Papers by Keyword: Double Exchange

Abstract: Structure, morphology, and magnetic properties of perovskite La_0.7Sr_0.2Ba_0.1Mn_1-xCr_xO₃ (x = 0; 0.03; 0.05; 0.07 and 0.1) synthesized via sol-gel method have been studied. X-ray diffraction studies confirm the Rhombohedral structure with R-3c space groups for all samples. The magnetization measurements clearly show that magnetization decreases with increasing Cr substitution concentration. The partial substitution of Cr³⁺ in the Mn site weakens the ferromagnetic double exchange interaction of the Mn³⁺–O–Mn⁴⁺ bond. It is caused by the emergence of the Cr³⁺–O–Cr³⁺ antiferromagnetic interaction. As known, the structural parameters affect the magnetic properties of perovskite manganese materials in the double exchange interaction. As the Cr³⁺ concentration increases, the average Mn-O bond length also increases, and the average Mn-O-Mn bond angle decreases, which dramatically weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction.

Abstract: In this work, (1-x) (Nd_0.67Sr_0.33MnO₃): x (TiO₂) composites with x = 0, 0.1, 0.2, 0.3 and 0.4 have been prepared to investigate the structural and electrical properties. Nd_0.67Sr_0.33MnO₃ (NSMO) was synthesised via the solid-state reaction method before incorporated with TiO₂. The addition of TiO₂ nanoparticle as the secondary phase in manganite composite would favour the spin-polarized tunnelling near to the grain boundary and thus enhance the extrinsic magnetoresistance. Nevertheless, nanoparticle addition might contribute to substitution and diffusion with manganite compound as reported in literature. The effect of the TiO₂ nanoparticle addition into NSMO composites has been examined by an X-ray diffractometer (XRD) and a four-point probe (4PP) system. From the thermogravimetric analysis (TGA), NSMO phase formation occurred in between 756.45 - 977.59 °C. XRD patterns showed that there is no peak shift when the TiO₂ concentration increases. It can be deduced that TiO₂ was segregated at the NSMO grain boundary region and its grain surface. However, a small amount of Ti atoms are expected to replace the Mn atoms in NSMO crystal system and has caused the increase in crystallite size. The electrical study showed that the presence of TiO₂ nanoparticle and substitution of Ti in Mn sites have weaken the double exchange (DE) mechanism and suppressed the metal-insulator transition temperature (T_MI). In addition, the insulating behaviour of TiO₂ has also caused the resistivity of composites to increase drastically.

Abstract: The peculiarities of the Faraday rotation in the imperfect garnet-type crystals for which the lattice irregularities are represented by the mixed valence clusters of transition metal ions are analyzed. The model takes into account the cubic and low symmetry crystal fields acting on the metal ions and migration of the “extra” hole. The electric dipole transitions in these types of clusters with migrating hole coupled to the spin core through the double exchange mechanism are shown to lead to a significant magnetooptical activity in the visible range of spectrum. The intensity of these transitions can be comparable or higher than those for the magnetic dipole ones in the same range of frequencies.

Abstract: A theoretical investigation of peculiarities in the formation of diffusion potential barrier for oxygen atoms in NiO1-δ crystal is performed. The approximation of pair interionic potentials set in analytical expressions is used for the evaluation of the potential barrier for the diffusing oxygen atom. It is shown that the potential energy of diffusing oxygen atom originates first of all from compensation between positive contribution of short-range interaction and negative contributions of the long-range Coulomb interaction. It is also influenced by effects of degenerate term splitting due to the low-symmetry crystal fields, created by the diffusing atom and vacancy, lattice relaxation and redistribution of excess charges in the vicinity of diffusing oxygen atom.

Abstract: The layered manganese oxides LaSr2Mn2-xTxO7 (T=Fe, Cr and Ni) has been prepared. The effect of substitution of transition metal ions for Mn on magnetic and magtoresistance properties has been studied. The compound has a tetragonal symmetry and the lattice parameters a decreases with increasing the doped content but the parameter c increases with x. It indicates that the MnO6 octahedron increase the distortion with x content. With the x increase, their ferromagnetic property was depressed. The Neel temperature TN was depressed gradually from 138K to 89K, 95K and 85K at x=0.3 for T=Fe, Ni and Cr respectively. Their magnetoresistance MR is gradually enhanced for all doped system. It suggests that the introduce of disorder in B-site have an important effect on the magnetic exchange and magnetoresistance because of the weaken of the double-exchange (DE) interaction.