V. Dyakonov

Comparative study of the pressure effect on critical parameters of GdBa2Cu4O8 and YBa2Cu4O8

Abstract The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (Tc, Hc1, jc) of YBa2Cu4O8 (Y124) and GdBa2Cu4O8 (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear of Tc with the growth of pressure was found, with dTc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd124, respectively. The difference in Tc increase on pressure, dTc/dp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites. It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hc1 under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc1 was observed.

Growth and properties of potassium holmium double tungstate KHo(WO4)2

Potassium holmium tungstate KHo(WO 4 ) 2 crystal, as other tungstates, shows the high temperature irreversible structural phase transition. Owing to this, KHo(WO 4 ) 2 single crystals were grown due to spontaneous crystallisation with use of the High Temperature Solution Growth technique, which allows to lower the temperature of crystallisation below the temperature of the phase transition. K 2 W 2 O 7 was used as a solvent. It provides a very wide temperature range of crystallisation and does not introduce additional impurities into the melt. The starting flux contained 20 mol% of KHo(WO 4 ) 2 dissolved in K 2 W 2 O 7 . It was found that potassium holmium double tungstate is pleochroic. The two different optical spectra: one spectrum for electrical vector of linearly polarised light parallel to optical Y axis (main spectrum) and second one for electrical vector perpendicular to Y axis were measured.

Electron paramagnetic resonance spectra of Er3+ in the monoclinic KY(WO4)2 crystal

Electron paramagnetic resonance (EPR) studies of Er3+ of single-crystal KY(WO4)2 have been carried out at X-band frequencies over a temperature range from 4.2 to 300 K. The main x- and z-axes of the g-tensor in the ac plane do not coincide with the crystallographic a-and c-axes, while the y- and b-axes are parallel. The EPR spectrum is characterized by a strong anisotropy of the g-factors (gz = 13.25 ± 0.05, gx = 0 ± 0.1, gy = 3.378 ± 0.005) and of the hyperfine interaction parameters (Az = −469 ± 3, Ax = 0 ± 3.5, Ay = −120.75 ± 0.8 in 10−4 cm−1). The temperature dependence of the g-factors is caused by an anharmonic part of the spin–phonon Hamiltonian as well as by a rotational contribution to this Hamiltonian. A spin–lattice relaxation is shown to result from two Orbach processes through the first and second excited levels of an electronic spectrum.

Crystal structure and magnetic properties of potassium erbium double tungstate KEr(WO4)2

Results of structural, magnetic and specific heat investigations of the potassium erbium double tungstate, KEr(WO 4 ) 2 , are presented. Potassium erbium double-tungstate KEr(WO 4 ) 2 single crystals have been grown by the top-seeded solution growth method (TSSG) and modified Czochralski techniques. It crystallizes in the monoclinic crystal structure (C2/c space group). The unit cell contains four formula units and is described by parameters a = 10.615(2) A, b = 10.316(2) A, c = 7.534(2) A, β = 130.73(3)°. From the x-ray diffraction measurements the fractional atomic coordinates, displacement parameters and interatomic distances have been determined. The specific heat C(T) of the KEr(WO 4 ) 2 crystal has been measured over a temperature range of 0.6-300 K. The susceptibility has been studied at T = 0.25-4.0 K. The magnetic phase transition was observed at a temperature of 0.48 K. The magnetization has been measured in the temperature region from 4.2 to 60 K and in magnetic field up to 1.6 T. A strong anisotropy of magnetic properties was found. The temperature and field dependences of susceptibility and magnetization data were used for both elucidation of character of the magnetic ordering and calculation of the exchange and dipole-dipole interaction energies as well as for determination of the possible magnetic structure of KEr(WO 4 ) 2 .

Jahn-Teller type structural transition in KDy(WO4)2

Abstract The low temperature structural phase transition in the KDy(WO 4 ) 2 single crystal has been first studied by means of both ESR method and specific heat C ( T ). A broad maximum of C ( T ) with peak at T c = 6.38 K and λ-shaped temperature dependence of the ESR line width ΔH ( T ) in the vicinity of transition may be explained as being due to the structural phase transition caused by the interaction of the electronic states of Dy ions with the lattice vibrations. An analysis of C ( T ) and ΔH ( T ) dependencies show that the second order structural phase transition is the transition of Jahn-Teller type and does not occur with a change of point symmetry.

