Permanent magnetism of dense-packed nanostructures

Authors

Ralph Skomski, University of Nebraska-LincolnFollow
Yi Liu, University of Nebraska-LincolnFollow
Jeffrey E. Shield, University of Nebraska-LincolnFollow
George C. Hadjipanayis, University of DelawareFollow
David J. Sellmyer, University of Nebraska-LincolnFollow

Date of this Version

2010

Comments

Published in JOURNAL OF APPLIED PHYSICS 107, 09A739 (2010). Copyright © 2010 American Institute of Physics. Used by permission.

Abstract

The effect of nanostructuring on magnetostatic interactions in permanent magnets is investigated by model calculations. Emphasis is on the energy product as a function of packing fraction of the magnetic phase, of the magnet’s macroscopic shape, and of the nanoscale feature size. The main difference between nanostructured and macroscopic magnetic bodies, namely, the transition between coherent and incoherent reversal, has a far-reaching impact on demagnetizing field and energy product. For small magnet sizes, the energy product is substantially enlarged, up to μ₀M_s²4 for soft magnetic materials, but this effect is difficult to exploit in real devices. In bulk magnets, the energy product depends on the packing fraction f of the soft phase and exhibits a maximum μ₀M_s²12 for f =23. Nanoscale magnetization processes involve demagnetizing factors different from the macroscopic ones used to determine the optimum shape of permanent magnets. Confusion of these two types of demagnetizing fields yields unphysical mechanisms, such as hysteresis-loop overskewing and the addition of self-interaction fields to the external field.