by Staff Writers
Bochum, Germany (SPX) May 22, 2013
Physicists at the Ruhr-Universitat Bochum (RUB) have found out how tiny islands of magnetic material align themselves when sorted on a regular lattice - by measurements at BESSY II. Contrary to expectations, the north and south poles of the magnetic islands did not arrange themselves in a zigzag pattern, but in chains.
"The understanding of the driving interactions is of great technological interest for future hard disk drives, which are composed of small magnetic islands", says Prof. Dr. Hartmut Zabel of the Chair of Experimental Physics / Solid State Physics at the RUB. Together with colleagues from the Helmholtz-Zentrum in Berlin, Bochum's researchers report in the journal "Physical Review Letters".
Complete chaos in the normal state
However, this is not the case. A magnetic material is only created when specific quantum mechanical forces are at work. Normally, the forces between the atomic dipoles are by far too weak to cause magnetic order. Moreover, even at low temperatures, the thermal energy causes so much movement of the dipoles that complete chaos is the result.
"However, the fundamental question remains of how magnetic dipoles would align themselves if the force between them was big enough", Prof. Zabel explains the research project.
Square lattice of magnetic islands
If you leave these dipoles to their own resources, at low temperatures you can observe the arrangement that results exclusively from the interaction between the dipoles. They assume the most favourable pattern in terms of energy, the so-called ground state. The islands serve as a model for the behaviour of atomic dipoles.
Using circularly polarised synchrotron light (X-ray photons), the photons stimulate specific electrons. These provide information on the orientation of the dipoles in the islands. The experiments were carried out at low temperatures so that the thermal movement could not interfere with the orientation of the dipoles.
Dipoles arrange themselves in chains
"In fact, you would expect a zigzag arrangement", says the Bochum physicist. Based on the chain pattern observed in the experiment, the researchers showed that higher order interactions determine how the magnetisation was oriented. Not only dipolar, but also quadrupolar and octopolar interactions play a role. This means that a magnetic island exerts forces on four or eight neighbours at the same time.
Magnetic islands in the hard drives of the future
For a functioning computer, you need a configuration in which the dipole islands interact as little as possible and can thus assume the states "0" and "1"independently of each other. For the technical application, a precise understanding of the driving interactions between magnetic islands is therefore crucial.
M. Ewerlin, D. Demirbas, F. Brussing, O. Petracic, A.A. Unal, S. Valencia, F. Kronast, H. Zabel (2013): Magnetic Dipole and Higher Pole Interaction on a Square Lattice, Physical Review Letters, DOI: 10.1103/PhysRevLett.110.177209
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