S. V. Telegin

Effect of cobalt deficiency on the structural phase transition in EuBaCo2–xO6–δ

The effect of cobalt deficiency on the structural phase transition in the double layered cobaltite EuBaCo2–xO6–δ has been investigated using differential scanning calorimetry and high-temperature X-ray diffraction. It has been shown that, upon introduction of 5% cobalt vacancies, the energy and temperature of the phase transition from the orthorhombic unit cell (space group Pmmm) to the tetragonal unit cell (space group P4/mmm) decrease by 6% and 11°C, respectively, as compared to the corresponding values typical of stoichiometric EuBaCo2O6–δ. A model of the formation of cobalt–oxygen vacancies has been proposed.

Unidirectional anisotropy of the resistivity in 112-type EuBaCo2O5.5 cobaltite

It is found that the resistivity of EuBaCo2O5.5 depends not only on the magnitude of an applied magnetic field but also on its sign. This phenomenon is new for cobalt and manganese oxides. The symmetric or shifted hysteresis loops (depending on the cooling method) for the resistivity correspond to similar loops for the magnetization. The angular dependence of the resistivity is described by the relation Δρ ~ sinθ, where the angle θ = 0 corresponds to the direction of magnetization, at which the sample has been cooled. The results are interpreted in terms of the exchange interaction between the ferromagnetic (FM) and antiferromagnetic (AFM) phases. At the cyclic variation of temperature or magnetic field, the resistivity of EuBaCo2O5.5 increases and does not return to the initial state. The metastable state of EuBaCo2O5.5 is related to the kinetic phenomena accompanying the first order FM–AFM phase transition.

Spin state of Co3+ ions in EuBaCo2–xO5.5–δ in the paramagnetic temperature range

The magnetic studies of EuBaCo2O5.50, EuBaCo1.93O5.40, and EuBaCo1.90O5.36 have been performed. Layered cobaltites EuBaCo2–xO5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature TC almost did not change as the number of Co3+–O–Co3+ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship TC > TN and TC < TN. At T > TC, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo2–xO5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo1.90O5.36 at T > 580 K demonstrated the transition of Co3+ ions to the HS state at lower temperatures as compared to the EuBaCo2O5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.

Optical properties and electronic structure of cobalt-deficit GdBaCo1.86O5 + δ single crystals

The optical properties of cobalt-deficit GdBaCo2 − xO5 + δ single crystals with different oxygen contents have been studied using the ellipsometry method in the spectral range of 0.16–4.8 eV. The optical conductivity spectrum has been characterized by a low-energy structure with the center at ∼1.3 eV and an intense broad absorption band at 1.8–4.0 eV. It has been found that the electronic transitions in the cobalt-deficit samples are partially blocked in a narrow spectral range of 0.5–0.8 eV due to weakening of the contribution from the interstitial Co(3d)-Co(3d) transitions. The ordered GdBaCo2O5.5 crystal exhibits a strong absorption in the energy band gap (Eg < 0.26 eV) due to the structural disorder, defects, and vacancies. A significant decrease in the interband absorption intensity in the tetragonal phase of the GdBaCo1.86O5.07 compound as compared to that in the case of the orthorhombic GdBaCo1.86O5.32 phase has been explained by the existence of only the pyramidal environment of a cobalt ion.

Effect of structural defects on the magnetic properties of the EuBaCo1.90O5.36 single crystal

The effect of structural defects in cobalt and oxygen sublattices with the constant average oxidation level 3+ of all cobalt ions on the magnetic properties of the EuBaCo1.90O5.36 single crystal has been studied. The magnetic properties of the single crystal and the polycrystalline sample of the corresponding composition are compared in the range T = 200–650 K. The results show that the cobalt-deficient EuBaCo2–xO5.5–δ samples demonstrate a three-dimensional XY ferromagnetic ordering of magnetic sublattices. The values of the effective magnetic moment at T > 480 K indicate the existence of the IS and HS states of Co3+ ions. The large difference of values of μeff of the EuBaCo1.90O5.36 single crystal and polycrystal can be due to that the magnetic ion spins lie in plane ab. The magnetic field directed along plane ab substantially influences the magnetic ordering at T < 300 K.

Anisotropic clusters with itinerant charge carriers in the layered EuBaCo1.9O5.36 cobaltite

The anisotropy of reflection spectra is studied for the single crystals of layered EuBaCo1.9O5.36 cobaltite within the temperature range of 80–295 K. The results involving the comparison with the magnetic and transport characteristics are analyzed. In the reflection spectra from the (001) and (120) planes measured at T = 295 K (below the temperature corresponding to the transition to the semiconducting state, TMI = 345 K), a contribution from itinerant charge carriers has been revealed. This contribution is associated with the existence of an inhomogeneous charge state. In the reflection spectrum from the (120) plane, the contribution from itinerant charge carriers holds down to T = 80 K. The difference between the reflection spectra from different planes and different characters of their changes with the temperature are attributed to the anisotropy of the clusters with itinerant charge carriers.

Evolution of the optical properties and the electrical resistivity of CaMnO3 during the substitution of Mo for Mn ions

The reflectance, the optical density, and the optical conductivity spectra of CaMn1 − xMoxO3 (x ≤ 0.07) polycrystals are studied over a wide spectral region. The substitution of Mo6+ ions for Mn4+ ions leads to a strong decrease in the first high-energy phonon band, which is related to vibrations in the MnO6 octahedron. The appearance of band-type charge carriers is detected in CaMn1 − xMoxO3 with x ≤ 0.04, and carrier localization is observed in CaMn0.93Mo0.07O3. This localization increases at T < 160 K, i.e., during the transition from the paramagnetic to the antiferromagnetic state with orbital ordering and a monoclinic structure. The optical conductivity spectrum of CaMn0.93Mo0.07O3 exhibits a strong shift of fundamental absorption bands, which is associated with a change in the band structure because of a decrease in the splitting of the 3d levels of Mn in the crystal field.

Mictomagnetic State in an EuBaCo2–xO5.5–δ Single Crystal

The magnetic properties of an EuBaCo1.9O5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo2O5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below TC = 242 K. At T < 300 K, all three spin states of the Co3+ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co3+ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co3+ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo1.9O5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo1.9O5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.

Spin State of Co 3+ Ions in Layered GdBaCo 2 O 5.5 Cobaltite in the Paramagnetic Phase

A new scheme interpreting the changes in the spin state of Co3+ ions in GdBaCo2O5.5 in the course of the metal–insulator transition is proposed. The transition occurs gradually within a wide (~100 K) temperature range. The changes in the spin state of Co3+ ions are revealed using the data on the linear thermal expansion. In the metallic state, less than one-half of Co3+ ions are in the high-spin (HS, S = 2) state in octahedra, whereas the remaining ions are in the low-spin (LS, S = 0) state. The transition to the nonmetallic state occurs owing to the transformation of the HS state to the LS state in octahedra and to the transformation of some part of LS Со3+ in pyramids to the intermediate-spin (IS, S = 1) state.

Determination of charge states of cobalt ions in nanostructured GdBaCo 2 O 5.5 cobaltites by means of X-ray absorption spectroscopy

X-ray absorption spectroscopy has been applied for a characterization of charge states of cobalt ions of nanostructured GdBaCo2O5.5 complex oxides. The analysis of experimental and theoretical data has allowed establishing a reduction in the oxidation degree from Co3+ to Co2+ for a part of cobalt ions in GdBaCo2O5.5 nanoceramics obtained by torsion under pressure.