Magnetic Ordering of Dy3+ Ions in RbDy(WO4)2 Single Crystal

The specific heat C(T) of the monoclinic RbDy(WO4)2crystal has been studied at very low temperatures 0.2≤T≤1.9 K and in magnetic fields 0≤H≤0.38 T. The Neel temperature was shown to be equal to TN = 0.818 ± 0.005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k = − 0.798 K. The C T) dependence below Neel temperature 0.5TN<T<0.99TN) is well described by 2D Ising model, whereas in the temperature region close above TN(1.01 TN<T< 2TN) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-TNdiagrams for fieldH ‖ aare in a reasonable agreement for simple quadratic lattice.

ESR-study of the low-temperature phase transition in KDy(WO4)2

Abstract The low-temperature structural phase transition of the Jahn-Teller type in a KDy(WO4)2 single crystal has been studied by means of the ESR method. The ESR line width near the point of transition (Tspt≈7 K) is shown to increase from 0.17 T up to a maximum value of 0.24T and then it decreases to 0.19 T. The width of transition area is about 8 K. The symmetry of g-tensor does not change at the point of the phase transition while the values of the g-tensor components change considerably: gz=3.13; gx≈0; gy=0.82 at T > Tspt; gz=1.98; gx≈0; gy=1.19 at T

Diagrammatic method for the theory of magnetic and resistive properties of manganites

A diagrammatic method for the theory of magnetic and resistive properties of manganites has been applied. The Holstein double-exchange model for a narrow band with the strong Hunds rule coupling was studied. In parallel with the Lang?Firsov unitary transformation of the zeroth Hamiltonian, we have realized the diagonalization of Hunds Hamiltonian neglecting the upper triplet. The diagram techniques taking into account the quantum spin fluctuations of the lower quintet and hole state with spin S = 3/2 were developed. The magnetic structure of the ground state and an influence of electron?phonon interaction have been analysed using the first nonvanishing approximation of the perturbation theory. Since a simple self-consistent equation for the Green function is lacking, the approximations for the effective interaction line with strong electron?phonon coupling were used. The influences of quantum fluctuations and electron?phonon interactions on magnetization, the Curie temperature and resistivity were investigated. The calculated temperature dependence of resistivity in the pure double-exchange model agrees well with experimental data.

Magnetic Properties of the Nanocrystalline DyMnO_{3} Compound

V. Dyakonov, A. Szytuła, S. Baran, Z. Kravchenko, E. Zubov, O. Iessenchuk, W. Bazela, M. Dul, A. Zarzycki and H. Szymczak Institute of Physics, PAS, al. Lotników 32/46, 02-668 Warsaw, Poland A.A. Galkin Donetsk Physico-Technical Institute, NANU, R. Luxembourg St. 72, 83114 Donetsk, Ukraine M. Smoluchowski Institute of Physics, Jagiellonian University, Reymonta 4, 30-059, Kraków, Poland Institute of Physics, Technical University, Podchorążych 1, 30-084 Kraków, Poland

Nanoparticle size effect on the magnetic and transport properties of (La0.7Sr0.3)0.9Mn1.1O3 manganites

Magnetic and transport thermal measurements of nanosize (La0.7Sr0.3)0.9Mn1.1O3 manganite are reported. The nanoparticles are synthesized with use of the co-precipitation method at different (800, 900, and 950°C) temperatures. Their crystal structure is determined to be perovskite-like with a rhombohedral distortion (the space group R3¯c). The phase composition and specific surface nanopowders are determined. The average size of synthesized nanoparticles (from 40to100nm) is estimated by both the method of low-temperature adsorption of argon and x-ray diffraction measurements. All the nanosize samples show ferromagnetic-like ordering with close phase transition temperatures. Their magnetization decreases with decreasing particle size. Comparison of experimental and calculated temperature dependences of the spontaneous magnetic moment shows that the spontaneous magnetization both in magnetic field and without field is well described in the framework of the double exchange model. The decrease of the magnetiza...