# Which metals can cause magnetic interference (passively)?

I am developing an application that uses the magnetometer inside smart-phones to detect orientation w.r.t. the Earth's magnetic field.

I have noticed that when the phone is held close to a metal structure, such as Iron/Steel railings, there is a huge deflection in the magnetometer reading. There is no externally-induced current flowing through the structure; it has induced warped the magnetic field by its own.

My question is, which types of metals can show this effect? Or more generally which type of elements can show this effect?

I suppose Ferromagnetic materials do show this effect. And paramagnetic materials (like Aluminium) will show a very weak effect. What about diamagnetic substances such as copper?

Update Changed the title to mention passively. I understand that a copper coil forced to carry current would induce a magnetic field that interferes with the Earth's magnetic field. But does the coil distort the Earth's magnetic field when no EMF is applied?

Well, I think that whatever distortion of the magnetic field is caused by paramagnetic materials - such as aluminum - is also caused, with the opposite sign, by diamagnetic materials - such as bismuth (whose effect should be almost exactly opposite to that of aluminum).

I am convinced you may neglect all those materials. Only paramagnetic materials and various steels and similar metals etc. are relevant. See the relative permeability in this table

http://en.wikipedia.org/wiki/Magnetic_permeability#Values_for_some_common_materials

and just ignore all the materials whose $\mu/\mu_0$ is very close to one, on either side. The only materials that have the opposite but comparably important effects as ferromagnetic materials are superconductors whose $\mu$ vanishes. ;-)

Ferromagnets may sustain their field even if the external source disappears: this is probably the biggest source of interference. However, as you point out, even materials without any permanent field could potentially influence the field that you measure by the mobile device.

The main materials that interfere are ferromagnetic and ferrimagnetic, especially those based on iron, nickel, cobalt and manganese.

Maybe not answer but...

This problem should not cause only metals. Any DC or AC current in wires should cause magnetic field. And this magnetic field isn't constant because the power consumption of such device isn't constant. Also any unshielded coil for switcher power supply should cause magnetic interference.

• Strange! I wonder how this magnetometers in smartphones (with all that leads with AC and DC close) work. – Georg Mar 8 '11 at 21:26
• You are right about current in wires causing a magnetic field effect, but I am interested in materials that cause passive interference. Your coil example may not be correct if the coil is made of a diamagnet like Copper. – HRJ Mar 9 '11 at 4:45
• @Georg My estimate is that the interference caused by the phone's circuits is on the order of 1 nano Tesla, while the Earth's magnetic field is on average 45 nano Tesla. So, there are errors in measurement but they are surmountable. – HRJ Mar 9 '11 at 4:48
• @HRJ Working as compass, those magnetometers have to have a resolution much smaller than those 45 nT . The problems with AC and all the currents in the smartphone is solved rather easily: Just filter out all signal frequencies above say, 1 Hz. This is elementary in electronics. – Georg Mar 9 '11 at 8:48

In the case of the coil carrying current, the diamagnetic properties of the wire are a factor in the overall mass of magnetic material. But the greater effect are the electrons moving along the wire independent of the material of the wire. These electrons curling around generate a magnetic field. Wrap a copper wire in a helix and pass a DC current through it and you create an electromagnet.