Polaron-like vortices, dissociation transition, and self-induced pinning in magnetic superconductors
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olaronLike Vortices, Dissociation Transition, and SelfInduced Pinning in Magnetic Superconductors1 L. N. Bulaevskii and S.Z. Lin Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545 USA email: [email protected]; [email protected] Received March 14, 2013
Dedicated to the memory of Professor Anatoly Larkin Abstract—Vortices in magnetic superconductors polarize spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaronlike and their velocities are determined by the effective drag coefficient that is significantly bigger than the Bardeen–Stephen (BS) one. As the current increases, vortices release polarization clouds and the velocity as well as the voltage in the I–V characteristics jump to values corresponding to the BS drag coef ficient at a critical current Jc. The nonuniform components of the magnetic field and magnetization drop as the velocity increases, resulting in weaker polarization and a discontinuous dynamic dissociation depinning transition. Experimentally, the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. As the current decreases, on the way back, vortices are retrapped by polariza tion clouds at the current Jr < Jc. As a result, the polaronic effect suppresses dissipation and enhances the crit ical current. Borocarbides (RE)Ni2B2C with a short penetration length and highly polarizable rare earth spins seem to be optimal systems for a detailed study of vortex polaron formation by measuring I–V characteristics. We also propose to use a superconductor–magnet multilayer structure to study polaronic mechanism of pin ning with the goal to achieve high critical currents. The magnetic layers should have large magnetic suscepti bility to enhance the coupling between vortices and magnetization in magnetic layers while the relaxation of the magnetization should be slow. For Nb and a proper magnet multilayer structure, we estimate the critical current density Jc ~ 109 A/m2 at the magnetic field B ≈ 1 T. DOI: 10.1134/S1063776113110071 1
1. INTRODUCTION
The conception of vortex as a polaron [1] was initi ated by experimental data on the critical current in Er borocarbide, and we first discuss these data. The fam ily of quaternary nickel borocarbides (RE)Ni2B2C (RE is a rare earth magnetic ion) is an interesting class of crystals that exhibit both singlet superconductivity and magnetic order at low temperatures [2–4]. A number of crystals in that family develop antiferro magnetic order below the Néel temperature TN, which is below the superconducting critical temperature Tc. Because the spatial periodicity of magnetic moments is well below the superconducting correlation length, superconductivity coexists quite peacefully with the antiferromagnetic order. By contrast, the ferromag netic order, antagonistic to Cooper pairing, leads to dramatic changes in both magnetic and superconduct ing orders in the coex
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