Magnetism: From Lodestones to MRI Scanners

The Oldest Known Force After Gravity

Magnetism is one of the first physical phenomena humans noticed and named. The ancient Greeks recorded that certain stones from the region of Magnesia in modern Turkey could attract iron. These lodestones — naturally magnetised pieces of the mineral magnetite — were humanity’s first encounter with a force that would eventually reshape civilisation.

Today, magnetism is understood as one aspect of electromagnetism, unified with electricity by James Clerk Maxwell in the 19th century. But its origins lie deep in quantum mechanics, in the behaviour of individual electrons within atoms.

The Atomic Origin of Magnetism

Every electron is a tiny magnet. This magnetism comes from two quantum mechanical properties:

Orbital angular momentum — As an electron moves around a nucleus, it creates a current loop, which produces a magnetic field just as a current in a wire does.

Spin — Electrons possess an intrinsic angular momentum called spin. This is not literally rotation, but a fundamental quantum property that generates a magnetic moment. Spin is what makes individual electrons behave like microscopic bar magnets.

In most atoms, electrons are paired with opposite spins, and their magnetic moments cancel. This is why most materials are not magnetic. But in certain elements — particularly iron, cobalt, and nickel — unpaired electrons with aligned spins create a net magnetic moment per atom.

Ferromagnetism: When Atoms Cooperate

Having magnetic atoms is necessary but not sufficient for a material to be a permanent magnet. The key is cooperation: in ferromagnetic materials, a quantum mechanical effect called the exchange interaction causes neighbouring atomic spins to align parallel to each other spontaneously.

This alignment occurs in regions called magnetic domains — typically microns to millimetres in size, each containing billions of atoms with their spins pointing the same way. In an unmagnetised piece of iron, these domains point in random directions, and their fields cancel. Applying an external magnetic field causes domains aligned with the field to grow at the expense of others, until the entire material is magnetised.

Above a critical temperature — the Curie temperature (770°C for iron) — thermal energy overcomes the exchange interaction and ferromagnetism disappears. Michael Faraday was among the first to study how heat affects magnetism.

Earth’s Magnetic Field

The Earth itself is a giant magnet, generated by convection currents of molten iron in the outer core — a process called the geodynamo. This field, about 50 microtesla at the surface, extends thousands of kilometres into space, forming the magnetosphere.

The magnetosphere shields the Earth from the solar wind — a stream of charged particles from the Sun. Without it, the solar wind would strip away the atmosphere over geological time, as appears to have happened on Mars. When solar wind particles do funnel along magnetic field lines near the poles, they excite atmospheric gases and create the aurora.

Earth’s magnetic field also provides navigation for compass needles, migratory birds, sea turtles, and bacteria that contain chains of magnetite crystals.

The field is not static: it wanders, fluctuates in strength, and has reversed polarity hundreds of times over geological history. Seismic studies and magnetic observations together help geophysicists model the dynamics of the Earth’s core.

Magnetism in Technology

Magnetism is woven into nearly every modern technology:

Electric motors and generators — Every motor converts electricity into motion using magnetic fields, and every generator does the reverse. The global economy runs on Faraday’s law of induction.

Data storage — Hard drives store information as patterns of magnetised and demagnetised regions on spinning platters. A read/write head detects the magnetic orientation of each tiny domain. While solid-state drives are replacing hard drives in many applications, magnetic storage remains dominant for large-scale data centres.

MRI scanners — Magnetic Resonance Imaging uses superconducting magnets cooled to near absolute zero to generate fields of 1.5–3 Tesla — 30,000 to 60,000 times stronger than Earth’s field. These fields align hydrogen nuclei in the body, and radiofrequency pulses probe how quickly they relax in different tissues. The result is detailed soft-tissue imaging without any ionising radiation.

Particle accelerators — The Large Hadron Collider uses over 1,200 superconducting dipole magnets, each generating 8.3 Tesla, to bend proton beams around its 27-kilometre ring. The proposed Future Circular Collider would require 16-Tesla magnets — pushing superconducting magnet technology to new limits.

Fusion reactors — Tokamaks confine superheated plasma using powerful magnetic fields. ITER’s central solenoid will be the most powerful superconducting magnet ever built, generating a field of 13 Tesla and storing enough magnetic energy to lift an aircraft carrier.

Magnetism at the Extremes

The strongest magnets in the universe are not human-made but astrophysical. Magnetars — a type of neutron star — possess magnetic fields of up to 10¹¹ Tesla, a hundred billion times stronger than the most powerful laboratory magnets. At these field strengths, the vacuum itself becomes birefringent, bending light differently depending on its polarisation. Matter near a magnetar is stretched into needle-like structures aligned with the field.

At the other extreme, researchers study materials where magnetism vanishes in unexpected ways. Quantum spin liquids — predicted by theory and now observed in certain materials — are magnetic systems where the spins remain disordered even at absolute zero, forming entangled quantum states rather than the ordered patterns found in conventional magnets. These exotic states connect magnetism research to quantum computing and topological physics.

From Ancient Stones to Quantum Frontiers

Magnetism is a thread that runs from the earliest curiosity of ancient civilisations to the frontiers of 21st-century physics. The same force that aligns a compass needle also confines plasma for fusion energy, images the brain in a hospital scanner, stores humanity’s digital knowledge, and bends particles to near the speed of light.

Understanding magnetism — from the quantum spin of a single electron to the dynamo of a planet’s molten core — remains one of the most rewarding journeys in all of physics.