No abstract

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

Strong vortex pinning by fission-induced uniformly splayed columnar tracks in anisotropic mercury cuprates is demonstrated to result from (re)scaling of the pinning landscape by a large superconducting anisotropy. The effective "narrowing" of the splay distribution restores variable range vortex hopping (VRH) motion expected for nearly parallel pins. VRH emerges as a distinctive peak in the vortex creep rate (ϳ12% at low fields at T ͞T c ϳ 0.5) of the most anisotropic HgBa 2 Ca 2 Cu 3 O 81d , a peak well described by a glassy dynamics with the characteristic exponent m ϳ 1͞3. [S0031-9007 (98)07526-7] PACS numbers: 67.40.Vs An outcome of an extensive effort [1] to counteract easy vortex wandering in high temperature superconductors [2] is the realization that a random array of parallel crystallographic columnar defects localizes vortices best over largest regions of the H-T space [3,4]. However, for such an array, thermally activated vortex motion (creep)-via nucleation and subsequent spreading of the doublekink vortex excitations [3]-takes advantage of the vast available phase space for hopping processes. Indeed, a variable-range vortex hopping (VRH) [5] (analogous to the variable range motion in doped semiconductors [6]) to the energetically equivalent (parallel) tracks can be surprisingly fast [4,5]. To inhibit the fast VRH channel, Hwa et al. [7] came up with an ingenuous theoretical idea relying on strong localizing action of columnar pins, namely forcing topological vortex entanglement [8] by splaying the track directions, i.e., giving them an angular spread. This would pin vortex kink pairs, and reduce hopping by reducing the number of parallel pinning sites.Recent tests of these ideas [9-12] demonstrate large supercurrent (J c ) enhancements and suppressed vortex dynamics by splayed columnar pins in a 3D (threedimensional, albeit anisotropic) YBa 2 Cu 3 O 72d (YBCO) [9,10]. There, the pinning is optimal only for sufficiently small splay angles [9] u Ӎ 10 ± , consistent with topologically entangled vortices [11,12]. However, the consequence of a large-u splay in a 3D system is severe-by destroying vortex coherence along the field direction [11] it reduces the pinning action to that of point defects [9,11]. On the other hand, large boosts of supercurrents, and especially large shifts of the irreversibility line by a large-angle (or uniform) splay in quasi-2D Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) [10,13], and by uniformly splayed tracks in highly anisotropic HgBa 2 CaCu 2 O 61d (Hg-1212) [14], are not well understood in the context of entanglement [7,10,11]. This invites a basic question about the interplay between strong correlated disorder, anisotropy, and structural (polycrystalline) disorder.Here we demonstrate that the complexity of randomly oriented columnar defects in a polycrystalline material is elegantly removed by a large superconducting anisotropy, restoring the generic physics and the relative simplicity of crystals with parallel pins. We show that strong vortex pinning by a uniform (large-u) splay in ani...

No abstract

We have developed a magnetic imaging scheme using the magnetoresistive spin valve head in a dc bias mode as a sensing element. By scanning the head in contact with the sample we obtain a submicron spatial resolution map of the normal component of the magnetic field in the temperature range 4.2–300 K. The writing element of the sensor can be used to alter the local magnetic structure in a controlled way. This technique was applied to image the magnetic domain structure down to 77 K in patterned thin films of La0.67Sr0.33MnO3, known for their colossal magnetoresistance. A reorientation of single or multiple domains in the films was accomplished by applying a local magnetic field with the writing element, while the effect on magnetotransport was monitored with the simultaneous measurement of current–voltage characteristics.