Introduction

In periodic classification of the elements, there is a group of metals referred to as transition metals which use their valence electron to form compounds with other elements. During the development and use of various technologies, these transition metals play an important role. Without the contribution of transition metals in developing these technologies, such as semiconducting, superconducting, spintronic, etc., is not possible. Spintronic covers varieties of oxide/non-oxide compounds having different forms, their properties and applications, such as manganese oxide based polycrystalline bulk (Doshi et al. 2011), nanostructures (Solanki et al. 2010), thin films (Solanki et al. 2011), heterostructures (Uma et al. 2012), multilayers (Vachhani et al. 2011), devices (Ashish et al. 2012) and composites (Cheng and Wang 2007), multiferroic based polycrystalline bulk (Ravalia et al. 2011), thin films (Ashish et al. 2013), devices (Megha et al. 2013), heterostructures (Gajek et al. 2006) and composites (Cheng and Wang 2007), high temperature superconductor (HTSC) based polycrystalline bulk (Rayaprol et al. 2004), thin films (Mavani et al. 2007), devices (Cassinese et al. 2004) and composites (Dong et al. 1998), diluted magnetic semiconductors (DMS) based polycrystalline bulk (Megha et al. 2011) and nanostructures (Megha et al. 2011), etc.

Manganese oxide based multiferroics, such as BiMnO3, YMnO3, HoMnO3, TbMnO3, etc., have been recently receiving an attention due to their fundamental physics (Lorenz et al. 2004; Aikawa et al. 2005; Singh et al. 2010) and potential for various spintronic applications (Gajek et al. 2007; Li et al. 2009), such as read and write heads, field and temperature sensors, spin valves, spin filters, tunnel junctions, p-n junction diodes, non-volatile random access memory devices, etc. In 1990s, YMnO3 (YMO) films were investigated for non-volatile ferroelectric memory devices, especially for metal–ferroelectric–semiconductor (MFS) devices (Fujimura et al. 1996a, b). YMO possesses hexagonal structure in its bulk form having electrical transition from ferroelectric (FE) to paraelectric (PE) at temperature TC is as high as ~1,000 K and magnetic transition from antiferromagnetic (AFM) to paramagnetic (PM) at temperature TN ~ 120 K. In hexagonal YMO compounds, the magnetoelectric (ME) coupling and AFM order are found to be very weak which in turn results in the negligibly small net magnetization at low temperature (Fiebig et al. 2002; Bertaur et al. 1963). By engineering the preparation method, one can synthesize the orthorhombic YMO which shows essentially different FE order and magnetic properties from hexagonal YMO compound, in addition to exhibition of significantly strong ME coupling (Kalashnikova and Pisarev 2003; Lorenz et al. 2007).

Many reports exist on the studies on Mn-site substitution of transition metals in YMO (Gutierrez et al. 2002; Moure et al. 2002; Veres et al. 2006; Aikawa et al. 2005). Gutierrez have observed the structural phase transformation from hexagonal to orthorhombic upon Cu2+ substitution which has been ascribed to the reduction in Jahn–Teller Mn3+ ionic density (Gutierrez et al. 2002). At the same time, same group has reported the effect of Cu2+, Ni2+ and Co2+ substitution at Mn3+ site on the structural behavior of doped YMO compounds and found that, the observed phase transition is governed by the structural parameters, such as size mismatch and tolerance factor (Moure et al. 2002). Veres et al. have reported the substitutional effect of Fe3+ at Mn-site on structural, transport and magnetic properties of YMn1-xFe x O3 (Veres et al. 2006). Studies from the view point of multiferroicity, Aikawa et al. have reported the substitutional effect of Ti4+ at Mn-site in YMO and found the structural phase transition and hence strong magnetodielectric behavior in the series of samples (Aikawa et al. 2005) Multiferroicity in doped YMO compounds has also been studied using the substitution of non-transition metal/non-magnetic Ga3+ at Mn-site (Zhou et al. 2005). Also, few reports are available on the studies on Y-site substitutional effect in YMO compounds (Lliev et al. 2005). At one hand, YMO oxides have been studied from the view point of substitution with transition metals and non-magnetic ions, on the other hand, very few reports are available on the multiferroic point of view studies on doped YMO oxides.

