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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. [ABSTRACT FROM AUTHOR]

The dependence of the exchange bias field and coercivity enhancement on ferromagnetic (FM) and antiferromagnetic (AF) layer thickness in exchange biased bilayers has been systematically investigated in CoFe/FeMn and CoFe/PdMn bilayers for digital encoding applications in biotechnology. A magnetic multilayer structure can be used as a digitally encoded tag if each (bi)layer has two magnetic states, positive and negative saturation, available at remanence and if each layer can be uniquely identified by its coercivity. We will demonstrate that by adjusting the AF and FM layer thickness in an AF/FM bilayer, both the bias field and the coercivity of the bilayer can be controlled. By contrasting CoFe/FeMn bilayers with CoFe/PdMn bilayers, it becomes apparent that the relative magnitudes of the coercivity enhancement and bias field depend on the particular AF material, although the qualitative behavior remains unchanged. In order to create a multilayer that can retain one of many magnetic states at remanence, a large coercivity enhancement but absent or small bias field are preferred. Analysis of the bilayers suggest that PdMn is a better choice of AF layer for this purpose and results on some multilayer films are shown which validate this claim. [ABSTRACT FROM AUTHOR]

We present the results of a systematic investigation of the coverage-dependent magnetic response and anisotropy of ultrathin Co75Fe25/Cu(110) films using the magneto-optic Kerr effect. A paramagnetic-ferromagnetic phase transition is found to occur at a critical thickness dc=4.0±0.25 ML. In the vicinity of dc, the paramagnetic susceptibility χ follows a power law with a critical exponent of γ=2.385±0.069 in excellent agreement with the theoretical value for a perfect two-dimensional geometric percolation phase transition. At the very onset of ferromagnetic order, a mixture of cubic and uniaxial anisotropy was observed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The magnetization reversal of individual 2 μm and 5 μm diameter polycrystalline Permalloy rings, with respective widths 0.75 μm and 1 μm, thickness 45 nm, has been investigated by focused magneto-optic Kerr effect (MOKE) magnetometry. Micromagnetic simulation of the reversal in the 2 μm diameter ring reveals that the onion-to-vortex state switching occurs by nucleation and subsequent annihilation of vortex walls that span the width of the ring, and that the vortex-to-reverse-onion state switching occurs by expansion of a reverse domain. The hysteresis loop shows good agreement with the experimental MOKE loop. Measurements of the switching through one-half of a 5 μm diameter ring enable the determination of the circulation of the vortex states accessed during one applied field cycle. The rings switch via one vortex state (either clockwise or anticlockwise) on both downward and upward applied field sweeps. The number of applied field cycles spent switching via one vortex state before changing to switch via the opposite vortex state is random, likely to be due to the history of the spin configuration and thermal fluctuations. [ABSTRACT FROM AUTHOR]

Thin epitaxial Co27Fe73 and Fe films have been grown on GaAs(001), with thickness of 8.5 and 10 nm, respectively. The mesofrequency switching dynamics have been investigated by ac magneto-optic Kerr effect magnetometry. Dynamic hysteresis loops have been measured along the four major in-plane crystallographic axes [100], [110], [010], and [1&1macr;0] in each film in the sinusoidal applied field frequency range of 0.2–2 kHz. The coercive field Hc for the CoFe film is 1.4–2.9 times larger than that for the Fe film, depending on the measurement axis, indicating a larger effective energy barrier to reversal. Examining the coercive field as a function of frequency IIc(f) in the CoFe than in the Fe film. As previously shown in epitaxial Fe films, a local minimum in Hc(f) can occur at the crossover from the wall propagation-dominated to the nucleation-dominated dynamic regime, and here we find that it is more pronounced for the CoFe than for the Fe film, indicating a reduction in domain-wall pinning. We conclude that the magnitude of the effective energy barrier to reversal sets the time scale for the domain nucleation and wall propagation processes and determines the characteristics of Hc(f) for the two films. [ABSTRACT FROM AUTHOR]

We present a magnetoresistance study of controlled spin switching and domain wall pinning effects in mesoscopic narrow ferromagnetic Permalloy ring structures containing notches. It is found that notches create an attractive potential well for the transverse domain walls present in these structures. The strength and size of this potential well is measured and found to stretch far beyond the physical dimensions of the notch. Measuring the resistance, a minimum at remanence is found to occur when a transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. Applying a field in the direction corresponding to a potential well edge, a magnetic state with a very wide domain wall is stabilized, leading to a dip in the magnetoresistance at such angles. Using notches of different sizes, it is demonstrated that controlled switching is achieved, which means that the circulation direction of the vortex state can be selected. [ABSTRACT FROM AUTHOR]

