M. Yu. Kameneva

Synthesis and oxygenation behavior of RBaCo4O7 + δ (R = Y, Dy-Lu)

We have studied phase formation processes in the R2O3-BaO-CoO = 0.5: 1: 4 (R = Y, Gd-Lu) systems at temperatures from 900 to 1100°C, optimized solid-state synthesis conditions for RBaCo4O7 + δ (R114), and obtained single-phase samples for R = Y, Dy, Ho, Er, Tm, Yb, and Lu. The behavior of R114 (R = Y, Dy, Lu) materials during thermal cycling and oxygenation has been investigated.

Spin transition and thermal expansion in the layered cobaltite GdBaCo2O5.5

The electrical conductivity and thermal expansion coefficient of GdBaCo2O5.5 samples have been measured in order to reveal the mechanism of the metal-insulator transition in cation-ordered cobaltites RBaCo2O5.5 (where R is a rare-earth element) and its relation to the change in the spin state of cobalt ions. It has been established that the unit cell volume considerably increases upon the transition to the metallic state at TMI ≈ 360–365 K and that the thermal expansion exhibits anomalies (which are two orders of magnitude weaker) due to the ferromagnetic and antiferromagnetic orderings. The data obtained confirm that the spin transition in Co3+ ions actually proceeds simultaneously with the metal-insulator transition and excludes the possibility of stepwise spin transitions occurring at lower temperatures.

Boundary line of the transition into the pseudogap state in thulium cuprates

Precision measurements of the heat capacity of 1-2-3 thulium cuprates with oxygen content ranging from 6.3 to 6.92 were performed in the temperature range 6–300 K. Analysis of the experimentl data showed anomalies in the temperature dependence of the electronic heat capacity. It is conjectured that the anomalies are due to a transition from the normal metallic into the pseudogap state.

Structural phase transitions in YBaCo4O7 + x cobaltate upon variations in oxygen content, according to X-ray diffraction data obtained using synchrotron radiation

According to thermogravimetric data, the equilibrium oxygen content in layered YBaCo4O7 + x cobaltate varies stepwise from x ≈ 1.4 to x ≈ 0 when temperature is varied in the range of 300–400°C. Detailed ex situ and in situ structural investigations using synchrotron radiation show that variation in the oxygen content in YBaCo4O7 + x is accompanied by substantial reordering of the crystal structure and has the character of a first-order phase transition between two rigorously determined phases that differ by the parameters and symmetry of the crystal lattice as well as by the oxygen content. In addition to uniquely high oxygen liberation upon heating, dx/dT, great mechanical stresses that lead to the material cracking and complicate its practical use are a consequence of the character of stepwise transitions in YBaCo4O7 + x.

Refinement of the composition and structure of YBaCo4−xAlxO7+δ crystals

YBa(Co4 − xAlx)O7 + δ (114Y) crystals have been grown in the Y-Ba-Co-O system by spontaneous crystallization from a slowly cooled nonstoichiometric melt. To change the oxygen content, the crystals were isothermally annealed in air in the range of 280–490°C. The crystals grown were characterized by scanning electron microscopy and powder X-ray diffraction. According to the data of an X-ray spectroscopic quantitative microprobe analysis, the average compositions of “as-grown” and oxygen-saturated crystals were Y1.04Ba1Co3.54Al0.50O7.8 and Y1.02Ba1Co3.55Al0.51O8.4, respectively. The refinement of the crystal structure after saturation on an automatic Bruker X8APEX diffractometer with a CCD detector (MoKα radiation, graphite monochromator, θmax = 32.54°, sp. gr. P63mc, a = 6.2746(9), c = 10.257(3) Å, V = 349.71(13) Å3, Z = 2, dcalcd = 5.220 g/cm3) reveals the location of Al in two independent positions of Co atoms and yields the general formula of the compound as YBaCo3.26Al0.74O7. Problems related to the difference in the compositions obtained by different methods are discussed.

