Magnetic Nanoscale Hybrids
Initial Support for this project is provided by the Texas Center for Superconductivity (Naugle), the Texas Advanced Research Program (Teizer) and the Welch Foundation (Naugle).
This new research program is focused on nanostructures in magnetic hybrids. In hybrid systems, fabricated from materials with different and even mutually exclusive states, a strong mutual interaction between subsystems can dramatically change the structure and properties of the constituent materials. This approach offers virtually unlimited horizons for qualitatively new science and technology. The first step will be to achieve new physical phenomena which are predicted to appear when two mutually exclusive states of matter, superconductivity and ferromagnetism, are combined in a single heterogeneous system. The interplay of superconductivity and ferromagnetism has been well studied theoretically and experimentally[i] for homogeneous systems. In such systems, both order parameters are homogeneous in space and suppress each other. As a result, in one or both the ordering is weak. However, by separating the superconducting (SC) and ferromagnetic (FM) subsystems in space, it is possible to avoid their mutual suppression due to the proximity effect. At the same time the magnetic field induced by the nonuniform magnetization of the FM textures penetrates into the SC via superconducting vortices (SV) providing a strong coupling between the SV and FM textures. These textures can be of structural origin (dots, rods) or, e.g. for homogeneous FM/SC bilayers, of topological origin like Domain Walls (DW). The TAMU team recently has pioneered theoretical studies of these Hybrid Magnet/Superconductor Nanostructures (HMSN).[ii],[iii],[iv],[v],[vi] Exciting predictions for HMSN configurations, which are important for our understanding of the fundamental properties of condensed matter, have resulted.
One example is the prediction of a ground state with a spontaneous vortex for a SC with an infinite FM nanorod with magnetization parallel to the rod axis. Although the infinite rod magnetization does not produce a magnetic field outside of the rod, similar to the confinement of the magnetic field inside an infinite solenoid, surprisingly, superconducting vortices can be spontaneously created due to the vortex magnetic field interaction with the rod magnetization. [15,18] As a result a spontaneous supercurrent can appear in the ground state of this system. Formally this contradicts Bloch’s theorem,[vii] or, in other words, time-reversal symmetry is broken. The TAMU team is now developing the detailed theory of such states. We are working with colleagues at U. Col. to produce films with a regular array of magnetic nanorods with diameters down to (10 nm) embedded in them. Another counter-intuitive example is the FM dot embedded in a SC film with magnetization normal to the plane of the film. The total magnetic flux from this dot through the SC film is exactly zero. However, it was shown by theorists in our team that this dot can create a co-called Pearl vortex in the SC film, which contains a net flux equal to the flux quantum hc/2e.
The influence of periodic magnetic arrays (dots, stripes) on superconducting films also will be studied. To avoid the proximity effect, insulating barriers will be grown between the superconductor and magnetic structures. With different patterns and periodicities, highly anisotropic and adjustable transport properties can be created, perhaps leading to new types of superconducting devices. The dependence of the phase diagram of the magnetic dot array on magnetic field, temperature, the coercive force of the dots, the magnetic moment direction, the geometry of the system (period of the array, film thickness), and the superconducting properties of the film (London penetration depth) will be studied. Preliminary theoretical analysis [16,17] predicts unusual behavior for vortex matter generated by magnetic dots on superconducting films. This includes formation of the vortex plasma with frozen and unbound vortices and versatile commensurate and incommensurate vortex lattices in an applied magnetic field. Extension of this study of magnetic hybrids to magnet/magnet and magnet/semiconductor nanostructures can have further important consequences, including combination of soft/hard magnet components to provide new “spring” magnets and strongly modulated magnetic fields in semiconductor or normal metal films.
In this project nanolithography will be used for nanopatterning of materials and for the production of miniaturized SQUID, which will be used for magnetic characterization of individual magnetic elements. Furthermore, magnetic force microscopy studies of flux lattice behavior, and field and temperature dependent measurements of the magnetic and transport properties of the hybrid structures will be employed.
[i] E.g. see O. Fischer, Magnetic Superconductors in Ferromagnetic Materials V. 5, K. H. J. Buschow and E. P. Wohlfarth, eds. (North-Holland, Amsterdam, 1990) pp. 465-549.
[ii] I. F. Lyuksyutov and V. L. Pokrovsky, Magnetization Controlled Superconductivity in a Film with Magnetic Dots, Phys. Rev. Lett. 81, 2344 (1998). http://prl.aps.org/
[iii] I. Lyuksyutov and D. G. Naugle, Frozen Flux Superconductors, J. Mod. Phys. Lett. B13, 491 (1999). http://ejournals.wspc.com.sg/journals/mplb/13/1315/S0217984999000622.html
[iv] D. E. Feldman, I. F. Lyuksyutov, V. L. Pokrovsky and V. M. Vinokur, Vortex Plasma and Transport in Superconducting Films with Magnetic Dots, Europhys. Lett. 51, 110 (2000). http://www.edpsciences-usa.org/articles/euro/abs/2000/13/a6144/a6144.html
[v] I. F. Lyuksyutov and V. L. Pokrovsky, Spontaneous supercurrents in magneto-superconducting systems, Modern Phys. Lett. B14, 409 (2000). http://ejournals.wspc.com.sg/journals/mplb/14/1412/S0217984900000574.html
[vi] I. F. Lyuksyutov, D. G. Naugle and V. L. Pokrovsky, Frozen Flux Superconductors, Proc. SPIE Vol. 4058, p. 376-387, Superconducting and Related Oxides: Physics and Nanoengineering IV, D. Pavuna and I. Bozovic, Eds. 2000.
[vii] N. Byers and C. N. Yang, Theoretical Considerations Concerning Quantized Magnetic Flux in Superconducting Cylinders, Phys. Rev. Lett. 7, 46 (1961); http://cornell.mirror.aps.org/abstract/PRL/v7/i2/p46_1
F. Bloch, Phys. Rev. B2, 109 (1970; Josephson Effect in a Superconducting Ring http://prola.aps.org/abstract/PRB/v2/i1/p109_1
Y. Imry, Inroduction to Mesoscopic Physics, (Oxford University Press, 1997).