Magnetic Materials?

[Jowik]

I have a couple questions about magnetic materials. 

Which materials exhibit different forces when near a magnetic field?  I know for example, some materials become magnetized like iron, and some materials are more permeable but don't magnetize, both are magnetic.  I assume, based on the different properties as this, that some materials will be attracted to a magnet stronger than other materials.  Which is the strongest known material(s)?  i.e. in comparing materials of the same size, and a magnet is placed near them in separate instances, which material would be drawn to the magnet from the furthest distance?

Thanks,

John

[PaulLowrance]

Hi John,

Probably the largest factor according to your question would be the shape of the magnetic material. The shape greatly changes effective permeability. As a simple rule, the effective permeability is equal to a k-factor * max permeability * length / diameter.  For example, consider material that has permeability of 5000, but its in the shape of a rod that is 10 cm long and 1 cm wide. The effective permeability for such a rod is ~63.  That's a long way from 5000. Yet if that same material was closed such as a toroid then the effective permeability is 5000. The term "permeability" by itself is referring to maximum effective permeability.  BTW, the effective permeability of the same 10m x 1cm rod for material that has maximum permeability of 100 is ~41.  As you can see, the effective permeability only went from 63 to 41 even though the material has 50 times less permeability.

Now if the shape is a horseshoe rather than a rod then the effective permeability drastically increases.

The amount of force between a PM (permanent magnet) and your material changes with effective permeability. So if the effective permeability doubles then the pulling force will also double.

Hysteresis and coercivity are another consideration. Hard iron for example has high coercivity while soft iron has low coercivity.  Highly pure iron can have a permeability over 1 million, but it has extremely low coercivity.  Hard iron has low permeability, but high coercivity.  All magnetic materials have coercivity. So in that sense all magnetic materials are PM's, even pure iron. What makes a good PM is high coercivity.

Another factor is heat. Most materials are not at peak permeability at room temperature. So generally the initial permeability increases as you increase the temperature, but something most people do not know is that the higher levels of permeability decrease. What I term the Hysteresis squatness factor decreases with temperature. So as the material heats up it's initial permeability might be increasing but it's over all permeability is actually decreasing. Also the saturation lowers as we increase the temperature. Hysteresis also lowers (improves) with temperature.

I think certain Metglas cores are a wonder material. Naudin did not use the best Metglas core. Some of these cores get in the million permeability range. It has high saturation, high frequency, high permeability, low Hysteresis. All of this spells strong attraction to a PM. Of course, if there is no PM, then one piece of Metglas won't really be too attracted to another piece of Metglas.

Extremely pure iron is even better according to you description, but both iron and Metglas are electrically conductive. So if you have fast changing magnetic fields then you'll have Eddy currents unless you use something like a tape wound core.  Although Metglas cores are better at higher frequencies then pure iron mostly due to their nano size domains.  These cores are ferromagnetic materials. Magnetite is a great ferromagnetic material. I believe it's permeability is around 25000, but it is non-electrically conductive. Magnetite works at very high frequencies, has low hysteresis and coercivity.

As for PM's, it's hard to beat neo magnets in most cases.

Yet there's always superconducting magnets.

In short, I'd say extremely pure Iron could be your best bet, but it must be extremely pure. Such iron has appreciably higher saturation than even Metglas, which will play a big factor in your magnetic attraction. If you are going to be working with such materials and need custom shapes and such then it might pay off to build equipment to make your own pure iron. You'll have to consult with some of the chemists too see how expensive this could be. Most chemists will probably give you a price number because they just buy equipment. I know there are method of making your own equipment really cheap. I've seen serious articles on using a microwave as a heating source. I know, microwaves and metals???  It's a little more complicated and they have a fancy setup of various types of materials that make it possible.

Hope this helps,
Paul Lowrance

[Jowik]

Hi Paul,

Thanks for the information.  It helps a bit.  I'll get to the point, I have a concept design for a toy that I've posted in the member section of the  GlobalTRB.org (The Global Technology Review Board) website and I've mentioned about it before in this forum as it uses Steorn's LEMA.  The toy does not generate electricity, nor is it designed to.

The toy in principle, needs magnetic material which will react to a magnet with the maximum possible force towards the magnet, the current design considers the shape of a ball, as rolling can be useful to reduce friction in this case, but design considerations could use a torus if that will result in a greater force regardless of friction.  Constant physical impact is a consideration, such that the material should not be brittle.  I suppose the best way to find the right material would be to purchase them or make an assortment of materials with different types and test them, but I don't even have a list other materials than the normal magnetic materials such as iron, netic, and Metglas (which to me by the name of it sounds brittle) .  If the best material is brittle, it must demonstrate the principle in a sufficiently long period of time.

Cheers,

John

[PaulLowrance]

Hi John,

The exact answer to your question really depends on the design.

1. How close will the PM come to the magnetic material?  If it comes close then you might need high saturation material such as pure iron as mentioned.

2. Does this need to generate "free energy?"  If yes, then the question suddenly becomes 1000's times more complex. If not then perhaps both objects could be PM's. There's no pulling force like two PM's unless you have a PM very close to iron or best yet ultra pure iron.

3. How expensive and elaborate can it be? If $ is not a big issue than it would pay to have ultra pure iron or if you want both objects to be PM's then use Neo magnets.


One thing is for certain is hard iron such as steel in all likely hood would not be your best choice unless you need to cut your costs. Ingot iron might be a moderate choice that's pure, but I don't think it's ultra most pure iron.

If you are concerned about Eddy currents then Magnetite is a good choice, but it's a hard and brittle material. You should have no problem buying a big chunk of magnetite real cheap at one of the nugget hunting (metal detecting) forums.

If the remanence from hard steel is too much and ultra pure iron is too costly then try ingot iron. If that does not work because of Eddy currents then I only know of the brittle ferrites such as Magnetite. You could always coat the Magnetite.

Paul Lowrance