Core iron barely boosting field strength

Question

I am a chemistry intern currently running a project that involves both chemistry and electromagnetism. Bottom line, I'm a little out of my depth and have much to learn, and I'm having a bit of trouble with the coils I constructed.

My setup will consist of four coils positioned horizontally around a microscope objective, and one underneath, such that the coils opposite eachother will create a homogenous field in the center, with a downwards gradient from the lower magnet. distance between opposite coils will be approximately 3 inches, so that's why I believed an iron core would be needed to achieve 15mT at the center.

I machined aluminum bobbins to act as a heat sink for the coil, as I'm fairly certain the aluminum shouldn't affect the field at all, although I do have delrin that I could use instead. For the cores, I bought some VIM VAR core iron (99.85-99.9% Fe, less than 0.01% C), machined them to press fit in the bobbins, and had the magnetically annealed in wet hydrogen to homogenize and maximize the relative permeability.

The relative permeability is supposed to reach as high as 15000 with the heat treating, imagine my surprise when the flux density was only 2.5x stronger with the core inserted. A current of 1.4A yields a flux density of 40 gauss at the mouth of the bobbin with air core, and 100 gauss with the iron core inserted.

My own calculations, as well as an online calculator I used from the company I bought the current amplifiers from, told me I should be easily able to achieve my desired field strengths, so this leads me to believe I either built something wrong, or was sold the wrong material.

Is this something that you guys would be able to help with, or is there a better forum for this?

EDIT: I did test several lower field strengths and a couple higher ones, the core isn't saturated, it is still in the linear region.

Answer 1

Keep the gaps around the objective to a minimum and link the outer ends of all the coils with your high permeability material.

You have to think of the magnetics as a circuit like an electrical circuit and to maximize the flux provide it with a low reluctance path as far as possible. As an example look at how a motor stator is constructed.

Answer 2

imagine my surprise when the flux density was only 2.5x stronger with the core inserted.

It's not a big surprise to me at all.

Any magnetic core material will barely concentrate flux in the presence of air gaps. The air gap will be the massively dominant factor in determining magnetic reluctance.

Yes, I can see your reasoning that adding a high permeability core material must significantly raise the flux by big amounts but, I'm sorry to disappoint you that this will not be the case.

EDIT: I did test several lower field strengths and a couple higher ones, the core isn't saturated, it is still in the linear region.

It will be nowhere near saturation in the presence of a big/significant air gap. The air gap is the weakest link of the chain when it comes to producing magnetic flux density.