PHYSICS
If you boil a magnet, does it lose its magnetism?
This was not inspired by any pagan ritual, but it is a very interesting question a student asked me years ago. It is also a refreshing one— in all my years in physics, it never occurred to me to boil a magnet! Jokes aside, I believe it’s important to create a bridge between the day-to-day and the scientific work performed in labs everywhere. After all, science is grounded in reality, and answering questions like these is a great way to remind ourselves of that.
Before delving into the answer, let’s clarify what magnets are and what makes them act as they do.
A most simplistic definition of a magnet is a material that attracts or repels other magnetic materials, such as iron. In fact, the properties of magnetic materials are very varied and constitute a field in and of itself.
So if we boiled a magnet, would it still attract iron, for example? As you might expect, the answer can be either yes or no, depending on the exact material the magnet is made of and what we really mean by boiling.
If the magnet were made of magnetite, a naturally occurring magnetic material, after we boiled it in water (100°C), it would still attract pieces of iron. This is because the Curie temperature of magnetite is about 585°C.
Let’s explore together how magnets work, what the Curie temperature is, and why it matters.
Magnetic Domains
Any piece of solid material is made of atoms, arranged more or less orderly. A chunk of magnetite, for example, could be thought of as being made of a combination of chunks, which we will call magnetic domains. In a simplistic view, inside these domains, the atoms are ordered. They are arranged in a lattice, which in turn, is composed of unit cells. Inside a unit cell, there can be one or more atoms, and repeating the unit cells in all three dimensions reproduces the ordered structure inside a domain. Each unit cell acts as a little magnet, and we will call the strength of this magnet a magnetic moment.
Inside a magnetic domain, the magnetic moments (or simply, moments) are aligned. This means they all point in the same direction. Therefore, the total moment of the magnetic domain will be the sum of the moments of each unit cell. In a magnetic material, these domains also align in the same direction, making the overall chunk of, for example, magnetite, attract or repel other magnets.
The Curie Temperature
When subjected to heat, the magnetic moments inside domains can start to vibrate and, if the thermal energy is large enough, they may become completely misaligned. Now pointing in random directions, the total magnetic moment of the chunk of magnetite could very well become zero. The temperature at which the thermal energy is high enough to agitate the moments to such a degree that they no longer work together to point in the same direction is called the Curie temperature. At this temperature, the magnet stops attracting or repelling other magnets or materials such as iron.
The Curie temperature is specific to each material and depends on the crystalline structure of said material — that is, the specific way the atoms are laid out within the unit cell and the pattern in which the unit cells repeat in all three spatial directions to reproduce the crystalline structure of the material. For example, the Curie temperature of iron is 770°C, but that of gadolinium (another ferromagnetic metal) is about 20°C. Therefore, boiling gadolinium in water would demagnetize it!
Is the process reversible?
Could we bring demagnetized gadolinium to its former glory? Sure we can! We just need to bring gadolinium to a temperature above its Curie point and place it in the magnetic field of a powerful magnet. Thermal energy does not only randomize the directions of the magnetic moments inside a domain, but it also offers the opportunity to re-align them; in this heated state, the moments are “malleable.” This means they can be manipulated and, by lowering the temperature below the Curie point, “frozen” back into a ferromagnetic — aligned magnetic — state.
Circling back to what we might mean by “boiling,” we’ve only referred to the conventional meaning of the word: placing an object in boiling water. But what is the boiling point? It is the temperature at which a liquid turns into a gas. Therefore, boiling a magnet could also mean bringing it to such a high temperature where it is already in liquid form and it now begins to vaporize.
However, at this point, the material has long lost its magnetic properties. For example, the boiling point of iron is 2,861°C (whereas its Curie temperature is only 770°C)! You might now be wondering about Earth’s liquid outer core made of iron and nickel, subjected to temperatures up to 5,500°C. How can a non-magnetic liquid generate Earth’s magnetic field?
The answer lies in the movement of this swirling hot liquid, which generates convective electrical currents. As you may know, moving electrical charges cause a magnetic field — an effect discovered all the way back in 1820 by Danish physicist Hans Christian Ørsted.
Fun facts
- Magnetite probably takes its name from Magnesia, a region of Ancient Greece, in Thessaly.
- The Curie temperature was named after the French physicist Pierre Curie, who was the first to observe a relationship between magnetism and temperature.
- The highest known Curie temperature is 1114.85°C, for elemental cobalt. That’s as approximately as hot as Hawaiian lava or a bonfire.
Summary
- Boiling a magnet in water may or may not demagnetize it, depending on the material the magnet is made of.
- Above its Curie temperature, a material loses its magnetic properties.
- The process of demagnetization can be reversed by heating the material above its Curie point in the presence of a strong magnet and subsequently cooling it back below its Curie temperature to “freeze” the magnetic moments in place.
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