F. J. Castaño

Magnetic behavior of lithographically patterned particle arrays (invited)

This article reviews recent progress in the fabrication, characterization, and analysis of large area arrays of sub-100-nm magnetic particles made by lithographic techniques. Particles are made by electrodeposition, evaporation and liftoff, or sputtering and etching, leading to a wide range of shapes, compositions, and microstructures. The remanent states, magnetic hysteresis, and uniformity of the particles and the interparticle interactions will be discussed.

Magnetic properties of template-synthesized cobalt∕polymer composite nanotubes

An approach to fabricate ferromagnetic∕polymer composite nanotubes has been developed. The surfaces of the pores in self-ordered porous alumina membranes are wetted with a polystyrene or poly-l-lactide layer containing a metallo-organic precursor. Decomposition of the precursor leads to the formation of thin-walled magnetic tubes with diameters of 160–450nm and wall thicknesses of a few nanometers. The magnetic properties of the tube arrays are interpreted as a result of the tube morphology and microstructure.

Anisotropy and magnetotransport in ordered magnetic antidot arrays

Magnetic films containing ordered arrays of holes (“antidots”) with period ∼200nm have been prepared using porous anodic alumina substrates with square and hexagonal symmetries. Large area (∼cm2) single-layer CoFe ordered antidot arrays show well-defined in-plane magnetic anisotropy related to the symmetry of the arrays, and the anisotropic magnetoresistance is smaller than that of a continuous film. For NiFe∕Cu∕CoFe antidot arrays, the giant magnetoresistance ratio of the patterned films is of similar magnitude to that of the unpatterned film, and shares the symmetry of the substrate. This behavior is attributed to the geometry of the antidots, which confine the magnetization of each layer parallel to the current flow.

Quantitative digital detection of magnetic beads using pseudo-spin-valve rings for multiplexed bioassays

We present a magnetic multiplexed assay technology which encodes the identities of target biomolecules according to the moment of magnetic beads to which they are attached. An active digital technique based on a microfabricated magnetoresistive ring-shaped sensor is demonstrated, which can distinguish the magnetic moments of micron-sized superparamagnetic beads. We propose that this development is key to combining nonvolatile magnetic labeling with biochemical libraries for high-throughput bioassays and rapid multiplexed detection.

Structure and thermomagnetic properties of polycrystalline Ni–Mn–Ga thin films

The structure and thermomagnetic properties of pulsed laser deposited polycrystalline Ni–Mn–Ga alloy films, grown onto thermally oxidized Si(100) wafers, have been investigated. The 300-nm-thick films were deposited at substrate temperatures between 500 and 600 °C from targets made from slices of a Ni2MnGa single crystal. The trends in the average composition suggest that samples deposited around 500 °C are composed of the Ni2MnGa (L21) phase as well as a nonmagnetic B2 phase. On increasing the temperature of the substrate, the Mn content as well as the saturation magnetization increase suggesting the appearance of a different magnetic phase that is tentatively ascribed to the Ni3(MnGa) (L11) magnetic phase.

Magnetization reversal in sub-100 nm pseudo-spin-valve element arrays

The magnetization reversal exhibited by arrays of 70-nm-wide pseudo-spin-valve (PSV) elements has been investigated by measurements of minor hysteresis loops. Samples were patterned from sputtered NiFe (6 nm)/Cu (3 and 6 nm)/Co (4 nm)/Cu (4 nm) magnetic thin film stacks. The overall room temperature magnetic behavior of the arrays can be understood by considering a distribution of switching fields for both the hard (Co) and soft (NiFe) magnetic layers. Such layers interact through exchange and magnetostatic coupling. Increasing the lengths of the elements leads to narrower switching field distributions and higher mean switching fields (particularly for the hard layer). On the other hand, decreasing the thickness of the Cu spacer leads to an increase of the switching field of the hard layer. Results obtained are well described by a model that treats each PSV as a coupled pair of rectangular single-domain films and uses the values of the interaction field between layers deduced from experimental minor loops.

Mesoscopic thin-film magnetic rings (invited)

The magnetic properties and magnetoresistance of thin-film circular and elliptical magnetic rings made from Co, NiFe, NiFe∕FeMn, and Co∕Cu∕NiFe have been explored. Single-layer rings show stable onion and vortex states and metastable twisted states containing a 360° wall. For NiFe rings, four-point magnetotransport results can be explained quantitatively by anisotropic magnetoresistance. NiFe∕FeMn exchange-biased rings show offset hysteresis loops, and the easy axis is determined by a combination of the ring ellipticity and the exchange coupling. In Co∕Cu∕NiFe multilayer rings the behavior is dominated by the magnetostatic coupling between the domain walls in the Co and NiFe. In the major loop the giant magnetoresistance varies between three distinct levels corresponding to combinations of onion and vortex states in the NiFe and Co layers.

Magnetism in multilayer thin film rings

The magnetic and magnetoresistive behaviour of circular and elliptical thin film multilayer rings is discussed. Rings are particularly interesting because they can adopt several stable and metastable magnetic states characterized by different numbers of domain walls. Electrically contacted rings made of Co/Cu/NiFe or IrMn/Co/Cu/NiFe, with diameters of ~0.5–5 µm and widths of ~100 nm were fabricated by electron-beam lithography, sputtering and liftoff processing. We show that multilayer rings switch their magnetization by a qualitatively different mechanism compared with that of a single-layer ring; how the magnetization circulation direction around the ring can be controlled; how the rings can be electrically contacted to show large fractional field-induced changes in resistance and how these structures may be used in magnetic random access memories or magnetic logic devices.

Magnetization switching in 70-nm-wide pseudo-spin-valve nanoelements

The magnetic domain structures and magnetization reversal of patterned 70-nm-wide pseudo-spin-valve (PSV) elements were studied by magnetic force microscopy (MFM). Both magnetically soft and hard layers form single-domain states at remanence, and can be magnetized either parallel or antiparallel to each other. The switching field of each layer, and the coupling between the layers, are quantified using MFM. Individual elements show well-defined switching fields, while the ensemble has a large switching field distribution due to variability between the PSV elements.

Patterning processes for fabricating sub-100 nm pseudo-spin valve structures

Interference lithography (IL) was used to pattern sputtered Co/Cu/NiFe layers into large-area arrays of pseudo-spin valve (PSV) elements. In order to precisely control size, aspect ratio, and shape uniformity of the elements, three methods of increasing complexity were developed. Pattern transfer was achieved by reactive-ion etching and ion milling, and was found to maintain the multilayered structure of the PSV film. The switching field of the PSV elements, and the remanent state, varied with the aspect ratio as expected. Furthermore, IL was employed to fabricate magnetic random access memory-type structures. Both sense and word lines were conductive, and the buried PSV elements had similar magnetic properties to PSV elements patterned in large-area arrays.