P.A.J. de Groot

Temperature control of Fano resonances and transmission in superconducting metamaterials.

Losses are the main evil that limits the use of metamaterials in practical applications. While radiation losses may be controlled by design, Joule losses are hereditary to the metamaterial structures. An exception is superconducting metamaterials, where Joule losses can be uniquely controlled with temperature in a very wide range. We put this in use by demonstrating temperature-dependent transmission in the millimeter-wave part of the spectrum in high-Tc superconducting cuprate metamaterials supporting sub-radiant resonances of Fano type.We demonstrate a millimeter-wave range metamaterial fabricated from cuprate superconductor. Two complementary metamaterial structures have been studied, which exhibit Fano resonances emerging from the collective excitation of interacting magnetic and electric dipole modes.

Superconducting plasmonics and extraordinary transmission

Negative dielectric constant and dominant kinetic resistance make superconductors intriguing plasmonic media. Here we report on the effect of extraordinary transmission through an array of sub-wavelength holes in a perforated film of high-temperature YBCO superconductor.

Giant magnetoresistance by exchange springs in DyFe2/YFe2 superlattices

Magnetization and magnetoresistance measurements are reported for antiferromagnetically coupled DyFe2/YFe2 multilayers in fields up to 23 T. It is demonstrated that the formation of short exchange springs ( ~20 A) in the magnetically soft YFe2 layers results in a giant magnetoresistance as high as 32% in the spring region. It is shown that both the magnitude of the effect and its dependence on magnetic field are in good agreement with the theory of Levy and Zhang for domain wall induced giant magnetoresistance.

Magnetic antidot arrays from self-assembly template methods

Using self assembly from lyotropic liquid crystalline phases and from colloidal suspensions of polystyrene spheres templates, we have prepared well-ordered, nanostructured magnetic materials. We present the results of electrochemical deposition of magnetic metals and alloys in the interstitial space between these templates. This technique has enabled us to create magnetic nanostructures with three-dimensional achitectures on length scales of 4 nm–1 μm. We find changes in coercive field, by more than 1 order of magnitude, dominated by the effects of the nanoscale shapes. Varying the parameters in the preparation allows us to produce materials with predetermined magnetic parameters. The templated electrodeposition technique offers the potential of a low-cost preparation method for submicron patterned magnetic media.

Scaling of the vortex pinning force in Bi2Sr2CaCu2O8+y

Abstract We have measured the magnetic behaviour of the vortex state in Bi2Sr2CaCu2O8+y over the range 6–50 K. By extrapo lating hysteresis loop measurements to zero sweep-rate we study the behaviour of the system in the metastable critical state. We find that above 30 K the critical current reduces to zero for the field range studied. By plotting the derived volume pinning force (Fpin) obtained from Jc we find that the pinning force curves scale. The shape of the curves closely follows the b 1 2 (1-b) 2 behav iour expected from Kramers theory. Using this relation we find that the point at which Fpin drops to zero defines a “depinning” line. The results are compared with predictions from the models of vortex motion based on vortex lattice melting and on thermally activated motion.

Negative coercivity in epitaxially grown (110) DyFe2/YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. Detailed magnetization curves have been obtained for YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×16 with w=45, 50, and 55 A. In particular, it is shown that the formation of magnetic exchange springs in the magnetically soft YFe2 layers, can be used to engineer multilayer samples with a negative coercivity. Further, by using asymmetric field cycling procedures, we have investigated the irreversible parts of the M–B loop, associated with the switching of the DyFe2 multilayers.

Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 A) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.

Electrodeposition of Ni-Si Schottky barriers

Electrodeposition is being used to fabricate magnetic microstructures directly on patterned n-type Si wafers of various substrate resistivities. The Ni-Si Schottky barrier is characterized and found to be of high quality for relatively low Si resistivities (1-2 /spl Omega//spl middot/cm), with extremely low reverse leakage. It is shown that a direct correlation exists among the electrodeposition potential, the roughness, and the coercivity of the films. A conductive seed layer or a back contact is not compulsory for electrodeposition on Si with resistivities up to 15 /spl Omega//spl middot/cm. This shows that electrodeposition of magnetic materials on Si might be a viable fabrication technique for magnetoresistance and spintronics applications.

Oscillatory thickness dependence of the coercive field in magnetic three-dimensional antidot arrays

Recent developments in magnetic applications, such as data storage, sensors, and transducers, are stimulating intense research into magnetism on submicrometer-length scales. Emerging self-assembly fabrication techniques have been proposed as viable, low-cost methods to prepare such submicron structures. In this letter we present studies on magnetic nanostructures with 3D architectures, fabricated using a self-assembly template method. We find that the patterning transverse to the film plane, which is a unique feature of this method, governs the magnetic behavior. In particular, the coercive field, a key parameter for magnetic materials, was found to demonstrate an oscillatory dependence on film thickness.

Shape induced anomalies in vortex pinning and dynamics of superconducting antidot arrays with spherical cavities

Using electrochemical deposition of Pb in the pores of templates prepared by self-assembly from colloidal suspensions of polystyrene latex spheres, well ordered superconducting antidot arrays with spherically shaped holes are fabricated. In contrast to conventional lithographic arrays the samples with spherical cavities demonstrate significantly reduced pinning strength favoring the formation of commensurate states. The ac-flux penetration acquires a hybrid intra- and intervalley regime. For high ac drives an unusual inversion to paramagnetic ac shielding is found at commensurate states.