Investigating the effect of composition on the magnetoelectric response in composite multiferroics

Mohsin Rafique1, Syed Qamar ul Hasan1, Muhammad Saif Ullah Awan2 and Sadia Manzoor1,2

1 Department of Physics, COMSATS Institute of Information Technology, Park Road, Islamabad, Pakistan

2Center for Micro and Nano Devices (CMND), Department of Physics, COMSATS Institute of Information Technology, Park Road, Islamabad, Pakistan

Abstract Magnetoelectric multiferroics exhibiting a strong coupling between magnetic (ferro/ferri/antiferro) and ferroelectric order have recently generated a sharply increasing number of research activities because of their scientific interest [1-4] and their significant promise in novel multifunctional devices [5-8]. Magnetoelectric coupling is the intrinsic property of some single phase compounds but they have very weak coupling between the two ferroic orders at device operating temperatures, which hinders their practical applications [9, 10]. However, in composites made of piezoelectric and magnetostrictive materials, a large ME effect can be observed at room temperature which is in fact several times higher than that in single phase multiferroics [4-7]. Individually the piezoelectric or magnetostrictive phases have no ME effect. The strain mediated ME effect in composites is the result of the product of the magnetostrictive effect (magnetic/mechanical effect) and piezoelectric effect (mechanical/electrical effect) and so is a product tensor property [10]. The magnetic field results in the magnetic order and hence the strain in the magnetostrictive phase which is transferred to the piezoelectric phase resulting in the polarization in the piezoelectric phase. The enhanced polarization in the piezoelectric phase can be detected by the increase in the capacitance of the composites in the presence of the magnetic field.

The role of magnetostrictive content and interface density of cobalt ferrite (CFO) and barium titanate (BTO) phases on the magnetoelectric response has been investigated in composites comprising of (0-dimensional) nanoparticles of the magnetostrictive material (cobalt ferrite) embedded in a continuous (3-dimensional) matrix of a piezoelectric material (barium titanate). These composites were fabricated by the sol-gel method, allowing the intermixing at molecular level, with four different molar concentrations (1:4, 3:7, 2:3 and 1:1) of the embedded phase with respect to the matrix. The obtained powders were calcined at 850° C for 2 hours in air. After calcination, these powders were pressed into pellets and then sintered at 1000° C for 12 hours. Silver paste was used to paint the electrodes onto the top and bottom sides of the pellets.

The crystalline phase and structure of the CFO-BTO composites studied by using X-ray diffraction (XRD) with Cu Kα radiation confirmed the presence of the spinel and perovskite phases representing the CFO and BTO respectively. No peaks corresponding to impurity phases were found in the diffraction pattern of all the samples.

Magnetic and ferroelectric properties were studied by using vibrating sample magnetometer and ferroelectric loop tracer system. The magnetocapacitance (MC) measured in the presence of static magnetic field of 1315 Oe. All samples display a positive magnetocapacitance, i.e. capacitance increases in the presence of magnetic field. When the sample capacitor is placed in the magnetic field, mechanical deformation is produced in the CFO phase because it is a strongly magnetostrictive material. This deformation, when transfers to the piezoelectric BTO phase due to the interfacial coupling, induces a piezoelectric stress in BTO phase, which leads to increase in the dielectric constant and so an increase in the capacitance as well.

Variation of MC with the increase of magnetic field at a fixed frequency of 0.5 MHz for different compositions of CFO-BTO composites is shown in figure 1a. The magnetocapacitance increases linearly with the increase of magnetic field for all samples. Our results are in close agreement with those obtained in ref. [11], in which a linear dependence of magnetocapacitance on the magnetic field has been obtained in Dy-doped nickel ferrite films and similar results were also found in references [12, 13]. This linear dependence is most likely due to a similar dependence of the magnetostriction on the applied magnetic field. Indeed the authors in [11, 12, 13] have found a close correspondence between the field dependences of magnetostriction and magnetocapacitance. Figure 1b shows that the magnetocapacitance increases with the increase of CFO content. The data show a clearly non-linear dependence of the MC on the CFO content. This, we believe is due to the compounded effect of the increase in the magnetostrictive phase content as well as increase in the interface density per unit volume in the composite with increasing CFO content.

Jang et al. [14] have shown that the magnetoelectric susceptibility coefficient can be calculated from MC data. Figure 3 shows the calculated magnetoelectric susceptibility for all samples. The values of calculated for our samples range from 0.02 to 0.8 mV.cm-1Oe-1. The sharp increase in the magnetocapacitance and the magnetoelectric susceptibility with CFO content is a direct consequence showing the role of the increasing number of CFO-BTO interfaces and the increased quantity of the magnetostrictive phase in promoting strong magnetoelectric coupling.

We on observed that the magnetocapacitance is not only dependent on the nature percentage of the magnetostrictive phase in the composite but is also dependent on the number density of the magnetostrictive/piezoelectric phases in the composites.

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Figures

Figure 1 The percentage MC vs. (a) H for different compositions of CFO-BTO composite, and (b) CFO content in CFO-BTO composites

Figure 2 Frequency dependence of magnetoelectric coefficient for different samples of CFO- BTO composites