Grain size-dependent magnetic and electric properties in nanosized YMnO3 multiferroic ceramics

Magnetic and electric properties are investigated for the nanosized YMnO3 samples with different grain sizes (25 nm to 200 nm) synthesized by a modified Pechini method. It shows that magnetic and electric properties are strongly dependent on the grain size. The magnetic characterization indicates that with increasing grain size, the antiferromagnetic (AFM) transition temperature increases from 52 to 74 K. A corresponding shift of the dielectric anomaly is observed, indicating a strong correlation between the electric polarization and the magnetic ordering. Further analysis suggests that the rising of AFM transition temperature with increasing grain size should be from the structural origin, in which the strength of AFM interaction as well as the electrical polarization is dependent on the in-plane lattice parameters. Furthermore, among all samples, the sample with grain size of 95 nm is found to have the smallest leakage current density (< 1 μA/cm2). PACS: 75.50.Tt, 75.50.Ee, 75.85.+t, 77.84.-s


Introduction
The hexagonal RMnO 3 (R = rare earth element or Y) compounds present opportunities for the industrial applications due to their unique nature of multiferroism [1]. Namely, the ferromagnetism, ferroelectricity and ferroelasticity occur simultaneously in the same material. The characteristics of multiferroism include a spontaneous magnetization which can be switched by an applied electric field, a spontaneous electrical polarization which can be reoriented by an applied magnetic field, and a strong coupling between these two properties [2]. Owing to the coupling between ferroelectric and magnetic domains, multiferroism is likely to offer a whole range of new applications and phenomena. Specific device applications that have been proposed for these multiferroic materials include the multiplestate memory elements, the transducer with magnetically modulated piezoelectricity, and the electric-field-controlled ferromagnetic resonance devices [2].
Most of hexagonal RMnO 3 exhibit ferroelectric (FE) transitions at high temperatures (T C ≈ 600 to 1,000 K) and antiferromagnetic (AFM) transitions at low temperatures (T N ≈ 70 to 130 K) with a frustrated triangular arrangement of Mn spins in the hexagonal c-plane [1][2][3][4]. Additional phase transitions at the temperature below 10 K were observed in the hexagonal RMnO 3 with the R 3+ ion of high magnetic moment, which is related to the R-R exchange correlations [5]. Several attempts have been directed towards the syntheses of new RMnO 3 compounds and the studies of their related properties [6,7]. In particular, the recent work on the hexagonal RMnO 3 compounds was focused on the following subjects: (1) the magnetic phases and the magnetic symmetry at low temperatures [8,9], (2) the coupling between the magnetic and FE orderings [10,11], and (3) the strong spin-lattice interaction of the geometrically frustrated Mn-spin system [12]. The studies on YMnO 3 , HoMnO 3 and LuMnO 3 indicated that the values of ordering temperatures are associated with the size of R 3+ ion. In addition, the size effects in yttriumbased manganites were also reported [13,14]. However, the size effects on the multiferroism remain unclear, and its understanding requires more experimental evidences. In this paper, we prepare a series of YMnO 3 samples with different grain sizes by a modified Pechini method to study systematically the effect of grain size on their magnetic and electric properties.

Experimental procedure
The nanosized samples of YMnO 3 were synthesized by a modified Pechini method using nitrates as metal precursors. First (1:1 molar ratio) were dissolved in distilled water. Citric acid (C 6 H 8 O 7 ) in 1:1 molar ratio with respect to the metal nitrates was added to the solution as a complexant, and the solution was adjusted to a PH value of 6.5 to 7 by adding ammonia. The mixture was dried at 120°C to form a gel, and then the obtained gel was burned until the combustion process was completed. After that, the precursory powders were reground and pressed into the pellets. Finally, the pellets were sintered at different temperatures ranging from 800°C to 1,050°C for 2 h, respectively. Electrodes were applied to both surfaces to measure electrical properties with silver paste.
The crystalline structure and the phase purity of the samples were examined with a typical X-ray diffraction (XRD), acquired by a Bruker D8 Advance X-ray diffractometer (Bruker UK Ltd., Coventry, Warwickshire, UK) equipped with a monochromatized Cu K α1 radiation and field emission scanning electron microscopy. The magnetization was measured with a Quantum Design superconducting quantum interference device (Quantum Design, Inc., San Diego, CA, USA) with an applied magnetic field of 500 Oe. For the dielectric measurements, a capacitance bridge (Agilent 4284A; Agilent Technologies, Inc., Palo Alto, CA, USA) hooked to a probe station with a closed-cycle low temperature system was used. The leakage currents of the samples were measured using a commercial FE test system (TF Analyzer, aixACCT Systems GmbH, Aachen, Germany). Figure 1 shows the XRD patterns of the YMnO 3 samples sintered at different temperatures from 800°C to 1,050°C. Based on the standard reference, all the observed peaks can be indexed on the basis of a hexagonal unit cell of space group P6 3 cm (JCPDS:25-1079), suggesting that all samples are pure phases without any impurity. In addition, with the increase in sintering temperature, there is a gradual intensity increasing and narrowing of the diffraction peaks, indicative of better crystallization and the grain growth. The lattice parameters were determined by Rietveld refinement method and shown in Figure 2. With increasing of sintering temperature, the value of c lattice parameter is slightly expanded, while the value of a lattice parameter decreased. The typical scanning electron microscopy (SEM) images of the YMnO 3 samples sintered at different temperatures are shown in Figure 3. From the images, it can be found that the grain size becomes larger as the sintering temperature increases. The estimated average grain size is about 25, 45, 95, and 200 nm for the samples sintered at 800°C, 850°C, 900°C, and 1,050°C, respectively.

