The effect of colossal magnetoresistance in SmCu3Mn4O12 perovskite-like oxide

Journal of Experimental and Theoretical Physics - The magnetoresistance effect in the SmCu3Mn4O12 compound with a perovskite-like structure is investigated for the first time. It is found that an...     

Structure and physical properties of layered double perovskite NdBaCo2O5.50 + δ (δ ≈ 0.25)

Magnetic, electric, and elastic properties of the crystal and magnetic structure of double layered perovskite NdBaCo₂O_{5.50 + δ} are studied by the neutron diffraction method at various temperatures. The data are analyzed using two models of crystal structure. In the first model, the sample consists of two crystal-structure phases with ordered and disordered arrangements of oxygen vacancies. In the second model, a new crystal-structure phase is formed in this compound, which is characterized by ordering of oxygen vacancies in the plane of the rare-earth ion in the 1c crystallographic position (0, 0, 1/2) of space group Pmmm. Two crystal-structure models correspond to different types of magnetic ordering (a mixture of a ferromagnetic phase and a G-type antiferromagnetic phase is presumed in the two-phase crystal-structure model, while a canted antiferromagnetic structure is presumed in the one-phase crystal structure model). The behavior of electric and elastic parameters is better described in the first model, while neutron diffraction studies are in better agreement with the second model.     

Magnetoresistive effect in RCu<Subscript>3</Subscript>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> perovskite-like oxides

The electrical properties of and the magnetoresistive effect in RCu₃Mn₄O₁₂ (R=rare-earth ion or Th) are studied. In all compounds of this series, the magnetoresistive effect amounts to 20% at liquid nitrogen temperature in the presence of a field of 0.9 T. An increase in the magnetoresistance with decreasing temperature and a high sensitivity to weak magnetic fields at low temperatures point to the intergranular nature of the effect. The magnetoresistance shows a peak in the vicinity of the Curie temperature T_C. Based on the dependences of the magnetoresistance on an external magnetic field, it is assumed that the magnetoresistance peak near T_C is related to the charge carrier scattering by magnetic inhomogeneities as in substituted orthomanganites. We believe that the magnetoresistance value near the magnetic ordering temperature depends on the synthesis conditions and the effect of the intergranular spacer on the transport properties of these compounds.     

Magnetic properties of superstoichiometric CaFe2O4 + δ obtained by thermobaric synthesis

The crystal structure and magnetic properties of the superstoichiometric (with respect to oxygen) CaFe₂O_{4 + δ} compound have been studied at temperatures of 5–300 K and in magnetic fields from 0 to ±10 T. The unit cell volume of this compound is found to exceed that of the initial stoichiometric CaFe₂O₄ composition. The results of studying the temperature and field dependences of the magnetization and magnetic susceptibility indicate the formation of ferrimagnetic ordering in CaFe₂O_{4 + δ} below the Neel temperature (180 K).     

Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals, La_2/3Ba_1/3(Mn_1−xCo_x)O₃ and La(Mn_1−xCo_x)O₃ ceramics has been performed. It was found that La(Mn_1−xCo_x)O₃ annealed at 800°C in the range 0.4x0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La_2/3Ba_1/3(Mn_1−xCo_x)O₃ solid solutions exhibit also an evidence for phase separation in the range 0.5x0.8. All the La(Mn_1−xCo_x)O₃ samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x=0.5. The La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co²⁺ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn³⁺–O–Mn⁴⁺, Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Co²⁺ and negative Mn⁴⁺–O–Mn⁴⁺, Co²⁺–O–Co²⁺, Co²⁺–O–Mn³⁺ superexchange interactions as well as Co²⁺ and Mn⁴⁺ ionic ordering.