Structure, spin glass, and spin state in perovskite GdCo/sub 1-x/Mn/sub x/O/sub 3/ (x /spl les/ 0.5)
Perovskite GdCo/sub 1-x/Mn/sub x/O/sub 3/ (x=0.2,0.3,0.4,0.5) compounds have been prepared by solid-state reaction. Structures were characterized using X-ray diffraction and Rietveld Refinement method. The compounds crystallized in orthorhombic with Plmn space group. Crystal lattices decreased with the increase of Mn doping level. DC magnetization and ac susceptibility were studied over a wide temperature range and different frequencies. A typical spin glass state was observed in all samples around the same temperature of 122 K where the compounds turned from paramagnetic to ferromagnetic states. For x<0.5, secondary transitions were observed both in dc and ac measurement which were independent of frequency but depended on doping level. The valences of Co or Mn with possible spin states were estimated.
Variation of charge-ordering transitions in R 1 / 3 Sr 2 / 3 FeO 3 ( R = La , Pr, Nd, Sm, and Gd)
The change of the electronic and magnetic properties as well as the charge-ordering (CO) transition related with lattice dynamics has been systematically investigated for crystals of ${R}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3} (R=\mathrm{La},$ Pr, Nd, Sm, and Gd) by transmission electron microscopy and measurements of transport, magnetic, and optical properties. In ${R}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3},$ the hybridization of O $2p$ and Fe $3d$ states, or the effective d electron hopping interaction, can be controlled to some extent by R-dependent lattice distortion. The ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{FeO}}_{3}$ with least rhombohedral lattice distortion undergoes a CO phase transition with ${T}_{\mathrm{CO}}=198 \mathrm{K}$ accompanying charge disproportionation into nominally ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Fe}}^{5+}$ sites, as well as antiferromagnetic spin ordering. When the R-site ion is changed from $R=\mathrm{La}$ to smaller-size R ion towards $R=\mathrm{Gd},$ and hence the $p\ensuremath{-}d$ hybridization interaction is decreased, ${T}_{\mathrm{CO}}$ is decreased and finally the CO transition disappears for $R=\mathrm{Sm}$ and Gd. The optical conductivity spectra for the $R=\mathrm{La}$ -- Nd compounds show a gap opening below ${T}_{\mathrm{CO}}$ and several activated phonon modes due to the periodic charge modulation. The spectral intensity of the new phonon modes shows a discontinuous increase at ${T}_{\mathrm{CO}}$ reflecting the first-order nature of the CO transition. In the cases of the $R=\mathrm{Sm}$ and Gd compounds with no CO transition, the gap feature is observed over a whole temperature region, while no phonon anomaly is discerned. These results imply that the strong $p\ensuremath{-}d$ hybridization as realized for $R=\mathrm{La},$ Pr, and Nd is necessary for stabilizing the specific valence-skipping charge-ordered state.
Magnetization distribution in the mixed-phase state of hole-doped manganites
The effect of ‘colossal magnetoresistance’ (CMR) in hole-doped manganites—an abnormal decrease of resistivity when a magnetic field is applied—has attracted significant interest from researchers in the past decade. But the underlying mechanism for the CMR phenomenon is not yet fully understood. It has become clear that a phase-separated state, where magnetic and non-magnetic phases coexist, is important, but the detailed magnetic microstructure of this mixed-phase state is so far unclear. Here we use electron microscopy to study the magnetic microstructure and development of ferromagnetic domains in the mixed-phase state of La1-xSrxMnO3 (x = 0.54, 0.56). Our measurements show that, in the absence of a magnetic field, the magnetic flux is closed within ferromagnetic regions, indicating a negligible magnetic interaction between separated ferromagnetic domains. However, we also find that the domains start to combine with only very small changes in temperature. We propose that the delicate nature of the magnetic microstructure in the mixed-phase state of hole-doped manganites is responsible for the CMR effect, in which significant conduction paths form between the ferromagnetic domains upon application of a magnetic field.