Frequency-dependent magnetoelectricity of CoFe₂O₄-BaTiO₃ particulate composites

Atchara KHAMKONGKAEO¹, Pongsakorn JANTARATANA ², Chitnarong SIRISATHITKUL³,

Teerapon YAMWONG⁴, Santi MAENSIRI⁵

1. Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand;

2. Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand;

3. Magnet Laboratory, School of Science, Walailak University, Nakhon Si Thammarat 80161, Thailand;

4. National Metals and Materials Technology Center (MTEC), Thailand Science Park, Pathumthani 12120, Thailand;

5. School of Physics, Institute of Science, Suranaree University of Technology,Nakhon Ratchasima 30000, Thailand

Received 6 January 2011; accepted 13 June 2011

Abstract: CoFe₂O₄-BaTiO₃ particulate composites were prepared by wet ball milling method, their magnetoelectric (ME) effect was studied as a function of their constituents and modulation frequency. The results show that the ME coefficient increases as a function of modulation frequency from 400 to 1000 Hz and the ME characteristics of ME curves are also modified because the electrical conductivity of the CoFe₂O₄ phase is sensitive to the increase in frequency between 400 and 1 000 Hz. The third phase Ba₂Fe₂O₅ formed during the sintering tends to reduce the ME effect.

Key words: ferrite; ferroelectric; CoFe₂O₄-BaTiO₃; magnetoelectric effect; modulation frequency

1 Introduction

Combination of both ferroelectric and magnetic orders in a single material leads to magnetoelectric (ME) effect which is activated by the mechanical coupling between magnetization and polarization [1-2]. Mechanical deformations induced by a magnetic field in a magnetostriction process give rise to an electric field by piezoelectric effect. Conversely, an application of electric field to ME materials can result in a magnetic field. ME materials are classified into single phase materials and multiphase composites. Their potential applications are in data storage, transducers and switching devices [1, 3]. Since the ME effect in single phase materials such as Cr₂O₃ and garnet films is too weak for applications, recent attentions have been turned to 2-phase ferrite-ferroelectric composites. In addition to composites with multilayered structures, a variety of particulate composites have been synthesized and characterized. Examples are combinations of various spinel ferrites (e.g. CoFe₂O₄, NiFe₂O₄, Ni_0.2Co_0.8Fe₂O₄ and Ni_0.8Co_0.1Cu_0.1Fe₂O₄) and piezoelectric materials (e.g. BaTiO₃, Ba_0.8Sr_0.2TiO₃, BaZr_0.08Ti_0.92O₃ and PbZr_0.5Ti_0.5O₃) [4-12].

The objective of this work is CoFe₂O₄-BaTiO₃ composites which have been investigated since 1974 [13]. The research on this ME composite is still active in recent experiments on particulate composites [10-12], superlattice films [14] and core-shell structures [15-16]. The ME coupling in self-assembled CoFe₂O₄-BaTiO₃ nanostructures has also been observed and analyzed [17-18]. According to Ref. [1], structural and electromagnetic properties of both magnetic and ferroelectric phases as well as their coupling at interfaces regulate the voltage response to the magnetic field. In addition to the applied static field, the modulation magnetic field is also required to generate ME response [19]. Resonance occurs when the modulation frequency coincides with magnetic, electric or mechanical eigen mode of the ME materials [1]. There have been theoretical suggestions that the modulation frequency has only moderate effect on the ME when it is lower than the electromechanical resonance frequency [20-21]. Hence, the frequency-dependent magnetoelectricity in a low frequency regime has received little attention. However, FETISOV et al [22] observed the maximum ME coupling between 400 and 1 000 Hz from measurements of Ni_0.2Co_0.8Fe₂O₄-PbZr_0.5Ti_0.5O₃ laminate composites up to 100 kHz. Since both theoretical and experimental works on the effect of modulation frequency are based on multilayered structures, this work is proposed to investigate a variation of ME in a low frequency regime in particulate composites with varying constituent phases of CoFe₂O₄ and BaTiO₃.

2 Experimental

Ferrite-ferroelectric composites (1-x)CoFe₂O₄- xBaTiO₃ in which volume fraction x varies as 0.3, 0.4, 0.5 and 0.6 were prepared by wet ball milling using CoFe₂O₄ (35-55 nm in diameter, 98% purity) and BaTiO₃ (85-128 nm in diameter, 99.6% purity) commercial nanopowders (Nanostructured & Amorphous Materials, Inc) as starting materials. The powders were mixed in ethanol and zircronia grinding media for 24 h. Then, the slurry was continuously stirred at 80 ¡ãC until dried powders were obtained. The composite powders were compressed using uniaxial pressing in a d16 mm die with an applied pressure of  200 MPa and then sintered at 1 200 ¡ãC for 24 h in air. The composite powders and ceramics were characterized for crystal phase identification by X-ray diffractometry (XRD) using Cu K_¦Á radiation with l=0.154 nm (PW3040 mpd control). Room temperature magnetic hysteresis loops were obtained by using a vibrating sample magnetometer (VSM) (Lake Shore VSM 7403) and ferroelectric hysteresis was examined by the Sawyer-Tower method. The ME effect was measured with a variation of magnetic field up to 366.07 kA/m and superimposed by a modulation magnetic field of 0.80 kA/m. These magnetic fields were supplied by an electromagnet (Walker Scientific HV-4H) with a modulated Helmholz coil and the voltage was detected by a lock-in amplifier (Stanford SR850).  The ME coefficient was calculated as a variation of the electric field in response to the applied magnetic field. The effect of the modulation frequency was studied by making ME measurements at 7 different frequencies from 400 to 1 000 Hz.

