Effect of contact geometry on magnetoresistance in CoPd-contacted carbon nanotubes

Abstract

We present local magnetoresistance measurements on carbon nanotubes contacted with the ferromagnetic alloy Co50 Pd50. A sample with contacts of dimensions thickness=10nm, normallength=4thinmathspaceμm exhibited the typical sharp switching expected at the coercive fields of the contacts. However, a sample with the same in‐plane geometry but a thickness of 40 nm displayed a complicated switching behavior indicative of gradual domain wall rotation. In both samples, the contact widths were varied to control shape anisotropy. Further characterization of the contacts and similar contact‐like structures via remanent state magnetic force microscopy suggested a single or few domain structure in the 10 nm thick contacts. The thicker contacts exhibited a complex domain structure characteristic of a hard or intermediate magnetic axis along the long axis of the contact, despite the effect of shape anisotropy. This result is discussed with respect to the influence of both thickness and in‐plane geometry on competing anisotropy contributions within the device. The characterization allows for the pinpointing of precise dimensions that will determine the magnetic easy axis of contacts, thereby controlling magnetotransport.

Impact of Tunnel-Barrier Strength on Magnetoresistance in Carbon Nanotubes

“…Furthermore, since the injection of charge carriers occurs only at the point where the contact ends [25], the local magnetic environment has a strong influence on the MR measurements. Importantly, pinning and moving of domain walls in contacts that incorporate several magnetic domains leads to unstable switching behavior of the contacts and a non-trivial MR signal [26]. For this reason, we optimized the shape of the contacts in such a way that the magnetization is in-plane along the long axis with a single or at most two domains [22].…”

Organic–Inorganic Hybrid Interfaces for Spin Injection into Carbon Nanotubes and Graphene

“…Following this publication, there have been some very interesting studies all from the Meyer's group in Jülich. [ 101–103 ] In particular, this team focused on the impact of the native tunnel barrier between CoPd contacts ($${P_{{\rm{CoPd}}}} \cong 25\% $$) [ 101,102 ] and a (single and multiwall) carbon nanotube on the amplitude of the MR effects. They showed that the MR increases when the contacts are resistive (>100 kΩ) especially at low temperatures compared to simple ohmic contacts whose resistance does not vary much with temperature.…”

Oscillatory Tunnel Magnetoresistance in a Carbon Nanotube Based Three-Terminal Magnetic Tunnel Junction

“…The weak spin–orbit coupling and hyperfine interaction in a carbon nanotube (CNT) − as well as the observation of long spin-flip scattering lengths of at least 130 nm in a multiwalled CNT and 1 μm in single-walled (SW) CNTs have prompted researchers to use these low dimensional materials as the spacer in next-generation MTJs. Despite the challenges in fabricating reliable magnetic contacts with CNTs, several groups have been successful in demonstrating a magnetoresistive effect in carbon nanotube based MTJs. ,− Measurements in two-terminal junctions reveal a considerable variation in magnetoresistance, ,− ,, which could be ascribed to the lack of chirality control in CNTs and difficulty in fabricating reproducible junctions. In the case of a three-terminal CNT-based MTJ, Sahoo et al first observed an electric field modulation of MR; an oscillation with a strong variation in the amplitude and width of the MR signal with gate bias was reported .…”