Permanent magnetism of dense-packed nanostructures

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 0 M s 2 / 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 0 M s 2 / 12 for f =2/ 3. 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.

Predicting the Future of Permanent-Magnet Materials

“…However, this reduces the coercivity, and the interplay between magnetization increase and coercivity decrease yields a volume fraction at which the energy product reaches a maximum. For highly idealized alnico, this analysis yield [40]. What would happen if there was some anisotropy associated with the predominant (001) [40], whereas the dashed line shows the effect of a positive .…”

Are there superspin glasses?

“…This contribution reduces the overskewing, because it amounts to a replacement of the demagnetizing factor D by D -1/3. The problem is multifaceted and includes a variety of issues, such as the distinction between the hard-demag field and well-known cavity fields, the striking involvement of self-interaction fields, 27 and the definition of the demagnetizing fields. The last must be done by properly taking into account that demagnetizing fields in extended perfect structures such as thin films and spheres, which are used for comparison, contain a curling-type selfinteraction field.…”

Ultrahard magnetic nanostructures

“…6 To investigate the effect of the orientation of the magnetic field (perpendicular and in-plane) on the hysteresis loop and dynamics of the magnetization, we performed micromagnetic simulations using the Nmag software package. 9 The system is modeled as a bilayer of FePt and Fe in a 50 Â 50 Â 22 nm 3 Figures 1 and 2 shows the spin structures for the Fe part of the Fe/FePt nanocomposite and the hysteresis loops for both field directions. In perpendicular fields, normal to the film plane and parallel to the c-axis, there is an abrupt drop of the soft phase's magnetization contribution at the softphase nucleation field H n .…”

“…1,2 This consideration is important for bulk applications, for example, in cars where magnet weight and volume matter. However, the magnet volume is not the main consideration in small-scale nanostructures 3 and in thin films for MEMS applications, and the question arises whether the energy product remains a valid figure of merit.…”