L. Gladczuk

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.

Superparamagnetic behavior of C60Fe

Abstract A large value of the Curie constant and field dependence of magnetization of C 60 Fe sample point to the existence of superparamagnetic interaction in the compound. The observed nonequilibrium effects can be interpreted within the framework of the superparamagnetic blocking model, if one assumes that the solid consists of clusters (composed of many C 60 Fe complexes) that have large magnetic moments. A wide temperature range of irreversible magnetization of the C 60 Fe samples and a broad maximum of the susceptibility temperature dependence suggest the distribution of the cluster size. Semiempirical quantum chemical calculations show that C 60 Fe complexes might adopt high spin state.

Pressure-induced oxygen-ordering processes in GdBa1.5Sr0.5Cu3O6+x

Abstract The influence of pressure on the superconducting properties of GdBa1.5Sr0.5Cu3O6+x (x = 0.63−0.93) ceramics has been studied. The pressure effect on Tc strongly depends on the oxygen content. Unlike YBa2Cu3O6+x the d T c d P value increases monotonically with decreasing x without the peak for x ≈ 0.7. The value d T c d P = 13.7 K (for x = 0.63) is the highest value for HTSCs of the 123 type. Under a prolonged action of pressure at room temperature the relaxation of the critical parameters (Tc, Hc1) as well as hysteretic phenomena in the case of pressure decrease have been observed. These effects are interpreted as being due to oxygen ordering in the CuO chains. The effect resulting from the oxygen ordering under pressure depends stronger on the oxygen index x than the intrinsic pressure effect connected only with the compression of the unit cell.

Magnetization studies on ErNi2B2C single crystals

Abstract The magnetization of single crystal ErNi 2 B 2 C has been measured in the temperature range from 2 K up to 200 K and in magnetic fields up to 5 T. Performed measurements reveal a striking anisotropy both in paramagnetic and ordered states. It has been suggested that the high susceptibility near the Neel temperature is responsible for anomalies observed in the temperature dependence of the first field for flux penetration and for the irreversibility field. A cascade of metamagnetic transitions has been observed for an external field applied perpendicular to the c -axis. These transitions are discussed in the framework of the incommensurate mean-field model.

Magnetic anisotropy of Au/Co/Au/MgO heterostructure: Role of the gold at the Co/MgO interface

The influence of Au interlayer on the out-of-plane magnetic anisotropy of Au/Co/Au/MgO/Au heterostructures with ultra-thin Co wedge-shaped film was investigated by means of ferromagnetic resonance. It was found that introduction of a single Au monolayer enhances the surface anisotropy of the Co/MgO interface. An occurrence of a large perpendicular anisotropy with respect to the Co overlayer suggests that the largest contribution to the interface anisotropy energy originates from the Au/Co interface as a result of strong Au and Co orbitals hybridization.

Temperature-induced magnetic-anisotropy crossover in a Co/MgO/Co heterostructure

Phenomena occurring at the interface may significantly modify the properties of magnetic thin films. In this paper, the effect of temperature on the magnetic properties of Co(001)/MgO/Co trilayers is studied. The trilayer structure was modified by partial coverage of Co layers by Au at the Co/MgO interface. It is found that when the quantity of deposited gold is small, formation of islands at the Co/MgO interfaces takes place, which is proved by the topological surface analysis and by the x-ray reflectivity. In the investigated trilayer structure, gradual changes of the easy magnetization axis from the in-plane to the out-of-plane direction are observed during lowering the temperature from 300 to 5 K. The analysis of the ferromagnetic resonance spectra within the multiple-magnetic-layer model system indicates that the surface anisotropy provides a significant contribution to the magnetic anisotropy of complete Co(001)/MgO/Co heterostructure.

Magnetostriction in the spin-Peierls state of CuGeO3 single crystals

Abstract The forced magnetostriction in the spin-Peierls phase of CuGeO 3 was measured in a magnetic field up to 12 T. The magnetostrictive strains were determined for magnetic field directed along the a,b and c axes. The strong increase of the magnetostriction constants is observed near the transition from spin-Peierls phase to incommensurate phase. The mechanism responsible for forced magnetostriction in CuGeO 3 is discussed.

Anisotropic magnetic and superconducting properties of ErNi2B2C single crystals

Abstract We report on studies of the magnetic and superconducting properties of ErNi 2 B 2 C single crystals in a wide range of magnetic fields (up to 5 T) and temperatures (2–200 K). It is shown that the magnetic behavior of this compound can be described in terms of the Ising-like model. The temperature dependence of the lower critical field and the irreversibility line clearly indicate an interplay between the superconductivity and the magnetism of the Er sublattice.

Magnetic anisotropy of YNi2B2C single crystals

Abstract Reversible and irreversible magnetization processes in YNi2B2C single crystal have been measured and analysed in terms of existing theories. Performed measurements suggest that anisotropy of the effective mass in YNi2B2C superconductor is rather small and similar to that observed in conventional superconductors. Effect of hydrostatic pressure on Tc is shown to be typical of low-temperature superconductors. It is suggested that the layered structure of YNi2B2C has some effect on the irreversible magnetization processes observed in this superconductor.

MgO thickness-induced spin reorientation transition in Co0.9Fe0.1/MgO/Co0.9Fe0.1 structure

Abstract The magnetic anisotropy (MA) of Mo/Au/Co0.9Fe0.1/Au/MgO(0.7–3xa0nm)/Au/Co0.9Fe0.1/Au heterostructure has been investigated at room temperature as a function of MgO layer thickness (tMgO). Our studies show that while the MA of the top layer does not change its character upon variation of tMgO, the uniaxial out-of-plane MA of the bottom one undergoes a spin reorientation transition at tMgO of about 0.8xa0nm, switching to the regime where the coexistence of in- and out-of-plane magnetization alignments is observed. The magnitudes of the magnetic anisotropy constants have been determined from ferromagnetic resonance and dc-magnetometry measurements. The origin of MA evolution has been attributed to a presence of an interlayer exchange coupling (IEC) between Co0.9Fe0.1 layers through the thin MgO film.