What are the examples of natural magnets
In general, a ferromagnetic metal is divided into areas with different directions of magnetization, so-called domains. If you get into the nano range, there is only a coherent spin structure. Such tiny magnetic particles are of fundamental physical interest for numerous applications.
A ferromagnetic metal is characterized by the fact that the intrinsic angular momentum of the electrons, the spins, are all in one direction. This leads to the formation of a macroscopically measurable magnetic field outside the magnet. In general, a ferromagnetic metal is divided into areas with different directions of magnetization, so-called domains. This weakens the magnetic field that can be measured outside the sample. If the dimensions of the magnet itself become smaller than the width of the domain wall in which the transition from one spin orientation to the other takes place, one comes into the size range in which the magnet only has one domain or a coherent spin structure. In extreme cases, such a system can also consist of a complex molecule.
Such tiny magnetic particles, which often measure only a few nanometers, are of fundamental physical interest in terms of their magnetic properties. But there is also tremendous interest in the properties of these nanomagnets from the application side. Mention should be made here of the ever greater packing densities and the ever smaller structures on magnetic storage materials such as the computer hard drives of a PC or the new types of non-volatile data storage devices, which do not lose the stored data even when the power is switched off.
It is not easy to measure the magnetic properties of an individual nanomagnet, since conventional methods of magnetometry require much more material to examine than a nanomagnet contains. One possibility is to combine nanomagnets with nanostructured semiconductors. The Hall effect can be used to experimentally investigate the switching of a nanomagnet. In order to obtain a measurable signal, however, the dimensions of the crossing area of the Hall sensor must not be significantly larger than the dimensions of the magnet itself. In this way, the stray field of the tiny magnet can be measured as a function of an external magnetic field. A hysteresis curve is obtained that documents the flipping of the magnetization of the particle from north to south and characterizes the magnetic material.
With such arrangements, the current through a single semiconductor nanostructure can finally be controlled - depending on the spin of the electrons in the current! In principle, a transistor can be built whose function is based on the spin of the electrons as a switchable variable. In connection with the single electron transistor, this would bring the realization of the smallest possible switch within reach, which is based on the flipping of a single electron spin.
Triangular islands of iron atoms
Probably the most elegant way to create large numbers of nanomagnets or other nanostructures is to use natural self-organization in the manufacturing process. For this purpose, one uses the laws of nature, the material properties and the freedom given by the manufacturing parameters so skillfully that the nanostructures with the desired properties arise “by themselves”.
The image, which was taken with a scanning tunneling microscope, shows a largely regular arrangement of small triangles made up of ordered iron atoms and with an edge length of only three nanometers. These iron islands have formed independently in this form on a double layer of copper atoms, which was vapor-deposited onto the densely packed (111) surface of a platinum crystal.
Why are such nanostructures formed? In the case of the iron islands, the following happens: The lattice structure of the copper layer does not quite match that of the platinum, which is usually the case with hetero-layer systems. The bond between platinum and copper atoms, however, forces the latter to adopt the lattice structure of the platinum at least in the first layer. The first copper layer thus grows "tense". In the present case, this tension is already relieved in the second copper layer through the formation of dislocations. Dislocations are disturbances of the normal lattice structure and usually separate areas with an ideal lattice structure from one another. In the present case they run as lines through the copper layer and, due to their great mobility, form a regular triangular network. The triangular shape results from the special choice of the substrate surface. The size of the triangles is determined by the bond conditions and the lattice spacing of the materials involved. In the example in Fig. 2, an additional layer of iron atoms has now been vapor-deposited. The iron atoms move around on the copper surface, but are repelled by the dislocation lines. They therefore form islands within each triangle of the dislocation network, the size of the likewise triangular islands being determined by the amount of evaporated iron atoms.
The self-organization shown in this example could now be used, for example, to produce ferromagnetic structures with atomic dimensions (one nanometer corresponds to three to four atomic diameters), these structures being arranged regularly over large areas and almost the same size. It remains to be seen whether specific applications can be developed from this. For fundamental questions of nanostructuring, however, this route is definitely a highly interesting alternative.
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