for the BNTF film. There are two possible reasons for the weak RT ferromagnetic property. On the one hand, Nd doping in BTF materials may tilt the iron–oxygen octahedra, and thus the latent magnetization locked in the antiferromagnetic state might be released. On the other hand, the small Ms at RT probably comes from the local ferromagnetic Fe–O clusters which are formed due to the random distribution of iron–oxygen and titanium–oxygen octahedra. As a result, a weak RT ferromagnetic property is observed over the dominating antiferromagnetic background. When the temperature is below 13 K, the antiferromagnetic interaction may change to ferromagnetic interaction, and thus the enhanced ferromagnetic property with large Ms of about 5.4 emu/cm3 is obtained at 4 K. Though small Ms and good ferroelectric property with large polarization are observed in BNTF films at RT, the coupling between them cannot be easily detected, probably due to the weak magnetic property. In order to enhance the coupling and the multiferroic properties of BNTF films, it is necessary to improve their magnetic characteristics by magnetic ion doping. Further investigation in this direction is in progress.

b Magnetization M (emu/cm3)

Fig. 5. Frequency dependence of dielectric constant and loss of the BNTF film measured at RT.

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Fig. 6. (a) Magnetization as a function of temperature measured under ZFC and 0.2 T FC cases. (b) Magnetization–field curve measured at RT for the BNTF film. The inset shows the magnetic hysteresis loop measured at 4 K for the BNTF film.

4. Conclusions In summary, polycrystalline BNTF films were prepared on Pt/ Ti/SiO2 /Si substrates by the metal–organic decomposition method. Large ferroelectric polarization and dielectric constant and small loss factor were obtained at RT. Though antiferromagnetic interaction dominated the magnetic properties of the BNTF films, a small Ms was obtained at RT probably due to the tilted iron–oxygen octahedra induced by Nd doping and/or the local Fe–O clusters formed due to the random distribution of iron–oxygen and titanium–oxygen octahedra. Moreover, an enhanced ferromagnetic property with large Ms of about 5.4 emu/cm3 was observed at 4 K. The present work highlights the possibility of using BNTF film as a RT multiferroic material, preferably via magnetic ion doping in order to enhance its magnetic properties. Acknowledgements This work was supported by the National Science Foundation (No. 50832002, 50972056), the 973 Project of MOST (Nos. 2006CB921804, 2009CB929501), NCET-06-0443, and NJU test foundation. References [1] M. Fiebig, J. Phys. D 38 (2005) R123. [2] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [3] T. Goto, T. Kimura, G. Lawes, A.P. Ramirez, Y. Tokara, Phys. Rev. Lett. 92 (2004) 257201.

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