Soft magnetic material for very low residual magnetism (low remanence) 1

[TJMAT]

Does anyone know what alloys / heat treatment would be good candidates for a solenoid core that is required to have very low residual magnetism (Br), high permeability and high saturation flux density (Bm)? The form of the core will be a long thin rod. The squareness (Br/Bm) is a good indicator of residual magnetism however it is not commonly provided by manufacturers of soft magnetic alloys. I believe that some Ni-Fe alloys (which exhibit low coercive force) can achieve near zero hysteresis for bipolar DC operation if properly annealed. My goal is to find an alloy that would yield a residual magnetism of 1% or less of the saturation flux. This is to insure that when the solenoid is de-energized from a fully saturated state that there is very little residual magnetism. I suspect that the long thin form of the rod will aid in this respect as the de-magnetization factor is relatively small. Seems that some of the nickel alloys have provided this performance however I cannot locate any information to confirm the performance and annealing used.

 

[EdStainless]

So you want high Br and low Hc.
There are a ton of these alloys out there.
Depending on the frequency of you signal you can select the correct alloy.
Any of the transformer lamination alloys would work.
For low frequency high purity Fe or Fe-Si alloys will work. There are even powdered metal grades used for some of this.
At higher frequency you start using Fe-Ni alloys.
If the field will switch very fast then you need good high frequency response.
You can find hysteresis loss curves for these magnetically soft alloys.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MagBen]

It is hard to obtain 1% of Bs, i.e. Br/Bs<1%. 10-20% is possible when you apply a magnetic field perpendicular to DOM during annealing. Or, apply a magnetic field along the DOM for something special alloy, say Fe-55%Ni, at some specifc annelaing temperature and time, the induced anisotropic constant is much less than magentocrystalline anisotropic constant. In both cases, however, the permeability will be sacrified.

Long thin rod will lead to the opposite direction you desired: with a higher Br. The reason is the same as you indicated, larger L/D will have a lower demag factor. but the result is a higher Br due to less demag.

 

[TJMAT]

EDStainless - Thank you for your assistance. I would like high Bs (don't laugh), low Br and high permeability (in order of importance). This usually results in a material that also has low Hc. The application is for an electromagnet than may be operated in the linear region (dipole moment proportional to current) or in the saturated region (maximum dipole moment). The intended operation is bipolar so the dipole moment direction can be in either direction dependent upon the direction of the excitation current. Operation is static (DC) and switching speed is not important. The design goal is to develop a magnet that can produce a moment of 500 A/m^2 at maximum rated linear excitation current (and a saturated moment of 600 A/m^2 at maximum excitation current). When the excitation current is reduced to zero the residual moment is desired to be less than 5 A/m^2. I've reviewed what I can find of the soft magnetic material data however Br is not always provided. Any suggestions for potential material sources would be appreciated. Seems like pure Fe, Ni-Fe, Fe-Si and stainless are candidates. Not so sure about the Fe-Co alloys as the Hc appears to be relatively higher than the Ni-Fe alloys. Are there any materials I can rule out. I need the material in rod form which would rule out he alloys that are only available in strip or sheet form. Cost is not a major concern for this application.

MagBen - Thank you for your assistance. You are right about the demagnetizing factor, I had its effect inverted. The long thin rod will have a small demagnetization factor which will only serve to slightly reduce the residual moment. What I have come to believe is that some reasonably common alloy coupled with a special annealing process will work. I believe that similar designs that provide residual moments on the order of 1% have used Ni alloys.

In general, is there a recognized order in the value of Br/Bs for the common soft magnetic materials? For instance, if Ni alloys are always better than Co alloys I can discard consideration of the Co alloys. Finally, does low carbon steel deserve consideration as a potential solution.

 

[MagBen]

in terms of Bs: FeCo>pure Fe>low C steel>FeSi> stainless steels > FeNi, He-50Ni can have higher Bs than stainless steel with high Cr.
in terms of perm: FeNi> FeSi > pure Fe > carbon steel > FeCo/stainless steel.

Br is not an intrinsic property. you can get a linear flat hysteresis loop for a rod simply by magnetizing perpendiculr to the length, the big shape anisotropy makes the magnetization a spin rotation process, rather than domain wall movement. the same principle applies when applying a magentic field perpendicular to magnetization direction during anneal, which leads to an induced anisotropy.

 

[TJMAT]

MagBen - Very interesting but this is at the pain threshold for my brain. So if I understand the spin rotation effect, the rod would be annealed while exposed to a transverse magnetic field (not sure how I would do this but lets move on). This would effectively magnetize the rod in the transverse direction. Then when an axial field was applied, the spin rotation effects would cause the alignment of the atoms dipole moment (electron spin would not be the primary mechanism, right?) resulting in temporary magnetization of the rod in the axial direction. When the applied axial field was removed, the atoms would return back to their previous orientation (or some non-aligned state) resulting in low residual magnetization along the axial direction but possibly with some residual magnetism in the transvers direction. The in-plane demagnetization of the cylinder is moderate acting to lower the transverse magnetic moment somewhat (maybe as much as 30%). Is this even close or do I need to find a consultant to explain it to me? What would happen when the rod was driven to deep saturation in the axial direction, would this erase the effect of the biased annealing? Finally, could I remove the transverse moment by alternating the direction of the magnetic bias along the rod during annealing (or perhaps using a helical magnetization pattern along the rod)? Thank you for your help and patience.

