The manganese-doped nanowires show very large negative magnetoresistance – that is their resistance decreases strongly as the materials are exposed to an increasing magnetic field, explains team leader Håkan Pettersson of Lund University. “What is more, the wires boast a very complex internal landscape with various magnetic interactions that result in the materials behaving like both paramagnets and so-called spin-glasses – an unusual situation,” he says.

The researchers measured the resistance of their nanowires by transferring them from the substrate that they had been grown onto a silicon substrate covered with a thin insulating silicon dioxide layer (SiO2) on which reference markers and macroscopic pads had already been predefined. Before transferring the wires, the team etched trenches in the SiO2 layer to align the wires. This meant that they could make more accurate measurements of the magnetoresistance.

“We then used electron beam lithography to define contacts connecting individual nanowires to the macroscopic contact pads (see figure),” says Pettersson. “We measured the magnetoresistance of the wires at different temperatures and with magnetic fields applied parallel and perpendicular to them.”

The team then performed ultrasensitive SQUID measurements by mechanically transferring large ensembles of the nanowires onto another SiO2-like substrate. These experiments allowed the researchers to unearth the complex physical mechanisms behind the large negative magnetoresistance in the nanowires.

“Our study has allowed us to better understand the transport mechanisms in magnetic semiconductor nanostructures,” Pettersson told nanotechweb.org. “One potential application that immediately springs to mind is sensitive integrated magnetic field sensors.”

The team, which includes researchers from Halmstad University in Sweden, Friedrich-Schiller-University Jena in Germany, Linneaus University in Sweden and National Donghwa University, Taiwan, says that it now plans to look at how ferromagnetism in the magnetic semiconductors is controlled by hole-mediated coupling between the manganese ions. “One possibility is to tune the hole concentration in the nanowires using an electric gate – something that will allow for electrically-gated ferromagnetism,” reveals Pettersson. “If successful such a strategy could help us develop new types of integrated magnetic devices such as transistors and memories.”

Tuning the magnetic phase on demand by an electric gate also offers us a completely new playground in which to study magnetic properties at the nanoscale, he added.

The work is detailed in Nano Letters DOI: 10.1021/nl402229r.

Further reading

Racetrack memory nears the finish line (Jan 2011)
Tailoring the magnetization processes of iron nanowires (Jul 2010)
Ion irradiation improves magnetic domain wall conduction (Aug 2013)