Yusuke Tomoda

Fabrication of nanogap electrodes by field-emission-induced electromigration

The authors report a simple and easy technique for the fabrication of nanogaps with separations of less than 10nm. This technique is based on electromigration induced by field emission current. Here, the authors investigated the dependence of tunnel resistance on the shape of nanogap electrodes and initial gap separation. The initial nanogap electrodes having asymmetrical shape with the separation of 30–60nm were fabricated by electron-beam lithography and lift-off process. In the nanogaps with asymmetrical shape, the tunnel resistance was controlled by the magnitude of the preset current during field-emission-induced electromigration and decreased from the order of 100TΩto100kΩ with increasing the preset current from 1nAto150μA. This tendency was quite similar to that of nanogaps with symmetrical shape. Furthermore, the tunnel resistance after the electromigration was less dependent on the initial gap separation and was completely determined by the preset current. This suggests that it is possible to con...

Magnetoresistance effect of planar-type ferromagnetic tunnel junctions

Planar-type Ni∕Ni oxide/Ni ferromagnetic tunnel junctions were fabricated by using nanolithography techniques. Ni oxide barriers were formed by scanning probe microscope local oxidation. For the observation of magnetoresistance (MR) difference across the junction, source and drain electrodes were patterned into different shapes by focused ion beam lithography, in order to induce magnetic shape anisotropy. The resistance of planar-type Ni∕Ni oxide/Ni ferromagnetic tunnel junction was changed by applying a magnetic field, and MR ratio exhibited above 100% at 16.3K. With increasing the bias voltage from 0.15to1.0mV at 16.3K, the MR ratio decreases from 103% to 5%. Furthermore, the MR ratio at 0.5mV decreases from 8.5% to 0.2% with increasing the temperature from 16.3to200K. This result implies that planar-type ferromagnetic tunnel junctions have potential use as nanoscale magnetoresistive elements.

Control of channel resistance on metal nanowires by electromigration patterning method

A simple method for fabricating nanoscale gaps between metal electrodes, based on the break of metal nanowires induced by electromigration of metal atoms, was investigated to control the channel resistance of the nanowires. In this article, the authors studied the control of the resistance of metal nanowires by introducing a constant-voltage (CV) mode and feedback-controlled electromigration (FCE) scheme. Ni nanowires were fabricated by electron-beam lithography on Si wafers coated with SiO2. The initial resistance of the nanowires was 0.8–1.2 kΩ. In CV mode, the channel resistance of 200-nm-wide nanowires was adjusted to be 10 kΩ, but nanowires with a width greater than 300 nm were not well controlled due to Joule heating. To improve the controllability of the channel resistance, they added the CV mode to the FCE scheme, based on resistance monitoring, to control the electromigration. This procedure created a nanogap with resistance ranging from a few-atom regime to a tunneling regime. These results imply that this technique can easily control the channel resistance of metal nanowires.A simple method for fabricating nanoscale gaps between metal electrodes, based on the break of metal nanowires induced by electromigration of metal atoms, was investigated to control the channel resistance of the nanowires. In this article, the authors studied the control of the resistance of metal nanowires by introducing a constant-voltage (CV) mode and feedback-controlled electromigration (FCE) scheme. Ni nanowires were fabricated by electron-beam lithography on Si wafers coated with SiO2. The initial resistance of the nanowires was 0.8–1.2 kΩ. In CV mode, the channel resistance of 200-nm-wide nanowires was adjusted to be 10 kΩ, but nanowires with a width greater than 300 nm were not well controlled due to Joule heating. To improve the controllability of the channel resistance, they added the CV mode to the FCE scheme, based on resistance monitoring, to control the electromigration. This procedure created a nanogap with resistance ranging from a few-atom regime to a tunneling regime. These results impl...

Magnetoresistance Properties of Planar-Type Tunnel Junctions With Ferromagnetic Nanogap System Fabricated by Electromigration Method

We report electromigration techniques for the fabrication of planar-type tunnel junctions with ferromagnetic nanogap system. In these techniques, by monitoring the current passing through the devices, we are easily able to obtain the planar-type Ni-Vacuum-Ni tunnel junctions. In this paper, magnetoresistance (MR) properties of the planar-type Ni-based tunnel junctions formed by stepwise feedback-controlled electromigration (SFCE) and field-emission-induced electromigration (activation) are studied. We performed the SFCE method for Ni nanoconstrictions connecting asymmetrical butterfly-shape electrodes. Furthermore, the activation technique was applied to Ni nanogaps with separations of 15-45 nm. MR ratio of the devices formed by the SFCE exhibited approximately 4% at 16 K . On the other hand, the devices fabricated by the activation showed MR ratio of above 300% at 16 K. These results suggest that it is possible to fabricate planar-type ferromagnetic tunnel junctions with vacuum barriers by electromigration techniques.