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Posted: Jul 05, 2011

Single-molecule nanomagnets enable novel graphene spintronics devices

(Nanowerk Spotlight) The field of single-molecule magnets is very promising, since an individual magnetic molecule represent the ultimate size limit to store and processing information. Magnetic molecules are considered very promising for spintronics – electronics exploiting also the spin of the electron – since they can store a bit of information in an extremely small volume. However, in order to make magnetic molecules work, one has to find a way to measure their magnetization, a task that has not been attempted so far.
Usual approaches are often to put the molecule inside two electrodes and pass a current through it. While this method may be efficient in order to address individual molecules, it is also very invasive and it may lead to a strong perturbation of the properties of the molecules.
A European research team has now designed and realized a novel hybrid spintronic nanodevice where the state of the molecule is measured 'indirectly', through a sensor coupled with the molecule.
"We have been able to detect extremely tiny magnetic signal, down to the single molecule level" Andrea Candini, a researcher at the Istituto Nanoscienze-CNR in Modena, Italy, tells Nanowerk. "To achieve this result, we used a special sensor made of a graphene nanostructure functionalized with magnetic molecules. In our new device, the magnetic state of the molecule is detected measuring the current passing through the graphene sheet. This is the first time that such a small signal is measured, beating the previous record by about a factor of 100."
Including collaborators from the Institut Néel – CNRS in France and the Karlsruhe Institute of Technology in Germany, the team used graphene as a kind of 'spider web' to chemically trap the molecules and detect their magnetization at the same time.
The researchers reported their findings in the June 7, 2011 online edition of Nano Letters (Graphene Spintronic Devices with Molecular Nanomagnets)
Graphene Spintronic Devices with Molecular Nanomagnets
(a) Schematic representation of the single molecule magnet (TbPc2). (b, c) Schematic view of the device, showing in (b) the molecule attached to graphene and in (c) the nano constriction contacted by source(S) and drain(D) electrodes. The magnetic moments of the single molecule magnet (hexyl and pyrenyl groups here omitted for clarity) on top of the constriction add another degree of freedom to tune the conductivity of the device. (d) False-color SEM image of the device presented in the text. SiO2 substrate and etched graphene are colored in purple. Graphene conductive regions are colored in green. Source and drain electrodes are indicated. (Reprinted with permission from American Chemical Society)
"We chose to use graphene as the sensor material since it is completely exposed to the external world and therefore it is very sensitive to its environment," explains Candini. "When a magnetic molecule is grafted on its surface, the conductivity of our graphene device showed to be sensitive to the reversal of the magnetization of the molecule."
This result is the outcome of a 3-year long research.
"We worked step by step, learning how to fabricate and obtain each single ingredient but the recipe for the final device was more difficult and since each single step is very difficult, the rate of success for the final device was very low – as usual in nanotechnology" says Candini.
To fabricate their novel spintronics device, the researchers first synthesized molecules suitable to graft the graphene honeycomb lattice. Then they tailored the graphene sheet with electron beam lithography and oxygen plasma etching, realizing a device of about 10 nanometers in size. Finally, they performed electrical measurements at very low temperatures, in order to limit the noise.
The device works similarly to the spin valve present in a reading head of today's hard disks, but it is much smaller.
There are several possible applications for this device. Foremost of course is data storage, for instance a hard disk where the information is stored in individual molecules, achieving a density of more than 100 terabits per square inch. According to Candini, similar devices could generally be used to detect individual magnetic objects, for instance in biosciences where magnetic markers are grafted to individual biological molecules.
"In order to make these devices work at normal ambient conditions, however, we still need to solve some technical problems, but we do not see any particular fundamental issue," he says.
The next step of this research will be to try to exploit the quantum nature of the molecules to manipulate their state in a coherent way and measure the signals through the graphene device.
Candini points out that, in order to achieve this point, it is important to achieve better control of the fabrication of the graphene nanostructure and of the coupling between the graphene device and the molecules grafted on its surface. The team is already working on these challenges.
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