Monthly archives: January, 2015

[Paper] Micromagnetics for the coercivity of nanocomposite permanent magnets

Our paper titled “Micromagnetics for the coercivity of nanocomposite permanent magnets” has been published in the proceedings of the 23rd  International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2014). The proceedings were not made available to the public but we are providing a PDF reprint here.

The work was presented by Johann Fischbacher on 19th August 2014 in Annapolis, Maryland.

Abstract:

Exchange spring permanent magnets may be a route towards high energy product permanent magnets with low rare-earth content. In composite magnets soft magnetic phases act as nucleation sites for magnetization reversal. We use micromagnetic simulations in order to understand the role of the size and shape of the soft inclusions on the magnetization reversal. We compare the switching field of magnetically soft spheroids, cuboids and cylinders embedded in a hard magnetic matrix. Whereas there is only little difference in the switching field for enclosed spherical or cubical soft shapes, prolate inclusions enhance the stability of the magnet.

Fig1

Fig. 1. Switching field of Nd 2 Fe 14 B cubes and
spheres with volume V

Fig2

Fig. 2. Switching field of alpha -Fe cubes (solid line)
and spheres (dashed line) with equal volume V s
in a Nd 2 Fe 14 B spherical shell. r denotes the
ratio of hard to soft magnetic volume.

Fig3

Fig. 3. alpha -Fe cubes (solid line) and spheres
(dashed line) enclosed by a 1 nm interlayer in a
Nd 2 Fe 14 B spherical matrix. The interlayer ex-
change constant A_i =fA_hard is reduced to decou-
ple inclusion and shell. Open markers refer to
the soft phase reversal field and filled markers
to the hard phase switching field.




[Paper] Enhanced Nucleation Fields due to Dipolar Interactions in Nanocomposite Magnets

Image from the paper

Magnetic reversal process: The pictures show the magnetic flux lines. The color denotes the magnetization direction (red: magnetization up, blue magnetization down).
The gap between the soft magnetic spheres (d incl = 8 nm) is 1 nm in the first two columns and 4 nm in the third column. The external field is applied in z-direction and its value is written next to each picture.
In the first column the soft magnetic inclusions are aligned perpendicular to the applied external field. The interaction with the outside inclusions is weakening the central sphere and forces it to switch first.
In the second and third column the soft magnetic spheres are aligned in a parallel manner to the applied external field. The two outside spheres reinforce the central one and therefore nucleation should not start in the center. But for gaps smaller than 4 nm a strong demagnetizing field in the location of the central sphere caused by the shell diminishes the strengthening effect due to dipolar interaction.

Our paper titled “Enhanced Nucleation Fields due to Dipolar Interactions in Nanocomposite Magnets” was presented by first author, Johann Fischbacher, at the JEMS 2012 conference and subsequently published in the The European Physical Journal B.

We are now making a PDF preprint of the resulting paper available here. The paper can be found on the journal webpage here.

Abstract:

One approach to construct powerful permanent magnets while using less rare-earth elements is to combine a hard magnetic material having a high coercive field with a soft magnetic material having a high saturation magnetization at the nanometer scale and create so-called nanocomposite magnets. If both materials are strongly coupled, exchange forces will form a stable magnet. We use finite element micromagnetics simulations to investigate the changing hysteresis properties for varying arrays of soft magnetic spherical inclusions in a hard magnetic body. We show that the anisotropy arising from dipolar interactions between soft magnetic particles in a hard magnetic matrix can enhance the nucleation field by more than 10% and strongly depends on the arrangement of the inclusions.Fischbacher et al., “Enhanced Nucleation Fields due to Dipolar Interactions in Nanocomposite Magnets”, Eur. Phys. J. B (2013) 86: 100
DOI: 10.1140/epjb/e2013-30938-1