Results and discussion
The temperature-dependent magnetization curves M (T) were measured in a magnetic field of 500 Oe under the conditions of zero-field-cooled (ZFC) and field-cooled (FC). Figure 4 displays the temperature dependence of magnetization for the powders with different grain sizes. Open symbols are the data with the ZFC mode, while the solid ones with FC mode. As can be seen, typical AFM to paramagnetic (PM) phase transition is observed for the sample with grain size of 200 nm, and the Néel temperature (T N ) is about 74 K. As the grain size decreases, the value of T N shifts to the lower temperatures and is equal to 52 K for the sample with grain size of 25 nm. This size-dependent T N is similar to the observation in the BiFeO 3 nanoparticles [15], where the increase in T N with increasing size has been discussed both in terms of phenomenological scaling relations and possible correlations with the decreasing electrical polarization. To further explore the magnetic properties of the samples, magnetic hysteresis loops for the YMnO 3 samples with different grain sizes have been measured at 5 K, as presented in Figure 5. For the samples with grain size of 25 and 45 nm, weak ferromagnetic (FM) behavior is observed with corresponding coercivity (Hc) about 395 and 260 Oe, respectively. The inset in Figure 5 shows the magnetic hysteresis curve for the sample with grain size of 25 nm has been measure at 55 K. It indicates the PM behavior which confirms that the FM component disappears above T N . Therefore, the weak FM component does not come from FM impurity phase. As the grain size increases, the weak FM behavior transforms into paramagnetism. Similar effect of grain size on magnetism was also reported in nanosized YMn 2 O 5 [16] and BiFeO 3 particles [17]. In fact, weak surface FM component is a universal feature for nanosized AFM systems, which is attributed to the deviation of the AFM arrangement to the disordered surface spin due to the lattice strain [17,18]. Based on the above consideration, the magnetic structure of the nanosized YMnO 3 can be considered as a core/shell system, where the inner part of the particle is AFM phase and the surface is FM component. Figure 6 shows the temperature-dependent dielectric permittivity ε(T) and loss tangent (tanδ) at 100 kHz for all measured YMnO 3 samples. In Figure 6a, the dielectric anomalies are observed at T* which is defined as the crossing point of two slopes as indicated by arrows. It shows that the T* shifts from 55 to 74 K with increasing grain sizes from 25 to 200 nm. As clearly apparent in Figure 6b, the positions of the broad peaks for the YMnO 3 samples with different grains sizes are near their T*. Moreover, the enhanced dielectric response observed for YMnO 3 with larger grains is similar to previously reported results for BaTiO 3 dielectrics [19]. The observed systematic shift in the temperatures of magnetic transition and dielectric anomaly demonstrates a strong correlation between magnetic ordering and electric polarization in nanosized hexagonal YMnO 3 ceramics. As to the coupling between antiferromagnetism and dielectric property, Katsufuji et al. [20] suggested that the dielectric anomaly was caused by the magnetic-ordering-dependent electronic excitation gap E g in ab-plane. According to this model, the change of AFM ordering pattern can induce dielectric anomaly via the change of E g , in a formula of ε = 1/E g 2 . Therefore, one can understand that the shift in the temperature of dielectric anomalies is related to the AFM interaction through the variation of Mn-O bond length with change the lattice Figure 1 The standard reference and the XRD patterns of YMnO 3 samples sintered at different temperatures. Samples were sintered at temperatures ranging from 800°C to 1,050°C for 2 h, respectively.   parameters. In addition, the systematical change in the lattice constant a plays an important role since the strength of AFM interactions strongly depends on the bond length of Mn-O. In general, the strength of AFM interaction can be written as [21]: where the sum is over the nearest neighbors andŜ i is a spin operator. The parameter J is proportional to the inverse of the distance between two nearest spins. Therefore, the reduction in a-parameter leads to the enhancement of J and hence to the rising of AFM transition temperature.
To further probe the electrical leakage effect, the leakage current were measured for all the samples at room temperature as shown in Figure 7. The leakage current density is large (> 100 μA/cm 2 ) for the sample with grain size of 25 nm. On the other hand, the leakage currents are much decreased by about four orders of magnitude for the samples with grain size larger than 45 nm. In addition, it is not expected that the sample with larger grain size of 200 nm is not the less leaky sample. As for the improvement of the leakage properties, it should be associated with the high denseness of the ceramics [22].

Conclusions
In summary, a series of hexagonal YMnO 3 samples with different grain sizes are synthesized by a modified Pechini method. The magnetic susceptibility indicates that with increasing grain size from 25 to 200 nm, the AFM transition temperature increases from 52 to 74 K. At the same time, a corresponding shift of the dielectric anomalies is observed, which suggests a strong correlation between the magnetic ordering and the electric polarization. Since the electronic excitation gap is inversely proportional to the dielectric permittivity and the spin structure influences the electronic excitation gap, we propose that the coherent shift in the magnetic ordering and the dielectric anomalies to high temperature with increasing grain size is related to the suppression of the in-plane lattice parameter.