Diamagnetism

Diamagnetism originates from the orbital motion of electrons in atoms. When an external magnetic field is applied, it alters the orbital angular momentum of electrons, inducing a magnetic moment that opposes the field according to Lenz's law¹. This induced moment is proportional to the field strength but opposite in direction, giving a negative magnetic susceptibility (χ < 0). The effect is weak because it arises from the classical response of paired electrons; materials with unpaired electrons typically exhibit stronger paramagnetism. The magnitude of diamagnetic susceptibility depends on the number of electrons and the average radius of their orbits. For most materials, χ is on the order of −10⁻⁵ to −10⁻⁶ (SI units). The phenomenon was first described by Michael Faraday in 1845, who observed that bismuth and antimony were repelled by a magnet².

Common diamagnetic materials include bismuth (the element with the largest diamagnetic susceptibility), water, graphite, noble gases (e.g., argon, helium), and most organic compounds³. Weak diamagnetism is also present in many biological tissues. Applications of diamagnetism include magnetic levitation: a strong enough magnetic field can levitate diamagnetic objects such as water droplets, organic materials, and even living organisms. In 1997, a frog was levitated using a 16 T magnet at the High Field Magnet Laboratory in Nijmegen. In MRI and NMR, diamagnetic susceptibility corrections are essential for image interpretation. Graphite exhibits strong diamagnetic anisotropy due to its layered structure, allowing it to levitate stably at room temperature⁴.

Superconductors are perfect diamagnets below their critical temperature, completely expelling an external magnetic field from their interior—a phenomenon known as the Meissner effect⁵. This results in a magnetic susceptibility of χ = −1 (SI units). In type-I superconductors, the field is expelled entirely up to a critical field, after which superconductivity is destroyed. Type-II superconductors exhibit a mixed state with partial flux penetration. The Meissner effect is exploited in magnetic levitation of superconductors, enabling frictionless bearings and maglev trains. The transition from normal to superconducting state involves a sharp change in diamagnetic response, which is used to study superconducting materials.

Diamagnetism is present in all materials, including living organisms. The levitation of biological samples (e.g., frogs, seeds, water) demonstrates that diamagnetism can overcome gravity in strong magnetic fields, a technique used in materials science and biology. Graphite exhibits significant diamagnetic anisotropy—its susceptibility varies by a factor of ~50 depending on orientation relative to the crystal planes, enabling stable levitation without active control⁴. In geophysics, the diamagnetic properties of minerals such as quartz and calcite influence the magnetic susceptibility of rocks, though they are usually overshadowed by paramagnetic or ferromagnetic contributions. Historically, Faraday's discovery of diamagnetism in 1845 led to the first classification of magnetic materials. A curious fact: pure water is diamagnetic, yet its susceptibility is so weak that a fridge magnet cannot lift it—but a 10 T magnet can repel it noticeably³.

Diamagnetism

A magnetic property producing a repulsive response to an external magnetic field, characterized by negative magnetic susceptibility.

Magnetic susceptibility

A dimensionless proportionality constant indicating the degree of magnetization of a material in response to an applied magnetic field.

Meissner effect

The expulsion of a magnetic field from the interior of a superconductor below its critical temperature, producing perfect diamagnetism.

Lenz's law

A law stating that an induced current flows in a direction that opposes the change in magnetic flux that produced it.

Orbital motion

The motion of electrons around an atomic nucleus, which gives rise to magnetic moments and diamagnetic effects.