We report a study of the effect of magnetocrystalline anisotropy in the magnetization reversal of submicron Co rings fabricated by nanoimprint lithography. For weak magnetocrystalline anisotropy, the complete reversal takes place via a transition from saturation at large negative fields, into a vortex configuration at small fields, and back to reverse saturation at large positive fields. When the anisotropy strength is increased to a critical value, the intermediate vortex configuration no longer exists in the magnetization reversal along the easy axis; instead, the reversal occurs through a rapid jump. However, when the applied field direction is far from the easy axis, the presence of the magnetocrystalline anisotropy favors local vortex nucleation, and this leads to a similar switching process as found for low anisotropy. Micromagnetic simulations indicate that the magnetization reversal process of the rings, starts from a buckling-like reverse domain nucleation, followed by local vortex formation and an avalanche process of local vortex nucleation. [ABSTRACT FROM AUTHOR]

Two major problems in spin electronics remain to be solved: room temperature spin injection at a source and spin detection at a drain electrode. The lateral size of magnetic contacts and the presence of a potential barrier at the interface are believed to have a key influence on the efficiency of both of these processes. We therefore aimed to clarify these issues by studying spin-polarized transport across epitaxially grown single crystal Fe (001)/GaAs nanoclusters and at the Schottky barrier formed at Ni[sub 80]Fe[sub 20]/GaAs interfaces. We observed a negative contribution to the magnetoresistance of an ultrathin (2.5 ML) discontinuous epitaxial Fe film as occurs in tunnel magnetoresistance. This result suggests that spin transport via GaAs is possible on the nanoscale. In the continuous NiFe/GaAs structures, circularly polarized light was used to create a population of spin-polarized electrons in the GaAs substrate and spin-polarized electron transport across the int! erface at room temperature was detected as an electrical response associated with the field-dependent photocurrent. Surprisingly, highly efficient spin transmission is observed at room temperature, indicating that there is no significant loss of spin polarization for electrons crossing the interface. This result unambiguously demonstrates that spin detection is possible at room temperature in a continuous ferromagnet/semiconductor contact in the presence of the Schottky barrier. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A simple but effective platform using on-chip current striplines placed on a ferrite magnet for bead manipulation is presented. The system has the potential to provide a simple but effective platform for handling magnetic tags in biological and biomedical applications.

Presents a study which used scanning magneto-optic Kerr effect microscopy to investigate the magnetic relaxation of a polycrystalline hcp cobalt film grown on silicon (111). Mechanism of magnetization reversal; Details on the orientation-dependent magnetic switching behavior for the cobalt/silicon(111) film.

Examines the ferromagnetic behavior of thin magnetic films down to the monolayer thickness layer. Discussion on the spin-polarized neutron reflection method; Information on the film thickness and temperature-dependent ferromagnetic behavior; Details of the ground-state magnetic moment behavior as a function of volume strain.

A new technique for high precision measurement of magnetic anisotropy fields in thin and ultrathin films called modulated field magneto-optical anisometry (MFMA) is described. MFMA can be performed by a simple extension to a conventional magneto-optical Kerr effect magnetometer, and is therefore experimentally simple. It can resolve an arbitrary combination of anisotropies of different symmetries with a very high precision, even when there is little magneto-optical signal (e.g., ultrathin in-plane magnetized films) and high optical noise. It is spatially resolving and readily suited to the ultrahigh vacuum environment. MFMA thus offers many advantages over existing anisotropy measurement methods. A quantitative comparison is made between anisotropy field measurements made by MFMA and by Brillouin light scattering on an ultrathin Fe(001) epitaxial film. Agreement is found to within a high precision. © 1997 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which examined the process by which the in-plane magnetization of an ultrathin epitaxial iron film reverses under the action of an external magnetic field. Experimental apparatus; Results and discussion; Conclusions.

Presents a study that investigated interlayer exchange coupling in epitaxial structures. Method of the study; Results and discussion; Conclusion.

Presents a study which developed a technique for engineering micron and submicron scale structures from magnetic films of transition metals using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic iron wires to be fabricated. Background on high-quality ultrathin magnetic films; Experimental details; Magnetization and magnetoresistance results.

The domain structure and the coercivity of epitaxial Fe(100) circular dot arrays of different diameters and separations have been studied using magnetic force microscopy (MFM) and focused magneto-optical Kerr effect (MOKE). The MFM images of the 1 [micro]m diameter single domain dot arrays show direct evidence of strong interdot dipole coupling when the separation is reduced down to 0.1 [micro]m. The coercivity of the dots is also found to be dependent on the separation, indicating the effect of the interdot dipole coupling on the magnetization reversal process. Index Terms--Fe dot arrays, interdot coupling, magnetic storage media, micromagnetism.

