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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 lets 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 |
Fe based 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 |
"Mu-metal" |
Large dM/dH for H » 0
ideally mr = 0 |
Ni/Fe/Cu/Cr » 77/16/5/2 |
| Hard Magnets |
| Permanent magnets |
Loudspeaker
Small generators
Small motors
Sensors
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Large HC (and MR) |
Fe/Co/Ni/Al/Cu »50/24/14/9/3
SmCo5
Sm2Co17
"NdFeB" (= Nd2Fe14B) |
| Data storage
analog |
Video tape
Audio tape |
Medium HC(and MR),
hystereses loop as rectangular as possible
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NiCo, CuNiFe,
CrO2
Fe2O3 |
| Data storage digital |
Ferrite core memory
Drum |
| Hard disc, Floppy disc |
| Bubble memory |
Special domain structure |
Magnetic
garnets (AB2O4, or
A3B5O12), 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 |
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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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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
Fe78B13Si19 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 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
