Room temperature two terminal tunnel magnetoresistance in a lateral graphene transistor.

We investigate the behavior of both pure spin and spin-polarized currents measured with four-probe non-local and two probe local configurations up to room temperature and under an external gate voltage in a lateral graphene transistor, produced using a standard large-scale microfabrication process. The high spin diffusion length of pristine graphene in the channel, measured both directly and by the Hanle effect, and the tuning of the relationship between the electrode resistance areas present in the device architecture allowed us to observe local tunnel magnetoresistance at room temperature, a new finding for this type of device. The results also indicate that while pure spin currents are less sensitive to temperature variations, spin-polarized current switching by an external voltage is more efficient, due to a combination of the Rashba effect and a change in carrier mobility by a Fermi level shift.

Effects of magnetic monopoles charge on the cracking reversal processes in artificial square ices.

In this paper we perform nanofabrication of square artificial spin ices with different lattice parameters, in order to investigate the roles of vertex excitation on the features of the system. In particular, the character of magnetic charge distribution asymmetry on the vertices are observed under magnetic hysteresis loop experiments. We then compare our results with simulation using an emergent Hamiltonian containing objects such as magnetic monopoles and dipoles in the vertices of the array (instead of the usual Hamiltonian based on the dipolar interactions among the magnetic nanoislands). All possible interactions between these objects are considered (monopole-monopole, monopole-dipole and dipole-dipole). Using realistic parameters we observe very good match between experiments and theory, which allow us to better understand the system dynamics in function of monopole charge intensity.

Experimental and theoretical evidences for the ice regime in planar artificial spin ices.

In this work, we explore a kind of geometrical effect in the thermodynamics of artificial spin ices (ASI). In general, such artificial materials are athermal. Here, We demonstrate that geometrically driven dynamics in ASI can open up the panorama of exploring distinct ground states and thermally magnetic monopole excitations. It is shown that a particular ASI lattice will provide a richer thermodynamics with nanomagnet spins experiencing less restriction to flip precisely in a kind of rhombic lattice. This can be observed by analysis of only three types of rectangular artificial spin ices (RASI). Denoting the horizontal and vertical lattice spacings by [Formula: see text] and [Formula: see text], respectively, then, a RASI material can be described by its aspect ratio [Formula: see text]. The rhombic lattice emerges when [Formula: see text]. So, by comparing the impact of thermal effects on the spin flips in these three appropriate different RASI arrays, it is possible to find a system very close to the ice regime.

Realization of Rectangular Artificial Spin Ice and Direct Observation of High Energy Topology.

In this work, we have constructed and experimentally investigated frustrated arrays of dipoles forming two-dimensional artificial spin ices with different lattice parameters (rectangular arrays with horizontal and vertical lattice spacings denoted by a and b respectively). Arrays with three different aspect ratios γ = a/b = [Formula: see text], [Formula: see text] and [Formula: see text] are studied. Theoretical calculations of low-energy demagnetized configurations for these same parameters are also presented. Experimental data for demagnetized samples confirm most of the theoretical results. However, the highest energy topology (doubly-charged monopoles) does not emerge in our theoretical model, while they are seen in experiments for large enough γ. Our results also insinuate that the string tension connecting two magnetic monopoles in a pair vanishes in rectangular lattices with a critical ratio γ = γ c = [Formula: see text], supporting previous theoretical predictions.

High power and low critical current density spin transfer torque nano-oscillators using MgO barriers with intermediate thickness.

Reported steady-state microwave emission in magnetic tunnel junction (MTJ)-based spin transfer torque nano-oscillators (STNOs) relies mostly on very thin insulating barriers [resulting in a resistance × area product (R × A) of ~1 Ωµm2] that can sustain large current densities and thus trigger large orbit magnetic dynamics. Apart from the low R × A requirement, the role of the tunnel barrier in the dynamics has so far been largely overlooked, in comparison to the magnetic configuration of STNOs. In this report, STNOs with an in-plane magnetized homogeneous free layer configuration are used to probe the role of the tunnel barrier in the dynamics. In this type of STNOs, the RF modes are in the GHz region with integrated matched output powers (P out ) in the range of 1-40 nW. Here, P o u t values up to 200 nW are reported using thicker insulating barriers for junctions with R × A values ranging from 7.5 to 12.5 Ωµm2, without compromising the ability to trigger self-sustained oscillations and without any noticeable degradation of the signal linewidth (Γ). Furthermore, a decrease of two orders of magnitude in the critical current density for spin transfer torque induced dynamics (J STT ) was observed, without any further change in the magnetic configuration.

Graphene field-effect transistor array with integrated electrolytic gates scaled to 200 mm.

Ten years have passed since the beginning of graphene research. In this period we have witnessed breakthroughs both in fundamental and applied research. However, the development of graphene devices for mass production has not yet reached the same level of progress. The architecture of graphene field-effect transistors (FET) has not significantly changed, and the integration of devices at the wafer scale has generally not been sought. Currently, whenever an electrolyte-gated FET (EGFET) is used, an external, cumbersome, out-of-plane gate electrode is required. Here, an alternative architecture for graphene EGFET is presented. In this architecture, source, drain, and gate are in the same plane, eliminating the need for an external gate electrode and the use of an additional reservoir to confine the electrolyte inside the transistor active zone. This planar structure with an integrated gate allows for wafer-scale fabrication of high-performance graphene EGFETs, with carrier mobility up to 1800 cm(2) V(-1) s(-1). As a proof-of principle, a chemical sensor was achieved. It is shown that the sensor can discriminate between saline solutions of different concentrations. The proposed architecture will facilitate the mass production of graphene sensors, materializing the potential of previous achievements in fundamental and applied graphene research.

Nanofabrication of 30 nm devices incorporating low resistance magnetic tunnel junctions.

In this paper the electron-beam lithography conditions and the nanofabrication process are described for current-perpendicular-to-plane (CPP) pillar devices with 30 nm critical dimensions. This work combines a RAITH-150 tool with a negative e-beam resist (AR-7520) so that dense nanopillar arrays are patterned fast into large area samples. The resist dilution and coating conditions are optimized, aiming at its thickness reduction down to 80 nm. The exposure parameters are tuned for different geometries and dimensions, so that features down to 30 nm are exposed with good accuracy (+/- 1.9 nm) and reproducibility. The complete integration of these nanoelements into CPP devices involved electron beam lithography, ion milling for pattern transfer and chemical-mechanical polishing (CMP). Results on devices incorporating very low resistance-area (R x A) MTJ films deposited by Ion beam assisted deposition are shown, for MTJ stacks with R x A down to 0.8 omega x microm2. Device characterization includes electrical measurement of the pillar resistance and the transfer curves under dc magnetic fields (TMR up to 40%).