Magnetostrictive hysteresis of TbCo/CoFe multilayers and magnetic domains

Magnetic and magnetostrictive hysteresis loops of TbCo/CoFe multilayers under field applied along the hard magnetization axis are studied using vectorial magnetization measurements, optical deflectometry and magneto optical Kerr microscopy. Even a very small angle misalignment between hard axis and magnetic field direction is shown to drastically change the shape of magnetization and magnetostrictive torsion hysteresis loops. Two kinds of magnetic domains are revealed during the magnetization: big regions with opposite rotation of spontaneous magnetization vector and spontaneous magnetic domains which appear in a narrow field interval and provide an inversion of this rotation. We show that the details of the hysteresis loops of our exchange-coupled films can be described using the classical model of homogeneous magnetization rotation of single uniaxial films and the configuration of observed domains. The understanding of these features is crucial for applications (for MEMS or microactuators) which benefit from the greatly enhanced sensitivity near the point of magnetic saturation at the transverse applied field.Comment: 10 pages, 11 figure

Geometrical edge barriers and magnetization in superconducting strips with slits

We theoretically investigate the magnetic-field and current distributions for coplanar superconducting strips with slits in an applied magnetic field H_a. We consider ideal strips with no bulk pinning and calculate the hysteretic behavior of the magnetic moment m_y as a function of H_a due solely to geometrical edge barriers. We find that the m_y-H_a curves are strongly affected by the slits. In an ascending field, the m_y-H_a curves exhibit kink or peak structures, because the slits prevent penetration of magnetic flux. In a descending field, m_y becomes positive, because magnetic flux is trapped in the slits, in contrast to the behavior of a single strip without slits, for which m_y =0.Comment: 11 pages, 5 figures, revtex

Hydrodynamic Instability of the Flux-antiflux Interface in Type-II Superconductors

The macroturbulence instability observed in fluxline systems during remagnetization of superconductors is explained. It is shown that when a region with flux is invaded by antiflux the interface can become unstable if there is a relative tangential flux motion. This condition occurs at the interface when the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in crystals. The phenomenon is similar to the instability of the tangential discontinuity in classical hydrodynamics. The obtained results are supported by magneto-optical observations of flux distribution on the surface of a YBCO single crystal with twins.Comment: 12 pages, 3 figures, submitted to Physical Review Letter

Flux penetration and expulsion in thin superconducting disks

Using an expansion of the order parameter over the eigenfunctions of the linearized first Ginzburg-Landau (GL) equation, we obtain numerically the saddle points of the free energy separating the stable states with different number of vortices. In contrast to known surface and geometrical barrier models, we find that in a wide range of magnetic fields below the penetration field, the saddle point state for flux penetration into a disk does not correspond to a vortex located nearby the sample boundary, but to a region of suppressed superconductivity at the disk edge with no winding of the current, and which is {\it a nucleus} for the following vortex creation. The height of this {\it nucleation barrier}, which determines the time of flux penetration, is calculated for different disk radii and magnetic fields.Comment: Accepted for publication in Physical Review Letter

Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d

Time-resolved local induction measurements near to the vortex lattice order-disorder transition in optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals shows that the high-field, disordered phase can be quenched to fields as low as half the transition field. Over an important range of fields, the electrodynamical behavior of the vortex system is governed by the co-existence of the two phases in the sample. We interpret the results in terms of supercooling of the high-field phase and the possible first order nature of the order-disorder transition at the ``second peak''.Comment: 4 pages, 3 figures. Submitted to Nature, July 10th, 1999; Rejected August 8th for lack of broad interest Submitted to Physical Review Letters September 10th, 199

Investigating The Vortex Melting Phenomenon In BSCCO Crystals Using Magneto-Optical Imaging Technique

Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi_2Sr_2CaCu_2O_8 (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H) or temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder or inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature / field, we have constructed maps of the melting landscape T_m(H,r), viz., the melting temperature (T_m) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.Comment: Paper presented at the International Symposium on Advances in Superconductivity & Magnetism: Materials, Mechanisms & Devices September 25-28, 2001, Mangalore, India. Symposium proceedings will be published in a special issue of Pramana - Journal of Physic

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of the MgB2 ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ~590 cm-1 related to the E2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas beyond this region the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ~ 5GPa exhibits a change in the slope as well as a "hysteresis" effect in the frequency vs. pressure behavior. These singularities in the E2g mode behavior under pressure support the suggestion that MgB2 may undergo a pressure-induced topological electronic transition.Comment: 2 figure

Superconductor strip with transport current: Magneto-optical study of current distribution and its relaxation

The dynamics of magnetic flux distributions across a YBaCuO strip carrying transport current is measured using magneto-optical imaging at 20 K. The current is applied in pulses of 40-5000 ms duration and magnitude close to the critical one, 5.5 A. During the pulse some extra flux usually penetrates the strip, so the local field increases in magnitude. When the strip is initially penetrated by flux, the local field either increases or decreases depending both on the spatial coordinate and the current magnitude. Meanwhile, the current density always tends to redistribute more uniformly. Despite the relaxation, all distributions remain qualitatively similar to the Bean model predictions.Comment: RevTeX, 9 pages, 9 figures, submitted to Supercond. Sci. Technol. Revision: MO image and more refs are adde

Large Predicted Self-Field Critical Current Enhancements In Superconducting Strips Using Magnetic Screens

A transport current distribution over a wide superconducting sheet is shown to strongly change in a presence of bulk magnetic screens of a soft magnet with a high permeability. Depending on the geometry, the effect may drastically suppress or protect the Meissner state of the sheet through the enhancement or suppression of the edge barrier critical current. The total transport current in the magnetically screened Meissner state is expected to compete with the critical current of the flux-filled sheet only for samples whose critical current is initially essentially controlled by the edge barrier effect.Comment: 6 figure

Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals

The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined from magnetization curves for different field sweep rates dH/dt and temperatures. The obtained results are consistent with theoretical reports in the literature about vortex creep over surface and geometrical barriers. The frequently observed low-temperature upturn of H_p is shown to be related to metastable configurations due to barriers for vortex entry. Data of the true lower critical field H_c1 are presented. The low-temperature dependence of H_c1 is consistent with a superconducting state with nodes in the gap function. [PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs