Magnetodielectric properties of Bi4NdTi3Fe0.7Co0.3O15 multiferroic system

Journal of Alloys and Compounds 622 (2015) 288–291

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Magnetodielectric properties of Bi4NdTi3Fe0.7Co0.3O15 multiferroic system X.Q. Chen a,b,⇑, Y. Xue a, Z.W. Lu a, J. Xiao a,b, J. Yao a, Z.W. Kang a, P. Su a, F.J. Yang a,b,⇑, X.B. Zeng c, H.Z. Sun d a

Faculty of Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, PR China Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan, Hubei 430062, PR China School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China d Seismological Bureau of Liaoning Province, Shenyang, Liaoning 110034, PR China b c

a r t i c l e

i n f o

Article history: Received 15 September 2014 Received in revised form 6 October 2014 Accepted 13 October 2014 Available online 19 October 2014 Keywords: Bi5Ti3FeO15 Ceramics Multiferroic Magnetodielectric

a b s t r a c t Bi4NdTi3Fe0.7Co0.3O15 polycrystalline samples were synthesized following a multicalcination procedure. X-ray analysis indicated a four-layer Aurivillius phase with an orthorhombic symmetry was obtained. The multiferroic properties of the sample at room temperature were demonstrated by the ferroelectric (2Pr = 7.23 lC/cm2, 2Ec = 35.72 kV/cm at applied electric field 70 kV/cm) and magnetic (2Mr = 232 m emu/g, 2Hc = 769 Oe at applied magnetic field 1.07 T) hysteresis loops. Obvious magnetodielectric effect which is dependent on frequency was observed at room temperature. It is found that the magnetodielectric effect exhibits a linear relation with M4 rather than M2 which may be due to the complicated spin-pair correlation interactions among next nearest neighbors. The present study suggests the possibility of magnetic ions doped Bi4NdTi3FeO15 as a potential candidate for novel multifunctional device application. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Multiferroic materials, which exhibit simultaneous ferroelectric and magnetic orders, have attracted a lot of interest in recent years due to their potential applications for developing novel storage media and spintronic devices [1,2]. In addition, the magnetoelectric (ME) coupling between the two orderings provides an additional degree of freedom in device design [3–5]. Although a number of materials exhibiting both ferroelectricity and magnetism were studied, unfortunately, single-phase multiferroic materials with ME effect at room temperature are very rare because not all the multiferroic materials possess ME effect [6]. And it is the ME coupling that can in principle permit data to be written electrically and read magnetically, then provide an additional degree of freedom in the design of novel multifunctional devices and realize the multi-state data storage. Thus, the search for multiferroic materials having a large ME effect at or above room temperature is ongoing. Recently, (BiFeO3)nBi4Ti3O12 (n = 1) (BTF), which can be viewed as inserting the well-known multiferroic BiFeO3 unit into the ⇑ Corresponding authors at: Faculty of Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, PR China. Tel.: +86 27 88663390. E-mail addresses: [email protected] (X.Q. Chen), [email protected] (F.J. Yang). http://dx.doi.org/10.1016/j.jallcom.2014.10.080 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

typical ferroelectric compound Bi4Ti3O12 (BTO), has been reconsidered as one of the candidates in single-phase multiferroics because of its potential to present both ferroelectric and ferromagnetic transitions at the same time in a single-phase [7–11]. To date, BTF based compounds have been extensively fabricated in either film or ceramic forms using different preparation routes, and the phase transition, electric and magnetic properties have been investigated. Ferroelectric and ferromagnetic curie temperatures were determined to be higher than the room temperature [12–14]. It has been proposed that the substitution of lanthanide ions for Bi ions could improve ferroelectric properties which can be attributed to the reduction in the oxygen vacancy concentration and the weakening of the defect mobility contributing to domain pinning due to the substitution [13,15,16]. The effect of magnetic ions doping on the magnetic properties of BTF was also studied and it is found that Co or Ni doped BTF systems exhibit enhanced magnetic performance at room temperature [17,18]. As for ME coupling effect, it can be measured by a magnetic response to an applied electric field or an electrical response to an applied magnetic field. For the latter scenario, change of the dielectric constant under an applied magnetic field, always described as ‘magnetodielectric (MD)’ or ‘magnetocapacitance’, is an important characterization of ME coupling effect in single-phase multiferroic materials. So far, although there are several reports about the influence of

