A.S. Andreenko

Crystal structure of Ru(Sr, Gd)2(Gd, Sr)Cu2O8 and Ru(Sr, Eu)2(Eu, Ce)2Cu2O10 phases

Abstract We have investigated the synthesis, crystal structure, resistivity and magnetic measurements of phases in the Ru(Sr,Gd) 2 (Gd,Sr)Cu 2 O 8− δ (Ru-1212) and Ru(Sr,Eu) 2 (Eu,Ce) 2 Cu 2 O 10− δ (Ru-1222) systems. Rietveld refinement of the structure was carried out. The refined compositions before and after oxygen treatment are Ru 1.00(1) (Sr 1− y Gd y ) 2 (Gd 1− z Sr z )Cu 2 O 8 ( y =0.03(1), z =0.19(1)) (Ru-1212; T c 1− x Cu x )(Sr 1− y Gd y ) 2 (Gd 1− z Sr z )Cu 2 O 8 ( x =0.05(1), y =0.02(1), z =0.16(1)) (Ru,Cu-1212; T c 1− x Cu x )(Sr 1− y Eu y ) 2 (Eu 1− z Ce z )Cu 2 O 10 ( x =0.04(2), y =0.09(2), z =0.44(3)) (Ru,Cu-1222; T c 1− x Ce x )(Sr 1− y Eu y ) 2 (Eu 1− z Ce z ) 2 Cu 2 O 10 ( x =0.08(2), y =0.15(2), z =0.25(3)) (Ru,Ce-1222; T c(onset) =40 K, T c(zero) =12 K). Ruthenium atoms in these structures are octahedrally coordinated as the (Ru 5+ O 6 ) 7− species. The formal charge of Cu (FC Cu) calculated from the electroneutrality condition of refined phase composition increases after oxygen treatment and achieves an optimal value (FC Cu∼2.14) for the superconducting (Ru,Ce)-1222 phase ( t =1000°C, p =1 GPa, τ =72 h).

Anomalies in the Young modulus at structural phase transitions in rare-earth cobaltites RBaCo4O7 (R = Y, Tm-Lu)

The elastic properties of rare-earth cobaltites RBaCo4O7 (R = Y, Tm-Lu) have been experimentally studied in the temperature range of 80–300 K. The strong softening of the Young modulus ΔE(T)/E0 ≈ −(0.1–0.2) of cobaltites with Lu and Yb ions has been revealed, which is due to the instability of the crystal structure upon cooling and is accompanied by an inverse jump at the second-order structural phase transition. The softening of the Young modulus and the jump at the phase transition decrease by an order of magnitude and the transition temperature Ts and hysteresis ΔTs increase from a compound with Lu to that with Tm. A large softening of the Young modulus at the structural transition in Lu- and Yb cobaltites indicates that the corresponding elastic constant goes to zero, whereas this constant in Tm cobaltite is not a “soft” mode of the phase transition. It has been found that the structural phase transition in Lu- and Yb cobaltites is accompanied by a large absorption maximum at the phase transition point and an additional maximum in the low-temperature phase and absorption anomalies in Tm cobaltite is an order of magnitude smaller.

Inverse magnetoresistance in (FeCoB)-(Al2O3) magnetic granular composites

The magnetoresistance, magnetization, and microstructure of granular composites with the general formula (Fe40Co40B20)x(Al2O3)100−x were studied for contents of the amorphous metallic component both above and below the percolation threshold (x≈43). The low-temperature transverse magnetoresistance of the composites is negative at x=41 and practically zero for x=49. For metal contents below the percolation threshold (x=31), a noticeable (7–8%) positive magnetoresistance, reached in magnetic fields of about 17 kOe, was observed. Possible mechanisms of the generation of inverse (positive) magnetoresistance are discussed.

Synthesis, crystal structure and properties of superconducting and non-superconducting RuSr2(Nd,Ce)2Cu2O10 phases»

Abstract In this paper we report on the structural, electrical and magnetic properties of a systematic work performed on the synthesis of RuSr 2 (Nd,Ce) 2 Cu 2 O 10− δ (Ru-1222) samples that differ for the number of annealings or sintering temperature in oxygen. This is done in order to highlight the different behaviours of properties as a consequence of the treatments. In particular, we deal with the development or loss of superconductivity after oxygen treatment. Further we discuss the influence of phase composition on superconductivity and magnetic ordering.

Magnetic properties of the intermetallic compounds RNi (R=Gd, Tb, Dy, Sm) and their hydrides

Hydrogen interaction with RNi intermetallic compounds and the influence of hydrogen on magnetic properties of these compounds were investigated. Ternary hydrides GdNiH3.2, TbNiH3.4, DyNiH3.4 and SmNiH3.7 were prepared by hydrogenation of the initial alloys at room temperature and hydrogen pressure up to 0.1 MPa. Hydrides possess orthorhombic CrB-type structure (S.G. Cmcm). The formation of hydrides results in substantial expansion of the metallic sublattices, weakening of the ferromagnetic interactions and decreasing of the paramagnetic Curie temperatures. The article was translated by the author.