By keeping in mind what has been discussed above, in the present communication, the results of dielectric behavior of sol–gel grown nanostructured Y0.95Ca0.05MnO3 (YCMO) oxides, sintered at various temperatures, have been discussed in the light of size effect.

Experimental details

Nanophasic Y0.95Ca0.05MnO3 (YCMO) were grown using simple and low cost sol–gel technique (Solanki et al. 2009; Kuberkar et al. 2012). High purity Yttrium Acetate [Y (CH3CO2)3 × 6H2O], Calcium Acetate [Ca (CH3CO2)2] and Manganese Acetate [Mn (CH3CO2)2 × 4H2O] were taken as starting materials in appropriate stoichiometric ratio. The precursor solution was prepared by dissolving the constituents (precursors/starting materials) in double distilled water (DDW) and acetic acid (AA) with desired stoichiometric composition. The optimum ratio of DDW and AA of 1:1 was maintained in proper volume to yield 0.3 M of solution. The solution was stirred at 90 °C using magnetic stirrer until a clean and transparent solution was obtained followed by the calcination of resultant brownish powder at 500 °C for 6 h in furnace. Final material product (in pellet form) was sintered at 700, 800, 900, 1,000 and 1,100 °C for 12 h in air. Hereafter, YCMO compounds sintered at 700, 800, 900, 1,000 and 1,100 °C are referred as YC7, YC8, YC9, YC10 and YC11, respectively. X-ray diffraction (XRD) patterns of YCMO compounds were recorded on Philips diffractometer (PW 3040/60, X’pert PRO) using CuKα radiation at room temperature (RT). Frequency dependent dielectric constant at various temperatures (in the range 80–300 K) was measured using Solatron impedance analyzer.

Results and discussion

XRD measurements were carried out on all the sol–gel grown nanostructured YCMO compounds sintered at various temperatures. Figure 1 depicts XRD patterns of YCMO compounds sintered at 700 °C (YC7) and 1,100 °C (YC11) which reveals that, all the compounds sintered at various temperatures are single phase in nature without any detectable impurities within the measurement range (Lorenz et al. 2004). All the compounds possess orthorhombic structure with the Pnma space group (no. 62). All the XRD peaks are identified for their respective (hkl) parameters. It is clear that, large amorphous background exists in YC7 while a clear background is present in YC11. Figure 2 depicts an enlarged view of (020) peaks of all the nanostructured YCMO compounds. The peak intensity increases while the peak broadening (full width at half maximum—FWHM) decreases with sintering temperature. Peak shifts towards lower 2θ, from 33.531° (YC7) to 33.412° (YC11), indicating an increase in cell parameters and cell volume. Also (020) peak is splitting in all the compounds which reveal the possibility of orbital ordering in the compounds (Doshi et al. 2009), which requires further investigations. Further, crystallite size (CS) can be calculated using the scherer’s formula: CS = 0.9λ/Bcosθ, where λ is the X-ray wavelength, B is the FWHM and θ is the angle. It is observed that, CS increases with sintering temperature. The calculated values of CS are 7.92 nm (YC7), 22.67 nm (YC8), 25.57 nm (YC9), 26.82 nm (YC10) and 70.43 nm (YC11). Figure 3 shows the variation in CS with sintering temperature. Observed increased peak intensity, reduced background, decreased FWHM and increased CS indicate the improved crystallinity with sintering temperature.

