Low-field Magnetic Relaxation Research Articles

Low field flux dynamics of Pr-doped Bi-2212 single crystals

Magnetic hysteresis M(H,T) and magnetic relaxation M(t) results on various Pr-doped single crystals Bi 2Sr 2Ca 1−xPr xCu 2O 8+δ are reported. Superconducting H-T phase diagram with applied field parallel to single crystal c-axis indicates that with progressive Pr doping from optimum composition (x = 0.11, T c = 88 K) to underdoped single crystal (x > 0.11), all characteristic field lines in H-T diagram shift to lower reduced temperature T/T c and lower field region with narrower second peak (H 2p) window. Low field magnetic relaxation data near H 2p indicates a distinct change of flux creep behavior from collective creep model for field smaller than second peak onset field (H on) to vortex glass model for H > H on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.

Spin-glass dynamics ofLa0.95Sr0.05CoO3

Experimental studies of dynamic magnetic properties of a highly homogeneous sample of ${\mathrm{La}}_{0.95}{\mathrm{Sr}}_{0.05}{\mathrm{CoO}}_{3}$ perovskite evidence the existence of a low-temperature spin-glass phase. A dynamic scaling analysis of ac susceptibility data according to conventional critical slowing down implies a finite spin-glass phase-transition temperature ${T}_{g}\ensuremath{\approx}14.6\mathrm{K}$ and a dynamic exponent $z\ensuremath{\nu}\ensuremath{\approx}10.3.$ Low-field magnetic relaxation experiments show essentially logarithmic magnetic relaxation and aging effects at all temperatures below ${T}_{g}$ and that temperature perturbations affect the relaxation function in a way that is characteristic for a low-temperature spin-glass phase. Finally, it is also shown that ``simple'' remanent magnetization vs temperature curves, measured after specific cooling protocols, can yield information on dynamic properties such as aging and memory phenomena.

Low-field flux creep behavior near second peak H 2p in Pr-doped Bi-2212 single crystal

Magnetic hysteresis M( H) and magnetic relaxation M( t) results on Pr-doped single-crystal Bi 2Sr 2Ca 0.89-Pr 0.11Cu 2O 8+ δ with optimum T c=88 K are reported. Superconducting H– T phase diagram with applied field parallel to single-crystal c-axis indicates that in the mixed state the anomalous second peak field and its onset field H 2p/ H on varies from 900/800 G at 15 K to 210/85 G at 40 K with a plateau around 20–30 K. Low-field magnetic relaxation M( t) data near H 2p/ H on indicates a distinct change of flux creep behavior. For H< H on, the relaxation mechanism follows the collective creep model M( t)= M 0[1+( μk B T/ U 0)ln( t/ t 0)] −1/ μ and crosses to the vortex glass model M( t)= M 0( t/ t 0) 1/[1−( Uo/ k B T)] when H> H on. Such a crossover behavior implies a change in vortex coupling mechanism from vortex quasi-lattice to entangled vortex glass.

Order director fluctuations contribute to the low-field magnetic relaxation of dilute solutes in a nematic solvent

High-resolution magnetic relaxation dispersion (HR-MRD) is used to measure the field dependence of the 19 F spin–lattice relaxation rates of C 6 F 6 dissolved in the nematic phase of p -methoxybenzilidene– p - n -butylaniline (MBBA). HR-MRD permits measurement of low concentration solute spectral densities, which can be interpreted in terms of the various contributions to the spin Hamiltonian and dynamical models of molecular motion. In this case, C 6 19 F 6 solute relaxation induced by magnetic dipole–dipole interactions reveal distinct components attributable to anisotropic rotational diffusion, and low-frequency order director fluctuations associated with the solvent.

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT conset as well asT c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ c ∼ exp[−ΔT c/T c0] and the flux pinning energyU ∼ ΔT c, where ΔT c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ c, highU yet low ΔT c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.

