Figure 5.5: Hysteresis loop for a randomly oriented array of SW particles. |
An array of non-interacting magnetic particles with a random distribution of anisotropy axes is a crude model for a real polycrystalline magnet. The hysteresis loop of such magnet is given in Fig. 5.5. The remnance for the array of particles is ½MS and the coercivity is 0.482 H a . If the anisotropy directions are distributed at random within the place, a case similar to particulate recording media, the remnance is 0.637 MS (=2/π) and the coercivity is 0.508 H a .
Another possible relation (Henkel plot) between two remnance curves for the system of non-interacting particles was pointed by Wohlfarth. The remnance on the initial magnetization curve, Mr , is obtained by applying a field H to the virgin state, and reducing it to zero. The remnance Mrd is obtained in a reverse field after saturating the magnetization. They are related by
|
(5.3) |
The deviations from a linear plot of Mri versus Mrd, known as Henkel plot, are the evidence for the existence of finite interacting particles.
Quiz:
(1) What is soft and hard magnetic maetrials?
(2) What is Henkel plot? How does it useful to study the interaction between the particles?
(3) Show that the coercivity of an ensemble of SW particles aligned with a common easy axis varies as 
?
(4) Who does the Stoner-Wohlfarth particles exhibit square hysteresis loop along the easy-axis direction?