Grain size-dependent magnetic and electric properties in nanosized YMnO3 multiferroic ceramics
© Han et al; licensee Springer. 2011
Received: 26 July 2010
Accepted: 8 March 2011
Published: 8 March 2011
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
The hexagonal RMnO3 (R = rare earth element or Y) compounds present opportunities for the industrial applications due to their unique nature of multiferroism . 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 . 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 multiple-state memory elements, the transducer with magnetically modulated piezoelectricity, and the electric-field-controlled ferromagnetic resonance devices .
Most of hexagonal RMnO3 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 [1–4]. Additional phase transitions at the temperature below 10 K were observed in the hexagonal RMnO3 with the R3+ ion of high magnetic moment, which is related to the R-R exchange correlations . Several attempts have been directed towards the syntheses of new RMnO3 compounds and the studies of their related properties [6, 7]. In particular, the recent work on the hexagonal RMnO3 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 . The studies on YMnO3, HoMnO3 and LuMnO3 indicated that the values of ordering temperatures are associated with the size of R3+ ion. In addition, the size effects in yttrium-based 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 YMnO3 samples with different grain sizes by a modified Pechini method to study systematically the effect of grain size on their magnetic and electric properties.
The nanosized samples of YMnO3 were synthesized by a modified Pechini method using nitrates as metal precursors. First, yttrium nitrate [Y(NO3)3·6H2O] and manganese nitrate [Mn(NO3)2·4H2O] in stoichiometric proportions (1:1 molar ratio) were dissolved in distilled water. Citric acid (C6H8O7) 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).
Results and discussion
where the sum is over the nearest neighbors and 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.
In summary, a series of hexagonal YMnO3 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.
The financial support of this work is from the National Science Council of Taiwan under the grant nos. NSC96-2112-M-390-003-MY3, 99-2112-M-390-005-MY3 and 98-2815-C-390-015-M.
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