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...     

Electronic structure of A- and B-site doped lanthanum manganites: a combined X-ray spectroscopic study

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.     

High-pressure effects on the crystal and magnetic structure of the Nd0.78Ba0.22CoO3 cobaltite

The crystal and magnetic structure of the Nd_0.78Ba_0.22CoO₃ cobaltite is studied by neutron diffraction at high pressures up to 4.2 GPa in the temperature range 10–300 K. The pressure dependences of structural parameters are obtained. Ferromagnetic ordering of the Co sublattice is observed at normal pressure below T _C ～ 140 K, and ferrimagnetic ordering of the Co and Nd sublattices with an antiparallel direction of magnetic moments appears at T _F ～ 40 K. The magnetic moment of Co and the temperature T _C change slightly under pressure, which points to the stability of the initial intermediate-spin (S = 1) state of Co³⁺ ions. This behavior differs considerably from the characteristic behavior of cobaltites that are close in chemical composition and structure and exhibit ferromagnetic ordering of only the Co sublattice. In these cobaltites, the magnetic moment of Co is substantially suppressed and T _C decreases under pressure, which is related to the change in the state of Co³⁺ ions from the intermediate spin state to the nonmagnetic low-spin state (S = 0). The interplay between the appearance of the magnetic interaction of the R-Co sublattices and the stability of the spin state of Co³⁺ ions in the Nd_0.78Ba_0.22CoO₃ cobaltite is discussed.     

Inelastic neutron scattering study of the lattice dynamics of LaCoO3

The lattice dynamics of lanthanum cobaltite is investigated using the coherent inelastic neutron scattering technique in a temperature range from 10 to 536 K, in which two phase transitions are observed: a change in the Co spin state and a metal-insulator phase transition. The measurements were performed using an IN8 high flux thermal neutron triple-axis spectrometer (ILL, Grenoble). Temperature changes in the phonon dispersion curves caused by the rearrangement of the electron and spin subsystems are found.     

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).     

Conditions favoring the polar weak ferromagnetic state in BiFeO<Subscript>3</Subscript>-type multiferroics

The crystal structure and magnetic properties of Bi_{1 − x} A _x FeO_{3 − x/2} (A = Ca, Sr, Pb, Ba), Bi_{1 − x} A _x (Fe_{1 − x} Ti _x )O₃, and Bi_{1 − x} A _x (Fe_{1 − x/2}Nb _x/2)O₃ solid solutions have been studied. It is shown that the homogeneous polar weak ferromagnetic state occurs in the vicinity of a morphotropic phase boundary in the systems where dopant ions lead to the reduction of the unit cell volume in the polar phase. In the case of A = Ca, the non-polar phase also exhibits weak ferromagnetism and the spontaneous magnetizations in the polar and nonpolar phases differ only slightly.     

Magnetic ordering in the perovskites Eu1−x CaxMnO3 (0⩻x⩻0.5)

The magnetic properties of Eu_1−x Ca_xMnO₃ have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)     

Investigation of magnetic phase transformations in a system of Nd1−x BaxMnO3−δ (0≤x≤0.50) depending on the conditions of preparation

Investigations are performed of the crystal structure and magnetic and electromagnetic properties of single crystals (0.23 ≤x≤0.34) and polycrystals (0 ≤x ≤0.50) of an Nd_1?x Ba_xMnO_3?δ system of solid solutions. It is found that, for samples prepared in the air, the maximal Curie temperature (T _C ) does not exceed 150 K, while, in the case of polycrystalline samples in the concentration range of 0.34 ≤x ≤0.50, prepared in a reducing medium (a gaseous mixture of argon and carbon monoxide), T _C increases to 320 K. As a result of the reducing medium effect on the compositions, the type of the magnetic phase transition to the paramagnetic state changes from the first to second order. The electrical resistivity of reduced polycrystalline samples (0.34≤x ≤0.50) decreases in magnitude and correlates with the behavior of magnetization. Both series of samples, prepared both in the air and in a reducing medium, exhibit a transition from the metal to dielectric state at a temperature below T _C . The temperature and field dependences of magnetization for the stoichiometric polycrystalline com-position of Nd_0.50Ba_0.50MnO₃ are measured under conditions of hydrostatic pressure. It is demonstrated that the hydrostatic pressure induces in Nd_0.50Ba_0.50MnO₃ the transition from the antiferromagnetic to ferromagnetic state. Based on the measurement results, hypothetical magnetic phase diagrams are constructed for the system of solid solutions being treated, depending on the concentration of barium and the method of preparation. It is found that no T _TC increase is observed in single crystals (0.23≤x≤0.34) such as is observed in polycrystals. It is assumed that the abrupt increase in T _C of polycrystalline samples prepared in a reducing medium is a result of the emergence of extended defects and microstresses in the crystal lattice.     

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.     

Transition from antiferromagnetic to ferromagnetic state of systems LaMnO3+λ and La1−x Srx(Mn1−x/2Nb x/2)O3

The crystal structure and magnetic and elastic properties of the system LaMnO_3+λ are investigated for various concentrations of oxygen. Upon an increase in the oxygen concentration, the orbital-ordered phase is transformed into an orbital-disordered phase via a two-phase crystal-structure state in the interval 0.04<λ<0.06. The transition is accompanied by a jumplike increase in the Curie temperature and spontaneous magnetization. An analysis of the magnetic properties in weak fields and of the temperature dependence of the Young modulus reveals the properties typical of the orbital-ordered antiferromagnetic phase up to λ=0.08. It is proposed that the two-phase state is associated with the martensite type of the orbital order-disorder phase transformation. The system La_1?x Sr_x(Mn_{1 ?x/2}Nb _x/2)O₃ in which all manganese ions are in the trivalent state exhibits a sequence of antiferromagnetic-ferromagnetic (x>0.2) and ferromagnetic-spin glass (x>0.4) transitions. In both systems, the orbital-disordered phases are ferromagnetic, indicating the crucial role of orbital ordering in the formation of magnetic properties.