ANALYSES OF WAGNER'S MODEL

To achieve a better understanding of the effects of the colossal magnetoresistivity in the mixed valence manganites,the model of a negative magnetoresistivity scaling proposed by Wagner et al is explored in both mathematical and physical analyses.The inconsistency in Wagners model is discussed and corrected. The behaviors of the large spin polarons in Nd 0.52 Sr 0.48 MnO 3 film at T c are studied and analyzed.The results show that only through necessary corrections can Wagners model become self consistent. A further exploration of the nature of the magnetically aligned clusters will make the model more convincing.

Stability of phases in (Ba, Gd)MnO_3 solid solution system

The existing phases in BaxGd1-xMnO3 solid solution system （0≤x≤ 1） were studied by analyzing the detailed crystal structure of each composition from the results of the Rietveld method using powder X-ray diffraction data. For a small substitution of Ba for Gd （0≤x〈0. 1）, the orthorhombic phase with a perovskite type structure （Pnma space group） was stably formed and this fact was supported by the electron diffraction data. There existed an intermediate phase of Ba0.33Gd0.67MnO3, which was characterized as the tetragonal phase with perovskite structure. The composition range of this phase was narrow and almost line compound. Between the regions of these phases, there existed two-phase region. There was also a two-phase region between the intermediate tetragonal phase and BaMnO3. Measurement of electrical conductivities of these orthorhombic solid solutions and tetragonal phases showed semiconducting behaviors for both phases and the existence of the phase transition at high temperature for the orthorhombic phase, The transition temperature decreased as the Ba content increased.

Structural Analysis of a New Phase of BaNdMnO_4

Single crystal of BaNdMnO4 with I4/mmm was synthesized by a floating zone method. A new phase of BaNdMnO4 was obtained by annealing the single crystal in a reducing atmosphere at 573 K for 10 d. The structural change was analyzed by four-circle X-ray diffractometry and transmission electron microscopy. The new phase has an orthorhombic unit cell (Fmmm) of a=0.5486(2) nm, b=0.5484(2) nm, and c=1.3059(2) nm.

Novel Phase Transition in BaPr_2Mn_2O_7

The rare earth manganite BaLn2Mn2O7(Ln = rare earth element) has a Sr3Ti2O7-type layered perovskite structure with double-block oxygen octahedra belonging to the Ruddlesden-Popper-type homologous series AO(ABO3)2. In Pr-manganite, a novel phase transition was first observed in situ at 1040 K by a thermal transmission electron microscope. At 1040 K, reflections began to split along the c-direction in the selected area diffraction pattern. This indicates that a new phase appears in the original phase matrix; the c length of the new phase is 1.5% longer than that of the original phase. After about 5 min, the original phase disappears. No change in the crystal symmetry (space group 14/mmm) is observed above 1040 K.

Influence of Cr-doping on the magnetic and electrical transport properties of Nd_(0.75)Na_(0.25)MnO_3

Polycrystalline Nd0.75Na0.25Mn1-xCrxO3（0≤x≤0.1） samples were prepared by sol-gel method and their magnetic and electrical transport properties were investigated experimentally.The parent compound with x=0 was a charge ordered manganite and the charge ordering transition occurred at 170 K.For lower Cr concentration（x0.03）,the samples preserved the charge ordering features of the parent compound,but the charge ordering transition temperature decreased with increasing x,in particular from 170 K for x=0 to 154 K for x=0.02.For the x≥0.03 samples,paramagnetic to ferromagnetic phase transition took place and the Curie temperature increased from 112.8 K for x=0.04 to 125.5 K for x=0.1.The electrical transport properties were also influenced by Cr doping.For x0.03,the samples displayed insulator characteristics below 300 K.For x=0.04,the sample showed insulator-metal transition around 75 K.For x≥0.05,the insulator-metal transition disappeared and the compounds showed insulator features again.The magnetic and electrical properties could be well explained by the phase separation scenario.

Critical parameters near the ferromagnetic-paramagnetic phase transition in La_(0.67–x)Y_xBa_(0.23)Ca_(0.1)MnO_3 compounds(x=0.10 and x=0.15)

The critical properties of the mixed manganite La0.67–x Y x Ba0.23Ca0.1Mn O3 with x=0.10 and x=0.15 around the paramagnetic（PM）-ferromagnetic（FM） phase transition were investigated through various techniques. These involved modified Arrott plots, Kouvel-Fisher method and Widom scaling relation. Magnetic data, analyzed in the critical region, using the above methods, yielded the critical exponents for（x=0.10） La0.57Y0.10Ba0.23Ca0.1Mn O3（β=0.312±0.002 and γ=1.147±0.003 at T C=299.23±0.05 K）. Moreover, the estimated critical exponents of（x=0.15） La0.52Y0.15Ba0.23Ca0.1Mn O3 were β=0.286±0.004 and γ=0.943±0.002 at T C=289.53±0.06 K. The critical exponents＇ values were close to the theoretical values of 3D-Ising model and tricritical mean-field model. These results suggested that the present composition should be close to a tricritical point in the La0.67–x Y x Ba0.23Ca0.1Mn O3 phase diagram. Expressing the field dependence as ΔS M∝H n allowed us to establish a relationship between the exponent n and the critical exponents of the material and to propose a phenomenological universal curve for the field dependence of ΔS M.