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applied magnetic field on dielectric constant in BTF systems [8,19,17,18], detailed report on MD effect of BTF compounds is still lack. Therefore, in this paper, we present a systematic study of MD properties of multiferroic Bi4NdTi3Fe0.7Co0.3O15 (BNTFC) ceramics prepared by a multicalcination procedure. 2. Experimental Polycrystalline samples with nominal composition Bi4NdTi3Fe0.7Co0.3O15 were prepared by two steps. Firstly, preparing BTO powders. Bi2O3 (analytic pure) with 15 wt% excess and TiO2 (spectral pure) powders were mixed by ball milling with ethanol and agate for 12 h, following calcinations at 760, 780, and 800 °C for 8, 16, and 24 h respectively to obtain BTO powders. Secondly, the obtained BTO powders were mixed with Fe2O3, Co2O3, Nd2O3 and Bi2O3 (analytic pure) agents in stoichiometry. Final mixtures were ball milled for 12 h, sintered at 640 °C for 8 h, and then ball milled again for 12 h. The presintered powders were pressed into pellets with a diameter of 10 mm and a thickness of 1 mm. The discs were sintered at 850 °C for 6 h in air and then furnace cooled to room temperature. The crystal structure of the sample was examined by a Bruker D8 ADVANCE Xray diffractometer (XRD) with Cu Ka radiation. The refinement of the lattice parameters was carried out by the Cohen least-squares method using our own computer program. For electrical measurement, the discs were lapped and polished to about 0.2 mm thick. Silver was pasted on both surfaces as electrodes. The ferroelectric properties were measured using a precision workstation ferroelectric tester system (Radiant Technologies) at room temperature. A vibrating sample magnetometer (VSM) was used to measure room temperature magnetic hysteresis loops. The MD properties of the capacitors were determined by low-frequency impedance analyzer (WK-6420), with an applied voltage of 0.5 V and applied magnetic field ranging from 0 T to 1 T.

3. Results and discussion Fig. 1 shows the XRD pattern of the BNTFC polycrystalline sample. The result indicates that the BNTFC ceramic is of Aurivillius phase with four-layer perovskite structure (Joint Committee on Powder Diffraction Standards (JCPDS) No. 38–1257). Assuming that the BNTFC has an orthorhombic structure, the lattice parameters can be calculated to be a = 5.4057 Å, b = 5.4169 Å, c = 41.5252 Å, which are slightly smaller than those of undoped BTF. This is caused by the ionic radius differences between bismuth and neodymium ions, iron and cobalt ions (r(Bi3+) = 0.103 nm, r(Nd3+) = 0.0983 nm, r(Fe3+) = 0.0645 nm, r(Co3+) = 0.061 nm) [20]. Fig. 2a gives the P–E hysteresis loops of the BNTFC sample measured at room temperature. The typical P–E loops although somewhat leaky have been observed. This leakage-related contribution was also confirmed in earlier reports [17,19,21] and seems to be intrinsic. However, as for BTF [19] and Bi5Ti3Fe0.5Co0.5O15 [17] systems, the positive-up–negative-down measurements confirmed that the ferroelectricity of BTF systems, although leaky, is intrinsic and natural and the leakage current does not play a major role in the ferroelectric behavior. For our BNTFC samples, Nd was selected to substitute for volatile Bi. And it is reported that the substitution

Fig. 1. X-ray h–2h diffraction pattern of BNTFC ceramic.

of rare earth elements such as La, Nd for Bi is effective to obtain superior ferroelectric properties including a low leakage current density [22]. Therefore, it is reasonable to refer that the contribution of leakage current to the ferroelectric behavior is small. The measured maximum remnant polarization 2Pr and coercive field 2Ec values are about 7.23 lC/cm2 and 35.72 kV/cm respectively. Fig. 2b shows the result of the magnetic measurement at room temperature. The sample exhibits a typical ferromagnetic M–H loop, with remnant magnetization (2Mr) of 232 m emu/g and coercive field (2Hc) of 769 Oe respectively. The measured 2Mr is much larger than that of undoped BTF (122 l emu/g) [23], which might attributed to the occurrence of strong direct Fe–O–Co interaction when the doping concentration of magnetic ion (here refers to Co) is 30% [24] and the release of large latent magnetization locked in the antiferromagnetic state because of the radii differences of Bi and Nd, Fe and Co ions. According to the results of Fig. 2(a) and (b), it can be concluded that BNTFC sample possesses room temperature multiferroic properties. Fig. 3 exhibits the dielectric constant (e) and loss tangent (tan d) as functions of frequency ranging from 20 Hz to 2 MHz for BNTFC sample under zero and 1 T magnetic field at room temperature. From Fig. 3a, it can be seen at the first glance that, regardless of the applied magnetic field, e drops dramatically with rising frequency which is defined as dielectric dispersion behavior, and it is also signified for the appearance of Debye-like dielectric loss peaks as shown in Fig. 3b. Except that, most importantly, the values of e and tand are enhanced by the application of magnetic field, and the increase is more obvious in the low frequency range than in the high frequency range, as shown in Fig. 3b and the insets of Fig. 3(a) and (b). The MD value, which is defined as eð0Þ MD ¼ Dee  100 ¼ eðHÞ eð0Þ  100, is determined to be about 0.12% at

