Seeking square law torroid core

Hi,
We are seeking a "square law" ferrite torroid to use as a fluxgate sensor for imbalance current detection. For 50Hz mains electricity.
No manufacturer seems to make these.
I tried Far-rite, Mag-inc, Ferroxcube and TDK, but no joy. Plenty of high permeability (eg 5000), but no "square law".
We need OD= >35mm, ID = >15mm
Please Do you know of any?

Hi Flyback,

Many years ago I did some work for PNI studying the earth's magnetic fields. They used a square flux core. I don't find then in my junk core pile. I will look more.
I also have some "mag amps" from coilcraft. No longer being make. I do have some power cores built to sharply saturate. magnetic amplifier: coilcraft has removed all information they had.

Not much help.

Thanks, is "nanocrystalline" possibly a "square law" core (some versions of it)? I believe ferrite cant be square law?

What I have has a sharp saturation point. It is designed to go from high inductance to very low fast.
Try this
ebay
Tape Wound?

Not sure that they consistute "square law", but amorphous iron (metallic glass) cores have much lower coercitivity and higher permeability than most (all?) other materials.

Try these guys: https://mhw-intl.com/amorphous-and-nickel-iron-cores/
Or these:https://www.hilltech.com/products/power_components/amorphous_nanocrystalline_cores.html