A new way to use a single molecule to create a magnetic field sensor

On January 26, 2015 Nature Nanotechnology published a paper “Tunable magnetoresistance in an asymmetrically coupled single-molecule junction” describing experiment on the use of a single molecule to detect magnetic field using tunneling current. We have asked one of the authors of this research, Dr. Cyrus Hirjibehedin from University College London, to comment on this work.

The Study

Shrinking the magnetic domains used to store data on today’s hard drives down to the atomic scale would dramatically increase the drives’ storage capacities. Reading the data stored in these domains, however, also requires developing magnetic field sensors on the molecular scale. To achieve this, we need to understand how the properties of a single molecule can be manipulated by a magnetic field. This manipulation, however, is difficult because a molecule’s responses to magnetic field changes are typically very weak. One way to address this challenge is to develop a way to magnify these effects.

To electrically access the magnetic properties of a single molecule, we placed it in a nanometer-sized gap between two metallic electrodes. In this experiment, however, the key step was to attach the molecule to each of the electrodes in an asymmetric manner, so that the electrical coupling to the molecule was much stronger with one electrode than with the other.

To do this, we separated the molecule from the two metal electrodes in two different ways. On one side we attached the molecule to an atomically-thin insulating layer made of a mixture of copper and nitrogen on top of a copper metal surface; the other metal electrode was the movable tip of a scanning tunneling microscope (STM), which could be positioned less than a nanometer above the molecule. Although the STM tip was not directly in physical contact with the molecule, electrical charge in the tip could pass through the gap to the molecule and into the underlying copper by quantum mechanical tunneling. This well-studied process, which is analogous to a person repeatedly walking into a wall and occasionally magically appearing on the other side, enables a very small current to flow through the molecule.

When a voltage is applied across the nanojunction, the orbitals of the molecule are shifted with respect to the energy of the free charges in the electrodes. When one of the orbitals aligns with the maximum energy of the free charges (the so-called “Fermi energy”) in either electrode, charge is transferred to or from the molecule. This changes the charge of the molecule, which then modifies its ability to conduct current. In some cases, the ability to conduct decreases, which results in a decrease in current with increasing voltage. This behavior is distinctly different from that of normal resistors that follow Ohm’s law – in which current increases with increasing voltage – and is called negative differential resistance (NDR). NDR effects are used to make a variety of modern electronic devices, including specialized oscillators.

When we examined the magnetic molecule iron phthalocyanine (FePc) in our asymmetric nanojunction, we found that about 12 percent of the molecules exhibited NDR. While NDR has been observed in other single molecule junctions, what was remarkable in this case was that the voltage at which the NDR occurred shifted by about 100 times more than expected when we applied a magnetic field to it. This dramatic increase in sensitivity is caused by the asymmetric coupling between the molecule and the two metallic electrodes. In this configuration, most of the voltage is applied between the STM tip and the molecule, leaving only a small fraction of the voltage to shift the orbitals of the molecule into alignment with the Fermi energy of the copper. Therefore, although the orbitals of the molecule shift by only a small amount in a magnetic field, a large voltage must be applied across the junction to bring them back into alignment. This dramatically increases the apparent sensitivity of the conductance in the junction to magnetic fields, producing a new kind of magnetic field sensor based on a current flowing through a single magnetic molecule.

Background

The magnetic properties of materials are exploited in a variety of different technologies. One of the most prominent ones is non-volatile data storage, in which the orientation of a magnetic domain is used to permanently encode information. The drive to increase storage capacity to handle the ever growing amounts of data that are generated in modern life has led to the exploration of the magnetic properties of ever smaller structures, down to single magnetic atoms and molecules.

For the past quarter century, much of the research on nanoscale magnetic systems has focused on magnetic molecules, and in particular on understanding how the structure of these molecules can be used to tune their magnetic properties. In many cases, placing a magnetic atom or molecule near a metal – which is necessary in many device proposals that use single magnetic molecules – can substantially weaken or even cancel out its magnetic properties. Therefore, this coupling was thought to be purely detrimental.

However, we are now learning that in some cases it is possible to control and even enhance the magnetic properties of magnetic atoms and molecules by modifying the electrical coupling to a nearby metal. For example, in 2014 we found that changing the strength of the electrical coupling between a magnetic atom and a metal could be used to tune its magnetic stability by modifying the energy required to change its magnetic orientation. Scientists at the Free University of Berlin have also shown that it is possible to protect the magnetic properties of a molecule by placing it on a superconducting surface. We are therefore very interested in exploring how such coupling can be used to engineer new magnetic properties in single molecule junctions and how these might be utilized in the ultimate limit of novel magnetic devices.

Future Direction

The size of the enhancement of the magnetic field sensitivity in our single molecule junctions is determined by the asymmetry of the coupling to the two electrodes. In the future, we will therefore explore how to further increase the asymmetry and thus the enhancement. This will be challenging because the best way to do this is to increase the coupling between the molecule and the surface, but this often weakens the magnetic properties of the molecule itself.

In the longer term, we have begun to explore how the properties of the single molecule junction can be modified or enhanced by introducing materials with more complex quantum mechanical properties. For example, over the last decade there has been a surge of interest in the properties of atomically thin materials, beginning with graphene (a single layer of carbon atoms arranged in a honeycomb lattice) and more recently progressing to other materials like silicene (the silicon analog of graphene) and a variety of transition metal dichalcogenides like molybdenum disulfide. In these materials the motion of the charge carriers is coupled with their magnetic properties, making them interesting candidates for use in nanoscale magnetic junctions.