From Ballistic Transport to Tunneling in Electromigrated Ferromagnetic Breakjunctions

Abstract

We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 nm to the atomic scale and eventually to the tunneling regime by means of electromigration. These devices are mechanically stable against magnetostriction and magnetostatic effects. We measure magnetoresistances ∼0.3% for 100 × 30 nm 2 constrictions, increasing to a maximum of 80% for atomic-scale widths. These results are consistent with a geometrically-constrained domain wall trapped at the constriction. For the devices in the tunneling regime we observe large fluctuations in MR, between -10 and 85%.

Anisotropic Magnetoresistance and Anisotropic Tunneling Magnetoresistance due to Quantum Interference in Ferromagnetic Metal Break Junctions

“…For this reason, our contacts are firmly attached to a non-magnetic silicon substrate and are measured entirely at low temperature to suppress thermally-driven surface diffusion of metal atoms. Similar structures have proven [8,9] to be much more mechanically-stable than previous samples which were measured at room temperature. We fabricate our devices using aligned steps of electron beam lithography to first pattern 20-nm-thick gold contact pads and then 30-nm-thick magnetic permalloy (Py = Ni 80 Fe 20 ) point contacts [9].…”

“…Similar structures have proven [8,9] to be much more mechanically-stable than previous samples which were measured at room temperature. We fabricate our devices using aligned steps of electron beam lithography to first pattern 20-nm-thick gold contact pads and then 30-nm-thick magnetic permalloy (Py = Ni 80 Fe 20 ) point contacts [9]. Each contact con- sists of two elongated electrodes which are connected by a 100-nm-wide bridge ( Fig.…”

“…PACS numbers: 72.25.Ba; 73.63.Rt; 75.75.+a The magnetoresistance properties of nanometer-scale magnetic devices can be quite different from those of larger samples. One aspect of this difference has been explored extensively in previous experiments -the resistance of magnetic domain walls created when the magnetic moment direction in one magnetic electrode is rotated relative to the moment in a second electrode [1,2,3,4,5,6,7,8,9]. Here we focus on a different aspect of the physics of magnetoresistance in nanoscale magnetic contacts -the anisotropic magnetoresistance (AMR) that arises when the magnetization throughout a device is rotated uniformly so as to change the angle between the direction of current flow and the magnetic moment.…”

Nanogap Electrodes

“…[83] While more recently, it has been well utilized to fabricate nanogap electrodes for nanodevices. [84][85][86][87][88][89] The passage of a large density current or application of a large direct current (dc) voltage to the thin metal wire predefined by electron-beam lithography would cause the electromigration of metal atoms and eventual breakage of the nanowire. This process can yield a stable electrode separation of 1 nm with high efficiency [84] and it can be monitored in real time by observing the current-voltage characteristics until only a tunneling signal is present.…”

Anisotropic magnetoresistance in nanocontacts

“…On the other hand, nanocontacts allow us to study AMR going from the contact ͑BAMR͒ to the tunnel ͑TAMR͒ regime in the same system, as shown in the case of both Ni and Py. 9,18 Ni nanocontacts have also been used as electrodes to explore the Coulomb blockade and the Kondo regimes. 19 The crux of the matter is to identify the necessary and sufficient conditions to expect large values of AMR in quantum transport.…”