Aluminum is a common metal, found in many everyday objects. While a refrigerator magnet won’t stick to an aluminum can, its magnetic properties are nuanced. Unlike iron, aluminum does not create its own persistent magnetic field or strongly attract to magnets. Understanding its behavior requires examining magnetism.
The Basics of Magnetism
Magnetism is a fundamental force arising from the motion of electrically charged particles, particularly electrons within atoms. Electrons possess both an orbital motion around the nucleus and an intrinsic property called “spin,” generating tiny magnetic fields, known as magnetic moments. In most materials, these individual atomic magnetic moments are randomly oriented or cancel each other out, resulting in no overall magnetic behavior.
Materials are categorized by how their atomic magnetic moments respond to an external magnetic field. Ferromagnetic materials, such as iron, nickel, and cobalt, exhibit strong magnetic properties because their atomic magnetic moments spontaneously align within small regions called magnetic domains. When an external magnetic field is applied, these domains align, leading to a powerful attraction that can persist even after the external field is removed. Other materials behave differently when exposed to magnetic fields.
Aluminum’s Response to Magnetic Fields
Aluminum is classified as a paramagnetic material, showing a very weak attraction to external magnetic fields. This subtle magnetic response is far less noticeable than the strong attraction seen in ferromagnetic substances.
The reason for aluminum’s paramagnetism lies in its atomic structure, due to unpaired electrons in its outer shells. These unpaired electrons have magnetic moments that can align slightly with an applied magnetic field. This alignment is only temporary and does not remain once the external magnetic field is removed.
Because this induced magnetism is extremely weak, everyday aluminum objects like foil or cans do not appear to stick to magnets. The slight attraction is typically imperceptible without highly sensitive instruments or very strong magnetic fields.
When Aluminum Interacts with Magnets
Despite not being strongly magnetic, aluminum can still interact with magnets under certain conditions, through eddy currents. Since aluminum is an excellent electrical conductor, a changing magnetic field moving near or through it can induce temporary electric currents within the metal. These induced currents, called eddy currents, flow in circular paths.
According to Lenz’s Law, these eddy currents generate their own magnetic fields that oppose the change in the original magnetic field that created them. This opposing magnetic field creates a resistive force.
For example, if a strong magnet is dropped through an aluminum tube, the eddy currents induced in the tube will create a magnetic braking effect, noticeably slowing the magnet’s fall. This interaction is temporary and does not result in the aluminum becoming permanently magnetized.
While pure aluminum is consistently paramagnetic, some aluminum alloys might contain trace amounts of ferromagnetic impurities like iron or nickel. These impurities can give the alloy a faint magnetic response, but this is not characteristic of pure aluminum and is generally insignificant.