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A careful study of the nonlinear susceptibility of multilayered two-dimensional Cu(14 at% Mn) spin glass layers separated by thick interlayers of Cu is reported. The experiments were performed in SQUID magnetometers using an ac susceptibility technique in the field range 0-0.5 kG and dc magnetization measurements in the field range 0-10 kG. The static 2D critical exponents have been determined from scaling analyses., INTERNATIONAL CONF ON MAGNETISM, EDINBURGH, SCOTLAND, SEP 02-06, 1991

A new method that uses superstructures of Cu(Mn) spin glass films separated by metallic interlayers to probe the decay with thickness of the RKKY interaction within the interlayers is discussed., INTERNATIONAL CONF ON MAGNETISM, EDINBURGH, SCOTLAND, SEP 02-06, 1991

The ac susceptibility of the reentrant Ising spin glass Fe0.62Mn0.38TiO3 has been investigated in a superconducting-quantum-interference-device magnetometer. The data show a transition from a paramagnetic to an antiferromagnetic state at T(N) = 32.8 K. At lower temperatures, the in-phase component of the susceptibility, chi’(omega), becomes frequency dependent and, simultaneously, a finite out-of-phase component of the susceptibility, chi”(omega)), appears. This behavior is interpreted to arise from a transition to a mixed spin-glass and antiferromagnetic state at T(g) = 25. 8 K. A dynamic scaling analysis of chi”(omega) yields critical exponents quite different from those found for the pure Ising spin glass Fe0.5Mn0.5TiO3. However, some apparent deviations from optimal scaling behavior are observed. Magnetic relaxation experiments on this reentrant spin glass reveal the existence of an aging phenomenon similar to that observed in ordinary spin glasses. It is also found that the behavior of the magnetic relaxation can be interpreted within the context of spin-glass domain theories.

Utilizing zero-field cooled magnetization measurements the nonequilibrium dynamics in a 2D spin glass film is investigated. The experiments show that aging is an inherent property of 2D spin glass systems with qualitatively similar features as in 3D systems., INTERNATIONAL CONF ON MAGNETISM, EDINBURGH, SCOTLAND, SEP 02-06, 1991

The reentrant Ising spin glass Fe0.62Mn0.38TiO3 has been investigated using SQUID magnetometry. The system is antiferromagnetic below T(N)(H = 0) = 33 K, and enters a mixed antiferromagnetic/spin glass phase below T(g)(H = 0) = 25.8 K. Within the concept of a recent mean field model, a ratio T(N)(0)/T(g)(0) close to unity indicates both intra- and interlayer frustration. The existence of an overlap length, l(DELTA-T), below which equilibrium spin correlations at temperatures T and T + DELTA-T are indistinguishable has been verified by magnetic relaxation experiments on this spin glass system., INTERNATIONAL CONF ON MAGNETISM, EDINBURGH, SCOTLAND, SEP 02-06, 1991

Thin films of YBa2Cu3O7 (YBCO) were deposited on single crystalline LaAlO3 substrates using d.c. magnetron sputtering. The quality of the films has been assessed by X-ray diffraction; YBa2Cu3O7 grows epitaxially with the c-axis parallel to the substrate normal. For some choices of the deposition parameters in the sputtering process, it was found that secondary phases were precipitated in the films. By a deliberate choice of deposition parameters, semi-coherently grown Y2O3 (yttria) inclusions could be obtained. They had square platelet or cubical shapes with sizes from 5 - 20 nm and thickness approximately 10 nm. The superconducting properties of the YBa2Cu3O7 films were investigated using SQUID magnetometry. Magnetization measurements on films with different densities of semicoherent Y2O3 inclusions indicate an enhanced flux pinning capability with increasing inclusion density., CONF ON CRITICAL CURRENTS IN HIGH T(C) SUPERCONDUCTORS, VIENNA, AUSTRIA, APR 22-24, 1992

