Quasi-2D behavior in bulk isotropic type-II superconductors: Shape effects

Abstract

The induced magnetization of an isotropic type-II superconductor with an anisotropic sample shape has been measured in an oblique magnetic field and separated into its reversible and irreversible contributions. The measurements unexpectedly show that the irreversible magnetization is oriented essentially parallel to the smallest sample dimension, and is almost independent of the angle of the applied field. This behavior can be understood by considering the critical-state model when the applied field is at an arbitrary direction with respect to the sample axes.PACS numbers: 74.60.Ge, 74.30.CiMuch recent work has focused on the anomalous effects of a magnetic field applied at an angle to the a-b planes of the anisotropic high-7V copper oxide superconductors [1-7]. Observed anisotropy in the irreversible (pinned) behavior is interpreted as being due to the intrinsic anisotropy of the material [1-5]. We have found, however, that when vortex pinning is present, even isotropic type-II superconductors exhibit unexpected results which can be explained as being due purely to sample shape considerations. For bulk parallelepipeds with geometrical aspect ratios as low as 2:1, the irreversible magnetization is oriented essentially parallel to the smallest dimension and does not change as the angle of the applied field is varied from perpendicular to greater than a 45° angle. This unexpected observation is shown here to result from the Bean critical-state model [8] modified to include an applied field at an angle with respect to the sample axes.The measurements were made on polycrystalline Nb and Pb-Bi alloys. The applied field H a was cycled between H C 2 and -H C 2. The induced magnetization density M, due to the macroscopic screening currents, was measured using a vibrating sample magnetometer to determine the component of M along H a , A/VSM, and a torque magnetometer to determine the component of M perpendicular to H a , M {or . All measurements were made at 4.2 K.There are two contributions to M: (a) a reversible part M r due to a screening current near the sample surface and associated with the expulsion of the magnetic field, and (b) an irreversible part M, associated with pinning of the vortices and the resultant vortex density gradient and screening current throughout the sample. The reversible contribution below H c \ equals -H/4K (the usual Meissner effect) and above H c \ is determined by the repulsive interactions of the vortices [9,10]. Demagnetization corrections due to geometry increase the absolute value of the initial slope above 1/4/r and cause M r to lie along a direction different from H a [9-11]. The irreversible contribution results from pinning of the vortices as they attempt to enter or leave the sample (in increasing or decreasing applied fields, respectively). A critical state is set up in which a vortex density gradient and a concomitant critical current J c flows throughout the sample, resulting in a magnetization A// [8,12]. The sign of the gradient, the direction of the current flow, and hence t...

History Effects and Phase Diagram near the Lower Critical Point inYBa2Cu3O7Zhukov1, Groot2, Kokkaliaris3 et al. 2001Phys. Rev. Lett.14190View full textAdd to dashboardCite

“…For crystal AS the transition fields of B umc 24 T and B lmc 5.5 T determine the upper and lower critical points, respectively, where the first-order transition terminates. Although the value of the upper critical field shows significant variations in different samples (15,19, and 24 T for ZY, D3, and AS, respectively), the lower critical point for all studied samples has the same value.…”

“…Because of the geometrical confinement of the shielding currents in thin samples [19] the direction of the moment essentially coincides with the c axis. Figure 1 shows typical behavior of the magnetization curves for the fully oxidized YBa 2 Cu 3 O y single crystal AS.…”

Flux-line lock-in to CuO planes in aLa1.9Sr0.1CuO4Bugoslavsky1, Zhukov2, Perkins3 et al. 1997Phys. Rev. B210130View full textAdd to dashboardCite

“…This result highlights the dominating influence in platelike samples of ''geometric lock-in'' effects, which have been studied in detail for the remanent magnetization, [23][24][25] and for the irreversible magnetization at high fields. 26,27 These are purely electrodynamic in origin, and cause the magnetic moment to tend to align closely parallel to the shortest sample dimension, independent of the superconductor type and structure. In Ref.…”

Angular dependence of the irreversible magnetization of YBa2Cu3

“…In this context, it was shown five years ago by Hellman et al that the irreversible magnetization of superconducting Nb samples is oriented perpendicular to the platelet ͑or film͒ plane when the aspect ratio is larger than 10/1. 10 However, it is only recently that the problem of flux penetration in thin films in the presence of magnetic fields perpendicular to the film plane has been solved analytically, assuming a field-independent critical current. 11,12 The deformation of the flux lines in the vicinity of the sample induces a curvature of the vortices inside the film.…”

“…There is only one assumption that can be made for the critical state: since the critical current has to circulate along the ab plane because of shape effect, in all cases the magnetization will be perpendicular to the film plane, as demonstrated by Hellman et al in the case of Nb thin films. 10 Possible currents with other geometries would in any case give negligible ͑nonmeasurable͒ contributions to the magnetization.…”