Tag: Work

“ROMEO” project featured on TV news

Our EU-funded FP7 project “Replacement and Original Magnet Engineering Options” (ROMEO) has recently been covered in the news in Slovenia, to promote the participation of our Slovenian collaborators.

News footage of the ROMEO project

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There is also now a project newsletter highlighting the aims and progress of the project. Please click here to download it.

 




[Paper] “Thermally activated coercivity in core-shell permanent magnets” published in vol. 117 of Journal of Applied Physics.

Our new paper “Thermally activated coercivity in core-shell permanent magnets” has been published today as J. Appl. Phys. 117, 17A733 (2015); http://dx.doi.org/10.1063/1.4916542 . In the paper we use numerical miromagnetics to calculate the performance of nanostructured core-shell-like permanent magnets, like the type that can now be produced by grain boundary diffusion of granular hot-deformed or sintered rare earth permanent magnets.

Figure 3 - FIG. 3. Reversal processes in the sin- gle grain models with (i) a pure NdFeB grain, (ii) a NdFeB grain with a soft outer defect, and (iii) NdFeB core, (Dy, Nd)FeB shell and an outer soft defect. Thermally activated coercive field values are indicated with the field direction (red arrows). The saddle point image is the configuration with the highest total energy, forming the peak of the energy barrier.

FIG. 3. Reversal processes in the sin- gle grain models with (i) a pure NdFeB grain, (ii) a NdFeB grain with a soft outer defect, and (iii) NdFeB core, (Dy, Nd)FeB shell and an outer soft defect. Thermally activated coercive field values are indicated with the field direction (red arrows). The saddle point image is the configuration with the highest total energy, forming the peak of the energy barrier.

The paper is free online for 30 days, after which the pre/re-print version will still be available here.




[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




New paper: “Grain-size dependent demagnetizing factors in permanent magnets”

Our new paper “Grain-size dependent demagnetizing factors in permanent magnets” has been published in Journal of Applied Physics (JAP). http://dx.doi.org/10.1063/1.4904854

UPDATED UPDATE: an updated reprint version that should be better for Google Scholar crawling is now available here

 

Abstract: The coercive field of permanent magnets decreases with increasing grain size. The grain size dependence of coercivity is explained by a size dependent demagnetizing factor. In Dy free NdFeB magnets, the size dependent demagnetizing factor ranges from 0.2 for a grain size of 55 nm to 1.22 for a grain size of 8300 nm. The comparison of experimental data with micromagnetic simulations suggests that the grain size dependence of the coercive field in hard magnets is due to the non-uniform magnetostatic field in polyhedral grains.

The article is free to download for 30 days, after which it will be available to journal subscribers. At that time I will post a reprint version on this site, which will be indexed by Google Scholar and other search engines.

Grain size paper: Fig. 5 Demagnetizing field of a uniformly magnetized cube evaluated at a distance of d =1.2Lex from the edge. Solid line: Component perpendicular to the easy axis. Dashed line: Component parallel to the easy axis.

Grain size paper: Fig. 5 Demagnetizing field of a uniformly magnetized cube evaluated at a
distance of d =1.2Lex from the edge. Solid line: Component perpendicular
to the easy axis. Dashed line: Component parallel to the easy axis.