Prednáška

Dňa 8. 4. 2010 sa na ElÚ SAV uskutočnila prednáška

Dr. Gorana Karapetrova s názvom:

Anisotropic Superconductivity and Vortex Dynamics in Magnetically Coupled F/S and F/S/F Hybrids

Abstrakt:
     The interplay between superconductivity and magnetism leads to a wealth of physical phenomena that are observed both in materials with intrinsic coexisting superconducting and magnetic phases and in engineered superconductor/ferromagnet heterostructures.
   In the first part we investigate the effect of individual atomic impurities on the superconducting state that they are embedded in. Using low temperature scanning tunneling microscopy and spectroscopy we could identify Co and Mn atoms in the CoxNbSe₂ and MnxNbSe₂ single crystals and observe the influence on the local electronic density of states at 0.4 K.
The magnetic impurities lead to pronounced peak effect in the magnetization of CoxNbSe₂ single crystals and MnxNbSe₂ single crystals. We correlate the peak effect in magnetization with the structure of the vortex lattice across the peak-effect region using scanning-tunneling microscopy.
   Magnetically coupled superconductor-ferromagnet hybrids offers advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductor-ferromagnet hybrids reveal rich superconducting phase diagram. Focusing on particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels.
   In order to explore the Abrikosov flux line ordering in F/S hybrids, we use a combination scanning tunneling microscopy and Ginzburg-Landau simulations [1]. The magnetic stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes non-linear due to a change in both the number of vortices and the confining potential.
   In F/S/F hybrids we demonstrate the evolution of the anisotropic conductivity in the superconductor that is magnetically coupled with two adjacent ferromagnetic layers [2]. Stripe magnetic domain structures in both F-layers are aligned under each other resulting in directional superconducting order parameter in the superconducting layer. The conductance anisotropy strongly depends on the period of the magnetic domains and the strength of the local magnetization. The anisotropic conductivity of up to three orders of magnitude can be achieved with spatial critical temperature modulation of 5% of T_c. Induced anisotropic properties in the F/S and F/S/F hybrids have a potential for future application in switching and non-volatile memory elements operating at low temperatures.

[1] G. Karapetrov, M.V. Milosevic, M. Iavarone, J. Fedor, A. Belkin, V. Novosad, and F.M. Peeters, Phys. Rev. B 80 (2009) 180506.
[2] A. Belkin, V. Novosad, M. Iavarone, R. Divan, J. Hiller, T. Proslier, J.E. Pearson, and G. Karapetrov, Appl. Phys. Lett. 96 (2010) 092513.