Enhanced Magnetoresistance in Molecular Junctions by Geometrical Optimization of Spin-Selective Orbital Hybridization

Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their ferromagnet–molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to ∼200% near the conductance quantum <i>G</i>₀. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal–molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal–molecule interfaces

The response of Human motor cortex to rfTMS.

<p>a) The cloverleaf coil is positioned over the motor cortex. Stimuli with constant strength are applied at 0.2 Hz. Individual muscle evoked potentials recorded from the first dorsal interosseous are shown in grey, and the averaged response in black. Rotating the coil by 50° in the plane of the cloverleaf coil and maintaining the center position does not change the amplitude of the muscle response. Data obtained from subject A.N. b) Extended experiment, testing 5 different orientations, controlled by a robotic device. For each orientation two opposite rotation directions of the rotating field were tested. The experiment was then repeated using a standard figure-eight coil, used to apply both biphasic pulses, as in rfTMS, and mono-phasic pulses. Under standard TMS no muscle response can be detected for orientations more than 45° away from the optimum. In contrast, rfTMS provides reliable stimulation independent of the coil's orientation. Data obtained from subject W.P.</p

Schematic of a culture whose cell's axons (red or black lines) are randomly orientated.

<p>a) A short magnetic pulse with a fixed single orientation (black arrow indicates direction of the stimulating field) stimulates only one cell whose axon (red line) is oriented parallel to the direction of the induced electric field is excited. b) A short rotating magnetic pulse (arc indicates the span of rotation of the stimulating field) stimulates all cells whose axons' orientations lie within the arc of the rotating electric field (red lines), leading to a population response of the network. c) Alternatively, when applying a long magnetic pulse with a fixed orientation, all cells with dendrites oriented parallel to the direction of the induced electric field (red circles) are excited, leading to a population response of the network. See also Note S1, S2 and S3 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086794#pone.0086794.s001" target="_blank">File S1</a>.</p

Results from neuronal culture and from rat motor cortex.

<p>a) The response of 2D Neuronal culture to rfTMS. Spiking activity in the culture was imaged through the viewing aperture (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086794#pone-0086794-g002" target="_blank">Figure 2b</a>). Fluorescent neurons are seen as white spots. The green rectangles indicate the regions of interest over which the signal was averaged. The dashed white line indicates the borders of the coverslip on which the culture was grown. b) The average calcium dependent fluorescence of the regions of interest outlined in a). Dashed lines mark events of magnetic stimulation using first the cross coil and then a single coil. The intensity of each stimulation pulse is noted in Tesla. Successful stimulation of a population response is indicated by a caret while intrinsic activity unrelated to magnetic stimulation is indicated by an asterisk. Note that the cross coil successfully triggered a response already with 0.8T (but not at 0.75T), while a single coil required around 1.3T. c) The response of rat motor cortex to rfTMS. Graphs of EMG recording of the Gastrocnemius when using the cross coil to stimulate a rat at different locations. Each location is illustrated to the right of the response trace with the black cross representing the cross coil. The last row was performed after cervical dislocation of the rat. Scale bar is <i>200 µV</i>. d) A comparison between the last two rows in c). The solid line is the average of <i>10</i> individual responses (green traces) of the rat to rfTMS over its head before dislocation and the dashed curve is the average of <i>5</i> individual responses (cyan traces) of the rat to rfTMS in a similar location over its head after dislocation.</p

Cross coil experiments.

<p>a) A photograph of the cross coil used in the experiment. The two coils interlock on perpendicular planes and connect to two independent stimulators. b) A photograph of the glass sphere that was custom made to fit inside the cross coil. The glass coverslip, on which the neuronal culture grows, and the fluid medium were inserted through a slot located at the top of the sphere. The coverslip lay on a flattened base at the bottom of the sphere and was observed via a viewing aperture, which was sealed with optically transparent glass. See also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086794#pone.0086794.s003" target="_blank">Video S1</a>. c) Schematic of the setup – the coverslip (red) was placed in a glass sphere inside the cross coil while an inverted epi-fluorescence microscope monitored neuronal activity. Scale bars in a–c are <i>2 cm</i>. d) Cross coil setup for rat experiments. The rat's head was positioned inside the cross coil (in place of the glass sphere, which was not used). EMG electrodes recorded muscle potentials from the Gastrocnemius. The EMG data was digitized and synchronized with the rfTMS pulses to assess the motor response to rfTMS. e) The induced electric field in the cross coil was measured using a pick-up coil oriented first on the plane of one of the coils (solid line) and then on the plane of the second coil (dashed line). The Magstim stimulator was loaded to <i>100%</i> and the HMS was loaded with <i>3.5 kV</i> (see details in the Methods section). f) A reconstruction of the effective electric field created from the sum of the two perpendicular components measured in e) with the field of coil #1 directed along the x-axis and the field of coil #2 along the y-axis. The effective field was reconstructed for a specific location just inside the poles of the cross coil (‘Neuronal culture’ arrow in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0086794#pone-0086794-g002" target="_blank">Figure 2d</a>). The effective field completes <i>¾</i> of a spiral cycle during the magnetic pulses cycle, as indicated by the black arrows.</p

Summary of magnetic stimulation response in both neuronal 2D cultures and anesthetized rats.

<p>Summary of magnetic stimulation response in both neuronal 2D cultures and anesthetized rats.</p

Cloverleaf coil experiment.

<p>a) Bottom and side X-ray images of the cloverleaf coil used in the experiment. The coils are coupled diagonally to form two figure of eight coils and each figure eight coil is connected to an independent power source. b) Neuro navigation software display. The position of the coil is tracked using the navigator and indicated by the central red dot and the yellow circle over an MRI scan of the brain of the subject. The coil shape is added offline to illustrate the actual position. The yellow sphere at the front is the nasion, the red sphere at the bottom left the left tragus, used in registering MRI and head position. The color scale indicates tentative field strength, calculated in real time assuming a spherical head model and a figure of eight coil. c) Electric field induced in a pickup coil positioned on 2 neighboring wings of the clover leaf coil. The coils were driven separately by 2 Magstim rapid² stimulators. d) A reconstruction of the effective electric field amplitude and direction during a rfTMS pulse of the clover leaf coil with the field of coil #1 directed along the x-axis and the field of coil #2 along the y-axis.</p