Fig. 1
figure 1

XRD patterns of YC7 and YC11 compounds

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Fig. 2
figure 2

Enlarged view of (020) peaks of all the YCMO compounds

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Fig. 3
figure 3

Variation in crystallite size (CS) with sintering temperature for nanostructured YCMO compounds

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To understand the effect of CS and sintering temperature on the dielectric behavior of presently studied sol–gel grown nanostructured YCMO compounds, temperature and frequency dependent complex impedance data were collected for all the YCMO compounds. The real and imaginary parts of complex permittivity or dielectric constant were calculated by inserting the obtained impedance data and sample dimensions into the standard equations given below:

$$\varepsilon^{*} = \varepsilon^{{\prime }} + i\varepsilon^{{\prime{\prime } }}$$
$$\varepsilon^{\prime } = - \frac{d}{{\omega \varepsilon_{0} A}}\frac{{Z^{\prime \prime } }}{{(Z^{\prime 2} + Z^{\prime \prime 2} )}}$$
$$\varepsilon^{\prime \prime } = - \frac{d}{{\omega \varepsilon_{0} A}}\frac{{Z^{\prime } }}{{(Z^{\prime 2} + Z^{\prime \prime 2} )}}$$

where, d is the thickness of pellet, A is area of electrode and \(\varepsilon_{0}\) is permittivity in vacuum. The frequency response of real part of dielectric constant (\(\varepsilon^{\prime }\)) at various temperatures obtained from above equations for all the YCMO compounds are shown in Fig. 4. The value of \(\varepsilon^{\prime }\) decreases with increasing frequency, which demonstrates a typical characteristic of FE materials (Ravalia et al. 2011). The observation can be understood by the phenomena of dipole relaxation where at low frequency the dipoles are able to follow the frequency of the applied field (Palkar et al. 2004). It is also clearly seen that, \(\varepsilon^{\prime }\) increases with temperature indicating the FE nature of YCMO compounds studied. At low temperatures, may be due to freezing effect on the atomic/ionic movement in the structure, dipoles cannot easily follow the field direction while at higher temperatures, most of the electrical dipoles get enough exciting thermal energy to be able to follow the changes in the external field direction, which enhances the contribution of the dipoles to the polarization leading to an increase in the value of \(\varepsilon^{\prime }\).

Fig. 4
figure 4

Temperature dependent \(\varepsilon^{\prime }\) vs. f plots of nanostructured YCMO compounds

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It is also evident from the Fig. 4 that, dielectric constant increases with sintering temperature, which can be understood as—with increase in sintering temperature, the CS increases and hence crystallites become more compact in higher sintered compounds as compared to lower one. The agglomeration between the crystallites and the removal of crystallite boundaries result in the interfacial electric/dipole polarization (Manoj and Yadav 2007) and hence the resultant dielectric constant increases with increase in CS/sintering temperature. Furthermore, it is well established fact that, with increase in sintering temperature, the oxygen deficiency gets increased in oxide materials. In the present case, YC7 possesses less oxygen vacancies compared to that in YC11. Oxygen vacancies convert the Mn4+ (generated due to 5 % Ca2+ substitution) to Mn3+ in YCMO in order to maintain the valance in the system. Thus, with increase in sintering temperature, the hopping conduction decreases (due to reduced Mn4+ ionic density) which results in the enhancement in dielectric permittivity.

The variation in imaginary part of dielectric constant (\(\varepsilon^{\prime \prime }\)) with frequency at various temperatures for all the YCMO nanostructured compounds is shown in the Fig. 5. It can be seen that, with increase in temperature, \(\varepsilon^{\prime \prime }\) increases. It is believed that, at higher temperatures, due to an increase in the mobility, enhanced conductivity causes an increase in \(\varepsilon^{\prime \prime }\), associated with the conduction losses. A dielectric anomaly (\(\varepsilon^{\prime \prime }\) peak) at low frequency is observed for all the compounds, which shifts towards lower frequency with temperature. The \(\varepsilon^{\prime \prime }\) is almost doubled than \(\varepsilon^{\prime }\) (the ratio is ~2) revealing high dielectric loss present in the compounds which may be due to the substitution of divalent Ca2+ in multiferroic YMO compound.

