Introduction to Nanoscience and Nanotechnology. Chris Binns

Introduction to Nanoscience and Nanotechnology - Chris Binns


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source of magnetism in materials is their constituent atoms, which consist of tiny permanent dipolar magnets whose strength is given by the magnetic moment1 of the atom (see Advanced Reading Box 1.1). In a material such as Fe, there is a strong interaction (the exchange interaction) between the atoms that line up the atomic magnets to produce a macroscopic magnetization. Note that the exchange interaction is a quantum mechanical effect and is not the normal interaction that you would see between two bar magnets, for example. For one thing, the interaction between bar magnets aligns them in opposite directions and for another, the exchange interaction is thousands of times stronger than the direct magnetic interaction.

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      The individual atoms of most elements have a permanent magnetic moment, so they generate a dipolar magnetic field similar to a simple bar magnet. The source of the atomic magnetic moment is twofold. It arises from the orbital motion of the electrons around the nucleus, which can be considered to constitute a simple current loop and also from the intrinsic angular momentum (spin) of the electrons. These two contributions generate an orbital and a spin magnetic moment and, for the elements Fe, Co, and Ni, the two contributions are simply added to obtain the total magnetic moment. The exchange interaction that acts between neighboring atoms arises from the Pauli exclusion principle. This tends to keep electrons apart if they have the same spins so that the Coulomb repulsion energy between the outermost electrons of neighboring atoms is reduced if the electrons align their spins in the same direction. This appears as a very strong magnetic interaction trying to align the spin magnetic moments, but it is an electrostatic effect produced by the quantum nature of the electrons. It is typically 3–4 orders of magnitude stronger than the direct magnetic interaction of the atomic magnetic moments assuming they are simple bar magnets.

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      The intelligence of evolution is highlighted here. If the particles are single‐domain particles, then they will stay magnetized forever, so forming a string of these ensures that the navigation system will naturally work. If the bacterium formed a single piece of the material the same size as the chain of particles, a domain structure would form and it would become magnetically dead. The nanoparticles are composed of magnetite (Fe3O4) rather than pure Fe but the argument is the same. There is currently research devoted to persuading the bacteria to modify the composition of the nanoparticles by feeding them


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