The effect of atomic volume on the Curie temperature and exchange integrals in amorphous R-Fe alloys

Abstract Curie temperatures and the dependence of exchange inegrals on the atomic volumes in amorphous R-Fe alloys were obtained by means of magnetization measurements under hydrostatic pressure and calculations based on molecular field theory. The hydrostatic pressure leads to decreasing of the ferromagnetic exchange interactions inside the iron magnetic subsystem and to increasing of the inferromagnetic exchange interactions between iron and the rare-earth subsystems. The results obtained for the amorphous alloys were compared with the results for the crystalline R-Fe compounds

Synthesis, structure, superconducting and magnetic properties of (Ru5+1-xMo6+x)(Sr, Gd)2(Gd0.7Ce4+0.3)2Cu2O10-δ (x = 0.0, 0.25, 0.50, 0.75, 1.0)

Abstract We report results of the structural, electrical and magnetic studies in synthesized (Ru1−xMox)Sr2(Gd0.7Ce0.3)2Cu2O10−δ (x=0.0, 0.25, 0.50, 0.75, 1.0) system. The RuSr2(Gd0.7Ce0.3)2Cu2O10−δ phase (x=0) may be both non-superconducting and superconducting with the transition temperature Tc(onset)=32 or 46 K depending on annealing conditions. Annealing under high oxygen pressure shifts Tc to higher temperatures that is caused by change of cation composition. In the (Ru1−xMox)Sr2(Gd0.7Ce0.3)2Cu2O10−δ system, lattice parameters of the different phases corresponding to the different composition decrease with increasing of Mo ions content. This is explained by tetrahedral coordination of Mo ions. The substitution of Mo for Ru suppresses the superconductivity: the phases with x=0.25 and 0.50 are superconductors with Tc(onset)=25 and 22 K, respectively, the phases with x=0.75 and 1.0 show a semiconductive behavior. A magnetic ordering is observed near by ∼95 K for all phases.

Magnetic properties and exchange interactions in amorphous and crystalline Y-Fe alloys

Abstract The effects of pressure on the magnetization and ac susceptibility of amorphous Y 0.19 Fe 0.81 alloy and crystalline Y 2 Fe 17 . The results show that two phase transitions para-ferromagnetic-spin-glass-like state transform under pressure to the single phase transition paramagnetic-spin glass in amorphous Y 0.19 Fe 0.81 alloy. The temperature range within which pressure induces the noncollinear ordering of Fe magnetic moments in crystalline Y 2 Fe 17 compound was obtained.

Ruthenium-Based Perovskitelike Phases: Synthesis, Structure, and Properties

This paper reports on synthesis and X-ray diffraction study of two groups of phases obtained from batch mixtures: (La,Srk+1(Ru,Cu)kO3k+1 and (La,Sr)3(Ru,Cu)3O6+δ (group I); RuSr2LnCu2O8−δ (Ln = Nd for the first time, Sm, Eu, Gd, (Gd,Y) for the first time), RuSr2(Ln,Ce4+)2Cu2O10−δ (Ln = Pr and Nd for the first time, Sm, Eu, Gd, (Gd,Y) for the first time), and (Ru,Cu)Sr2(Ln,Ce4+)2Cu2O10−δ (Ln = Tb and Y for the first time) (group II). In group I, phases with K2NiF4, Sr3Ti2O7, and cubic perovskite type structures are typically formed; in group II, these are respectively (Ru,Cu)(Sr,Ln)2(Ln,Sr)Cu2O8−δ (1212 type), (Ru,Cu)(Sr,Ln)2(Ln,Ce4+)2Cu2O10−δ (1222 type), and cubic perovskite type structures (the content of the latter depends on the type of Ln). Variation of the formal charge (f.c.) of Ru (group I) and Cu (group II) was evaluated in relation to the cation composition of the phases (groups I and II) and the content of superstoichiometric oxygen (group II). Phases of 1222 type with Ln = Nd, Sm, Eu, Gd, and (Gd,Y) and phases of 1212 type with Ln = Gd exhibited superconducting properties with Tc max ∼ 40 K.

Peculiarities of Elastic Properties of RE Cobaltites RBaCo4O7 (R = Dy - Er, Y) at Magnetic Phase Transitions

It has been found that the temperature dependences of the Young’s modulus for rare-earth cobaltites RBaCo4O7 (R = Dy - Er, Y) exhibit significant hysteresis and irreversible effects over a wide range (80–280) K between the structural and magnetic phase transition temperatures. At the magnetic phase transition temperature, there are weak and smoothed anomalies of the Young’s modulus, which correlate with the low dimensionality and frustration of the exchange interactions in the Kagome lattice of the Co subsystem for the studied rare-earth cobaltites.