Guide Tang

Structural and magnetic properties of self-doped perovskite manganites La0.8-xSr0.2MnO3−δ

Self-doped perovskite manganites with nominal composition La0.8−xSr0.2MnO3−δ (0 ≤ x ≤ 0.20) have been prepared by the sol-gel method. The highest heat treatment temperature used was 1073 K. The x-ray diffraction (XRD) patterns indicate that the samples had a single phase with the ABO3 perovskite structure when the doping content was x ≤ 0.10 and that when the doping content was x ≥ 0.15 the samples had two phases with the ABO3 perovskite structure being the dominant phase and Mn3O4 being the minor phase. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions were estimated for the perovskite phases in which there are Mn2+ ions and no vacancies at the A sites. The ion contents have been corrected by Rietveld fitting of the powder sample x-ray diffraction data. Magnetic measurements indicated that the evolution of the Curie temperature (TC) vs. the Mn4+ ion content ratio at the B sites of the ABO3 structure is in accord with experimental results for La1−xSrxMnO3 samples.

Influence of temperature on vertical Bloch lines in the domain walls of three kinds of hard domains

The temperature stability of vertical Bloch lines in the domain walls of three kinds of hard domains in uncompressed states is investigated experimentally. It is found that there exist three critical temperature ranges in which vertical Bloch lines are unstable for each kind of hard domain, i.e., [T01,(T0)OHB] for ordinary hard bubbles (OHBs), [T01,(T0)ID] for the first kind of dumbbells (IDs), and [T01,(T0)IID] for the second kind of dumbbells (IIDs). Here T01 is the initial critical temperatures in which vertical Bloch lines, in the walls of ordinary hard bubbles, the first kind of dumbbells and the second kind of dumbbells, are annihilated, while (T0)OHB, (T0)ID, and (T0)IID are the final critical temperatures at which vertical Bloch lines in their corresponding hard domains annihilate completely.

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〉.

Enhanced room temperature magnetoresistance in maganites La0.60Sr0.25−yNa0.15Δy MnO31

Abstract Bulk polycrystalline samples of La0.60Sr0.4MnO3 and La0.60Sr0.25−yNa0.15Δy MnO3 with 0.00 ≤ y ≤ 0.15 (“Δ” representing cation vacancy) were successfully synthesized using the sol-gel method. The structural, magnetic, and electrical properties of the polycrystalline of the samples were investigated. The results of X-ray powder diffraction patterns show that these compounds crystallize in a distorted rhombohedral structure with the space group R C. The measurement shows that, with vacancy content y increasing, the unit cell volume V of samples increases, furthermore, the Curie temperature Tc decreases. The temperature dependence of resistivity shows that all samples undergo a metal-semiconductor transition accompanying a ferromagnetic to paramagnetic transition with the increase of temperature. Under an applied filed of 1.8 T, a maximum room temperature magnetoresistance (MR) of 20% is obtained at 293 K for the compound La0.60Sr0.1Na0.15Δ0.15MnO3. The MR peak value of La0.60Sr0.1Na0.15Δ0.15MnO3 increases 2 times more than that of La0.60Sr0.40MnO3(MRp = 6.4%, TMR = 373 K), and the MR peak is shifted from 373 K to room temperature.