The manganites of the type RMnO3 (R=La, Nd, Pr, Sm) are antiferromagnetic and the end (n= µ) members of the so-called Ruddlesden-Popper series, Rn+1MnnO3n+1. These oxide materials illustrate many interesting properties like colossal magnetoresistance (CMR)1-13, charge ordering (CO)14-21, magnetic field induced structural and ferromagnetic transitions22-24 when R is partially substituted by divalent cation A (=Ca, Sr, Ba, Pb) as R1-xAxMnO3. According our results of resistivity (r) and specific heat (C) on the Pr1-xCaxMnO3 series, we confirmed the Pr0.65Ca0.35MnO3 is the good choice to investigate the interplay of double exchange (DE) interaction and charge(CO)/orbital(OO) ordering. A systematic study of r, magnetization (M) and C on polycrystalline Pr0.65Ca0.35-xSrxMnO3 (x=0-0.35) perovskite manganites has been reported. The T-x phase diagram presenting their electrical and magnetic properties is prevailed. The Pr0.65Ca0.25Sr0.1MnO3 (for x=0.1) sample is particularly unique showing a CO transition at TCO ~ 200K, an antiferromagnetic (AFM) ordering transition at TN ~ 175K, a metal-insulator (MI) transition at TMI ~ 80K and a ferromagnetic (FM) ordering transition at TC ~ 45K in the absence of magnetic fields. However, the C data of it do not show any anomaly at TMI for MI transition but illustrates a much smaller anomaly than expected one at TC and is suppressed by magnetic fields. This may indicate that the FM ordering in it, commonly related to MI transition, is of meta-stable characteristic and is ascribed to electronic and magnetic instability induced by spin fluctuations. This is established from the T-H phase diagram, as well as the thermal and magnetic hysteresis in r, M and C.

The hole-doped perovskite manganese oxide such as Ln1-xAxMnO3 (Ln = La, Nd, Pr, and A = Ca, Sr, Ba, Pb) is one of the most studied topics in the recent years due to the observation of colossal magnetoresistance (CMR). Basically, LaMnO3 has an almost insulating behavior and on antiferromagnetic arrangement. By substituting a divalent cation (A2+) in place of La3+, LaMnO3 can be driven into metallic and ferromagnetic state. Mixed valence of Mn 3+ / Mn4+ is needed for both metallic behavior and ferromagnetism in these materials. The CMR characteristic occurs in the ferromagnetic state. A systematic investigation of the structural, magnetic and electrical properties in the perovskite colossal magnetoresistance materials La0.7-xLnxPb0.3Mn1-yMeyO3 (Ln=Pr, Nd and Y, Me=Fe and Co) has presented in this thesis. By subatituting Nd, Pr, Y for the La and Co, Fe for the Mn, the substitution effects on the crystallographic deformation, magnetotransport behavior and electrical properties in these compounds have been studied. According to the results of this research, crystallographic distortion is induced by the substitution of smaller ions, Pr or Nd, onto the La-site. Powder $x$-ray diffraction patterns show a crystallographic transition from rhombohedral symmetry (R-3c) to orthorhombic (Pbnm) crystal structure as the doping content is increased. The increase of deformation from R-3c to Pbnm decreases the bond angle of Mn3+¡ÐO2-¡ÐMn4+ , increases the cant of Mn spin, weakens the double-exchange interaction and results in decrease of ferromagnetism, low ferromagnetic transition temperature Tc, eg electron bandwidth and conductivity. However, the great quantity of decrease in resistivity by an external field leads to the increase in the magnetoresistance ratio. We also find that the increase of saturation magnetization results from the contribution of magnetic ion of Pr or Nd. In addition. in contrast to substitution La by magnetic ion of Pr and Nd, the saturation magnetization is decreased as Y content is increased. The zero-field-cool (ZFC) and field-cool (FC) magnetic measurements indicate that the range of spin ordering for Y one is shorter than Pr one or Nd one with the same doping content. It is because of the small ionic radius of Y, which results in larger distortion, increases the bond angle of Mn3+¡ÐO2-¡ÐMn4+, and corresponds low ferromagnetic transition temperature. The distortion induced by Mn-site substitution is not obvious due to the similar radius of Mn, Co and Fe. Powder x-ray diffraction patterns show a single phase of rhombohedral symmetry (R-3c) for Co doped ststem and a slight crystallographic transition from rhombohedral (R-3c) to orthorhombic (Pbnm) symmetry for Fe doped system. Values of temperature dependence of magnetization indicate that the ferromagnetic double-exchange interaction is gradually substituted by the superexchange interaction. The ZFC-FC curves also indicate that long-range spin ordering is progressively substituted by the short-range spin ordering. The substitution of Mn by Co and Fe supresses the double-exchange interaction, decreases the ferromagnetism and the ferromagnetic transition temperature. Due to the synthesis of the substitution of Nd, Pr, Y for La and Co, Fe for Mn, the mechanism of substitution effects are proved different. The substitution of Nd, Pr and Y for La distorts the crystal, decreases the Mn3+¡ÐO2-¡ÐMn4+ bond angle, and results in the transition of properties, while the substitution of Co and Fe for Mn decrease the percentage of ferromagnetic Mn3+¡ÐO2-¡ÐMn4+. The purpose of this thesis is to clear up the role functions of all elements in these compounds and properties of these compounds. Based on the knowledge of these compounds, it would be helpful to control the physical mechanism and improve the characteristics on preparing their thin film devices.

Les oxydes non-stoechiométriques de type Ruddlesden-Popper, tel que Pr2NiO4+δ, peuvent être dopés en trous par substitution du strontium au praséodyme ou par insertion d’oxygène. Ces modes de dopage s’accompagnent de mises en ordre complexes impliquant la distribution des ions oxygène excédentaires, des ordres de charge et/ou de spin. Le diagramme de phase de Pr2-xSrxNiO4+δ a été exploré par diffraction des rayons X (en laboratoire et par rayonnement synchrotron) ainsi que neutronique. Pour la phase Pr2NiO4.25, nous avons mis en évidence une sur-structure incommensurable avec des réflexions satellites au 6ème ordre, produisant un spectre de diffraction très complexe avec 4 individus et 8 domaines incommensurables. Par diffractions synchrotron et neutronique, un ordre de charge de type échiquier a été identifié dès la température ambiante, suivi en dessous de 170 K par un ordre de type rubans ; un ordre de spin incommensurable s’établit au-dessous de 99 K. Ce travail a permis de révéler un ensemble complexe de phases ordonnées structuralement et électroniquement, gouvernées par des variations subtiles de stoechiométrie en strontium et oxygène. / Non-stoichiometric oxides from the Ruddlesden-Popper series, such as Pr2NiO4+δ, can be hole-doped by substituting strontium to praseodymium or by oxygen insertion. This leads to complex structural ordering involving oxygen-, charge- and spin ordering. The complex phase diagram of Pr2-xSrxNiO4+δ was explored using X-ray (laboratory and synchrotron) as well as neutron diffractions. For the doped phase of highest oxygen content (Pr2NiO4.25), we could evidence an incommensurate structure with satellite reflections of 6th order, yielding a very complex diffraction pattern of up to four twin-individuals and eight incommensurate domains. Checkerboard-type charge ordering was identified already at ambient temperature, while stripe charge ordering was observed below 170 K by synchrotron and neutron diffraction; incommensurate spin ordering appears below 99 K. This thesis reveals the existence of many complex oxygen and electronically ordered phases going along with small variations of the oxygen/strontium stoichiometry.