Nucleation of superconductivity and vortex matter in superconductor–ferromagnet hybrids

Abstract

Abstract. The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially-separated conventional low-temperature superconductor (S) and ferromagnet (F), is reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field Hext, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affect the nucleation of superconductivity leading to an exotic dependence of the critical temperature Tc on Hext. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this "soft" S/F hybrids requires rearrangements of both, superconducting and ferromagnetic characteristics, as compared with "hard" S/F structures. Some figures in this paper are in color only in the electronic version.

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

“…Thus, our calculations explain the asymmetry observed in several magnetization loops of SCs with periodic arrays of magnetic dots or antidots. 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][2][3] One of the lines of research that is attracting more attention is the possibility of increasing the critical-current density J c of the SC by placing ferromagnetic components near or inside the SC. 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 .…”

Continuously tuneable critical current in superconductor-ferromagnet multilayers

“…This is consistent with the observation that jc enhancements occur when the magnetization in the Ni films reverses and a network of 'one-dimensional' magnetic domain walls is present. The interaction between a vortex line and a finitesized magnetic particle is described by [15] U r M r B r r d r ,…”

Doppler-scanning tunneling microscopy current imaging in superconductor-ferromagnet hybrids

“…11,12 On the other hand, if the two materials are electrically isolated, the resulting system's properties are governed only by the magnetic interactions. [13][14][15][16] In particular, the properties of the superconductor can be strongly altered due to the orbital interaction with the ferromagnet's stray field. This could lead to a variety of interesting opportunities such as the ability to guide the supercurrent by altering the magnetic domain structure in the ferromagnet, 17 control vortex mobility, 18 and induce the nucleation of filamentary superconductivity.…”