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Nanomagnetism, Electronic Structures & Surfaces

Noncollinear magnetism


Contrary to ferro- or antiferromagnetic structures, in a noncollinear magnetic arrangement the direction of the magnetization may change from atom to atom by arbitrary angles. Due to topological frustration such structures can be formed on triangular lattices. The image shows a periodic Néel state. Here co-planar spins form 120° angles between nearest neighbors. A unit cell shown in red contains 3 atoms. The total magnetization M in the unit cell is zero. In this picture, the spins are oriented in the surface plane. The orientation of the spin with respect to the lattice is determined by the spin-orbit interaction.

(P.Kurz, G. Bihlmayer, S.Blügel).

Spin-orbit coupling and magnetocrystalline anisotropy


Magnetic recording would not be possible without a relativistic effect called spin-orbit coupling. It determines the orientation of the spins with respect to the crystal lattice, or the magnetocrystalline anisotropy. The strength of this effect determines the storage density on an magnetic storage device. Therefore, finding new materials with large magnetocrystalline anisotropies is of substantial importance and the theoretical understanding of this material-dependence is achieved by extensive ab-initio calculations. The picture on the right shows a Pt(111) step edge decorated with Co atoms. The formed Co wires are an example of a low-dimensional magnetic system that has an highly anisotopic out-of-plane magnetization.

(X.Nie, A. Shick, G. Bihlmayer, S.Blügel).

Electric fields on surfaces


In many experimental techniques (like in scanning tunneling microscopy) an electric field acts on a surface. How the electronic structure of a surface or interface (e.g. in a tunneljunction) is changed by this field can be studied on a ab-inito (quantum mechanical) basis. The figure shows the screening charge induced in the a Xe covered Ag(001) surface when a field is applied. We can see that the screening still occurs at the metal surface and not where the adsorbate sits and that the adsorbate atom becomes polarized.

(S.Clarke, M. Weinert, S. Blügel).

Magnetic surfaces with spin-polarized STM


Direct observation of the two-dimensional atomic scale antiferromagnetic structure of a monolayer magnetic film by spin-polarized scanning tunneling microscopy (SP-STM). All atoms of the monolayer film (red and green) are of the same chemical species (Mn) and differ only by the orientation of their magnetic moment. Using a magnetic probe tip it is possible to measure an SP-STM image (see the height profile) showing one kind of magnetic atoms (red) as hills and the other kind (green) as valleys, i.e. a direct image of the atomic scale magnetic order within the magnetic film.

(S. Heinze, D. Wortmann, S. Blügel).