The influence of oxygen vacancies on the magnetic state of La<Subscript>0.50</Subscript>D<Subscript>0.50</Subscript>MnO<Subscript>3−γ</Subscript> (D=Da,Sr) manganites

The crystal structure and magnetic and electric transport properties of polycrystalline La_0.50D_0.50MnO_3?γmanganites (D=Ca, Sr) were studied experimentally depending on the concentration of oxygen vacancies. The La_0.50Sr_0.50MnO_3?γ system of anion-deficient compositions was found to be stable and possess a perovskite structure only up to the γ=0.25 concentration of oxygen vacancies, whereas, for the La_0.50Ca_0.50MnO_3?γ system, we were able to obtain samples with the concentrations of oxygen vacancies up to γ=0.50. The stoichiometric La_0.50D_0.50MnO₃ (D=Ca, Sr) compositions had O-orthorhombic (Ca) and tetragonal (Sr) unit cells. The unit cell of the anion-deficient La_0.50Sr_0.50MnO_3?γ manganites also became O-orthorhombic when the concentration of oxygen vacancies increased γτ;0.16). Oxygen deficiency in La_0.50Sr_0.50MnO_3?γ first caused the transition from the antiferromagnetic to the ferromagnetic state γ～0.06) and then to the spin glass state γ～0.16). Supposedly, the oxygen vacancies in the reduced La_0.50Sr_0.50MnO_{3? γ} samples with γ≥0.16 were disordered. The special feature of the La_0.50Ca_0.50MnO_3?γ manganites was a nonuniform distribution of oxygen vacancies in the La_0.50Ca_0.50MnO_2.75 phase. In the La_0.50Ca_0.50MnO_2.50 phase, the type of oxygen vacancy ordering corresponded to that in Sr₂Fe₂O₅, which led to antiferromagnetic ordering. The specific electric resistance of the La_0.50D_0.50MnO_3?γ anion-deficient samples increased with increasing oxygen deficiency. The magnetoresistance of all samples gradually increased as a result of the transition to the magnetically ordered state. Supposedly, the La_0.50Ca_0.50MnO_3?γ manganites in the range of oxygen vacancy concentrations 0.09≤γ≤0.50 had a mixed state and contained microdomains with different types of magnetic ordering. The experimentally observed properties can be interpreted based on the model of phase layering and the model of superexchange magnetic ordering.     

Paramagnetic-ferromagnetic and insulator-metal phase transitions in La<Subscript>0.88</Subscript>MnO<Subscript>2.95</Subscript>

We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La_0.88MnO_2.95. This compound exhibits an insulator-metal (IM) phase transition at T _IM ≈ 253 K (above the Curie temperature, T _C ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T _C , isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La_0.88MnO_2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.     

Crystal structure and magnetic properties of Ba-ordered manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd)

The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln_0.70Ba_0.30MnO_3?δ (Ln = Pr, Nd) with a cation ratio Ln³⁺/Ba²⁺ ? 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln_0.70Ba_0.30MnO₃ synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T _C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size <D> of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O₂] = 10^?4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn₂O₅ phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln₂O₃ (P $\bar 3$ m1, Z = 1) and (3) MnO (Fm $\bar 3$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size <D> = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn₂O₅ exhibit ferrimagnetic properties with Néel temperatures T _N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn₂O₆, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T _C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln_0.90Ba_0.10MnO_3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T _C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.