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Magical magnetism, electron “spin”, and easy iron paths

Q: Why are some things magnetic? What makes them magnetic? (Jamar, Charlottesville, Virginia)

Q: I have no clue how magnetism gets "frozen" in a magnet — there’s no current flowing, right? (Kevin, Penryn, California)

A: A moving electrical charge (electric current) produces a magnetic field — macroscopic flow in wires, for example.

Our polarized planet Earth is an example of really macroscopic flow. Earth has magnetic poles because of charged-particle currents roiling deep within its molten core.

But, there’s more than macroscopic flow to magnetism, as your insightful question implies. How magnetism gets "frozen" in a magnet takes us to the microscopic level where some flow does occur — electrons orbiting the nucleus. However, the major contributor to a magnet’s magnetism is the inherent magnetic property of electrons that we call, somewhat misleadingly, "electron spin."

The property "is an inherent quantum property with no classical analog," says Rod Nave, physics professor at Georgia State University.

The term misleads because electrons don’t actually spin.

We can easily show that, says Nave. "Any scaling of an electron size and spin rate would require speeds on its perimeter far in excess of the speed of light. So electron spin can’t really produce the magnetic property we call ‘electron spin.’"

Instead, every electron is a tiny magnet due to its inherent magnetism (what we call electron spin).

Furthermore, the alignment of the electron spins makes a hunk of iron (magnetite) into a magnetic lodestone.

All atoms have electrons with electron spin and magnetic fields due to their orbits about the nucleus. But not all material is magnetic like the lodestone (ferromagnetic). If the electron spins of an atom’s electrons are aligned oppositely, their magnetic fields cancel. That’s what happens with tissue paper, flesh, or other non-ferromagnetic substances.

Each iron atom, on the other hand, has four electrons whose spin magnetism doesn’t cancel. They line up. Aligned magnetic fields make matter magnetic.

The top figure shows the un-aligned domains of an un-magnetized piece of iron. The bottom figure shows the same iron piece in the presence of a magnetic field. Note how the field has aligned the iron’s domains.

Iron is a peculiar, remarkable substance. Its aligned-field electrons spontaneously couple and form small long-lasting domains. The spins inside these microscopic domains are almost perfectly aligned. Most domains, though, aren’t aligned. In common un-magnetized iron, many domains are randomly oriented. See figure.

Simply bringing a magnet (and its magnetic field) near iron, however, will align its domains or cause those aligned with the external field to grow at the expense of their neighbors. With a stethoscope, we can hear little domains click into position! This domain alignment produces an overall net magnetism.

When we take the magnet away from pure iron, its magnetism goes away shortly because ordinary thermal motion of its atoms jiggles the domains out of alignment. On the other hand, we can make permanent magnets out of many special iron alloys that are different in composition. Once the domains of these iron alloys are aligned, they are much harder to randomize. Instead, the domains stay aligned "for years instead of minutes," says Nave.

Further Reading:

Georgia State University: HyperPhysics – Magnetism by Rod Nave

Florida State University, Molecular Expressions: Demonstration of how iron filings line up with magnetic field lines — a click away.

Q: Is there any material or substance that blocks or interrupts a magnetic field? I am trying to find something that, when placed between the ends of two bar magnets, will keep them from attracting each other. (Mike, Benton, Tennessee)

A: Magnetic field lines form closed loops that go out from one pole (for example, the north pole), arc back into the magnetic material at the south pole, and close the path inside the magnet by returning to the north pole. See figure. The field lines go blithely through most materials in their path — even many metals — unperturbed and largely undiminished.

A bar magnet and its magnetic field. The field lines through air close the loop with the lines inside the magnet. [Rod Nave, Georgia State University]

The only passive way to block a magnetic field is to shunt the return flux lines away from the realm you want to shield. You can’t absorb the flux since the field lines must complete the loop. Instead, force the stray flux lines through iron (or some like material). The iron provides a path of least resistance for the returning flux lines and this significantly reduces the field’s effect in the protected region.

So, to answer your question — place a heavy iron structure (for example, a thick iron bar considerably longer and higher than the magnets) between the two bar magnets to greatly reduce their mutual attraction.

The flux lines of one magnet will hop aboard the iron bar — crowding in — since it’s the easy iron path to return to the magnet, leaving few to struggle through air complete their loop. Thus fewer flux lines will travel through air, reaching and affecting the other magnet.

Further Reading:

Magnetic Shield Corporation: Do I need shielding?

(Answered April 1, 2005)