Discovery of a new magnetic order

Jul 31, 2011
Atomic-scale Magnetic Lattice of Skyrmions
Physicists at Forschungszentrum Juelich and the universities of Kiel and Hamburg are the first to discover stable magnetic skyrmions on a surface as opposed to in bulk materials. The tiny cycloidal spirals, made up of just 15 atoms with their spins, form a regular lattice. The diagram shows simulations of magnetic measurements with the aid of spin-polarized scanning tunneling microscopy in black and white. The orange, red and green arrows show the upward or downward orientation of the spins. The cube-shaped "peephole" designates a single skyrmion. Credit: University of Hamburg

Physicists at Forschungszentrum Jülich and the universities of Kiel and Hamburg are the first to discover a regular lattice of stable magnetic skyrmions – radial spiral structures made up of atomic-scale spins – on a surface instead of in bulk materials. Such tiny formations could one day form the basis of a new generation of smaller and more efficient data storage units in the field of information technology.

The scientists discovered the spirals, each made up of just 15 atoms, in a one-atomic-layer of iron on iridium. They present their results in the current issue of the scientific journal Nature Physics.

The existence of magnetic skyrmions was already predicted over 20 years ago, but was first proven experimentally in 2009; a group of research scientists from the Technische Universität München (TUM) had identified lattices of magnetic vortices in manganese silicon in a weak magnetic field. Unlike these structures, the ones now discovered by at Jülich, Kiel and Hamburg exist without an external magnetic field and are located on the surface of the materials examined, instead of inside them. Their diameter amounts to just a few atoms, making them at least one order of magnitude smaller than the skyrmions which have been identified to date.

"The magnetically-stable entities that we have discovered behave like particles and arrange themselves like atoms in a two-dimensional lattice", explains Prof. Stefan Blügel, Director at the Peter Grünberg Institute and the Institute for Advanced Simulation in Jülich. "This discovery is for us a dream come true". Already in 2007, the same scientific team had discovered a new type of magnetic order in a thin manganese film on tungsten and demonstrated the critical significance of the so-called Dzyaloshinskii-Moriya interaction for the formation of its wave-like structure. The same interaction is also necessary for the formation of the spiral-shaped skyrmions.

The scientists did not discover the skyrmion lattice at first attempt. Originally, they wanted to prepare a one-atomic layer of chromium on iridium, in order to investigate the presumed existence of a different magnetic state. As the experiments were unsuccessful, they then tried with other metals. Using spin-polarized scanning tunnelling microscopy in studies of iron on iridium at the University of Hamburg, the researchers noticed regular magnetic patterns that were not consistent with the crystalline structure of the metal surface. "We were sure straightaway that we had discovered skyrmions", says Blügel. Intricate calculations undertaken by the Jülich supercomputers subsequently proved him right.

The result is a model describing the formation of the spin alignment through a complex interplay of three interactions: the chiral Dzyaloshinskii-Moriya interaction, the conventional interaction between spins plus a non-linear interaction involving four spins. The model should help, in the future, to selectively influence magnetic structures on surfaces. "We are now planning to investigate the effect of electricity on skyrmions; how do the electron spins of an electric current "ride" the spirals, how do they influence resistance and how are the spirals affected?", says Blügel.

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More information: "Spontaneous atomic-scale magnetic skyrmion lattice in two dimensions," Nature Physics, published online: 31.07.2011; DOI: 10.1038/NPHYS2045

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Caliban
5 / 5 (4) Jul 31, 2011
I wonder how much pressure would be required to collapse these vortices? And could this principle be used to manufacture, for instance, zero-friction bearings? Passive mag-lev?
lqtoai
5 / 5 (3) Jul 31, 2011
I like your comment, Caliban. But I guess that for practical issues, this discovery has very very long way to go. The same problem happens to graphene, because when these few-layer materials are in contact with other materials, their properties will be influenced significantly. Therefore, to sell the recipe of this newly discovered magnetic order to manufacturers, I guess more researches are required, for example about the reproducibility, or the life time of this magnetic state.
Caliban
not rated yet Jul 31, 2011
Yeah Iqtoai,

That's essentially what I was asking. Even at that- just imagine what a difference this would make in the service lifetime of small electronic applications, like disk drives and cooling fans, for instance.

If ways could be found to adapt these principles for more heavy-duty purposes -well, the sky's the limit.

antialias_physorg
5 / 5 (1) Aug 01, 2011
And could this principle be used to manufacture, for instance, zero-friction bearings? Passive mag-lev?

Unlikely as the effect is a surface effect and therefore the field strengths are exceedingly small. The cumulative magnetic field over a macro (or even micro) scopic area seems to be zero.

This does not seem to be an applicable for bulk/strong magnetic materials but for very localized fields (like we need for magnetic data storage).
Imagine a sheet, one atom thick, which could be used as a solid state storage solution. Now imagine stacks and stacks of these. The information density could be enormous if we can find an elegant way to read/write fom these.
dutchman
not rated yet Aug 01, 2011
In order for this to be useful in data storage, one has to be able to change the state of the location. I wonder if that is even theoretically possible. I did not see anything in the article that would hint at such a possibility.
jselin
not rated yet Aug 01, 2011
I wonder if these have mobility at room temperature or if the order was formed at the higher temperatures seen during the monolayer deposition. If its possible to write data to it the "annealing" temperature will have to be high enough above room temperature to preserve it.