Marco Gobbi

Room-temperature spin transport in C60-based spin valves.

A IO N Spintronics, or the possibility of performing electronics with the spin of the electron, has been fundamental for the exponential growth of digital data storage which has occurred in the last decades. Indeed, hard-disk drives read-heads are the maximum exponent of what is currently being called fi rstgeneration spintronic devices. Current read-heads, although technologically very complex, are scientifi cally based simply on the tunnel magnetoresistance effect (TMR; magnetoresistance being the change in electrical resistance of a device under the application of an external magnetic fi eld). A tunnel magnetoresistive vertical spin valve is composed of two ferromagnetic layers separated by a thin (around 1 nm) insulating layer, and the resistance of the structure can be switched between two different values upon the application of a magnetic fi eld capable of rotating the magnetization vector of the ferromagnetic layers from parallel to antiparallel. [ 1 ] For the eventual success of a second-generation of spintronic devices, more complex mechanisms than the nanometre-distance spin transport in metallic or insulating materials have to be obtained. In particular, coherent spin transport at distances above a few nm and spin manipulation are unavoidable requirements for the production of sophisticated prototypes of, for example, spin transistors or spin light-emitting diodes. [ 2 , 3 ] Organic semiconductors (OS) have emerged as promising materials for advanced spintronics applications. Their spin relaxation mechanisms, mainly represented by spin orbit interaction and hyperfi ne interaction with protons, [ 4 ] are very small, and long spin lifetimes have been consistently detected. [ 5 ] Moreover, in spite of the relatively low carrier mobility of these materials, organic vertical spin valves with semiconducting channels thicker than 100 nm have been demonstrated. [ 6–11 ] In parallel, OS ultrathin layers perform successfully as spin tunnel junctions, and extremely high ( > 300%) magnetoresistance (MR) values have been obtained at low temperatures. [ 12 ] Regarding possible applications of spin transport in OS, a basic operational requirement is the room temperature (RT) operation of the devices. So far, only organic spin tunnel junctions have shown any signifi cant MR effect at RT. [ 13–17 ] By contrast, most devices employing thicker organic layer ( > 15 nm) show a clear decay of the MR well below RT. [ 6–11 , 18 ]

On-chip manipulation of protein-coated magnetic beads via domain-wall conduits.

For this reasonmanipulationatthenanoscaleofsurfacefunctionalizedmagneticbeads in suspension is of paramount importance in biotechnol-ogy, nanochemistry, and nanomedicine as it leads to a precisecontrol of the tagged biological entity.In the past few years many approaches have been developedboth for the manipulation and transport of a massive particlepopulation or of a single particle, e.g., microfabricated current-carrying wires,

Domain wall displacement in Py square ring for single nanometric magnetic bead detection

physical properties of a domain wall DW localized at a geometric corner. The concept illustrated in the present paper relies on a previous experimental work made on square rings of Permalloy Py for application in magnetic storage of information. 11 In this design head-to-head and tail-to-tail DWs having a transverse structure Neel type 11 can be positioned at a given corner, and their position can be read electrically thanks to the AMR effect: when a DW is present between two sensing leads a reduction in the resistance is observed since some of the magnetization of the DW points perpendicularly to the current flow. Otherwise, when there is no DW present between the two sensing leads, the magnetization follows the direction of the perimeter of the ring and the resistance is higher. In this work we adapted this device to demonstrate a detection concept suitable for the detection of magnetic nanobeads. Panel a of Fig. 1 shows the scanning electron microscopy image of the structure used in the present experiment. The 30 nm thick Py square ring structures have been lithographically patterned on top of 20 nm thick and 100 nm wide Au contacts, previously fabricated on a SiO2 /Si substrate. The outside size of the rings is 1.0 1.0 m 2 , the width of each segment is about 180 nm, and the slit is about 80 nm wide. For the magnetoresistance measurements presented here, the voltage drop was measured

Nanosized corners for trapping and detecting magnetic nanoparticles

We present a device concept based on controlled micromagnetic configurations in a corner-shaped permalloy nanostructure terminated with two circular disks, specifically designed for the capture and detection of a small number of magnetic beads in suspension. A transverse head-to-head domain wall (TDW) placed at the corner of the structure plays the role of an attracting pole for magnetic beads. The TDW is annihilated in the terminating disks by applying an appropriate magnetic field, whose value is affected by the presence of beads chemically bound to the surface. In the case where the beads are not chemically bound to the surface, the annihilation of the TDW causes their release into the suspension. The variation of the voltage drop across the corner, due to the anisotropic magnetoresistance (AMR) while sweeping the magnetic field, is used to detect the presence of a chemically bound bead. The device response has been characterized by using both synthetic antiferromagnetic nanoparticles (disks of 70 nm diameter and 20 nm height) and magnetic nanobeads, for different thicknesses of the protective capping layer. We demonstrate the detection down to a single nanoparticle, therefore the device holds the potential for the localization and detection of small numbers of molecules immobilized on the particles functionalized surface.