Fig. 5
figure 5

Temperature dependent \(\varepsilon^{\prime \prime }\) vs. f plots of nanostructured YCMO compounds

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To understand the relaxation mechanism in the nanostructured YCMO compounds and how CS affects the relaxation in the YCMO, the variation in dielectric modulus \(M^{\prime \prime }\) with frequency in the temperature range of 80–350 K has been measured (Fig. 6). It is found that, with increase in temperature (from 80 to 350 K) and sintering temperature (700–1100 K), anomaly peak in \(M^{\prime \prime }\) shifts towards higher frequency. From the peak shifting behavior of YCMO compounds, observed in \(M^{\prime \prime }\) vs. f plots, one can find the activation energy for all the samples using the formula:

Fig. 6
figure 6

Temperature dependent \(M^{\prime \prime }\) vs. f plots of nanostructured YCMO compounds

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$$f_{m} = f_{0} exp\left( { - \frac{{E_{a} }}{{K_{B} T}}} \right)$$

where, fm is the frequency corresponding to the loss peak, f0 is the pre-exponential factor, Ea is the activation energy for relaxation, KB is the Boltzmann constant and T is absolute temperature. Figure 7 shows the plots of ln fm vs. 1,000/T (T 200–350 K) for YCMO nanostructured compounds. Values of activation energy, Ea, can be calculated using the slope obtained from the straight line fitting of the plots as shown in Fig. 7. The calculated values of Ea are 41.49, 70.89, 83.37 and 104.5 meV for YCMO samples sintered at 800, 900, 1,000 and 1,100 °C, respectively. It can be seen that, with increase in sintering temperature, Ea increases, which can be attributed to the sintering temperature induced enhancement in oxygen deficiency/vacancies and hence decrease in carrier hopping resulting in the enhancement in activation energy.

Fig. 7
figure 7

Plots of ln fm vs. 1000/T using \(M^{\prime \prime }\) vs. f data for nanostructured YCMO compounds

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Effect of sintering temperature and crystallite size on the dielectric behavior of nanostructured YCMO compounds has been discussed in detail. Temperature dependent physical state of the materials can be understood as: mixed valent manganese oxides are intrinsically disordered systems having small scale (nanoscale) phases, such as ferromagnetic metallic (FMM), paramagnetic insulating (PMI), antiferromagnetic insulating (AFMI), charge-ordered insulating (COI), etc., depending upon the temperature, coexisting in single phase compound resulting in the electrical inhomogeneities (Kuberkar et al. 2012). The effect of sintering temperature on these temperature dependent nanoscale phases can be ascribed to the agglomeration of smaller crystallites forming the larger crystallites at higher sintering temperatures and related boundary modifications (Solanki et al. 2009). In the present case, the effect of sintering temperature on the inhomogeneous electronic phases can be understood, in the context of non-compositional oxygen content, as: in lower sintered samples, the boundaries are less oxygen deficient while in higher sintered samples the boundaries are more oxygen deficient which in turn results in the non-stoichiometric inhomogeneous phases in the samples. Across the boundaries, the oxygen deficiency results in the conversion of Mn4+ (generated due to the substitution of Ca2+) to Mn3+ which controls the conductivity (leakage current) in the compounds. The larger the oxygen deficiency the smaller the conductivity (leakage) resulting in the stronger dielectric nature in the samples sintered at high temperatures.

Conclusions

In summary, Y0.95Ca0.05MnO3 (YCMO) nanostructured compounds have been successfully synthesized using low cost and easy sol–gel route employing acetate precursor route. Increase in \(\varepsilon^{\prime }\) and \(\varepsilon^{\prime \prime }\) with temperature and CS has been ascribed to the thermal activation to the dipoles and interfacial polarization across the crystallite boundaries of the compounds sintered at higher temperatures, respectively. It is observed that, with increase in sintering temperature, the oxygen vacancies play an important role in governing the dielectric behavior, i.e. increase in Ea with sintering temperature is attributed to the enhancement in oxygen deficiency/vacancies and hence decrease in carrier hopping and increase in Ea. In conclusion, sintering temperature and hence CS is the key factor to maintain/govern the activation energy and electric dipole behavior in presently studied YCMO nanostructured compounds.