Now two groups have developed independent techniques to image the magnetic structure of surfaces and interfaces in great detail. The methods promise to greatly improve our understanding of magnetism at the nanometre scale.
Naively it might be thought that the magnetic field next to the surface of a material made from magnetic atoms can simply be inferred from its physical and chemical structure using well-known theories. However, this is not the case and there have been many notable surprises. For example, it was discovered 15 years ago that the variations of the magnetic field along a magnetic nonmagnetic interface is much smaller than that expected from the structural profile of this interface.
Ideally, we would like lo be able to simultaneously characterize the physical, chemical and magnetic properties at the atomic or nanoscale level. However, the main problem with realizing this task is that nanostructured magnets have inherently small volumes, meaning that very sensitive probes are needed.
A particularly interesting phenomenon that is still not fully understood, even though it has been studied extensively in magnetic bilayers is the so-called exchange bias. Bilayer structures consist of two materials that exhibit different magnetic properties such as ferromagnetism and antiferromagnetism. In a ferromagnet, the spins of all the individual atoms line up in the same direction, while the neighbouring atoms in an antiferromagnet point in opposite directions.
In a ferromagnet, the hysteresis loop - a plot of the magnetization versus
the applied magnetic field as the field is increased and decreased - is symmetric about the zero-field line. However, when the same material is placed in contact with an antiferromagnet the hysteresis loop shifts away from zero. The origin of this exchange bias, and its dependence on properties including the spin orientation and surface roughness, is currently not understood. It is not even known whether there are one or more mechanisms at work.
Many theoretical models have been developed to understand this phenomenon, and several can explain some of the features found experimentally. However, they all assume that the magnetic structure of the interface is the same as the bulk material. Clearly an experimental determination of the magnetic structure is essential to advance the theory further. There are essentially three major ways to image magnetic materials at the atomic level: neutron and electron scattering; direct imaging using scanning probe microscopy; and interactions with polarized electromagnetic radiation. Although neutron scattering is ideal for this purpose, it has only been used sporadically because large areas of material are needed for measurements. The two recent studies have used the other complementary techniques to determine the structure of a much smaller area of the surface of an antiferromagnet, and the relative orientation of the spins at the interface between a ferromagnet and an antiferromagnet.
Matthias Bode and co-workers at the University of Hamburg and the Institute for Solid State Research in Germany used scanning probe microscopy to image the physical and magnetic structure of a single layer of manganese atoms deposited on a tungsten substrate. First they moved a sharp non-magnetic tungsten tip across the antiferromagnetic manganese surface. Electrons tunnel from the tip to the surface to reveal the location of the atoms. They then repeated the experiment using a tip coated with iron, which allowed them to image the magnetic structure of the surface. The findings show that the magnetic structure has twice the periodicity of the physical structure, a result that is in good agreement with earlier theoretical predictions.
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