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M. Donolato1, R.Bertacco1, M. Gobbi1, V. Metlushko2, M.Deryabina3, P. Vavassori

1 LNESS - Dipartimento di Fisica Politecnico di Milano, Via Anzani 42, 22100 Como, Italy
2 Department of Electrical and Computer Engineering, University of Illinois at Chicago, Chicago ,Illinois 60607
3 DTU Nanotech, Department of Micro- and Nanotechnology, Technical University of Denmark,
├śrsteds Plads, DK-2800 Kgs Lyngby,
Cornell Nanofabrication Facility, Cornell University, Ithaca, New York 14853
4 CIC nanoGUNE Consolider, 20018 Donostia-San Sebastian, Spain and CNR- INFM S3, CNISM and Dipartimento di Fisica, Universit`a di Ferrara, 44100 Ferrara, Italy

We present a method for the manipulation (capture, transport , accumulation, and release) of nanometric magnetic particles in wet environments with active control of position (at the nm scale) and time. In the last years many approaches have been developed both for the manipulation and transport of massive particle population or of a single magnetic beads, e.g., microfabricated current carrying wires, micromagnets, and magnetic tweezers. However, none of these approaches combines the true single nano-particle manipulation ability at the nanoscale and the compatibility with lab-on-chip applications, which are the most prominent features of our approach.
In very narrow ferromagnetic wires (nanowire), shape anisotropy restricts the magnetization to lie parallel to the wire axis. In such a nanowire, a domain wall (DW) is a mobile interface, which separates regions of oppositely aligned magnetization. Nanowires made from a soft magnetic material such as Permalloy (Py, Ni80Fe20) have been shown to form an excellent conduits for DWs that can be nucleated and manipulated in a controllable way; this phenomena has recently attracting much interest for data storage applications.
In our system magnetic beads are captured by the stray field created by a DW in magnetic strips and their transport and release is obtained via precise control over DW nucleation, displacement, and annihilation processes through application of external magnetic fields. This can be obtained in conduits made of rectilinear segments, implementing a stepper motor for the digital motion of magnetic particles, or in curved conduits allowing for a continuous motion with precise control of the particle position with a precision of the order of 100 nm. Pulsed magnetic fields, or continuous rotating fields in the case of circular rings, are applied through an electromagnet monitoring in real time the beads displacement using an optical microscope.
The coating of the chip┬┤s surface have been investigated in order to avoid the sticking with a bead functionalized with different proteins (Protein A, streptavidin, biotin). Because magnetic particles with functionalized surfaces are commonly used as molecule carriers or labels, our approach holds potential for several intriguing applications including single molecule or cell manipulation, bioseparation and biomagnetic sensing. The possibility to design many parallel conduits on the same chip allows to sort and manipulate different targets (molecules, cells...) and probe information about the statistical distributions of biological systems. The easy integration on chip of nano-transport lines and sensors of domain walls and particles provides a direct pathway to the realization of complete functional devices for molecular manipulation, analysis and synthesis, with continuous remote control of process.