K. D. D. Rathnayaka

Giant change in infrared light transmission in La0.67Ca0.33MnO3 film near the Curie temperature

Yu. P. Sukhorukov,, E. A. Gan’shina, B. I. Belevtsev, N. N. Loshkareva, A. N. Vinogradov, K. D. D. Rathnayaka, A. Parasiris, D. G. Naugle Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia Physics Department, Moscow State University, 119899, Moscow, Russia B. Verkin Institute for Low Temperature Physics & Engineering, Ukrainian Academy of Sciences, Kharkov, 61103, Ukraine Department of Physics, Texas A&M University, College Station, TX 77843-4242, USATransport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La0.67Ca0.33MnO3 film which shows colossal magnetoresistance. The correlations between these properties are presented. A giant change in infrared light transmission (more than a thousand-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic field were observed as well. The giant changes in transmittance and the large magnetotransmittance can be used for development of IR optoelectronic devices controlled by thermal and magnetic fields. Required material characteristics of doped manganites for these devices are discussed.

Transport and Magnetic Anisotropy in CMR Thin Film La1-xCaxMnO3 (x 1/3) Induced by a Film-Substrate Interaction

We present a study of anisotropy of transport and magnetic properties in a La 1-x Ca x MnO 3 (x1/3) film prepared by pulsed-laser deposition onto a LaAlO 3 substrate. We found a non-monotonic dependence of magnetoresistance (MR) on magnetic field H for both H perpendicular and parallel to the film plane but perpendicular to the current. In the longitudinal geometry (when H is parallel to both the current and the film plane) the MR was negative at all fields below 20 kOe, as expected for colossal-magnetoresistance manganites. This rather complex behavior of MR manifests itself at rather low temperatures, far below the Curie temperature T c , which was close to room temperature. Two main sources of MR anisotropy in the film have been considered in the explanation of the results: (1) the existence of preferential directions of magnetization (due to strains stemming from the lattice film-substrate mismatch or other reasons); (2) dependence of resistance on the angle between current and the magnetization, which is inherent in ferromagnets. The transport and magnetic properties of the film correspond well to this view. In particular, the following angle dependence of MR is found: R(θ)/R(0) = 1 + δ an (T,H) sin 2 θ (where θ is the angle between the field and current directions in the plane normal to the film but parallel to the current). The temperature and magnetic field dependences of δ an (T,H) were recorded and analyzed. A clear magnetization anisotropy, that generally favors the magnetization in the film plane is also found. At the same time the recorded magnetization curves (as well as the MR data) indicate, that the film crystal structure should be inhomogeneous in such a way that various parts of the films have non-identical magnetic properties (with different directions of spontaneous magnetization). This hypothesis is supported by X-ray diffraction which revealed that the film is inhomogeneous in strain, lattice parameter and lattice orientation. This peculiar macroscopic-scale disorder is caused by a film-substrate interaction. The possible reasons for formation of such structure and its effect on MR anisotropy are considered.

Point-contact spectroscopy investigation of superconducting-gap anisotropy in the nickel borocarbide compound LuNi2B2C

Point contacts are used to investigate the anisotropy of the superconducting energy gap in LuNi 2B2C in the ab plane and along the c axis. It is shown that the experimental curves should be described assuming that the superconducting gap is nonuniformly distributed over the Fermi surface. The largest and the smallest gaps have been estimated by two-gap fitting models. It is found that the largest contribution to the point-contact conductivity in the c direction is made by a smaller gap and, in the ab plane by a larger gap. The deviation from the one-gap BCS model is pronounced in the temperature dependence of the gap in both directions. The temperature range, where the deviation occurs, is for the c direction approximately 1.5 times more than in the ab plane. The G parameter, allowing quantitative estimates of the gap anisotropy by one-gap fitting, in the c direction is also about 1.5 times greater than in the ab plane. Since it is impossible to describe satisfactorily such gap distribution either by the one- or two-gap models, a continuous, dual-maxima model of gap distribution over the Fermi surface should be used to describe superconductivity in this material.

Thermomagnetic hysteresis effects in NiMn and NiMnPd thin films

dc magnetization measurements, for zero-field cooled (MZFC) and field-cooled (MFC) cases, have been carried out for flash-evaporated Pd-doped NiMn thin films. These included reentrant phases (Ni76−xPdx)Mn24, for 0⩽x⩽5, and Ni75Mn23Pd2, a pure spinglass phase. The studies were performed over the temperature range 3–300 K. Low-field magnetization measurements show the irreversibility effect (MZFC and MFC diverge) at temperatures below the Curie temperature Tc. In Ni75Mn23Pd2, MZFC falls below MFC, as usually observed. However, in reentrant compositions, MZFC crosses MFC upon warming into the ferromagnetic regime, where it stays above MFC at temperatures below Tc. This unusual behavior is attributed to a model of Imry and Ma in which, in a ferromagnet with antiferromagnetic impurities, the impurities can couple to the host ferromagnetic alignment and force the system to break into domains antiferromagnetically coupled to each other. Field-cooled hysteresis measurements indicate the uniaxial anisotropy in the...

