Shape of a magnetic resonance line in a thin film on the surface of anisotropic superconductor with irregularly distributed Abrikosov’s vortices

Abstract

538.945The form of an electron paramagnetic resonance (EPR) in a thin paramagnetic film (λ/10, λ -London's depth of magnetic field penetration into superconductor) overlying the surface of an anisotropic superconductor is calculated taking into account the local magnetic field non-uniformity of an irregular Abrikosov's vortex lattice. It is shown that the form of EPR is noticeably varied with the degree of irregularity of the superconductor vortex lattice. It is suggested that an inclusion of this circumstance into consideration may essentially change the conclusions made on the lattice type and parameters of this superconductor, which are typically derived from the analysis of the EPR form.For the investigation of magnetic properties of high-temperature superconducting materials (HTSC) a wide use is made of the methods of magnetic resonance [1] and scanning of magnetic field on the surface of superconductor [2, 3], as well as of the experiments on muonic spin rotation [4]. Magnetic probing on the superconductor surface occupies a special place among the methods of superconductor studies. The probe typically used is a film of an organic compound of diphinil picryl hydrazyl (DPPH) in the case of EPR [5,6] or that of silicon oil in the case of nuclear magnetic resonance (NMR) [7]. In both cases, an examination is made of the temperature dependence of non-uniform linewidth of magnetic resonance under transition of a specimen into either non-supeconducting state or into vortex state in the case of the external uniform field sweeping. These experiments provide an estimation of the depth of magnetic field penetration into superconductor, λ. When the EPR-and NMR-probes were thin films (≤1000 Å) of controllable thickness, then it was possible to obtain more detailed information on superconductor parameters from the analysis of the magnetic resonance line shapes.This study presents the results of calculations of the shapes of lines of magnetic resonance in a thin magnetic field of a calibrated thickness (≈0.1 λ), coated onto the surface of an anisotropic superconductor. It is well known that an Abrikosov vortex lattice is formed in HTSC in an external magnetic field Н (Н с1 < H < H c2 , H c1 , H c2 are the first and second critical fields). However, the distribution of local magnetic field has so far been calculated for either regular vortex lattices [8 -10] or for the case of complete chaos in the distribution of Abrikosov's vortices -random non-correlated distribution of vortices [11], with no signs of a regular vortex lattice. Nonetheless, the case of a irregular vortex lattice is quite common. This can be presented as a lattice of vortices in which every i-th vortex is located not exactly in the node of a regular lattice but rather displaced by a certain arbitrary vector a i from its regular position in the lattice, provided that a i << L (L -average spacing between the vortices). These displacements a i can result both from vortex pinning and from quite slow thermal vibrations of the vortex lattice. It is this very ...