Features of the low-temperature specific heat in underdoped YBa2Cu3O6 + x single crystals

The behavior of the low-temperature specific heat C(T) for YBa2Cu3O6 + x single crystals with the doping level corresponding to the normal phase has been studied by the relaxation technique at different values of the applied magnetic field. It has been found that the C(T)/T plot exhibits such an anomaly as a steep increase with decreasing temperature from T about 4 K down to T ≤ 2 K (the minimum temperature value accessible in the experiment). The applied magnetic field as high as 9 T inverts this anomaly and leads to a sharp drop in C(T)/T during cooling within the same temperature range. A model involving the Schottky-type centers formulated in this work and the data on spin correlation functions has allowed us to calculate the temperature dependence of the specific heat, which fits the experimental curves quite well.

Anomalies of the electronic heat capacity of thulium cuprates in the pseudogap phase region

Precision measurements of the heat capacity of thulium 1-2-3 cuprate with oxygen concentrations x=6.92 and x=6.7 are made in the temperature range 6–300 K. After the lattice components of the heat capacity of the samples are subtracted off, a comparison of the results makes it possible to determine the contribution to the heat capacity due to the formation of the pseudogap phase for the sample with x=6.7. The temperature dependence of this contribution has an anomaly near the boundary of the transition from the pseudogap phase to the normal metallic phase. The experimental results correlate with the theoretical ideas about the rearrangement of the electronic structure upon the transition of the system to the pseudogap phase region.

Magnetoresistance of lightly doped TmBa2Cu3Ox crystals. Reorientation of the antiferromagnetic structure in a magnetic field

The magnetoresistance of lightly doped TmBa2Cu3Ox single crystals is investigated in the temperature range 4.2–300 K for magnetic fields up to 12 T. For the antiferromagnetic sample (x=6.3), when the current and field lie in the ab plane, the magnetoresistance is the sum of an anisotropic and a background component. The existence of the anisotropic component is attributed to the restructuring of the antiferromagnetic domain structure in a magnetic field.

Effect of dilution in the cobalt subsystem on phase transitions in rare-earth cobaltites RBaCo4–x M x O7 (R = Dy–Er, Yb, Y; M = Al, Zn)

We have experimentally studied the structural and elastic characteristics of rare-earth cobaltites RBaCo4–xMxO7 (R = Dy–Er, Yb, Y), in which cobalt ions are partly substituted by diamagnetic Al or Zn ions. It was found that small substitution of Co3+ ions by Al3+ ions in the YbRBaCo4–xMxO7 system (x = 0.1, 0.2, 0.5) leads to a rapid decrease and smearing of ΔE(T)/E0 anomalies of the Young’s modulus in the region of the structural phase transition, which is accompanied by increasing hysteresis. Pure rare-earth cobaltites RBaCo4O7 (R = Dy–Er, Y) exhibit a correlation between the room-temperature structure distortion and hysteresis on the ΔE(T)/E0 curve in a temperature interval of 80–280 K. In Zn-substituted cobaltites RBaCoZn3O7, both the hysteresis and ΔE(T)/E0 anomalies disappear, as do low-temperature sound absorption maxima. This behavior is evidence of the suppression of structural and magnetic phase transitions and the retention of only short-range correlations of the order parameter in Zn-substituted samples.

Elastic, magnetic, and magnetoelectric properties of the CaBaCo4O7 multiferroic

The structural, elastic, magnetic, and magnetoelectric properties of the CaBaCo4O7 multiferroic are experimentally studied and compared with the properties of the related YBaCo4O7 cobaltite, where Y3+ ions substitute for Ca2+ ions. Unlike the frustrated YBaCo4O7 magnet, the softening of Young’s modulus and the hysteresis in the ΔE(T)/E0 curve of ferrimagnetic CaBaCo4O7 in the paramagnetic region are weak, and the anomaly during the magnetic transition increases by almost an order of magnitude. This difference can point to different characters of the development of a long-range magnetic order in these two cobaltites. The distortion of the crystal structure that removes the frustrations of exchange interactions is found to correlate with the magnetic behavior of the cobaltites under study. The magnetization curves of the Ca cobaltite have two steps below 15 K, which can point to the presence of a metastable state in a high magnetic field. The study of the longitudinal and transverse magnetoelectric effects in a pulsed magnetic field demonstrates that their magnitudes are maximal near TC and change their character from linear to quadratic during passage through this temperature.