Master of Science – Research
Insititute for Superconducting and Electronic Materials
Liu, Peng, Doping effects on the crystal structural, magnetic, and dielectric properties of multiferroic hexagonal RMnO3 (R = Er and Ho), Master of Science – Research thesis, Insititute for Superconducting and Electronic Materials, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3267
Due to the magnetoelectric coupling between ferroelectric and magnetic orderings, multiferroic materials are a promising family with great potential applications, such as multistate memory elements and magnetic field sensors. However, weak coupling and extremely low coupling temperature are two insurmountable problems for single phase multiferroic materials, and the hexagonal manganites are no exception. A two-dimensional triangular magnetic structure is a typical characteristic of the hexagonal manganite family. Hence, this family is an ideal system to investigate multiferroic properties with two-dimensional frustrated magnetic structure. In this thesis, so as to improve these two key drawbacks of the hexagonal manganites, the doping effects on ErMnO3 and HoMnO3 compounds are systematically investigated through Cu and Fe doping at the Mn site of ErMnO3, as well as Er doping at the Ho site of HoMnO3. All the compounds were prepared by the solid state reaction method, which can be indexed to the hexagonal structure with the P63cm space group. Another two common features of the hexagonal manganites were also confirmed in this study. On the one hand, no magnetic transitions were observed in the temperature dependent magnetic susceptibility data, because the antiferromagnetic ordering moments of Mn3+ ions are masked by the paramagnetism from rare earth ions. On the other hand, all transition metal ions in the as-prepared samples remain stable in the high spin states, indicating that no effective changes of the trigonal bipyramidal crystal fields are induced to modify the spin states.
As for single phase ErCuxMn1-xO3 (0 ≤ x ≤ 0.1) compounds, the doping effect on the crystal structure, magnetic properties, and dielectric properties were systematically studied. Rietveld refinement indicates that the a lattice parameter increases and the c lattice parameter decreases with increasing Cu concentration. The magnetic moments of doped samples are enhanced, which is due to modification of the frustrated spin arrangement by the superexchange interaction between Cu2+ ions and Mn3+ ions. The specific heat capacity data show a peak at the antiferromagnetic transition temperature, which decreases from 77 K for x = 0 to 61 K for x = 0.1. The frequency dependence of the dielectric constant and the dielectric loss both increase with the higher doping levels, indicating the enhancement of corresponding conductivity. A large negative magnetocapacitance effect was observed in paramagnetic-state ErCu0.05Mn0.95O3 at 300 K, which shows that the dielectric constant decreases with increasing magnetic field at fixed frequencies. For example, the dielectric constant changes from 38.8 to 12.3 at 1 MHz, a decrease of 68.3%.
In the single phase hexagonal ErFexMn1-xO3 (0 ≤ x ≤ 0.15) system, lattice parameters a and c first decrease with doping, but this is followed by a subsequent increase at higher doping levels. Although the magnetic susceptibility data do not show any clear magnetic transition, intrinsic antiferromagnetic ordering is indicated by the negative values of the Curie-Weiss temperature, which changes in agreement with the doping evolution of lattice parameter a. The specific heat capacity data show that the antiferromagnetic transition temperature increases from 77 K for ErMnO3 to 80 K for ErFe0.15Mn0.85O3. Furthermore, the magnetizations of these compounds slightly decrease with increasing doping level. The real part of complex impedance of these samples increase for higher doping levels over the whole frequency range from 100 Hz to 4 MHz, indicating enhanced insulativity with the more Fe contents.
The Ho1-xErxMnO3 (0 ≤ x ≤ 1) compounds are an Er and Ho mutual doping system, of which samples with x = 0.5 and 0.6 show the coexistence of ErMnO3 and HoMnO3. The Er doping effects on single phase compounds are investigated in terms of crystal structures, magnetization, specific heat, dielectric constant, and ferroelectric hysteresis loops. The a lattice parameter decreases as the Er content increases, whereas the c lattice parameter nearly keeps constant with pretty minor fluctuations. The slight doping evolution of the structure of the Mn-O5 trigonal bipyramid polyhedron is consistent with the stability of the high spin state of Mn3+ ions. The isothermal magnetization is reduced with increasing Er content. Specific heat data show that the antiferromagnetic transition temperature rises from 72 K for HoMnO3 to 76 K for ErMnO3. Dielectric data and ferroelectric hysteresis loops show all the samples are not only ferroelectric materials with leaky nature, but also with dielectric relaxation related to oxygen vacancies.