3 Results and discussion

In Fig. 1, XRD patterns of milled CoFe₂O₄-BaTiO₃ powders with varying volume fractions are shown with references to CoFe₂O₄ and BaTiO₃ peaks. The peaks are indexed as cubic spinel ferrite and tetragonal perovskite ferroelectric phases. The intensity of either ferrite or ferroelectric peaks predictably increases with their fractions in the composites. The absence of other unidentified peaks in Fig. 1 means that no chemical reaction occurs between these phases during milling. After sintering, the XRD patterns of all ceramics in   Fig. 2 still display both CoFe₂O₄ and BaTiO₃ phases, and Ba₂Fe₂O₅ peaks are also observed in the case of x=0.3 or 0.4. The third phase of Ba₂Fe₂O₅ is likely formed by the replacement of Fe²⁺?Fe³⁺ in Ti³⁺, which is of a minor fraction during the prolonged sintering. In the case of x=0.5 or 0.6, the replacement is suppressed because Fe²⁺?Fe³⁺ is no longer excessive. Only one weak Ba₂Fe₂O₅ peak appears in the case of x=0.6 and no third phase is observed at all when the amounts of constituents are in balance.

Fig. 1 XRD patterns of (1-x)CoFe₂O₄-xBaTiO₃ powders

Fig. 2 XRD patterns of (1-x)CoFe₂O₄-xBaTiO₃ ceramics after sintering at 1 200 ¡ãC in air for 24 h

From hysteresis loops of CoFe₂O₄-BaTiO₃ ceramics at room temperature in Fig. 3, the ferrimagnetism exists in all composites with the saturation magnetization under magnetic field of 795.80 kA/m. According to the appearance of hysteresis loops, the investigated composites can be divided into 2 groups, i.e., the high magnetic fractions (x=0.3 and 0.4) and the low magnetic fractions (x=0.5 and 0.6). The dependence of saturation magnetization (M_s) and coercivity (H_c) on x is shown in Fig. 4. The highest saturation magnetization of about 4.7¡Á10^{-2 A}¡¤m² is predictably observed in the case of x=0.3. With increasing the BaTiO₃ fraction, the magnetization is decreased because each BaTiO₃ grain acting like a void for the magnetic phase reduces the magnetic moment per unit volume [6, 9, 16, 23]. The composites with x=0.5 or 0.6 have significantly lower saturation magnetization compared with with those x=0.3 or 0.4, resulting in a variation in magnetization, which does not follow a straight line predicted by Bruggeman¡¯s effective medium theory [6, 9, 16, 23]. On the other hand, the increase in the coercivity with increasing x is not abrupt. This can be understood by considering the origins of enhanced coercivity, which are the reduction in interactions between CoFe₂O₄ grains and the domain wall pinned by surrounding phases [12]. In this case, the pinning arises from the combination of antiferromagnetic Ba₂Fe₂O₅ and ferroelectric BaTiO₃ phases which are gradually increased. It follows that the coercivity of the composite with x=0.5 is lower than the extrapolation by a straight line due to the absence of pinning by Ba₂Fe₂O₅.

With an exception of x=0.5, voltage up to 10 kV leads to a electrical breakdown in the composites. The internal discharge in ferrite-ferroelectric composites is attributed to the more conductive ferrite phase. It is fitting that ferroelectric loops of CoFe₂O₄-BaTiO₃ composites of equal fractions shown in Fig. 5 are elliptical in shape. This indicates the lossy characteristics which tend to change with the driving frequency.

Fig. 3 Magnetic hysteresis loops of (1-x)CoFe₂O₄-xBaTiO₃ ceramics

Fig. 4 Saturation magnetization and coercivity of (1-x)CoFe₂O₄-xBaTiO₃ ceramics

Fig. 5 Ferroelectric hysteresis loops of (1-x)CoFe₂O₄-xBaTiO₃ ceramics at varying frequency when x=0.5

