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Posted: Sep 27, 2016
Electrically accessing complex magnetism in rare earth atoms
(Nanowerk News) A new way to electrically access the magnetic properties of rare earth atoms, which are of crucial importance in many modern technologies, has been revealed by a team of researchers from the London Centre for Nanotechnology at UCL, the University of Stuttgart, the University of Nottingham, and the Forschungszentrum Jülich. Reporting their work in Nature Communications ("Sub-molecular modulation of a 4f driven Kondo resonance by surface-induced asymmetry"), the scientists show that encasing the rare earth atom dysprosium inside the framework of a single molecule allows the dysprosium to interact magnetically with electrical charge passing through the molecule, a crucial step in developing the smallest possible devices based on rare earth elements.
Topographic STM images of dysprosium double-decker phthalocyanine molecules on a copper surface. As seen on the left, the molecules normally appear symmetric. However, as shown on the right, within a certain range of energy the asymmetric electronic and magnetic interactions between the molecule and the underlying copper surface cause the physically symmetric molecule to appear asymmetric. (Image courtesy of Ben Warner, Fadi El Hallak, Henning Prüser, and Cyrus Hirjibehedin)
Rare earth metals, such as neodymium or dysprosium, are a central ingredient in creating materials that have important technological applications, ranging from making stronger magnets to more efficient light bulbs. Materials made with rare earth elements are also scientifically interesting because they exhibit intriguing quantum mechanical states. The set of rare earth elements behaves very differently from traditional metals like copper and iron, mainly because of the more complex nature of the orbitals that carry unpaired (i.e. magnetic) electrons.
In the past two decades, scientists have explored magnetism in traditional metals at the atomic scale using techniques that are capable of observing and manipulating individual atoms and molecules, such as scanning tunnelling microscopy (STM). However, the more contracted nature of the magnetic orbitals in rare earth metals has made it much more difficult to access the unique magnetic properties of such atoms with these powerful probes.
The focus of this new work is a molecule that incorporates a single dysprosium atom between two phthalocyanines, which are chemically similar to naturally occurring molecules such as heme in blood or chlorophyll in plants. This dysprosium-based molecule is already known to have interesting magnetic properties arising from the rare earth atom at its core.
By depositing this “double-decker” magnetic molecule on a copper surface, the researchers were able to use STM to observe an exotic quantum mechanical phenomenon called the Kondo effect. This effect arises when the magnetism of a single atom is affected by the electrical charges, which are also magnetic, in a nearby metal and indicates that the two are coupled.
One of the most beautiful manifestations of this occurs when imaging the molecule with STM. At most energies, the molecule appears symmetric, reflecting its symmetric physical structure. However, as seen below, in a small window of energy the Kondo effect modifies this appearance to make it asymmetric, reflecting the asymmetric electronic and magnetic coupling of the molecule to the underlying copper.
Dr Fadi El Hallak, the researcher who conceived of the study while at UCL and who is now a Senior R&D Manager at the data storage company Seagate Technology, said that “We saw the first hints of this result almost six years ago, and we wanted to be sure that this was due to the unique properties of rare earth atoms”.
To do this, team members performed state-of-the-art calculations to simulate the dysprosium molecule on the copper surface. Researcher Dr Nicolae Atodiresei at the Forschungszentrum Jülich explained that the results of these calculations demonstrate that “the molecular environment can be used to directly control access to normally isolated magnetic properties of rare earth atoms”.
“This work will open new frontiers in research into the important fundamental properties and commercial applications of rare earth atoms at the atomic scale”, said Dr Cyrus Hirjibehedin, the project’s lead scientist.
Some of the data obtained in this work contributed to the citizen science crowdsourcing project “Feynman’s Flowers", in which members of the public to help unlock the secrets of magnetism at the molecular scale.