8 - Nanoscale magnetism

Summary

Nanoscale magnets have at least one dimension in the nanometre range. They exhibit size-specific magnetic properties such as superparamagnetism, remanence enhancement, exchange averaging of anisotropy and giant magnetoresistance when the small dimensions become comparable to a characteristic magnetic or electrical length scale. Thin films are the most versatile magnetic nanostructures, and interface effects such as spin-dependent scattering and exchange bias influence their magnetic properties. Thin-film stacks form the basis of modern magnetic sensors and memory elements.

Matter behaves differently down in the nanoworld, where the length scales of interest range from about 1 nm up to about 100 nm. The atomic-scale structure of matter can usually be ignored, but the mesoscopic dimensions of the magnetic nano-objects are comparable to some characteristic length scale, below which the physical properties change. We have already encountered one important nanoscale object in bulk magnetic material – the domain wall. It is extended in two directions, but not in the third; the domain wall width δ_w is one of the characteristic lengths that concern us here.

The number of small dimensions in a nanoscale magnet may be one, two or three. Some examples of each are illustrated in Fig. 8.1. The one-small-dimension class includes magnetic thin films, which are at the heart of many modern magnetic devices. Magnetic and nonmagnetic layers can be stacked to make thin-film heterostructures, such as spin valves and tunnel junctions. The films are usually grown on a macroscopic substrate.