Why Do Superconductors Expel Magnetic Fields? Understanding the Meissner Effect
After becoming interested in superconductors, I noticed something familiar while watching Avatar: Unobtanium, the fictional material that appears in the movie.
Seeing it suspended in the air made me wonder:
“Could this be something like a superconductor?”
As I began looking into how real superconductors interact with magnetic fields, I came across one of their most fascinating properties: the Meissner effect.
So what exactly is the Meissner effect, and why is it so important to understanding superconductivity?
The Meissner Effect: Expelling Magnetic Fields
In my previous article, I explored how superconductivity was discovered in 1911, when the electrical resistance of mercury was found to disappear at extremely low temperatures.
Zero electrical resistance is probably the best-known property of a superconductor.[2]
But it is not the whole story.
Superconductors also behave in a remarkable way when exposed to magnetic fields.
In 1933, German physicists Walther Meissner and Robert Ochsenfeld discovered that when certain materials enter the superconducting state, they expel magnetic fields from their interior.[1]
This phenomenon became known as the Meissner effect.
In simple terms, when a material is cooled below its critical temperature and becomes superconducting, it does more than conduct electricity without resistance. Its magnetic behavior also changes dramatically.
What Happens to the Magnetic Field?
Every magnet produces a magnetic field around it.
In an ordinary material, an external magnetic field can penetrate through the material. A superconductor behaves differently once it is cooled below its critical temperature.
Currents flow near the surface of the superconductor and generate a magnetic field that opposes the applied field. As a result, the magnetic field is largely excluded from the superconductor's interior.
That is why illustrations of the Meissner effect often show magnetic field lines bending around a superconductor rather than passing straight through it.
Learning about this immediately reminded me of the floating Unobtanium in Avatar.
The movie is fiction, of course, but the visual similarity made the real physics even more interesting to me.
Why Is the Meissner Effect So Important?
The Meissner effect matters for a reason that goes beyond the striking image of a floating magnet.
It tells us that a superconductor is not simply a perfect conductor with zero electrical resistance.
When superconductivity was first discovered in 1911, the disappearance of electrical resistance was its most obvious feature.
More than two decades later, the discovery of the Meissner effect revealed that the superconducting state also has a distinctive magnetic property.
So there are two key ideas to remember when thinking about superconductivity:
Electrical resistance drops to zero.
And:
Magnetic fields are expelled from the interior of the material under the appropriate superconducting conditions.
That second property is the Meissner effect.
So Why Does a Magnet Float?
One of the most memorable demonstrations of superconductivity is a magnet floating above a superconductor.
It looks almost impossible at first.
A magnet creates a magnetic field, while a superconductor responds to that field through its unusual magnetic properties. Under the right conditions, this interaction can create an upward magnetic force strong enough to balance the downward pull of gravity.
The magnet then appears to float.
Gravity has not disappeared. The magnet is simply in a situation where magnetic force and gravity are in balance.
This is one reason the floating Unobtanium in Avatar caught my attention. The material itself may be fictional, but real superconductors can produce their own remarkable levitation effects through their interaction with magnetic fields.
Do Superconductors Always Expel Magnetic Fields Completely?
This is where things become a little more complicated.
It is tempting to think that a superconductor simply pushes out every magnetic field under all conditions. In reality, its behavior depends on factors such as the type of superconductor, its temperature, and the strength of the applied magnetic field.
This is especially important for Type-II superconductors.
Within a certain range of magnetic fields, a Type-II superconductor can allow magnetic flux to penetrate parts of the material in the form of quantized structures known as magnetic vortices, or flux lines.[1]
So the simple statement that “superconductors completely expel magnetic fields” does not describe every situation.
And this detail turns out to be important when we look at some of the most impressive superconducting levitation experiments.
Why Doesn't the Magnet Simply Get Pushed Away?
This was the next question that caught my attention.
If a superconductor repels a magnetic field, why doesn't the magnet simply move away?
In many superconducting demonstrations, something much more interesting happens.
A magnet or superconductor can remain suspended at a stable height. It may stay in position even when tilted, and in some demonstrations, a superconductor can glide along a magnetic track while remaining suspended above it.
What keeps it so stable?
The answer involves another fascinating phenomenon: flux pinning.
In a Type-II superconductor, magnetic vortices can become pinned at tiny defects or irregularities within the material. This can help lock the relative position of the superconductor and the magnetic field, producing the remarkably stable levitation seen in many demonstrations.
That is why the Meissner effect and flux pinning should not be treated as exactly the same thing.
Before learning more about superconductivity, I also assumed that a floating magnet could be explained simply by the Meissner effect.
But the more I looked into it, the more I realized that there was another piece of physics behind those striking demonstrations.
In the next article, I’ll take a closer look at flux pinning and how it helps superconductors remain stably suspended in a magnetic field.
References
American Physical Society — Superconductivity,[1] J. R. Schrieffer & M. Tinkham
American Physical Society source
[2]CERN — Superconductivity
CERN superconductivity overview
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