200 kHz. The value is smaller than the reported value of 0.39% measured at 100 kHz in Bi5Fe0.5Co0.5Ti3O15 thin film due to the higher measuring frequency of 200 kHz [17]. What’s more, the frequency corresponding to the dielectric loss peak does not shift with the application of magnetic field which means the applied magnetic field has no effect on interfacial polarizations in the low frequency. Fig. 4 shows the magnetic field dependences of MD response at 200 kHz together with the changes in the M2 (Fig. 4a) or M4 (Fig. 4b). From Fig. 4, we can see that the observed MD effect increases with increasing of the magnetic field. The origin of anomaly in e on the magnetic order of ferroelectromagnets could be explained in the framework of the Ginzburg–Landau theory for the second-order phase transition [25]. In a ferroelectromagnet, the thermodynamic potential U can be written in the form

b 2

U ¼ U0 þ aP2 þ P4  PE þ a0 M2 þ

b0 4 M  MH þ cP2 M 2 2

where P, M, E and H are the polarization, magnetization, electric field and magnetic field, respectively and a, b, a0 , b0 and c are the coefficients of Landau free energy terms and their nature can be dependent on type of magnetic phase transition. The term cP2M2 describes the ME coupling between P and M. According to the investigation by Kimura et al. [26] and considering e / oU2/oP2, one can obtain De  c M2. According to the relation, the MD effect should trace the M2 in a changing magnetic field and show a linear relation with M2. However, from Fig. 4(a) and (b), it can be observed that the MD effect traces the M4 rather than the M2. What’s more, as shown in the inset of Fig. 4(a) and (b), it is a higher order magnetization M4 not M2 that holds the linear relation with MD. Such a linear relation between M4 and MD was also observed in double perovskite La2 CoMnO6 nanoparticles [27] and Bi2NiMnO6–La2NiMnO6 superlattices [28]. It is reported that below the ferromagnetic transition temperature, a higher order coupling would take place due to the

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(a)

(b)

10

M (memu/g)

P (μC/cm 2 )

5 0 -5

300 200 100 0 -100 -200 -300

-10

-60

-30

30

0

60

-10

0

-5

E (kV/cm)

5

10

H (kOe)

Fig. 2. (a) P–E hysteresis loops and (b) magnetic hysteresis loop of BNTFC ceramic measured at room temperature.

2.5

tanδ

304 302

1000

0.12

3.0

ε

1500

ε

(b) 3.5

H=0T H=1T

306

MD (%)

(a) 2000

500k

0.04 0.00

1.5

700k 1.4M

f (Hz) H=0T H=1T

1.0

1M 1.5M2M

f (Hz)

500

2.0

0.08

0.5 0.0

0

100

1k

10k

100k

1M

100

f (Hz)

1k

10k

100k

1M

f (Hz)

0.12

0.04 0.00 0.05

M2

0.09

0.10

0.06 0.03 0.00 -10

-5

0

5

(b)

10

0.024 0.021 0.018 0.015 0.012 0.009 0.006 0.003 0.000

0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

0.10

MD

MD

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0.08

M 4(emu/g) 4

0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

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M 2(emu/g)2

(a)

0.05 0.00 0.008

0.016

0

5

M4

-10

H (kOe)

-5

MD (%)

Fig. 3. The frequency dependences of dielectric constant e (a) and loss tangent tan d (b) for BNTFC capacitor under zero and 1 T magnetic field. The inset of (a) is magnified high frequency part, while the inset of (b) is the frequency dependence of MD.

10

H (kOe)

Fig. 4. Magnetic field dependences of (a) the M2 and (b) the M4 together with the MD effect measured at 200 kHz. The insets of (a) and (b) are the variations of MD with the M2 and the M4.

complicated spin–pair correlation interactions among next nearest neighbours [27]. Combined with the excellent magnetic hysteresis loop (shown in Fig. 3b) and the ferromagnetic transition temperature of 497 K in Bi4NdFe0.5Co0.5Ti3O15 ceramic, similar composition with BNTFC, reported by us [13], it can be inferred the present BNTFC sample is of the ferromagnetic state at room temperature. Certainly, further theoretical and experimental study would be required to understanding clearly the origin behind such a higher order coupling of MD effect in ferromagnetic regime. 4. Conclusion In summary, we have prepared Bi4NdTi3Fe0.7Co0.3O15 polycrystalline samples by a multicalcination procedure. Single phase of four-layered Aurivillius structure with an orthorhombic symmetry was obtained. The sample exhibits room temperature multiferroic properties with 2Pr = 7.23 lC/cm2, 2Ec = 35.72 kV/cm, 2Mr = 232 m emu/g, and 2Hc = 769 Oe. The dielectric constant increases with increasing the magnetic field but decreases with rising frequency

measured at constant magnetic field. It is found that the MD effect exhibits a linear relation with M4 rather than M2 which may be due to the complicated spin-pair correlation interactions among next nearest neighbors. The present study suggests the possibility of magnetic ions doped Bi4NdTi3FeO15 as a potential candidate for novel multifunctional device application. Acknowledgements The authors are grateful for financial support from the National Nature Science Foundations of China under Grant Nos. 51002047, 51202062 and 11274101. References [1] M. Fiebig, J. Phys. D Appl. Phys. 38 (2005) R123. [2] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L.M. Ardabili, T. Zhao, L.S. Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Science 303 (2004) 661. [3] T. Lottermoser, M. Fiebig, Phys. Rev. B 70 (2004) 220407.

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