Magnetic flux patterns in superconductors deposited on a lattice of magnetic dots: A magneto-optical imaging study

Abstract

We investigate the flux penetration in Pb films, of different shapes, deposited on top of a periodic array of Co/ Pt dots with perpendicular anisotropy by means of magnetization and magneto-optical measurements. A clear dependence of the critical current density on the magnetic state of the dots and their polarity with respect to the direction of the applied magnetic field is observed by both techniques. The magnetic state of the dots changes the flux penetration from smooth to channelling. Additionally, in the fully magnetized state, an anisotropic current distribution is observed in circular-shaped samples. The flux penetration is dominated by avalanches only for configurations which correspond to a high critical current, irrespective of its origin, be it low temperature, magnetization state of the dots, or angle between the lattice of dots and the edge of the sample.

Tunability of the critical-current density in superconductor-ferromagnet hybrids

“…1 A particularly interesting experiment performed in Ref. 7 showed a hysteresis loop of a superconducting Pb film grown on an array of Co/Pt magnetic dots, with the general characteristics described in Fig. 1.…”

“…Many experiments are being carried out to study the effect of ferromagnetic parts on the J c of the SC, both in the transport case, that is, when a current is fed in the SC via an external source, [4][5][6] and in the magnetic case, in which current in the SC is induced by an external magnetic field H a . [7][8][9][10] Considering the magnetic case, it is known that in SCs the width of the hysteresis loop M͑H a ͒ is directly associated to the critical-current density J c ͑H a ͒ of the SC, 11,12 according to the critical-state model ͑CSM͒. 13 Obtaining a desired value of J c at a given field H a is, therefore, equivalent to finding a hysteresis loop with the adequate shape as to yield this J c value at H a .…”

“…Due to the geometry and unless otherwise specified, the magnetization has only z-component. For this work we assume an exponential de- 17 with typical values J c0 = 4.44ϫ 10 8 A / m 2 and 0 H 0 = 5.6ϫ 10 −4 T. 7 In order to understand the effect of a PM in the hysteresis loop M͑H a ͒ of a SC we first consider, in Fig. 1͑a͒, a case of a superconducting film on top of a thin magnetic substrate.…”

Imaging the Statics and Dynamics of Superconducting Vortices and Antivortices Induced by Magnetic Microdisks

“…The inevitable transformation of the magnet's continuous stray field outside the superconductor into quantum flux-line bundles inside the superconductor gives rise to several interesting properties such as topological instabilities in superconductor-ferromagnet bilayers [2], field-induced superconductivity [3], domain-wall superconductivity [4], composite self-organized critical states [5,6], or quantized levitation height [7,8]. It is also crucial to take into consideration the interaction of quantized flux lines with ferromagnets in order to understand more practical systems such as magnetic force microscopy measurements of superconducting samples [9,10], the enhancement of the superconducting critical current in superconductorferromagnet hybrid systems [11][12][13][14], or why superconducting levitating trains do not derail in sharp curves [15,16].…”

Vortex patterns in a superconducting-ferromagnetic rod

“…Mesoscopic superconductors have new and interesting properties [4], and also provide an interesting playground to understand the coexistence of magnetism and superconductivity [5,6]. For instance magnetic dots on top of superconducting film have been investigated both theoretically and experimentally [7][8][9][10].…”