D. M. Silevitch

Magnetic trapping and self-assembly of multicomponent nanowires

Magnetic nanowires suspended in fluid solutions can be assembled and ordered by taking advantage of their large shape anisotropy. Magnetic manipulation and assembly techniques are demonstrated, using electrodeposited Ni nanowires, with diameter 350 nm and length 12 μm. Orienting suspended nanowires in a small magnetic field H≈10 G promotes self-assembly of continuous chains that can extend over several hundred μm. The dynamics of this process can be described quantitatively in terms of the interplay of magnetic forces and fluid drag at low Reynolds number. In addition, a new technique of magnetic trapping is described, by which a single magnetic nanowire can be captured between lithographically patterned magnetic microelectrodes. The use of three-segment Pt–Ni–Pt nanowires yields low resistance, Ohmic electrical contacts between the nanowires and the electrodes. This technique has potential for use in the fabrication and measurement of nanoscale magnetic devices.

Magnetotransport properties of bent ferromagnetic nanowires

Magnetotransport measurements were performed on individual multisegmented Pt–Ni–Pt nanowires fabricated by electrochemical deposition in nanoporous alumina templates. The nanowires were removed from the template, and precipitated onto substrates from liquid suspension. The Pt end segments provide an oxide-free interface to the magnetic central segment of interest. Centrifugation prior to precipitation induces sharp bends in the nanowires. The angular dependence of the magnetoresistance of both straight and bent nanowires was used to observe domain switching. The magnetic response of straight nanowires is well described by the curling model of domain reversal. In the case of the bent nanowires, the general behavior of each individual straight segment is also consistent with this model, but evidence for interactions between the segments is also observed.

Imaging and magnetotransport in superconductor/magnetic dot arrays

Magnetoresistance and scanning Hall probe microscopy studies of Nb-film/Ni-dot structures are reported. The dots act as pinning sites for superconducting vortices. The transport measurements focus on the effects of introducing disorder into the positions of the pinning lattice near the superconducting critical temperature Tc in structures with 250 nm diameter Ni dots randomized about an ideal square lattice with lattice constant a=560 nm. Features observable in the ordered arrays at higher multiples of the matching field H0=Φ0/a2 are washed out in the disordered arrays, but those at H0 remain. Scanning Hall probe microscope images were taken of the vortex configurations at fields up to 1.2H0 in ordered arrays of 1-μm-diameter dots on a 5.2 μm×4 μm rectangular lattice. These show that despite the relatively weak pinning of the magnetic dots, ordering commensurate with the dot lattice occurs even for fields below H0. Both transport and imaging studies point to the importance of interstitial vortices in dete...

Disorder and correlations in extended superconducting nanostructures

Abstract Experiments are presented on the magnetic properties of two types of extended superconducting nanostructures where disorder can be introduced in a controlled way. Magnetotransport measurements on Nb films overlaying arrays of 250-nm diameter Ni dots show that the superstructure observed at higher multiples of the matching field H 0 = Φ 0 / a 2 , where a =560 nm is the dot lattice constant, are systematically suppressed as disorder is introduced into the dot arrays. In arrays of superconducting rings in external fields corresponding to half-integral numbers of flux quanta per ring, flux quanta trapped in individual rings repel each other due to the magnetic coupling between rings, and the system is analogous to an Ising antiferromagnet. Disorder enters through small, random variations in ring sizes, and plays the role of a random field in the Ising model. SQUID magnetometry and scanning Hall microscopy (SHM) were used to probe the dynamics and specific magnetic configuration of square, honeycomb, kagome, and triangular lattice arrays containing up to 10 6 micron-size Al rings. The dynamics are dominated by a temperature-dependent energy barrier E B and hysteresis in the flux state of the ring populations. This population hysteresis is directly observed in ∂ M /∂ T measurements. SHM measurements at Φ 0 /2 per ring show antiferromagnetic correlations that can be suppressed by going to higher flux fractions due to increases in the effective random field.