Adding or removing one atom from a solid containing about 1023 atoms does not make much of a difference but this is not the case in nanostructures, explains team member Samir Lounis of the Institut für Festkörperforschung in Jülich. Here, the magnetic structure changes dramatically across the entire nanowire depending on whether the total number of atoms in the wire is odd or even.
"Changing the parity of a wire by adding or subtracting a single atom leads to this incredible effect – a switch of the magnetic behaviour from one state to a completely different one," Lounis told nanotechweb.org. "This peculiar nano-effect does not occur in micro or macrostructures and is totally reversible."
"Frustration" effect
The effect occurs thanks to "frustration": for example, when a suspended antiferromagnetic wire is put on a ferromagnetic surface, there is a frustration between the local antiferromagnetic structure and the underlying ferromagnetic substrate. This is because neighbouring atoms in the wire prefer to couple antiferromagnetically but are prevented from doing so by the topology of the substrate. In such a system, the magnetic interaction between neighbours is strong but the magnetic structure as a whole is fragile and small changes to it can make a big difference.
The researchers obtained their results using ab initio quantum-chemical computer simulations based on a refined version of Density Functional Theory (for which Walter Kohn and John Pople received the 1998 Nobel Prize in Chemistry). This type of calculation relies only on the basic laws of quantum mechanics without additional adjustable parameters.
By tuning the strength of the magnetic interaction within the wire and between the wire and a surface, Lounis and co-workers succeeded in constructing a phase diagram from which they predicted where the parity effect occurred on a wire length.
Magnetic switch
"The effect is nothing else but a magnetic switch between two different magnetic states switched by the addition or removal of an atom," said Lounis. "In hard disks, the magnetic information is stored in binaries (0) and (1) that can be represented by the two magnetic solutions found in these nanowires."
As well as switching applications, the behaviour observed in the simulations could be useful for building logic gates made of magnetic wires. This is because perturbations at the edge of the wire travel along the whole wire until they reach the other side, explains Lounis. "This is a wonderful example of magnetic information transport between two points separated by just a few nanometres."
The team is now planning to look at chiral magnetic nanostructures deposited on surfaces. "These are particularly interesting because spin-polarized electrons may transfer a torque on the wire and the magnetisation of the wire might rotate," added Lounis.
The work was reported in Physical Review Letters.