Puzzled by magnetized aluminum!

Question

For a school project I bought a set of magnets and an aluminum strip. The magnets are places along both sides of the aluminum strip (all in the same direction). Now the aluminum strip is exhibiting magnetization. I was under the impression that aluminum cannot be magnetized. What is the explanation? The strip seems to have opposite polarity vs the magnets near it.

Answer 1

Most practical aluminum alloys are significantly paramagnetic. For instance, if you try to use a permanent magnet to search for a lost steel nut in aluminum machine tool chips ( such as from aluminum automotive wheels after drilling the bolt holes ) the smaller machine tool chips will actually smother the permanent magnet and align with the magnetic field. The result was that I had to go out to the car and search in my small bottle of the special metal nuts for fluorescent fixtue reflectors.

The mechanism of action is that when say manganese is diluted in the proper amount such as with oxygen or copper, the manganese changes from parmagnetic to ferromagnetic. That is, at a proper average atomic spacing ferromagnetism of manganese reaches a peak.

Similarly, practical aluminum alloys contain small amount of silicon, magnesium, copper, and so forth to improve the mechanical properties. Go over to Alcan's website and look up the composition of Stabiloy(R) aluminum wire. The different alloy components create slightly magnetizable domains and atomic spacing in aluminum alloys that optimises paramagnetism but not nearly as strong as for steel.

The matter that some aluminum alloys are good paramagnets makes these alloys the ideal material for the disc of a kilowatthour or varhour meter. The magnetic conductivity and electrical conductivity of certain aluminum alloys makes these alloys produce the most amount of torque in an eddy current motor or brake. Steel cannot be used for the disc of a KWHr meter because it acts as a magnetic short circuit.

The same aluminum alloys are used for the platter of disc drives because the aluminum shields the upper and lower surfaces from each other and also acts as magnet keepers to keep the magnetic domains from drifting around, essentially a form of solid through solid diffusion.

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I forgot to add that paramagnetic aluminum alloys have essentially zero hysteresis loss and about twice the magnetic saturation of steel. These 2 properties also make certain aluminum alloys the ideal material for the disc of a KWHR meter or eddy current brake.

These aluminum alloys also make for excellent electromagnetic shields hence the better flexible coaxial cables ( cable TV, closed circuit television ) have one or more layer of aluminum ally foil in addition to copper braid - the aluminum foil both closes the holes in the copper braid and confines small magnetic fields to the interior or exterior of the cable. Copper braiding only shields of coaxial cables can be used to make a shielded air core radio frequency transformer but the copper plus aluminum foil cables cannot be used for this.

The copper plus aluminum foil cables cannot be used for a radio transmitter above a few watts. This application requires plain copper braid coaxial cable or aluminum sheathed "hardline" coaxial cable.

Answer 2

Aluminium is paramagnetic, so external magnets will induce a magnetic field in it. However the magnetic susceptibility is only 2.2 $\times$ 10$^{-5}$ so I'd be surprised if the induced magnetism would be high enough to be noticable. Normally you can only measure paramagnetism in the lab with sensitive equipment.

Does the aluminium bar remain magnetic when you remove the magnets? If not that would suggest it is induced magnetism, but if so there must be some ferromagnetic material present. Either way a noticable magnetic field suggests your aluminium isn't pure (as Anna suggests in a comment).