A. Ionescu

Characterization of Ni thin films following thermal oxidation in air

The authors study the thermal oxidation of nickel thin films (50 nm) fabricated by conventional thermal evaporation, resulting from annealing in air at 300, 325, 350, 400, and 700 °C. The characterization is performed by x-ray diffraction, Raman spectroscopy, superconducting quantum interference device magnetometry, and scanning electron microscopy. These techniques show that the oxidation increases with annealing temperature. The formation of granular films of coexisting Ni and NiO is confirmed after annealing at 400 °C. The magnetic measurements indicate coexisting ferromagnetism and antiferromagnetism, corresponding to Ni and NiO contributions. The magnetic hysteresis loops reveal exchange bias in the samples annealed at 235, 350, and 400 °C due to the competition between the exchange interactions at the Ni/NiO interfaces.

Manipulation and tracking of superparamagnetic nanoparticles using?MRI

The use of magnetic fields in magnetic resonance imaging (MRI) for the tracking and delivery of chemotherapeutics bound to superparamagnetic nanoparticles offers a promising method for the non-invasive treatment of inoperable tumours. Here we demonstrate that superparamagnetic magnetite nanoparticles fabricated by an easily scalable method can be driven and tracked in real time at high velocities in vitro using MRI hardware. Force balance calculations are consistent with the magnetic properties of individual 10 nm diameter particles that move collectively as micron sized agglomerates with hydrodynamic diameter similar to that inferred from zero-magnetic-field dynamic light scattering measurements.

Characterisation of ferromagnetic rings for Zernike phase plates using the Aharonov-Bohm effect

Holographic measurements on magnetised thin-film cobalt rings have demonstrated both onion and vortex states of magnetisation. For a ring in the vortex state, the difference between phases of electron paths that pass through the ring and those that travel outside it was found to agree very well with Aharonov-Bohm theory within measurement error. Thus the magnetic flux in thin-film rings of ferromagnetic material can provide the phase shift required for phase plates in transmission electron microscopy. When a ring of this type is used as a phase plate, scattered electrons will be intercepted over a radial range similar to the ring width. A cobalt ring of thickness 20 nm can produce a phase difference of π/2 from a width of just under 30 nm, suggesting that the range of radial interception for this type of phase plate can be correspondingly small.

Thickness-dependent magnetic properties of oxygen-deficient EuO

We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu.

Magnetic properties of ceramics from the pyrolysis of metallocene-based polymers doped with palladium

Solution processing is a facile method to generate magnetic thin films. Polyferrocenylethylmethylsilane (PFEMS) was doped with palladium (II) acetylacetonate using two methods: sublimation of Pd(acac)2 to form Pd nanoparticles in the PFEMS films and direct mixing of Pd with the PFEMS polymer precursor prior to film deposition. These polymer composites all exhibit paramagnetic behavior, with increasing magnetic susceptibility for increasing Pd content. Pyrolysis of the precursors yields ferromagnetic ceramics at room temperature. The effect of the pyrolysis temperature and atmosphere on the magnetic properties, chemical composition, and crystalline structure of the ceramics was explored. For ceramics containing Pd, FePd alloys are observed to form pyrolyzed under argon at 1000 °C. The formation of these alloys results in enhanced coercivity, remanent magnetization, and saturation magnetization of the ceramics.

Electrical determination of the spin relaxation time of photoexcited electrons in GaAs

Spin-dependent transport for photoexcited electrons in an epitaxial Fe/GaAs interface was characterized from 5 to 300 K. The presence of spin-dependent transport was confirmed at all the measured temperatures and the spin polarization across the interface is found to increase with decreasing temperature. A time-of-flight-type model based on the Dyakonov–Perel (DP) spin relaxation mechanism was employed to explain the temperature dependence, providing that the estimated spin relaxation time in GaAs is 62 ps at 5 K. This short spin relaxation time can be explained by the stronger efficiency of the DP mechanism for hot-electrons.

High Throughput Biological Analysis Using Multi‐bit Magnetic Digital Planar Tags

We report a new magnetic labelling technology for high‐throughput biomolecular identification and DNA sequencing. Planar multi‐bit magnetic tags have been designed and fabricated, which comprise a magnetic barcode formed by an ensemble of micron‐sized thin film Ni80Fe20 bars encapsulated in SU8. We show that by using a globally applied magnetic field and magneto‐optical Kerr microscopy the magnetic elements in the multi‐bit magnetic tags can be addressed individually and encoded/decoded remotely. The critical steps needed to show the feasibility of this technology are demonstrated, including fabrication, flow transport, remote writing and reading, and successful functionalization of the tags as verified by fluorescence detection. This approach is ideal for encoding information on tags in microfluidic flow or suspension, for such applications as labelling of chemical precursors during drug synthesis and combinatorial library‐based high‐throughput multiplexed bioassays.

Effect of MgO barriers on ferromagnetic metallic layers studied by polarized neutron reflectivity

The effect of MgO tunnel barriers on the magnetic moment of ultrathin magnetic Fe and Co layers is studied by polarized neutron reflectivity. The MgO barriers were grown by molecular beam epitaxy using either direct evaporation from a source crystal or evaporation of Mg in an oxygen background atmosphere. A decrease of the magnetic moment is observed for both Fe and Co, and atomic intermixing of 1.5–3.5 ML at interfaces between the Fe, Co, and MgO is determined. Evaporation from a MgO source crystal yields a tunnel layer which displays better stoichiometry and epitaxy than is obtained using the other growth method.

Design and fabrication of SU8 encapsulated digital magnetic carriers for high throughput biological assays

A design of a biological molecule carrier is presented for the application of high throughput multiplexing biological assays. This carrier contains a bit addressable “magnetic barcode” made of either Permalloy or cobalt thin films, sandwiched between two planar SU8 protective layers. We describe how the design of the magnetic carriers is optimized by engineering the coercivity of each barcode element, allowing the number of available signatures to be increased. Fully encapsulated digital magnetic carriers which carry a 5 bit addressable barcode were also fabricated and are presented. Writing and reading of digital carriers were both performed after releasing in dried solution.

Magnetic properties of Fe/AlOx/GaAs(001) structures

We report results on the effect of the AlOx barrier layer on the magnetic moment of Fe using polarized neutron reflection. The sample has nominal composition 25nmAu∕5nmFe∕1.3nmAlOx∕GaAs(001). The metal deposition was done in ultrahigh vacuum and the Al oxidation was performed in two steps. From the fit to the neutron data we obtain for the magnetic moment of Fe the value (2.164±0.026)μB, very close to the bulk value (2.185μB at room temperature). We conclude that there is no dead layer at the AlOx interface.