The longitudinal component of the magnetization of a single crystal of Bi2.2Sr1.7CaCu2O8 has been investigated by SQUID-magnetometry when the magnetic field is applied at different angles 0-degrees-90-degrees with respect to the crystallographic c-axis. The anisotropy of the system, deduced from the magnetization and relaxation rate data, indicates a mass anisotropy m(c)/m(ab) of order 20, a strikingly low value compared to what has been found from torque and magnetoresistance measurements m(c)/m(ab) almost-equal-to 3000., 6TH INTERNATIONAL WORKSHOP ON CRITICAL CURRENTS, CHURCH COLL, CAMBRIDGE, ENGLAND, JUL 07-11, 1991

The time dependence of the low field susceptibility of a Bi2.2Sr1.7CaCu2O8 single crystal has been investigated in the field range 0.1-300 gauss and at temperatures 0.25 less-than-or-equal-to T/T(c) less-than-or-equal-to 0.9. The relaxation of the magnetization is found to change from a logarithmic to a non-logarithmic form as the applied field is increased, implying that a conventional flux creep model cannot sufficiently describe the behaviour., SATELLITE CONF TO THE 19TH INTERNATIONAL CONF ON LOW TEMPERATURE PHYSICS : HIGH TEMPERATURE SUPERCONDUCTIVITY, QUEENS COLL, CAMBRIDGE, ENGLAND, AUG 13-15, 1990

The field, time and temperature dependence of the longitudinal component of the magnetization of a single crystal of BiSrCaCuO has been investigated by SQUID-magnetometry as a function of the tilt angle between the crystallographic c-axis and the applied magnetic field. We here discuss the results on the field and angular dependence of the magnetic response in zero field cooled magnetization experiments at low fields., INTERNATIONAL CONF ON MATERIALS AND MECHANISMS OF SUPERCONDUCTIVITY HIGH TEMPERATURE SUPERCONDUCTORS 3, KANAZAWA, JAPAN, JUL 22-26, 1991

The nonlinear ac susceptibility chi’-nl of the short-range Ising spin glass Fe0.5Mn0.5TiO3 has been measured using a superconducting-quantum-interference-device magnetometer. The spin-glass temperature, T(g), and the critical exponent gamma were estimated from the temperature dependence of the quadratic field term of chi’nl, yielding T(g) = 20.70 and gamma = 4.0. Static-scaling analyses, using different scaling equations, gave similar results. Using gamma and results from previous dynamic-scaling analyses, a number of critical exponents have been obtained through different scaling relations, e.g., delta = 8.4 and nu = 1.7. The results support the existence of a finite-temperature phase transition in a three-dimensional Ising spin glass.

The time dependence of the zero-field-cooled and thermoremanent magnetization of the isotropic extreme type-II superconductor PbMo6S8 has been measured at temperatures 0.7 less-than-or-equal-to T/T(c) less-than-or-equal-to 0.95 and for fields 1 less-than-or-equal-to H less-than-or-equal-to 300 G. The results show that the time dependence of the decay can be divided into three different field regimes: (1) A low-field regime with a closely logarithmic decay and a well-established critical state. The relaxation rate [partial M/partial ln(t)] increases as H-5 for the zero-field-cooled magnetization and as H-2.4 for the thermoremanent magnetization. The reduction in magnetization over 5 decades in time (0.3-3 X 10(4) s) is, at most, a few percent. (2) A crossover-field regime with a change from a logarithmic to a nonlogarithmic decay. (3) A high-field regime where the time dependence of the decay is well described by the expression delta-M approximately [ln(t/tau-0)]-alpha. The reduction in magnetization in the crossover- and high-field regimes is as large as 25-30% in the measured time interval. The results in the high-field regime are interpreted within the theory of collective flux creep. Comparisons are made with the magnetic relaxation found in anisotropic high-T(c) superconductors.