The ME coefficient as a function of an applied magnetic field is dependent on the BaTiO₃ fraction (x). In Fig. 6, all ME curves at 400 Hz exhibit a similar trend with a high sensitivity to changing magnetic field of less than 7.96 kA/m. Beyond this low field regime, the increase in the applied magnetic field gradually raises the ME coefficient until reaching the maximum of around    47.75 kA/m. This peak corresponds to the saturation in magnetostriction of the ferrite phase [9]. With further increasing magnetic field up to 366.07 kA/m, the ME is monotonically decreased because the mechanical stress in CoFe₂O₄ can no longer be enhanced. The composites with higher BaTiO₃ fractions tend to exhibit lower ME effect which is in contrast to a previous report in CoFe₂O₄-BaTiO₃ particulate composites [10]. It is commonly known that the increase in a more conductive ferrite phase results in a substantial leakage current in the composites and tends to limit the effect of the poling.  However, the larger grains of ferrite can promote favorable interface coupling for the ME. This interface effect may outweigh the reduction in resistivity, giving rise to the increased ME in more conductive composites [12]. Interestingly, the composite without Ba₂Fe₂O₅ (x=0.5) shows an unusually high ME coefficient, indicating that the third phase is not only responsible for the magnetic properties but also detrimental to the ME effect. Due to its antiferromagnetic nature, the ME effect is not induced in the Ba₂Fe₂O₅ phase and the transfer of mechanical stress between CoFe₂O₄ and BaTiO₃ is disrupted by its existence.

The effect of modulation frequency on the ME response is exemplified in Fig. 7. In the case of x=0.3, the ME curves at 500-1 000 Hz share some characteristics with that measured at 400 Hz. The low field sensitivity is reduced from 500 to 700 Hz but becomes more pronounced at frequencies higher than 700 Hz. Moreover, the ME curves at 800 and 900 Hz exhibit anomalous fluctuations with another sharp response at a field larger than 79.58 kA/m. Their origin is not clear and deserves further investigations because of its existence in every composition. At every frequency, the peaks around 47.75 kA/m can still be observed. The ME coefficients from such peaks are plotted in Fig. 8 to compare the effect of the frequency and the BaTiO₃ fraction (x). The ME coefficients are approximately  2-7 mV/A. In all composites, the ME coefficients exhibit roughly linear variations with the frequency from 400 to 700 Hz. The linearity does not continue into 800 Hz at which the ME characteristics anomalously change. Since the increase in ME coefficient in Fig. 8 resembles that of laminate composites [22], this frequency-dependent ME can also be explained in terms of the variations of the electrical conductivity of the ferrite phase in the low frequency regime.

Fig. 6 Comparison of ME coefficients as function of magnetic field modulated at 400 Hz

Fig.7 Comparison of ME coefficients as function of magnetic field when x=0.3

Fig.8 Variations of ME coefficient at peak around 600 Oe in Fig. 7 with different modulation frequencies and x values

4 Conclusions

1) The ME composites (1-x)CoFe₂O₄-xBaTiO₃ are successfully prepared by wet ball milling. In addition to cubic spinel CoFe₂O₄ and tetragonal BaTiO₃, Ba₂Fe₂O₅ is clearly observed in XRD patterns of ceramics, the x value of which is 0.3 or 0.4 due to the replacement of Fe²⁺?Fe³⁺ in Ti³⁺ after sintering at 1 200 ¡ãC in air for 24 h. This third phase affects the magnetic and ME properties of the composites.

2) The saturation magnetization is decreased whereas the coercivity is increased with the increase in BaTiO₃ fraction in the ME composites because this nonmagnetic phase reduces the magnetic moment per unit volume and the interaction between CoFe₂O₄ grains.

3) The ME coefficient, ranging between 2 and     7 mV/A, increases with increasing modulation frequency from 400 to 1 000 Hz. The low field response is also sensitive to the change in frequency. This frequency-dependent magnetoelectricity is related to the variation of electrical conductivity of the CoFe₂O₄ phase.

Acknowledgements

This collaboration was supported by the Industry/University Cooperative Research Center (I/UCRC) in HDD Component, the Faculty of Engineering, Khon Kaen University and National Electronics and Computer Technology Center (NECTEC), National Science and Technology Development Agency (NSTDA). The TGIST scholarship (TG-33-12-51-048D) is granted to the first author by the National Metal and Materials Technology Center (MTEC) of NSTDA. The authors would like to thank the Ubon Ratchathani University and Department of Chemistry, Khon Kaen University for providing XRD and VSM facilities.

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Atchara KHAMKONGKAEO¹, Pongsakorn JANTARATANA ², Chitnarong SIRISATHITKUL³,

Teerapon YAMWONG⁴, Santi MAENSIRI⁵

1. Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand;

2. Department of Physics, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand;

3. Magnet Laboratory, School of Science, Walailak University, Nakhon Si Thammarat 80161, Thailand;

4. National Metals and Materials Technology Center (MTEC), Thailand Science Park, Pathumthani 12120, Thailand;

5. School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand

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(Edited by FANG Jing-hua)

Corresponding author: Chitnarong SIRISATHITKUL; Tel: +66-75-673230; Fax: +66-75-672004; E-mail: schitnar@wu.ac.th

DOI: 10.1016/S1003-6326(11)61033-9