Tag: <span>Research</span>

[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.




Paper “Hard Magnet Coercivity” published in proceedings of REPM2014

This August Prof. Dominique Givord of Institut Néel – CNRS presented our paper titled “Hard Magnet Coercivity” during the 23rd International Workshop on Rare earth and Future Permanent Magnets and Their Applications (REPM2014) in Annapolis, Maryland.

The manuscript was included in the conference proceedings and we would now like to make the reprint available to the wider public: Please click here for the PDF file.

Abstract: Based on a critical analysis of the experimental coercive properties, general considerations on the reversal mechanisms in RFeB magnets are recalled. By plotting together the experimental parameters obtained in various magnets, common features of the reversal processes are demonstrated. Modeling provides an almost quantitative description of coercivity in these materials and permits connecting the defect characteristic properties to reversal mechanisms.

 

Annapolis, Image in Public Domain

Annapolis, Image in Public Domain




“Battenberg” structured magnets, new paper published in APL

Our new, cake-themed paper on nanostructured permanent magnets has now been published in Applied Physics Letters.

In the paper we present results from micromagnetic simulations that assess the performance of multi-phase nanostructured permanent magnets, whose cross-section resemble that of a Battenberg cake. By including a super-hard outer shell we are able to counteract the effects of thermal fluctuations and surface defects, both of which are detrimental to the performance of such permanent magnets.  Such magnets are important for the motors in electric vehicles and for the generators in wind turbines, and these machines usually operate at elevated temperatures.

Click here to download the author-formatted reprint (free) or here for the official, journal-formatted version ($).

A home made Battenberg Cake, end view, with a slice leaning on it [Creative Commons Attribution 3.0 Unported – Author:Henrycooksey, Wikimedia Commons]

In case you are wondering, the soft cores in our magnet are like the sponge, the interdiffusion layer is like the jam-sponge mix you get at the surfaces of the sponge, the hard matrix is the jam and the super hard shell is like the marzipan. The difference is that our model has 3D periodicity while a typical Battenberg cake has only 2D periodicity, like a checkerboard prism. Battenberg cakes can be made in a number of different configurations, and so can Battenberg magnets! Stay tuned for more cake-inspired magnetic nanostructures in the future!

Proposed microstructure of a nano-composite per- manent magnet. Each grain is composed of soft magnetic inclusions embedded in a hard magnetic phase. Both phases are coupled through a thin inter-diffusion layer with proper- ties that may be different from either of the two phases. A grain may have a super-hard shell with an anisotropy field exceeding that of the hard magnetic matrix phase. At the surface a ferromagnetic layer with small or zero anisotropy is assumed (defect layer).

Fig. 1 Proposed microstructure of a nano-composite per-
manent magnet. Each grain is composed of soft magnetic
inclusions embedded in a hard magnetic phase. Both phases
are coupled through a thin inter-diffusion layer with proper-
ties that may be different from either of the two phases. A
grain may have a super-hard shell with an anisotropy field
exceeding that of the hard magnetic matrix phase. At the
surface a ferromagnetic layer with small or zero anisotropy is
assumed (defect layer).

Left: Data and fit of the energy barrier of a single grain as function of the applied field without the super-hard shell (hollow diamonds and dashed line) and with (filled circles and solid line). Right: B(H) loops for a single grain at 450 K. The circles indicate the working point where the energy product reaches its maximum. Dashed lines: Fe 65 Co 35 /Nd 2 Fe 14 B, solid lines: Fe 65 Co 35 /Nd 2 Fe 14 B/Sm 2 Fe 17 N 3 .

Fig. 2 Left: Data and fit of the energy barrier of
a single grain as function of the applied field without
the super-hard shell (hollow diamonds and dashed line)
and with (filled circles and solid line). Right: B(H)
loops for a single grain at 450 K. The circles indicate
the working point where the energy product reaches its
maximum. Dashed lines: Fe 65 Co 35 /Nd 2 Fe 14 B, solid lines:
Fe 65 Co 35 /Nd 2 Fe 14 B/Sm 2 Fe 17 N 3 .