Nanoengineered Magnetic-Field-Induced Superconductivity

Abstract

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor / ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled. When the applied magnetic field exceeds a certain critical value, superconductivity is suppressed due to orbital and spin pair breaking effects. This very general property of superconductors sets strong limits for their practical applications, since, in addition to applied magnetic fields, the current sent through a superconductor also generates magnetic fields, which can lead to a loss of zero resistance. Materials that are not only able to withstand magnetic fields, but in which superconductivity can even be induced by applying a magnetic field, are very rare and up to now only (EuSn)Mo 6 S 8 [1, 2], organic λ-(BETS) 2 FeCl 4 materials [4,5] and HoMo 6 S 8 [3] show this unusual behavior. The appearance of magnetic-fieldinduced superconductivity (FIS) in the former two compounds was interpreted in terms of the Jaccarino-Peter effect [6], in which the exchange fields from the paramagnetic ions compensate an applied magnetic field, so that the destructive action of the field is neutralized. Here we report that FIS can also be realized in hybrid superconductor / ferromagnet nanostructured bilayers. The basic idea is quite straightforward (see Fig. 1): a lattice of magnetic dots with magnetic moments aligned along the positive z-direction is placed on top of a superconducting film. The magnetic stray field of each dot has a positive z-component of the magnetic induction B z under the dots and a negative one in the area between the dots. Added to a homogeneous magnetic field H, see Fig. 1(b), these dipole fields enhance the z-component of the effective magnetic field µ 0 H ef f = µ 0 H + B z in the small area just under the dots and, at the expense of that, reduce H ef f everywhere else in the Pb film, thus providing the condition necessary for the FIS observation. This new field compensation effect is not restricted to specific superconductors, so that FIS could be achieved in any superconducting film with a lattice of magnetic dots. To implement the idea of the nanoengineered FIS, we have prepared a sample, which reminds us of other systems used during the last decade for studying flux pinning by periodic arrays of magnetic dots

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

“…1͒ but in the space between them, where the dots field has the opposite polarity. 21,22 Although the geometry of our simulated system is not exactly the same as in the experimental case ͑we consider infinitely long PMs instead of an array of magnetic dots and only consider a fragment of SC on top of two PMs͒ when taking into account a equivalent space between dots with reversed field, we show in Fig. 2 a calculated M͑H At this point we can ask whether the described behavior is the only possible effect that a PM can create on the hysteresis loop of a SC.…”

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 flipping in superconductor/ferromagnet spin-valve structures

“…So far, it has been established experimentally that there can be three ways to modify the superconducting properties in S/F structures: first by the usual proximity effect (i.e., Cooper pair penetration into the ferromagnet), second by the inverse proximity effect), [1][2][3] and third by stray fields coming from the ferromagnet Bloch domain walls or magnetic poles from the sample edges. 4 For the case of the proximity effect, when Cooper pairs penetrate into the ferromagnet, they experience the ferromagnet's exchange field generating the so-called Fulde-FerrelLarkin-Ovchinnilov (FFLO) state. 5,6 In this state, the real part of the order parameter penetrates the ferromagnet a distance on the order of the coherence length ξ F and oscillates due to dephasing by the exchange energy of the ferromagnet.…”