Sidewall control of static azimuthal bistable nematic alignment states

Stable azimuthal alignment states have been created in the plane of a homogeneous layer of nematic liquid crystal by the action of one or more sawtooth sidewalls. The alignment states in devices with two sawtooth sidewall structures, either in-phase or in anti-phase, and with one sawtooth wall opposite a flat wall have been investigated as a function of the sawtooth pitch. The optical textures of the observed states are in excellent agreement with the predictions of nematic Q-tensor theory. The frequencies of occurrence of the different states are broadly consistent with the expected inverse correlation with the Q-tensor predictions for their energy

Evidence for hot electron magnetocurrent in a double barrier tunnel junction device

Copyright © 2005 American Institute of PhysicsHot electron transport has been studied in three terminal Ta/TaOx/Co/AlOx/Ni81Fe19 structures fabricated by magnetron sputtering through shadow masks. With the Co base and Ta collector connected together via a small resistor, the collector current contains contributions first from hot electrons injected from the Ni81Fe19 emitter, and second from a geometrical artifact that leads to tunneling from the Fermi level in the base. Both sources of collector current lead to a room temperature magnetocurrent effect. The hot electron contribution begins to dominate as the emitter-base voltage −Veb exceeds 0.3 V

Origin of large magnetocurrent in three-terminal double-barrier magnetic tunnel junctions

Copyright © 2005 American Institute of PhysicsDouble-barrier magnetic tunnel junctions (DBMTJs) of composition Co/AlOx/Co/AlOx/Ni81Fe19 have been fabricated by magnetron sputtering through shadow masks. Two terminal measurements made upon the individual tunnel barriers revealed nonlinear I–V curves and significant room-temperature tunnel magnetoresistance (TMR) in all cases. Measurements were also performed with connections made to all three electrodes. The TMR of a particular tunnel barrier within the DBMTJ can be strongly modified by applying a bias voltage to the other barrier, while the TMR measured across the two barriers in series decreases more slowly with increasing bias voltage than for a single barrier. With zero bias applied between the central Co base electrode and the Co collector electrode, the collector current was measured as electrons were injected from the Ni81Fe19 electrode. For structures grown on Si/SiO2 substrates, the collector current showed a nonmonotonic dependence upon the emitter-base bias voltage, and collector magnetocurrent values in excess of 100% were observed at nonzero emitter-base bias values. For structures grown on quartz the collector current increased while the magnetocurrent decreased with increasing emitter-base voltage. We suggest that the enhanced TMR and magnetocurrent effects can be explained by substrate leakage and geometrical artifacts rather than by transport of spin-polarized hot electrons across the base layer

Spin polarization and barrier oxidation effects at the Co/alumina interface in magnetic tunnel junctions

Copyright © 2004 American Institute of PhysicsThe electronic structure and polarization in magnetic tunnel junctions prepared with varying degrees of barrier-layer oxidation have been studied using x-ray absorption spectroscopy across the Co L2,3 absorption edges. It was found that the Co electronic structure near the Co∕alumina interface tended to that of cobalt oxide as the barrier oxidation time was increased. However, the net Co 3d spin polarization, determined from x-ray magnetic circular dichroism, increased for moderate oxidation times compared to that obtained for an under-oxidized Co∕Al interface. It is proposed that the expected dilution of the measured polarization due to the formation of (room temperature) paramagnetic cobalt oxide, is offset by an increase in the Co 3d spin-polarization of the interface layer as the interface bonding changes from Co–Al to Co–O with increasing oxidation times

Emerging Chirality in Artificial Spin Ice

Artificial spin ice, made up of planar nanostructured arrays of simple ferromagnetic bars, is a playground for rich physics associated with the spin alignment of the bars and spin texture associated with the magnetic frustration at the bar vertices. The phase diagram is exotic, showing magnetic monopole-like defects and liquid and solid phases of spins arranged in loop states with predicted chiral order. We show that magnetotransport measurements in connected honeycomb structures yield the onset of an anomalous Hall signal at 50 kelvin. The temperature scale can be attributed to the long-range dipolar ice phase. The topological Hall signal arises because chiral loops form at the sample edges, indicating a generic route to exotic states via nanoarray edge structure

Exploring the phases of 3D artificial spin ice: From Coulomb phase to magnetic monopole crystal

Artificial spin-ices consist of lithographic arrays of single-domain magnetic nanowires organised into frustrated lattices. These geometries are usually two-dimensional, allowing a direct exploration of physics associated with frustration, topology and emergence. Recently, three-dimensional geometries have been realised, in which transport of emergent monopoles can be directly visualised upon the surface. Here we carry out an exploration of the three-dimensional artificial spin-ice phase diagram, whereby dipoles are placed within a diamond-bond lattice geometry. We find a rich phase diagram, consisting of a double-charged monopole crystal, a single-charged monopole crystal and conventional spin-ice with pinch points associated with a Coulomb phase. In our experimental demagnetised systems, broken symmetry forces formation of ferromagnetic stripes upon the surface, a configuration that forbids the formation of the lower energy double-charged monopole crystal. Instead, we observe crystallites of single magnetic charge, superimposed upon an ice background. The crystallites are found to form due to the intricate distribution of magnetic charge around a three-dimensional nanostructured vertex, which locally favours monopole formation. Our work suggests that engineered surface energetics can be used to tune the ground state of experimental three-dimensional ASI systems

The non-random walk of chiral magnetic charge carriers in artificial spin ice

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics

Limitations in artificial spin ice path selectivity: the challenges beyond topological control

Magnetic charge is carried through nanowire networks by domain walls, and the micromagnetic structure of a domain wall provides an opportunity to manipulate its movement. We have shown previously that magnetic monopole defects exist in artificial spin ice (ASI) and result from two bar switching at a vertex. To create and manipulate monopole defects and indeed magnetic charge in general, path selectivity of the domain wall at a vertex is required. We have recently shown that in connected ASI structures, transverse wall chirality (or topology) determines wall path direction, but a mechanism known as Walker breakdown, where a wall mutates into a wall of opposite chirality partially destroys selectivity. Recently it has been claimed that in isolated Y-shaped junctions that support vortex walls, selectivity is entirely determined by chirality (or topology), the suggestion being that vortex wall chirality is robust in the Walker breakdown process. Here we demonstrate that in Y-shaped junctions, magnetic switching in the important topologically protected regime exists only for a narrow window of field and bar geometry, and that it will be challenging to access this regime in field-driven ASI. This work has implications for the wider field of magnetic charge manipulation for high density memory storage

Use of microscale coplanar striplines with indium tin oxide windows in optical ferromagnetic resonance measurements

Copyright © 2005 American Institute of PhysicsIt is shown that a coplanar stripline structure containing indium tin oxide windows can be used to perform optical ferromagnetic resonance measurements on a sample grown on an opaque substrate, using a pulsed magnetic field of any desired orientation. The technique is demonstrated by applying it to a thin film of permalloy grown on a Si substrate. The measured precession frequency was found to be in good agreement with macrospin simulations. The phase of the oscillatory Kerr response was observed to vary as the probe spot was scanned across the coplanar stripline structure, confirming that the orientation of the pulsed field varied from parallel to perpendicular relative to the plane of the sample