4.4.1 Applications and Magnetic Materials

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 M_S, M_R, and H_C 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 M_R Small H_C 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 m_r = 0	Ni/Fe/Cu/Cr » 77/16/5/2
Hard Magnets
Permanent magnets	Loudspeaker Small generators Small motors Sensors	Large H_C (and M_R)	Fe/Co/Ni/Al/Cu »50/24/14/9/3 SmCo₅ Sm₂Co₁₇ "NdFeB" (= Nd₂Fe₁₄B)
Data storage analog	Video tape Audio tape	Medium H_C(and M_R), 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
Quantum devices	MRAM	Special spin structures in multilayered materials

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.

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