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General Overview |
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What are typical applications for magnetic materials? A somewhat stupid question
– after all we already touched on several applications in the preceding subchapters. |
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But there are most likely more applications than you (and I) are able to name. In addition,
the material requirements within a specific field of application might be quite different, depending on details. |
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So let's try a systematic approach and list all relevant applications together with some key
requirements. We use the abbreviation MS, MR, and HC for saturation, remanence, and coercivity, resp., and low w, medium w, and high w with
respect to the required frequency range. |
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| Field of application | Products |
Requirements | Materials |
| Soft Magnets |
Power conversion
electrical - mechanical |
Motors
Generators
Electromagnets |
Large MR
Small HC
Low losses = small conductivity
low w | Febased materials, e.g.
Fe + »(0,7 - 5)% Si
Fe + » (35 - 50)% Co |
| Power adaption | (Power) Transformers |
| Signal transfer |
Transformer
("Überträger") | Linear M - H curve |
| LF ("low" frequency; up to » 100 kHz) |
Small conductivity
medium w |
Fe + » 36 % Fe/Ni/Co » 20/40/40 |
| HF ("high" frequency up to » 100 kHz) |
Very small conductivity
high w | Ni - Zn ferrites |
| Magnetic field screening |
"Permalloy"
"Mu-metal" |
Large dM/dHfor H » 0
ideally mr=0 |
Ni/Fe/Cu/Cr »77/16/5/2 |
| Hard Magnets |
| Permanent magnets |
Loudspeaker
Small generators
Small motors
Sensors
| Large HC
(and MR) | Fe/Co/Ni/Al/Cu »50/24/14/9/3
SmCo5
Sm2Co17
"NdFeB" (=Nd2Fe14B) |
| Analog data storage | Video tape
Audio tape |
Medium HC (and MR),
hystereses loop as rectangular as possible
| NiCo, CuNiFe,
CrO2
Fe2O3 |
Digital data storage | Ferrite core memory
Drum |
| Hard disc, Floppy disc | | Bubble memory | Special domain structure |
Magnetic garnets
(AB2O4, or A3B5 O12), e.g.
with A= Yttrium
(or mixtures of rare earth), and B=mixtures of Sc, Ga, Al
Most common: Gd3Ga5O12 |
| Specialities |
| Quantum devices |
GMR
reading head | Special spin structures in multilayered materials |
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| MRAM (for recent developments
see also here) |
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As far as materials are concerned, we are only scratching the surface here. Some
more materials are listed in the link |
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Soft Ferromagnets |
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The general range of applications for soft magnets is clear from the table above.
It is also clear that we want the hystereses loop as "flat" as possible, and as steeply inclined as possible.
Moreover, quite generally we would like the material to have a high resistivity. |
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The requirements concerning the maximum frequency with which one can run through
the hystereses loop are more specialized: Most power applications do not need high frequencies, but the microwave community
would love to have more magnetic materials still "working" at 100 GHz or so. |
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Besides trial and error, what are the guiding principles for designing soft magnetic
materials? There are simple basic answers, but it is not so simple to turn these insights into products: |
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Essentially, remanence is directly related
to the ease of movement of domain walls. If they can move easily in response to magnetic fields, remanence (and coercivity)
will be low and the hystereses loop is flat. |
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The essential quantities to control, partially mentioned
before, therefore are: |
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The density of domain walls. The fewer domain
walls you have to move around, the easier it is going to be. |
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The density of defects able to "pin" domain walls. These are not just
the classical lattice defects encountered in neat single- or polycrystalline material, but also the cavities, inclusion
of second phases, scratches, microcracks or whatever in real sintered or hot-pressed
material mixtures. |
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The general anisotropy
of the magnetic properties; including the anisotropy
of the magnetization ("easy" and "hard"
direction, of the magnetostriction,
or even induced the shape of magnetic particles
embedded in a non-magnetic matrix (we must expect, e.g. that elongated particles behave differently if their major
axis is in the direction of the field or perpendicular to it). Large anisotropies generally tend to induce large obstacles
to domain movement. |
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A few general recipes are obvious: |
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Use well-annealed material with few grain boundaries and dislocations. For Fe
this works, as already shown before. |
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Align the grains of e.g. polycrystalline Fe-based material into a favorable
direction, i.e. use materials with a texture. |
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Doing this by a rather involved process engineered by Goss for Fe and Fe-Si alloys was a major break-through around 1934.
The specific power loss due to hystereses could be reduced to about 2.0 W/kg for regular textured Fe and to
0.2 W/kg for (very difficult to produce) textured Fe
with 6% Si (at 50 Hz and B » 1 T) |
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Use isotropic materials, in particular amorphous
metals also called metallic glasses, produced by extremely fast cooling from
the melt. Stuff like Fe78B 13Si 19 is made (in very thin very long ribbons) and used.
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Total losses of present day transformer core materials (including eddy current
losses) are around 0,6 W/kg at 50 Hz which, on the one hand, translates into an efficiency of 99,25 %
for the transformer, and a financial loss of roughly 1 $/kg and year – which is not to be neglected, considering
that a big transformer weigh many tons. |
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Reduce the number of domains. One solution would be to make very small magnetic particles
that can only contain one domain embedded in some matrix. This would work well if the easy direction of the particles would
always be in field direction, i.e., if all particles have the same crystallographic orientation pointing in the desired
direction as shown below. |
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This picture, by the way, was calculated and is an example of what can be done with theory.
It also shows that single domain magnets can have ideal soft or ideal hard behavior, depending on the angle between an easy
direction and the magnetic field. |
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Unfortunately, for randomly oriented particles, you only get a mix – neither here nor
there. |
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Well, you get the drift. And while you start thinking about some materials of
your own invention, do not forget: We have not dealt with eddy current losses yet, or with the resistivity of the material. |
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The old solution was to put Si into Fe. It increases the resistivity
substantially, without degrading the magnetic properties too much. However it tends to make the material brittle and very
hard to process and texture. |
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The old-fashioned way of stacking thin insulated sheets is still used a lot for
big transformers, but has clear limits and is not very practical for smaller devices. |
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Since eddy current losses increase with the square
of the frequency, metallic magnetic materials are simply not possible at higher frequencies – i.e., as soon as
you deal with signal transfer and processing in the kHz, MHz or even GHz region. We now need ferrites. |
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© H. Föll (MaWi für ET&IT - Script)
