From high school, I remember that Aluminium has 13 electrons and thus has an unpaired electron in the 3p shell. This should make Aluminium magnetic. However, the wiki page of Aluminium says its non-magnetic at one place(with a citation needed tag though) and at another place says it's paramagnetic. Doing a google search shows up some contradictory results. So what is the truth?
Note:The context of the question is this answer on scifi.SE about magneto.
It really depends on what you mean by "magnetic," because there are different kinds of magnetic properties.
Materials like iron are ferromagnetic, which means that once you align the individual magnetic dipoles in the material, they will tend to stay aligned even without an external magnetic field. Ferromagnetic materials are the ones that permanent magnets are made out of, and they are probably what most people think of when they imagine a magnetic material. There are only three elements (as far as I know) that are ferromagnetic: iron, cobalt, and nickel, although other elements can be combined to make ferromagnetic polyatomic crystals.
Other materials that aren't ferromagnetic can (and typically do) have interesting magnetic properties, though - in other words, just because a material isn't a ferromagnet doesn't mean it doesn't interact magnetically at all. Paramagnetism is one such interaction. When you put a paramagnetic material in a magnetic field, its individual dipoles tend to align with the magnetic field, and thus with each other, thereby making the material magnetic. When this happens, the paramagnetic material is attracted to the magnetic field. The difference is that when you take the external magnetic field away, the individual dipoles in a paramagnetic material don't retain their orientation. Instead, thermal motion takes over and reorients them randomly. So a paramagnetic material only has a net magnetic moment while it is in an external magnetic field.
If Magneto is able to control magnetic fields, then that would potentially allow him to control all sorts of magnetic materials - not just ferromagnets (iron etc.) but also all paramagnetic and perhaps diamagnetic materials, since he can create the external field necessary to magnetize those materials. In fact, all materials, even non-metals, are diamagnetic to some (small) extent. However, paramagnetism and especially diamagnetism are generally much weaker effects than ferromagnetism, so it stands to reason that Magneto would have a harder time controlling non-ferromagnetic materials.
The closest thing to a scientific explanation for Magneto's abilities that I can come up with is that he's able to generate magnetic fields that are strong enough to have a significant effect on ferromagnetic and some of the more paramagnetic materials, but with diamagnetic materials, the magnetic fields he can produce are not strong enough to override other natural forces acting on those materials. Of course, I'm sure that wouldn't really hold up if you really look at the comics or the movie closely... but with comic books you probably don't want to ask too many questions ;-)
Short answer: Aluminum is considered "paramagnetic." Most laymen would consider that to mean "non-magnetic."
Longer answer (and greatly simplifying): Materials fall into three categories, dependent upon their relative permeability (called 'Ur' in this post). Almost all materials have a relative permeability which is very, very close to the relative permeability of free space --> Ur = 1. The classification is as follows:
Diamagnetic: Ur is VERY close to 1, but slightly less than 1 (e.g., Ur = 0.99997). Paramagnetic: Ur is VERY close to 1, but slightly more than 1 (e.g., Ur = 1.00003). Ferromagnetic: Ur can be orders of magnitude greater than 1 (e.g., Ur = 7283.5).
~ALL~ materials have a diamagnetic effect. This is a small magnetic field, which is only present if another magnetic field is induced on the material, and serves to OPPOSE the induced field. If no other effect swamps out this effect, then the material is considered diamagnetic. Some examples of diamagnetic materials are: bismuth, copper, lead, mercury, silver, gold, and diamond.
Some materials have a paramagnetic effect which overcomes the diamagnetic effect. This is a small magnetic field, which is only present if another magnetic field is induced on the material, and serves to STRENGTHEN the induced field. Paramagnetism (unlike diamagnetism) is temperature dependent, being stronger at lower temperatures. A material which has this, but not ferromagnetism, is considered paramagnetic. Examples of paramagnetic materials are: aluminum; magnesium; tungsten; and titanium.
Ferromagnetic materials can have their own magnetic field, independent of an applied magnetic field. In addition, they can have very large relative permeabilities. Examples of ferromaagnetic materials are: iron; nickel; and cobalt. When ferromagnetic materials are heated above their "curie temperature," they lose their ferromagnetic properties and become paramagnetic.
Then there are ferrimagnetic (used in ferrite beads) and anti-ferromagnetic materials, which we won't get into.
So, the big breakpoint is between ferromagnetic materials and anything else. It does not matter much to most laymen whether Ur is 0.99999 (diamagnetic) or 1.00001 (paramagnetic). Both types of materials have a Ur of almost exactly 1. They do not interrupt or divert magnetic lines of force. Unless you have a very, VERY highly concentrated magnetic force (as in an MRI exam), there is no observable effect on these materials. The layman considers them "non-magnetic."
Magneto: Something to think about is that most metals do conduct electricity very well. Inducing a circular current in a piece of metal would make that piece produce its own magnetic field (electromagnetism). That field could then be acted upon by external forces to manipulate the object.
Aluminum is not magnetic without an external magnetic field, however when an external magnetic field is applied or in presence of it Aluminum becomes "slightly" magnetic as its electron align to the magnetic field however due to thermal motion as described by Vintage the alignment of the electrons within the material (aluminium) is randomized thus its net magnetic force will cancel out, in other words it will become non-magnetic.
That effect is known as Paramagnetism, normally paramagnetic materials have magnetic permeability that is greater than or equal to 1 which means they are attracted to magnetic fields. That in mind, they are slightly weaker that ferromagnetism effect and tend to posses linear $B(H)$ characteristic.
To see how aluminium behaves in an magnetic field watch this clip: https://www.youtube.com/watch?v=J6dqO5FG3D4
and to know about the overall behavior of paramagnetic materials (without math, nor too much in depth explainations) watch this to get the gist of paramagnetic behavior when exposed to an magnetic field: https://www.youtube.com/watch?v=xAURp5V7zoQhttps://www.youtube.com/watch?v=xAURp5V7zoQ
In the context of the Question. YES. Aluminum is magnetic.
I have witnessed it first hand. I will not bore you with the scientific details, nor where or how I came to see this.
But suffice it to say 20+ years ago i saw an aluminum electromagnet that had the ability to attract OR repel aluminum. It could shoot a can about 6 feet in the air.
As far as I can tell, Aluminium is only magnetic when influenced by an external magnetic field.
