4.4 Applications of Magnetic Materials

4.4.1 Everything Except Data Storage

General Overview

What are typical applications for magnetic materials? A somewhat stupid question - after all we already touched on several applications in the preceding subchapters.
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.
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.
 
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
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
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
MRAM
 
As far as materials are concerned, we are only scratching the surface here. Some more materials are listed in the link
Data storage is covered in a separate module, here we just look at the other applications a bit more closely.
 

Soft Ferromagnets

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.
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.
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:
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.
The essential quantities to control, partially mentioned before, therefore are:
The density of domain walls. The fewer domain walls you have to move around, the easier it is going to be.
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.
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.
A few general recipes are obvious:
Use well-annealed material with few grain boundaries and dislocations. For Fe this works, as already shown before.
Align the grains of e.g. polycrystalline Fe-based material into a favorable direction, i.e. use materials with a texture.
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)
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.
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.
 
Hystereses of single domain
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.
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.
Unfortunately, for randomly oriented particles, you only get a mix - neither here nor there.
 
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.
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.
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.
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.
 
 
Questionaire
Multiple Choice questions to 4.4.1
 
 

To index Back Forward as PDF

© H. Föll (Electronic Materials - Script)