Study of magnetic and electrical properties of Zn0.9Mn0.1Fe2O4-BaTiO3 multiferroic composites

Abstract

Generally, magnetic and electric dipoles are mutually exclusive in crystalline materials. But the composite multiferroics offers a way to attain magnetic and ferroelectric ordering simultaneously. Also, the composites are important for the advancement of magneto-electronic devices, magnetoelectric sensors, random access memory, etc. [1]. In the present work, the room temperature multiferroic property was verified through magnetic hysteresis (M(H)) and ferroelectric hysteresis (P(E)) loop measurements. The composites (1-x) BaTiO 3 +(x) Zn0.9Mn0.1Fe2O 4 (x=0, 0.10, 0.20 and 1) were synthesized by using the nanoparticles of Zn0.9Mn0.1Fe2O4 (ZMF) and BaTiO3 (BTO) by sintering at 1200 °C for 6 hours. The nanoparticles of BTO and ZMF were synthesized by hydrothermal method and by co-precipitation method respectively as described in our earlier works [2], [3]. The crystal structure of the nanoparticles of ZMF and BTO are cubic spinel and tetragonal phase respectively as confirmed by X-ray diffraction (XRD) analysis (Fig.1a). The XRD of the composite (x=0, 0.10, 0.20 and 1) samples confirm the co-existence of BTO and ZMF phases. It is observed from the field emission scanning electron microscope (FESEM) micrographs (Fig.1a inset) that the composite system consists of two kinds of regions: one corresponds to ferromagnetic (FM) phase and the other one to ferroelectric (FE) phase. Fig. 2(a) shows the magnetization vs. magnetic field (M(H)) plots for the FM-FE composites at 300K with x=0.10, 0.20 and 1 respectively. An increase in magnetization is observed with the inclusion of ZMF. The samples exhibit magnetic hysteresis behaviour with very low coercivity values indicating their soft ferromagnetic nature with much lower magnetization value than pure ZMF (inset of Fig. 2(a)). This indicates the existence of non-magnetic FE phase along with FM phase. The ferroelectric particles surrounding the magnetic particles influence the magnetic coupling among the magnetic particles. The presence of M(H) hysteresis loop confirms the magnetic ordering in the FM-FE composites. Fig. 2(b) shows the change of saturation magnetization (Ms) and coercivity (Hc) as a function of ZMF percentage (x %) for all the FM-FE composites. The gradual increase in Ms-values with the increase in ZMF content indicates that the Ms-values for the composites follow the mixture rule. Therefore, the magnetic response of composites depends on ZMF percentage [3]. Also, the increase in coercivity observed with decreasing ferrite percentage may be due to the diamagnetic nature of BTO. The observed multiple resonance in the ferromagnetic resonance (FMR) spectra (Fig.2(c)) of the composites is attributed to the coexisting magnetic states of the cations and cation distribution between A and B sublattices of spinel ferrites. It can also be attributed to the presence of heterogeneities in the composites. The decrease in resonance field (Hr-value) is related to the increase in internal magnetic field which indicates the increase in magnetic response with increasing ZMF percentage. Fig. 1(b) shows the ferroelectric hysteresis (P(E) loop) at room temperature for FM-FE composites $(\mathrm {x}=0.10$ and 0.20). The shape of P(E) loop confirms the ferroelectric ordering of the composites at room temperature. The positive curvature of the P(E) loop shows that the leakage current contribution is minimal. The remnant polarization and coercive field increases with increase in ZMF percentage. Since in the FM-FE composites, the ferroelectric grains are surrounded by the ferrite grains or vice versa. Therefore, the heterogeneous microstructure may be a possible reason for alteration in the interaction among the internal poles of the FM-FE composite. The FM-FE composites show a decrease in dielectric constant $(\varepsilon )$ value (solid lines in Fig.1c) with an increase in dielectric loss $(\tan \delta )($ symbol lines in Fig.1c) with an increase in ZMF-content. It is observed that the dielectric constant and loss values decrease with increasing frequency approaching to a lower saturation value at high frequencies. The variation of $\varepsilon -$value with frequency is attributed to the fact that the electric dipoles are unable to follow the fast alternate electric field oscillations under high frequencies. At lower frequencies, the FM-FE composites have higher $\varepsilon $ and loss values. The FE-FM distribution makes two types of inter grain connectivity impling two types of ionic relaxations in the low frequency region confirming the results from FESEM micrographs. It is clear from the results that magnetic, electric and dielectric properties of composites strongly depend on the microstructure of the sample. And the multiferroic properties of ferromagnetic-ferroelectric composites are due to the induced electric polarization in the magnetic order or vice versa.