ferrite powder/ magnetite powder difference

[reidabook1]

Hi
Please excuse my ignorance, I'm new to magnetic materials and no matter how much reading I do still can't get my head around a few things.
I'm wondering if anyone can clear up the differences in the magnetic properties of magnetite and ferrite powders for me?
Am I correct in thinking that ferrite powder has to be magnetized by applying an external magnetic field whereas magnetite powder does not?
i.e. if a remanence of 140-150mT for a ferrite powder is being quoted, is this after it has been magnetized in an external field?
Would it be possible to detect ferrite powder using a magnetometer if no external magnetic field had been applied first?
Cheers Chris

 

[IRstuff]

Magnetite is a naturally occurring iron oxide that may have been naturally magnetized
Ferrite is a different iron oxide usually with other materials

If the ferrite powder has not been previously magnetized, then a magnetometer will see only the distortion of the earth's magnetic field caused by the ferrite, but I would think that's a pretty small effect, unless you have a lot of ferrite powder. Even if it's been magnetized previously, unless all the particles were magnetized in-place, you'd probably still only see the field distortion and not a field directly from the powder.

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[EdStainless]

First you need to ask how they were made, natural or manufactured. And how the properties were measured. The testing is always done with some kind of applied external field, that is how you measure the resonse. What was the test frequency?
With that low of a remanence this is a magnetically soft ferrite, the kind that is used as cores or chokes. These are meant to nearly transparent to magnetic fields at some frequency, but not at others.
Because it is feromagnetic you will see it with a magnetometer, but how much depends on the frequency and field intensity. If both ate low then I doubt that you would see it.

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[MagBen]

Normally, you cannot detect a field from ANY magnet using a magnetometer if it is not magnetized. Earth's magnetic field is an enternal field, which could magnetize a soft magnet more or less, but the field is pretty weak (0.5 Gs). If you got a remanence of 140-150mT, it must be magnetized.

Ferorite magnet includes hard and soft ferrite. "ferri" means ferrimagnetic, vs ferromagnetic, so the magnetization is relatively low, whne compared with ferromagnetic materials. The remanence of a hard ferrite is normally larger than 200 mT, however, if it is a powder form, the remanence can be much smaller due to lack of anisotropy (possibly) or demagnetization factor.