Overlap length scale of a re-entrant ferromagnet

Dynamic properties of the ferromagnetic (FM) and re-entrant spin-glass (RSG) phase of the re-entrant ferromagnet (Fe 0.20Ni 0.80) 75P 16B 6Al 3 have been experimentally studied by low field magnetic relaxation measurements. An aging phenomenon occurs at constant temperature both in the RSG and the FM region. A fragility of an imposed aging state to magnetic field perturbations discloses the existence of an overlap length scale. The threshold field for an observable disturbance on an aging state acquired during a wait time at constant temperature, shows a notable decrease when passing from the low-temperature RSG region to the FM phase.

Low-field magnetic relaxation study of a single crystal with planes

We have measured magnetic relaxation M versus time in a twin-free single crystal of for magnetic fields applied parallel to a - b planes. Magnetic relaxation is studied as a function of temperature (T) for a fixed field (H) and at a fixed T as a function of H. Long-time M versus t curves were mainly obtained for the partial field penetrating regime. All log M versus log t curves obtained in the partial-field regime present a kink at time separating two distinct regions. The kink gradually disappears as the applied field fully penetrates the sample. The value of is constant for all field values. Similar but more accentuated differences are observed when the effective activation energy U extracted from the data is plotted against M(t). A linear behaviour of U with M is observed in the full field penetrated regime. We estimate that the Bean penetration depth increases exponentially with H in the partial-field regime. We also determined the range of temperatures for which surface barriers are important at the initial stage of relaxation.

Chaos in the Ferromagnetic Phase of a Reentrant Ferromagnet.

The inherent dynamic properties of the ferromagnetic phase of a reentrant ferromagnet, (F${\mathrm{e}}_{0.20}$N${\mathrm{i}}_{0.80}$${)}_{75}$${\mathrm{P}}_{16}$${\mathrm{B}}_{6}$A${\mathrm{l}}_{3}$, have been experimentally investigated by low field magnetic relaxation and ac susceptibility measurements. A prominent aging behavior and an apparent fatal fragility to temperature fluctuations imply that the ferromagnetic phase of the reentrant system has a chaotic nature similar to the spin glass phase, but in striking contrast to the robust nature of the regular ferromagnetic phase.

Anti-Meissner effect and low field magnetic relaxation in sintered Bi-2212

A detailed study of the low field magnetic properties of some sintered Bi-2212 samples has been performed. For sufficiently low applied magnetic fields, typically for fields H < 0.2 Oe, the field-cooled magnetisation becomes positive at some temperature below T c (anti-Meissner effect), contrary to the usual Meissner effect. The field dependence of this effect is comparable to the predicted behaviour for a system in an orbital glass state. Additionally, the relaxation of the zero-field-cooled magnetisation displays an unexpected field dependence, the relaxation rate S = ∂M/ ∂ log( t) increases as H α with α ≈ 0.9 at very low fields. In contrast to the behaviour found in sintered YBCO, this result is not accounted for in conventional flux creep models, where the prediction is α ≥ 2.

Low field magnetic relaxation of superconducting C 60Rb 3

The relaxation of magnetization M in superconducting C 60Rb 3 have been measured at different temperature ranges in low magnetic fields below T c to explore the flux pinning. The measurements were performed with a superconducting quantum interference magnetometer. The applied magnetic field was 300 G – 600 G and the measured time at each temperature was 1 × 10 4 sec. M versus 1nt shows a linear behavior for t ≥ 3600 sec. The data were analyzed in the framework of the flux pinning model of Anderson-Kim. The analyses give the activation energy of U(5K) ≈ 20 mev, and the critical current density based on J c(T) ∝ [dM/dlnt] × [U/kT] shows J c ∝ (1−t) m, with the exponent m ≈ 2.0. The results are discussed.

Low field magnetic relaxation phenomena in YBaCuO

The relaxation of the magnetization of a sintered sample of YBa 2Cu 3O 7 has been investigated in a SQUID-magnetometer. The relaxation may adequately be described by a power law t -a at all fields (1–100 gauss) and temperatures (20–90 K) investigated. A marked field dependence of the exponent a and a corresponding field dependence in the relaxation rate (1/H ∂M/∂ln t) is observed. Three rather distinct field regimes with different values of the exponent a and different field dependences of the relaxation rate may be distinguished: a low field region where a logarithmic relaxation ( a ∼ 0) is found, an intermediate regime with a ∼ 0.04 and a high field region with a ∼ 0.07.