MAG3110 - Long term effect in strong magnetic field

[Deejay]

Hi,

It's a very generic question.

Any idea what effect a really strong magnet would do long-term to a co-located magnetometer (e.g. MAG3110 or any magnetometer in general)?

Just interested to hear if anyone may have observed problems with any potential magnetometer pre-configuration if used for a long period in strong fields.

Thanks.

Devan

[TomasVaverka]

Hi Devan,

Freescale uses a magnetic sensor technology called "TMR" (Tunnelling Magneto Resistive). The native sensors for this technology are "in-plane". These handle X and Y portions of the 3D vector naturally. In order to get "Z", we utilize a flux-guide which redirects Z axis field such that it can be sensed with an in-plane sensor. If the magnetic field exceeds specifications (10 mT), that flux-guide will retain some residual magnetism (your offset) when you take the original source away. This is known as "perming". If you reapply the opposite side of the same magnet, you will probably see the offset shift a similar amount the other side of the baseline. There is no internal mechanism to reverse flux guide perming.

There is a second physical mechanism that can apply to all axes, as it affects the free layer of the MJT. The maximum field here is the maximum range of the sensor (1 mT for the MAG3110). There is a software reset bit (AUTO_MRST_EN) which is recommended before every read cycle that can reset the MJT.

Regards,

Tomas

[adamelias]

Hello Tomas,

On a related note, where does the 100,000µT maximum (100mT) come from in the datasheet?

What will happen if a magnetic field is applied greater than this value? Does this effect the X, Y and Z axis the same way?

Will powering down the device prevent perming or is it a material property of the flux guide.

I've witness the perming you mentioned above in the Z direction on some MAG3110 sensors and it does seems to be permanent until the polarity of the interrogator magnet changes.   Is perming in the X and Y direction as impactful as the Z direction?

Thanks,

ADam

[michaelestanley]

Adam,

I'll take a stab at your questions, as I discussed these topics some time ago with our local magnetics expert.  But before that, please understand, NXP does NOT guarantee operation beyond maximums in our datasheets.  Discussion of the background beyond those specifications in these forums is FYI only, and does not constitute NXP relaxing those specs for a given application.   Now that I've got that out of the way:

There are THREE different mechanisms in play on these sensors:

• The integrated "software reset" resets the MJT (Magnetic Junction Transistor) free magnetic layer, and we recommend that it be applied before each measurement.  This applies to all channels
• The Z-axis flux guide was mentioned above.  If you exceed the operating range of the device (+/-1000 microTeslas for the MAG3110), this becomes an issue.  There is no mechanism within the device to correct for any perming effects in the flux guide.  As mentioned above, it can be reversed with a large magnetic field in the opposite direction.  We refer to this as a "magnetic reset".  But it is not something I expect the average user can do in practice (at least in any kind of precise fashion).   The 10K microTesla spec in the datasheet refers to this perming effect.
• The 100K microTesla number refers to the point at which the "pinned" layer in the MJT is corrupted.  When that happens, the part is toast and needs to be replaced.  This applies to X, Y and Z channels.

All of the above apply whether the part is powered or not.

I hope that helps.

Mike