First magnetization curves in rings of 1 [micro]m diameter have been computed. The most important feature in the curve is the critical field for vortex annihilation. It is found that this field can be controlled accurately by changing the thickness and the width of the ring. Index Terms--Computational icromagnetics, hysteresis, rings, vortex annihilation.

Permalloy ([Ni.sub.80][Fe.sub.20]) wire-based structures (30 nm thick and 1 [multiplied by] w [multiplied by] 10 [micro]m wide) with junctions (crosses, networks, H-shapes, rectangular chains and ring chains) prepared on a GaAs (100) substrate were observed in both their demagnetized and remanent states by magnetic force microscopy (MFM) in order to investigate the role of junction geometry in domain formation. Except in ring chains, two classes of domain configuration are found at the junction: (i) a domain wall-like feature due to abrupt spin rotation and (ii) a triangle-shape domain consistent with a flux closure configuration. Ring chains, on the other hand, form vortex domains at every other junction. The MFM observations are compared with micromagnetic calculations which qualitatively support the magnetic domain configurations. Index Terms--Mesoscopic structures, magnetic domain configuration, magnetic force microscopy, micromagnetic calculation.

The magnetization reversal process in mesoscopic permalloy ([Ni.sub.80][Fe.sub.20]) wire structures has been investigated using scanning Kerr microscopy, magnetic force microscopy (MFM) and micromagnetic calculations. We find that the junction offers a site for reversed domain wall nucleation in the narrow part of the wires. As a consequence, the switching field is dominated by the domain nucleation field and the junction region initiates reversal by the wall motion following the nucleation of domains. Our results suggest the possibility of designing structures that can be used to 'launch' reverse domains in narrow wires within a controlled field range. Index Terms--Magnetization reversal, mesoscopic wires, nucleation process, switching field.

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in ferromagnet/semiconductor hybrid structures. The Schottky characteristics at the interface were varied by using NiFe, Co and Fe as the ferromagnet. The Schottky characteristics were clearly observed with NiFe and Co/GaAs, while almost ohmic I - V characteristics were seen with Fe/GaAs. At negative bias a helicity-dependent photocurrent, dependent upon the magnetization configuration of the film and the Schottky barrier height, was detected upon modulating the polarization from right to left circular. For the magnetization along or perpendicular to the surface normal, the helicity-dependent photocurrent [I.sup.n] or [I.sup.o], respectively, was measured. The asymmetry P = ([I.sup.n] - [I.sup.o]) / ([I.sup.n] + [I.sup.o]) of the helicity-dependent photocurrent decreases upon increasing the doping density of the GaAs substrates. P also decreases with photon energy hv as found for the polarization of photoexcited electrons in GaAs. In NiFe/GaAs samples for hv = 1.59 eV, P = 16% for [n.sup.+] = [10.sup.23] [m.sup.-3] and P = -23% for [p.sup.-] = [10.sup.25] [m.sup.-3] doped substrates, i.e. P is comparable in magnitude to the theoretically predicted spin polarization of 50% for the optically pumped conduction band in GaAs. The results provide unambiguous evidence of spin-polarized electron transport through the ferromagnet/semiconductor interface and show that the Schottky barrier height controls the spin-polarized electron current passing from the semiconductor to the ferromagnet. The asymmetry data indicates that spin-polarized electrons are transmitted from the semiconductor to the ferromagnet with a high efficiency. Index Terms--Ferromagnet/semiconductor interface, photon helicity, Schottky barrier, spin-polarized electron.

The effect of strain on the magnetic anisotropy of epitaxial Co/Ni/Cu/Si(001) structures was studied using polar magneto-optic Kerr effect (MOKE) magnetometry and reflection high energy electron diffraction (RHEED). Co has the effect of reducing the perpendicular magnetic anisotropy of Ni for the whole range of Ni thicknesses (30 [Angstrom] to 120 [Angstrom]) while a variation of the perpendicular Co anisotropy constants with the Ni thickness is observed. The first and second order magnetoelastic coupling coefficients of Co were obtained from this strain-related variation, and are found to have opposite signs. We show that by varying the Ni thickness it is possible to sensitively control the magnetic anisotropy of the Co layer. Index Terms--Magnetic anisotropy, magnetoelastic effects, strain, thin films.

The interface magnetic and electronic properties of two Fe/III-V semiconductor systems, namely Fe/GaAs and Fe/InAs, grown at room temperature have been studied. A 'magnetic interface', which is essential for the fabrication of magneto-electronic (ME) devices, was realized in both Fe/GaAs and Fe/InAs systems with suitable substrate processing and growth conditions. Furthermore, Fe/InAs was shown to have favorable interface electronic properties as Fe forms a low resistance ohmic contact on InAs. Two prototypes of ME device based on Fe/InAs are also discussed. Index Terms: ultrathin films interface magnetism, Fe/GaAs, Fe/InAs, magnetoelectronics