Impurity-assisted tunneling magnetoresistance under a weak magnetic field.

Injection of spins into semiconductors is essential for the integration of the spin functionality into conventional electronics. Insulating layers are often inserted between ferromagnetic metals and semiconductors for obtaining an efficient spin injection, and it is therefore crucial to distinguish between signatures of electrical spin injection and impurity-driven effects in the tunnel barrier. Here we demonstrate an impurity-assisted tunneling magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect reflects on-off switching of the tunneling current through impurity channels by the external magnetic field. The reported effect is universal for any impurity-assisted tunneling process and provides an alternative interpretation to a widely used technique that employs the same ferromagnetic electrode to inject and detect spin accumulation.

Flexible spintronic devices on Kapton

Magnetic tunnel junctions and nano-sized domain-wall conduits have been fabricated on the flexible substrate Kapton. Despite the delicate nature of tunneling barriers and zig-zag shaped nanowires, the devices show an outstanding integrity and robustness upon mechanical bending. High values of bending angle (ru2009=u20095u2009mm) have been achieved without degradation of the device performance, reaching room-temperature tunneling magnetoresistance ratios of 12% in bended Co/Al2O3/NiFe junctions. In addition, a suitable route to pattern high-quality nanostructures directly on the polyimide surface is established. These results demonstrate that Kapton is a promising platform for low-cost, flexible spintronic applications involving tunnel junction elements and nanostructurization.

How reliable are Hanle measurements in metals in a three-terminal geometry?

We test the validity of Hanle measurements in three-terminal devices by using aluminum (Al) and gold (Au). The obtained Hanle and inverted Hanle-like curves show an anomalous behavior. First, we measure Hanle signals 8 orders of magnitude larger than those predicted by standard theory. Second, the temperature and voltage dependences of the signal do not match with the tunneling spin polarization of the ferromagnetic contact. Finally, the spin relaxation times obtained with this method are independent of the choice of the metallic channel. These results are not compatible with spin accumulation in the metal. Furthermore, a scaling of the Hanle signal with the interface resistance of the devices suggests that the measured signal is originated in the tunnel junction.

Magnetic nanostructures for the manipulation of individual nanoscale particles in liquid environments (invited)

The manipulation of geometrically constrained magnetic domain walls (DWs) in nanoscale magnetic strips attracted much interest recently, with proposals for prospective memory and logic devices. Here we demonstrate that the high controllability of the motion of geometrically constrained DWs allows for the manipulation of individual nanoparticles in solution on a chip with the active control of position at the nanometer scale. Our approach exploits the fact that magnetic nanoparticles in suspension can be captured by a DW, whose position can be manipulated with nanometer scale accuracy in specifically designed magnetic nanowire structures. We hereby show that the precise control over DW nucleation, displacement, and annihilation processes in such nanostructures allows for the capture, transport, and release of magnetic nanoparticles. As magnetic nanoparticles with functionalized surfaces are widely used as molecule carriers or labels for single molecule studies, cell manipulation, and biomagnetic sensing, t...

Determination of energy level alignment at metal/molecule interfaces by in-device electrical spectroscopy

The energetics of metal/molecular semiconductor interfaces plays a fundamental role in organic electronics, determining the performance of very diverse devices. So far, information about the energy level alignment has been most commonly gained by spectroscopy techniques that typically require experimental conditions far from the real device operation. Here we demonstrate that a simple three-terminal device allows the acquisition of spectroscopic information about the metal/molecule energy alignment in real operative condition. As a proof of principle, we employ the proposed device to measure the energy barrier height between different clean metals and C60 molecules and we recover typical results from photoemission spectroscopy. The device is designed to inject a hot electron current directly into the molecular level devoted to charge transport, disentangling the contributions of both the interface and the bulk to the device total resistance, with important implications for spintronics and low-temperature physics.

C60-based hot-electron magnetic tunnel transistor

A C60-based magnetic tunnel transistor is presented. The device is based on the collection of spin-filtered hot-electrons at a metal/C60 interface, and it allows an accurate measurement of the energy level alignment at such interface. A 89% change in the collected current under the application of a magnetic field demonstrates that these devices can be used as sensitive magnetic field sensors compatible with soft electronics.