Using a Single Molecule to Create a New Magnetic Field Sensor

A new way to use a single molecule as a magnetic field sensor has been discovered by LCN researchers at UCL and the University of Liverpool. In the study, published in Nature Nanotechnology, the scientists show how magnetism can manipulate the way electricity flows through a single molecule, a key step that could enable the development of magnetic field sensors for hard drives that are a tiny fraction of their present size.

Magnetoresistive effects, in which the electrical conductivity of a substance is manipulated by a magnetic field, have many applications today, including the magnetic field sensors in computer hard drives and unconventional types of computer memory.

In hard drives, magnetised areas on spinning disks are used to store information. As the magnetised areas pass a magnetic sensor, they trigger fluctuations in electric current flowing through the sensor, allowing the data to be read. Making these areas smaller increases a hard drive’s storage capacity without making it bigger, but also requires a smaller sensor.

“However, making smaller sensors isn’t trivial” says Fadi El Hallak, the researcher who conceived of the study and who is now a Magnetics Specialist at hard drive manufacturer Seagate Technology. “It is difficult to use magnetism to control the current flowing through objects the size of single molecules because the response to changes in the magnetic field is often very weak. The magnetic field enclosed by a single molecule is very small and these fields can only induce very small shifts in the molecule’s energy levels, making it very hard to detect.”

To get around this problem, the researchers developed a method of magnifying the effect of the magnetism on the flow of current in the detector.

First, the researchers created a junction in which a single magnetic molecule was weakly coupled to two metallic leads. The barriers between the molecule and the nearby metals were high enough that electrical charge in the metals could not flow over the barriers. However a small fraction of the charge can effectively go through the barriers by undergoing quantum mechanical tunnelling. This well-studied process, which is analogous to a person repeatedly walking into a closed door and occasionally magically appearing on the other side, enables a very small current to flow through the molecule when a voltage is applied across it.

To control the molecule’s energy levels, the scientists configured the experiment so that the molecule was much more strongly connected to one metal lead than the other. “This creates a large lever arm in which small shifts in the molecule’s orbitals caused by the magnetic field are magnified” says Cyrus Hirjibehedin, the project’s lead scientist. “The effect we observed is like that of a very lopsided see-saw, where a small change at one end causes a dramatic change at the other end, ” adds Ben Warner, the first author of the paper.

The team’s research demonstrates a new kind of sensor for magnetic fields that can be created in systems as small as a single molecule. The technology is promising for creating new computer technologies, and further refinement is being explored for commercial applications.

Publication:

Tunable magnetoresistance in an asymmetrically coupled single-molecule junction, Ben Warner et al., Nature Nanotechnology (2015) 

Image:

Topographic image generated by a scanning tunnelling microscope (STM) of an iron phthalocyanine molecule separated from an underlying metallic copper surface by a thin barrier of copper nitride. Since the coupling between the molecule and the copper surface is much stronger than that between the molecule and the STM tip above the molecule, the electrical response of the molecule to a magnetic field can be enhanced to create new magnetoresistive effects. Image: Ben Warner, Fadi El Hallak & Cyrus Hirjibehedin

Further Details

In their experiments, the researchers created the single molecule junction by depositing iron phthalocyanine molecules – synthetic molecules that are similar to the heme component of human blood – onto a copper surface capped with an atomically-thin insulating layer of copper nitride and then positioning the metallic tip of a scanning tunnelling microscope (STM) above it. The copper nitride and the vacuum between the STM tip and the molecule form the two tunnelling barriers, with the former being much weaker than the latter.

When the researchers applied a voltage across the junction, they observed regions in which the molecule exhibited so-called “negative differential resistance”, when the total current flowing will decrease with higher applied voltage rather than the normal increase. Combining the lever-arm effect of almost two orders of magnitude and the negative differential resistance, the researchers demonstrated two new types of magnetoresistance: one allows for arbitrarily large values of the ratio of the change in magnetoresistance, while in the other the sign of the resistance shift changes above a critical field, creating a binary switch.

A commentary article about this has been posted on Serious Science’s website:

http://serious-science.org/news/2676

This research has also been named as the result of the month by Scienta Omicron, an innovator in Surface Science that provides top capabilities for the research community through its technology leadership in electron spectroscopy, scanning probe microscopy and thin film deposition:

http://www.scientaomicron.com/en/result-of-the-month/107

LCN Author(s): 
Other contributors: 
Fadi El Hallak (Former LCN); Henning Prüser (Former LCN); John Sharp (University of Liverpool); Mats Persson (University of Liverpool)