 

[MagBen]

From an applcation standpoit, you seemed to complicate the situation. Rotation over wall movement is just one of the physica reasons to expain why hysteresis loop can be linear, and Br can be lower. Trying to expain a magnetism phenomenon is never easy.
For a rod, if you magnetize it along the transverse direction, you would probably get a linear loop, no need annealing under an external field. But I donot know why people would use the rod material this way. I cannot imagine how you are going to use rods for solenoid applicaitons with magnetization direction perpendicular to the length.

For strip or sheet products, one of my projects was to apply an external field perpendicular to magentization direction (flux flow direction) such that the loop is linear, and so i got a constant permeability at some field range. The purpose was to decrease harmonic distortion.

Magnetic anneal can be very trick. It does not work for all types of magnetic alloys. For a same alloy when it works, even applied a field at the same direction as flux flow direction, one can obtain either a square loop, or a flat loop, depending on the annelaing temperature and time.

 

[TJMAT]

I guess that there are not many simple answers. Maybe I'm looking at the problem all wrong. I am developing a solenoid to produce a dipole moment that will result in a torque when immersed in an external field. A saturable ferromagnetic core is used to minimize the power required to develop the dipole moment. When the solenoid is not energized it is desired to have as small a residual torque (and I assume as small a residual dipole moment) as practical. Various soft magnetic materials have been used (iron, Ni-Fe, Stainless, Hiperco, etc.) and result in a solenoid that has very low residual torque (1% of the torque at saturation). I've assumed that this is due to the annealing process used to prepare the core material. Am I missing something here and is there some other phenomena that acts to reduce the magnetic dipole moment when the solenoid is not energized that I'm not properly accounting for? As you mentioned previously, a flat hysteresis loop would be the type needed for this type of operation (see attachment). So how do you achieve a flat hysteresis loop?

 

[hacksaw]

Ben, good luck with this one, the irreconcilable needs and perceived solutions defy explanation

seems that the op needs to be more specific about what functionality is being sought , clearly not just a materials issue

 

[MagBen]

1% torque doensot mean Br/Bs=1% due to 1. torque is proportional to B^2, 2. it is kind of open circuit, residual magnetization < Br. Also, the total usable Bs = Bs from rod + B from the coil(wire), the latter may be negligible if N.I is small.

I am not aware there are obvious effects of magnetic annealing on pure Fe, stainless steel. Combining cold roll percentage and annealing temperature, we can make anisotropic (more square loops) Fe-Ni and Fe-Si. You may try to anneal Fe-(50-55)Ni rod with the present of an external DC field (10-20Oe) perpedicular to length (temp needs < Curie point) to induce an anisotropy transverse to the rod. A simple while feasible way may be applying a small reverse current to demag the rod. Note it is almost no way to fully demag the rod, but since the reverse current is such small, even a reversly magnetized field can be small.

 

[TJMAT]

MAGBEN, maybe HACKSAW is correct, perhaps my version of the problem statement is misleading. In simple terms (the only kind I can even hope to master) I am developing a linear electromagnet (a solenoid around a rod core). The purpose of the electromagnet is to produce a magnetic dipole moment that is dependent upon the excitation current applied. The dipole magnitude is approximately linear up to saturation where it levels off. The direction of the dipole is dependent upon the direction of the excitation current. When the electromagnet is active within an external magnetic field a torque will result that is the cross product of the dipole moment with the Bext field. The same principle as a compass needle in the Earth's field. These electromagnets have been built using stainless steel (430F), high purity Fe, Ni-Fe alloys and I believe Fe-Co alloys. These electromagnets have demonstrated linear torque performance with respect to excitation current to better than 1% with respect to the torque at saturation (its not clear that all of these core materials can achieve low residual torque however the Ni-Fe and Fe-Co have demonstrated low residual torque). This means that at zero excitation current the residual torque is less than 1% of the torque at saturation. The details of the construction of these electromagnets are not available however special annealing is noted as the reason for the demonstrated performance. So if a circular rod made of a ferromagnetic material were driven to saturation by an enclosing solenoid and then the excitation current was reduced to zero (but not below zero), the residual torque (in an external field ) would be less than 1% of the torque at saturation. The torque at zero current is due only to the residual moment of the rod since the solenoid is at zero current. I believe that your suggestion for anneal under orthogonal magnetic bias has merit. Modification of the B-H curve of the core material toward a flat shape (as opposed to to a square loop) will reduce the hysteresis (improve linearity and minimize residual magnetism). I'm sure what you are referring to when you note that torque is proportional to B^2, isn't it proportional to the dipole moment given a fixed external field. Do you know of any services that can anneal under magnetic bias? THANK YOU for your assistance!

 

[MagBen]

Try Arnold Magnetic Technology at Marengo IL facility. They do a lot of magnetic annealing for AlNiCo, but i am not sure if they are willing to do this for you except for their production.