Theory of the Heisenberg Superparamagnet

The concept of uniform bulk magnetization in superparamagnetic particles is examined critically. By a straightforward idealization of the conditions met in actual superparamagnetic samples, it is shown that this concept is related to a fundamental problem of the physical statistics of magnetic first-order phase transitions. Thus, the fact that the concept is applicable to experimental data on superparamagnets has far-reaching consequences with respect to the nature of the magnetic transition at zero field.