D.H. Ji

Quantum-mechanical method for estimating ion distributions in spinel ferrites

A quantum-mechanical method for estimating the cation distribution in spinel ferrites is proposed, by which the ionization energy of the cations and the Pauli repulsion energy is considered, together with the magnetic ordered energy and the tendency toward charge density balance. Using this method, not only can the difference between the observed and the traditional theoretical magnetic moments of the spinel structure ferrites MFe2O4 (M=Mn,Fe,Co,Ni,Cu) be explained, but also the dependence of the magnetic moments of the ferrites M1−xZnxFe2O4 (M=Mn,Fe,Co,Ni,Cu) on the doping level x can be fitted.

Study of the free energy of the La1−xCaxMnO3 manganites based on the temperature dependence of the crystal cell volume

A study of the free energy of the perovskite manganites was performed by fitting experimental results for the temperature dependence of the crystal cell volume using a detailed semiclassical model of the free energy. The fitted results coincide with the experimental results for the samples La1−xCaxMnO3 (x=0.2, 0.25, 0.3, and 0.5). The free energy includes the energy corresponding to thermal vibration of the crystal lattice, the ionic cohesive energy, and additional energies produced by electrical carriers from the double exchange mechanism and the thermal activation of small polarons.

Study of the dependence of the magnetic moment of La1−xSrxMnO3 on the Sr doping level x

The dependence of the magnetic moment of the perovskite manganites La1−xSrxMnO3 on the Sr doping level, x, is fitted using an oxygen 2p itinerant electron model for magnetic oxides (i.e., the IEO model), which is similar to the O 2p hole model proposed by Alexandrov et al. (Phys. Rev. Lett., 96 (2006) 117003). According to this IEO model, the itinerant electrons in these manganites are oxygen 2p electrons (or 2p holes) rather than 3d electrons, as assumed in the double-exchange interaction model. Furthermore, the proposed IEO model describes the process by which the itinerant electrons transit along the O2−–Mn2+–O2−–Mn3+–O1− ion chains in La1−xSrxMnO3 (x < 0.15), wherein a 2p hole is present in the outer orbit of the O1− anion and the magnetic moment of the Mn3+ cation is opposite to that of the Mn2+ cation.

Behavior of Mn2+ in perovskite manganites with nominal composition La0.6−xNdxSr0.1MnO3

The fact that there are Mn2+ at the A sites in the ABO3 perovskite phase of manganites with the nominal composition La0.6−xNdxSr0.1-MnO3 showed by detailed experimental study and theoretical calculations. The magnetic moments of these Mn2+ are antiparallel to those of the Mn ions at the B sites. The content of the Mn2+ increases as the average ionic radius, 〈rA〉, of the ions at A sites decreases, resulting in the experimentally observed phenomenon that the content of the Mn3O4 phase in the manganites decreases with decreasing 〈rA〉.