The low-temperature macroscopic phase separation in La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>CoO<Subscript>3 − δ</Subscript> cobaltite

La_0.5Ba_0.5CoO_2.87 has been characterized with neutron and synchrotron powder diffraction and magnetization measurements. This compound was shown to have a cubic crystal structure at temperatures above 200 K whereas slightly above the Curie point T _c ∼ 170 K the structural separation into two different pseudocubic phases gradually develops upon cooling. The structural transformation is reversible. At 2 K the sample consists of G-type antiferromagnetic oxygen-poor and ferromagnetic oxygen-rich phases (approximately 33 and 66%, respectively).     

Mn site substitution of La0.67Ca0.33MnO3 with closed shell ions: Effect on magnetic transition temperature

Mn site is substituted with closed shell ions (Al, Ga, Ti, Zr and a certain combination of Zr and Al) and also with Fe and Ru ions carrying the magnetic moment (S=5/2 and 2 respectively) at a fixed concentration of 5 at %. Substitution did not change either the crystal symmetry or the oxygen stoichiometry. All substituents were found to suppress both the metal-insulator and ferromagnetic transition temperatures (T _p(ρ) and T _C, respectively) to varied extents. Two main contributions identified for the suppression are the lattice disorder arising due to difference in the ionic radii between the substituent (r _M) and the Mn³⁺ ion (r _Mn ³⁺) and in the case of the substituents carrying a magnetic moment, the type of magnetic coupling between the substituent and that of the neighboring Mn ion.     

Structural and magnetic phases of Bi<Subscript>1−x</Subscript>A<Subscript>x</Subscript>FeO<Subscript>3−</Subscript><Subscript>δ</Subscript> (A = Sr,Pb) perovskites

The Bi_1−xA_xFeO_{3− δ} (A = Sr, Pb) systems have been studied using the X-ray, neutron powder diffraction and magnetization measurements in a magnetic field up to 14 T. It was found that around x ∼ 0.06 the crystal symmetry changes from a rhombohedral (space group R3c) to pseudo-tetragonal. In the composition range 0.07 ≤ x ≤ 0.14 the phases with different symmetry of the unit cell coexist independent of synthesis conditions. The neutron powder diffraction shows that the iron ions have average oxidation state close to 3+. The magnetic structure for Bi_0.5Sr_0.5FeO_{3− δ} is found to be G-type antiferromagnetic with magnetic moment of about 3.8 μ_B/Fe³⁺. The weak ferromagnetic state due to magnetoelectric interactions was revealed in the lightly doped rhombohedrally distorted compositions. No evidence for a spontaneous magnetization was observed for the pseudo-tetragonal phases. These compositions show irreversible nonlinear magnetization vs. field behavior apparently due to small local deviations from the collinearity of the magnetic moments.     

Study of Doped Complex Cobalt Oxides by Neutron Diffraction and Methods Based on Synchrotron Radiation

We present the results of studying oxygen-deficient doped complex cobalt oxides by means of neutron diffraction and methods based on synchrotron radiation in the high energy range at low temperatures and a high external pressure. It is shown that the magnetic interactions in these compounds can be interpreted in terms of the model of superexchange between cobalt ions with different oxidation states.     

Crystal structure and magnetic properties of the LaCo0.5Fe0.5O3 perovskite

The magnetic and crystal structures of the LaCo_0.5Fe_0.5O₃ perovskite are investigated. It is established that the unit cell of this compound at room temperature is characterized by rhombohedral distortions. As the temperature decreases, the compound undergoes a structural phase transition from the rhombohedral phase to the orthorhombic phase in the temperature range 200–300 K. The LaCo_0.5Fe_0.5O₃ perovskite has an antiferromagnetic structure with the G _z spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo_0.5Fe_0.5O₃ perovskite are interpreted within a model according to which the ground state of Co³⁺ ions is a low-spin state and the existence of the weak ferromagnetic component is associated with the exchange interactions between the Fe³⁺ ions.     

Positive magnetoresistance effect in rare earth cobaltites

The structure, magnetic, and magnetotransport properties of the Pr_0.5Sr_0.5Co_{1 ? x} Fe _x O₃ system have been studied. The ferromagnet-spin glass (x = 0.5)-G-type antiferromagnet (x = 0.7) transitions and the metal—insulator transitions (x = 0.25) have been revealed. It has been established that the magnetoresistance of the metallic ferromagnetic cobaltites changes sign from positive to negative as the external magnetic field increases. The positive component increases and the negative component decreases with decreasing temperature. The negative magnetoresistance increases sharply in the insulating spinglass phase. Possible causes of the low-magnetic-field positive magnetoresistance in the rare earth metallic cobaltites are discussed.     

Spin state and magnetic interaction of cobalt ions in niobium-doped cobaltites

The magnetic properties and electrical conductivity of La_1?x Sr_xCo_1?x/2Nb _x/2O₃ solid solutions with trivalent cobalt ions are studied. These solid solutions are found to be spin glasses with T _f ～ 25 K. The ferromagnetic component is most pronounced in the composition with x = 0.15. The electrical conductivity decreases with increasing strontium content. The results obtained are interpreted within a model according to which cobalt ions located in the vicinity of strontium ions reside in an intermediate-spin state and the Co³⁺-O-Co³⁺ super-exchange interaction is ferromagnetic because of the local dynamic orbital correlations.     

Temperature evolution of the crystal structure of Bi1 − x Pr x FeO3 solid solutions

The crystal structure of solid solutions in the Bi_{1 ? x} Pr _x FeO₃ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 ≤ x ≤ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi_{1 ? x} Pr _x FeO₃ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO₃ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi_{1 ? x} Pr _x FeO₃ solid solutions. A three-phase structural state has been revealed for the solid solution with x = 0.125 at temperatures near 400°C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.