Magnets: revealing their disturbing nature

Few natural phenomena arouse as much fascination as a magnet. Try distracting a child from their favorite screen by putting a couple of magnets within reach. It is very likely that he feels the same sense of wonder that Albert Einstein reported when, at the age of four or five, his father showed him a compass.

Why are there two magnetic poles? Why is it that if a magnet is broken, each fragment has two poles again? Can this division be repeated indefinitely? If you have ever asked yourself these questions, you may not know that it has taken us thousands of years to answer them.

Two decades after feeling that childhood fascination, that same child, Albert Einstein, contributed to developing the two great pillars of Physics, Quantum Mechanics for the small and the Theory of Relativity for the large, which have allowed us to understand the world and, also, understand how a magnet works.

Looking for the smallest magnets

The path that leads to the understanding of magnetism is worthy of a mystery novel. For centuries he wandered among effluvia and animist schools that gave magnetite a soul.

The intimate relationship between magnetism and electricity began to be clarified in the seventeenth century. In 1681, for example, a ship had to sail towards Boston following the south of its compass, after the ship’s sailors noticed how lightning had reversed the poles of the magnetic needle.

Almost a century and a half later, in 1820, Francois Arago, French scientist and politician, finally showed that an electric current behaves like a magnet. In 1874, G. Johnstone Stoney he proposed that electricity is transmitted in discrete units or electrons, and only a few decades later the great conceptual edifice of the microscopic world that we call Quantum Mechanics was forged. In it, electrons are elementary particles carrying electric charge.

Despite lacking an internal structure, it was soon postulated that an electron behaves like a tiny magnet, the smallest possible magnet. This intrinsic property is pictorially associated with a rotation or spin of the particle in two possible directions, which are described with an arrow pointing in two directions that we call up and down states.

Today we know that all elementary particles have spin, and that when the number of states is greater than one they behave like tiny magnets. It is “as if” this multiplicity of states were equivalent to the charge of the particles having an intrinsic movement, creator of the magnet. But what we didn’t know, until now, is for what eerie reason these particles line up one way or another.

The big is necessary to understand the small

In one of the greatest blows in the history of Science, Paul Dirac demonstrated in 1928 that the spin of the electron and the magnitude of its tiny magnet arose naturally by integrating Einstein’s Theory of Relativity with Quantum Mechanics: Science of the greatest to the aid of that of the smallest! The rest, also paraphrasing Dirac, “it’s just calculation”.

Thus, the spins of the different electrons of a magnetite atom, molecule, or crystal interact following the rigid and counterintuitive laws of Quantum Mechanics. If they line up parallel, the tiny electronic magnets get stronger, and we have a permanent magnet. If they are randomly oriented or anti-parallel aligned, the individual magnets cancel.

One of those rigid laws, which shapes the world we observe like no other, is the so-called Pauli exclusion principle, formulated by Wolfgang Pauli in 1925. It states that we cannot place two identical particles with an even number of spin states in the same place in space. We will not be able to have two electrons above in the same place, but we can have one above and one below.

In this way, electrons of the same spin seem to repel each other very strongly. We all check these Pauli repulsions daily. They are the ones that prevent us from going through a wall like ghosts, and the ones that determine, as we have recently shown, how elementary magnets fit together in a moleculethus helping to reveal one of the scientific unknowns of magnetism.

In the bowels of molecular magnets

Despite the fact that we “only” have to solve the equations of Quantum Mechanics to predict whether the different spins of a material will be constructively reinforced or not, even the most powerful computers available do not allow us to find sufficiently precise solutions.

For this reason, chemists and physicists use simplified models. With them we have designed all modern magnetic devices and synthesized extraordinary molecules that behave like permanent magnets. By manipulating molecular magnets we can dream of memory sticks of unimaginable capacity or future quantum computers.

The simplest case, and therefore the best studied, is that of molecules containing two magnetic centers, for example two metal atoms. The joint task of the synthetic chemist and the theoretical chemist consists in predicting what environment these two atoms must have in order for their respective spins to strengthen, or on the contrary to cancel. In many cases this coupling can be “tuned” and become temperature dependent, or illuminating the system with light of a certain color.

Resolved decades-old scientific dispute

Since Science is a human enterprise, when we travel from equations to models, different minds can arrive at divergent models, which end up uniting supporters and opponents. This scientific partisanship, similar in nature to politics, can last for years or decades, and has been particularly intense in the field of molecular magnetism.

The controversy is usually resolved when a new integrative model settles the dispute, usually showing that the various currents are nothing more than particular corners of a more general reality. Well, what we have just shown is that it is the Pauli barriers created by the electrons that bridge the metallic atoms that determine the type of alignment of their spins.

How this happens can be understood with an analogy. Let us associate each of the possibilities of electronic spin with a color, red and blue, for example. Suppose electrons hate having partners of the same color (Pauli repulsion). Now imagine two distant electrons, which do not distinguish the color of their neighbor.

Red-red and red-blue combinations are equivalent. Let us now put, halfway between these two electrons, a new pair made up of a red electron and a blue one, whose color already distinguishes the previous ones. Whatever the arrangement of these two intermediate electrons, red and blue or blue and red, the initial electrons will choose different colors. The Pauli repulsion has favored an antiparallel alignment of the two electrons.

We have all experienced how, after overcoming a hill and reaching a higher peak, new landscapes unseen from the previous observation point are revealed. We hope that from the newly found new vantage point, as yet unexplored avenues will be opened to design new molecular magnets.

Font: Angel Martin Pendas / THE CONVERSATION

Reference article: https://theconversation.com/magnets-revealing-their-disturbing-nature-178716