Time-dependent susceptibility measurements have been performed on multilayered two-dimensional (2D) 40-angstrom-wide Cu0.865Mn0.135 spin-glass films, separated by Cu interlayers of width 10 < w(il) < 1200 angstrom. It is found that samples with w(il) < 30 angstrom display 3D dynamic behavior. For 30 < w(il) < 600 angstrom a gradual decoupling of the spin-glass layers occurs, and for w(il) greater-than-or-equal-to 600 angstrom typical 2D behavior is observed. The effect of a finite interlayer coupling on the dynamic susceptibility is a distinct crossover from 2D dynamic behavior at short observation times to 3D dynamic behavior at long times. A clear influence on the spin-glass dynamics is still observable for interlayer thicknesses of order 500 angstrom; i.e., multilayered spin-glass films are extremely sensitive probes of the effective range of Ruderman-Kittel-Kasuya-Yosida interaction mediated via metallic interlayers. The results also yield information on the relation between time scale and length scale in relaxation experiments, a fundamental concept of domain theories of the spin-glass phase.

High precision magnetic relaxation measurements have been performed on BiSrCaCuO single crystals with the field H applied parallel to the c-axis. At temperatures T/T(c) > 0.5, the data exhibit two different field regimes. One for H < H* (H* corresponds to the field where full flux penetration of the sample is achieved) where the ZFC and RM magnetization relaxes logarithmically with time in agreement with the conventional flux creep model; a second one for H > H* where the relaxation follows a [ln(t/t0)]-alpha dependence. This relaxation law is predicted in the collective flux creep theory for J << J(c) as well as in the vortex-glass model. In the temperature range 0.25 < T/T(c) < 0.4, there is a change in character of the measured decay of the magnetization. This change in character is interpreted to arise from thermal fluctuations of the flux-line lattice giving rise to a considerable reduction in the pinning strength above the depinning temperature.

19TH INTERNATIONAL CONF ON LOW TEMPERATURE PHYSICS, BRIGHTON, ENGLAND, AUG 16-22, 1990

The zero-field dynamics of a Cu(10 at% Mn) spin glass system has been investigated by means of zero-field-cooled magnetization measurements. By subjecting the sample to specific temperature variations just before the field application, new information about the nature of the spin glass phase is obtained. The results are in accord with some predictions from domain theories on the behaviour of the magnetic relaxation in spin glasses. Specifically, the experimental results support the prediction that the spin glass dynamics is governed by an interplay between a growing domain size and the overlap length.

19TH INTERNATIONAL CONF ON LOW TEMPERATURE PHYSICS ( LT-19 ), BRIGHTON, ENGLAND, AUG 16-22, 1990

34TH ANNUAL CONF ON MAGNETISM AND MAGNETIC MATERIALS, BOSTON, MA, NOV 28-DEC 01, 1989

19TH INTERNATIONAL CONF ON LOW TEMPERATURE PHYSICS ( LT-19 ), BRIGHTON, ENGLAND, AUG 16-22, 1990

10TH INTERNATIONAL CONF ON NOISE IN PHYSICAL SYSTEMS, UNIV BUDAPEST, BUDAPEST, HUNGARY, AUG 21-25, 1989

INTERNATIONAL CONF ON MATERIALS AND MECHANISMS OF SUPERCONDUCTIVITY : HIGH TEMPERATURE SUPERCONDUCTORS 2, STANFORD UNIV, CA, JUL 23-28, 1989

INTERNATIONAL CONF ON MATERIALS AND MECHANISMS OF SUPERCONDUCTIVITY : HIGH TEMPERATURE SUPERCONDUCTORS 2, STANFORD UNIV, CA, JUL 23-28, 1989

INTERNATIONAL CONF ON MATERIALS AND MECHANISMS OF SUPERCONDUCTIVITY : HIGH TEMPERATURE SUPERCONDUCTORS 2, STANFORD UNIV, CA, JUL 23-28, 1989