Granular superconductivity in polycrystalline ruthenocuprate RuSr2(Gd1.5Ce0.5)Cu2O10−δ: magnetoresistive and magnetization studies

Granular superconductivity effects in polycrystalline samples of RuSr2(Gd1.5Ce0.5)Cu2O10??, as prepared (by a solid-state reaction method) and annealed (12?h at 845??C) in pure oxygen at 30?atm, are presented. The resistive transition to the superconducting state of the as-prepared sample is found to be considerably affected by granularity. In particular, an evident kink in the temperature dependence of the resistance R(T) is seen at the temperature, Tc0?34?K, at which grains become superconducting. The resistive transition depends strongly on the applied current. The family of R(T) curves taken for different transport currents is branched with a branching point at TcJ?23.2?K. Below this temperature the intergrain Josephson coupling starts to develop. For low current, R decreases with decreasing temperature below TcJ as expected for the transition to the superconducting state, whereas R(T) curves for higher current form a minimum at T?17.3?K, showing a quasi-re-entrant behaviour. The influence of the granular structure of the as-prepared sample shows itself also in the temperature behaviour of the magnetization, M(T), in low field. Application of low magnetic field (below 400?Oe) leads to a broadening of the resistive transitions below TcJ, similar to that caused by increasing the current. Both the current and magnetic field depress the Josephson coupling between the grains, producing a dramatically large effect on the resistive transition. The R(T) and M(T) dependences of the annealed sample show a fairly sharp superconducting transition far less affected by granularity. The results obtained imply that oxygen annealing improves the intergranular connection considerably, but it does not exert much influence on the intragrain superconductivity. No indication of intragrain granularity has been found in the samples studied. The influence of ageing (due to deoxidation) of samples for different conditions of storage is considered briefly as well.

Competition of multiband superconducting and magnetic order in ErNi2B2C observed by Andreev reflection

Point contacts (PC) Andreev reflection dV/dI spectra for the antiferromagnetic (TN6 K) superconductor (Tc11 K) ErNi2B2C have been measured for the two main crystallographic directions. The observed retention of the Andreev reflection minima in dV/dI up to Tc directly points to an unusual superconducting order parameter (OP) vanishing at Tc. The temperature dependence of the OP was obtained from dV/dI using the recent theory of Andreev reflection including the pair-breaking effect. For the first time the existence of two superconducting OPs in ErNi2B2C is shown. A distinct decrease of both OPs as temperature is lowered below TN is observed.

Point-contact spectroscopy of the nickel borocarbide superconductors RNi2B2C (R = Y, Dy, Ho, Er, Tm, Lu)

An overview of the recent efforts in point-contact (PC) spectroscopy of the nickel borocarbide superconductors RNi 2 B 2 C in the normal and superconducting (SC) state is given. The results of measurements of the PC electron-boson(phonon) interaction spectral function are presented. Phonon maxima and crystalline-electric-field (CEF) excitations are observed in the PC spectra of compounds with R = Dy, Ho, Er and Tm, while for R = Y a dominant phonon maximum around 12 meV is characteristic. Additionally, non-phonon and non-CEF maxima are observed near 3 meV in R = Ho and near 6 meV in R = Dy. Directional PC study of the SC gap gives evidence for the multi-band nature of superconductivity in R = Y, Lu. At low temperature the SC gap in R = Ho exhibits a standard single-band BCS-like dependence, which vanishes above T*c ≃ 5.6 K < T c ≃ 8.5 K, where a specific magnetic ordering starts to play a role. For R = Tm (Tc ∼ 10.5 K) a decrease of the SC gap is observed below 5 K.

Transport, thermal, and magnetic properties of RuSr2(Gd1.5Ce0.5)Cu2O10−δ, a magnetic superconductor

Resistivity, thermoelectric power, heat capacity, and magnetization for samples of RuSr2(Gd1.5Ce0.5)Cu2O10−δ were investigated in the temperature range 1.8–300K with a magnetic field up to 8T. The resistive transitions to the superconducting state are found to be determined by the inhomogeneous (granular) structure, characterized by the intragranular, Tc0, and intergranular, Tcg, transition temperatures. Heat capacity, C(T), shows a jump at the superconducting transition temperature Tc0≈37.5K. A Schottky-type anomaly is found in C(T) below 20K. This low-temperature anomaly can be attributed to splitting of the ground term S7∕28 of paramagnetic Gd3+ ions by internal and external magnetic fields.

Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

Abstract The thermal conductivity and thermopower are reported for a hole doped Eu 1.5 Ce 0.5 RuSr 2 Cu 2 O 10+ δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T c =45 K superconductivity and weak ferromagnetism coexist ( T m =180 K). Weak features in the thermopower, S ( T ), and thermal conductivity, κ ( T ), are observed both at T m and at T * =140 K. The thermopower begins to decrease sharply toward zero at T c , and there is an extremely sharp increase of about 30% in the thermal conductivity at T c . This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T c . A small shoulder is observed in κ ( T ) in the temperature range T =5–13 K.

Changes in resistivity behavior of metallic glass Fe70Ni12B16Si2 due to molybdenum substitution for nickel

The changes in the resistivity behavior of metallic glass Fe70Ni12−xMoxB16Si2 are reported as a function of Mo substitution (x=0, 2, 4, and 6) for Ni at temperatures between 300 and 1.5 K.