Diamagnetism

Levitating pyrolytic carbon

Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. According to Lenz's law, this opposes the external field. Diamagnets are materials with a magnetic permeability less than $\mu _{0}$ (a relative permeability less than 1).

Consequently, diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. It is generally quite a weak effect in most materials, although superconductors exhibit a strong effect.

Diamagnetic materials cause lines of magnetic flux to curve away from the material, and superconductors can exclude them completely (except for a very thin layer at the surface).

History

In 1778 S. J. Bergman was the first individual to observe that bismuth and antimony were repelled by magnetic fields. However, the term "diamagnetism" was coined by Michael Faraday in September 1845, when he realized that all materials in nature possessed some form of diamagnetic response to an applied magnetic field.

Diamagnetic materials

Notable diamagnetic materials

Material	$\chi _{v}\left(\times 10^{-5}\right)$ (SI units)
Bismuth	-16.6
Carbon (diamond)	-2.1
Carbon (graphite)	-1.6
Copper	-1.0
Lead	-1.8
Mercury	-2.9
Silver	-2.6
Water	-0.91
Superconductor	-105

Diamagnetism is a very general phenomenon, because all electrons, including the electrons of an atom, will always make a weak contribution to the material's response. However, for materials that show some other form of magnetism (such as ferromagnetism or paramagnetism), the diamagnetism is completely overpowered. Substances that mostly display diamagnetic behaviour are termed diamagnetic materials, or diamagnets. Materials that are said to be diamagnetic are those that are usually considered by non-physicists to be "non-magnetic", and include water, wood, most organic compounds such as petroleum and some plastics, and many metals including copper, particularly the heavy ones with many core electrons, such as mercury, gold and bismuth. The diamagnetism of various molecular fragments are called Pascal's constants.

Diamagnetic materials have a relative magnetic permeability that is less than 1, thus a magnetic susceptibility which is less than 0, and are therefore repelled by magnetic fields. However, since diamagnetism is such a weak property its effects are not observable in every-day life. For example, the magnetic susceptibility of diamagnets such as water is $\ \chi _{v}$ = −9.05×10⁻⁶ (SI units). The most strongly diamagnetic material is bismuth, $\ \chi _{v}$ = −1.66×10⁻⁴ , although pyrolytic carbon may have a susceptibility of $\ \chi _{v}$ = −4.00×10⁻⁴ in one plane. Nevertheless, these values are orders of magnitudes smaller than the magnetism exhibited by paramagnets and ferromagnets.

A superconductor acts as an essentially perfect diamagnetic material when placed in a magnetic field and it excludes the field, and the flux lines avoid the region

Superconductors may be considered to be perfect diamagnets ($\ \chi _{v}$ = −1), since they expel all fields (except in a thin surface layer) due to the Meissner effect. However this effect is not due to eddy currents, as in ordinary diamagnetic materials (see the article on superconductivity).

Additionally, all conductors exhibit an effective diamagnetism when they experience a changing magnetic field. The Lorentz force on electrons causes them to circulate around forming eddy currents. The eddy currents then produce an induced magnetic field which opposes the applied field, resisting the conductor's motion.

Demonstrations of diamagnetism

Curving water surfaces

If a powerful magnet (such as a supermagnet) is covered with a layer of water (that is thin compared to the diameter of the magnet) then the field of the magnet significantly repels the water. This causes a slight dimple in the water's surface that may be seen by its reflection.

Diamagnetic levitation

A live frog levitates inside a 32 mm diameter vertical bore of a Bitter solenoid in a magnetic field of about 16 teslas at the Nijmegen High Field Magnet Laboratory. Various videos

Diamagnets may be levitated in stable equilibrium in a magnetic field, with no power consumption. Earnshaw's theorem seems to preclude the possibility of static magnetic levitation. However, Earnshaw's theorem only applies to objects with positive moments, such as ferromagnets (which have a permanent positive moment) and paramagnets (which induce a positive moment). These are attracted to field maxima, which do not exist in free space. Diamagnets (which induce a negative moment) are attracted to field minima, and there can be a field minimum in free space.

A thin slice of pyrolytic graphite, which is an unusually strong diamagnetic material, can be stably floated in a magnetic field, such as that from rare earth permanent magnets. This can be done with all components at room temperature, making a visually effective demonstration of diamagnetism.

The Radboud University Nijmegen, the Netherlands, has conducted experiments where water and other substances were successfully levitated. Most spectacularly, a live frog (see figure) was levitated.

In September 2009, NASA's Jet Propulsion Laboratory in Pasadena, California announced they had successfully levitated mice using a superconducting magnet, an important step forward since mice are closer biologically to humans than frogs. They hope to perform experiments regarding the effects of microgravity on bone and muscle mass.

Recent experiments studying the growth of protein crystals has led to a technique using powerful magnets to allow growth in ways that counteract Earth's gravity.

A simple homemade device for demonstration can be constructed out of bismuth plates and a few permanent magnets that will levitate a permanent magnet.

Theory of diamagnetism

The Bohr–van Leeuwen theorem proves that there cannot be any diamagnetism or paramagnetism in a purely classical system. Yet the classical theory for Langevin diamagnetism gives the same prediction as the quantum theory. The classical theory is given below.

Langevin diamagnetism

The Langevin theory of diamagnetism applies to materials containing atoms with closed shells (see dielectrics). A field with intensity $B$, applied to an electron with charge $e$ and mass $m$, gives rise to Larmor precession with frequency $\omega =eB/2m$. The number of revolutions per unit time is $\omega /2\pi$, so the current for an atom with $Z$ electrons is (in SI units)

$I=-{\frac {Ze^{2}B}{4\pi m}}$.

The magnetic moment of a current loop is equal to the current times the area of the loop. Suppose the field is aligned with the $z$ axis. The average loop area can be given as $\pi \langle \rho ^{2}\rangle ,$ where $\langle \rho ^{2}\rangle$ is the mean square distance of the electrons perpendicular to the $z$ axis. The magnetic moment is therefore

$\mu =-{\frac {Ze^{2}B}{4m}}\langle \rho ^{2}\rangle$.

If the distribution of charge is spherically symmetric, we can suppose that the distribution of $x,y,z$ coordinates are independent and identically distributed. Then $\langle x^{2}\rangle =\langle y^{2}\rangle =\langle z^{2}\rangle =\langle r^{2}\rangle /3$, where $\langle r^{2}\rangle$ is the mean square distance of the electrons from the nucleus. Therefore $\langle \rho ^{2}\rangle =\langle x^{2}\rangle +\langle y^{2}\rangle =(2/3)\langle r^{2}\rangle$. If $N$ is the number of atoms per unit volume, the diamagnetic susceptibility is

$\chi ={\frac {\mu _{0}N\mu }{B}}=-{\frac {\mu _{0}NZe^{2}}{6m}}\langle r^{2}\rangle$.

Diamagnetism in metals

The Langevin theory does not apply to metals because they have non-localized electrons. The theory for the diamagnetism of a free electron gas is called Landau diamagnetism.

External links

• Video of a museum-style magnetic elevation train model which makes use of diamagnetism
• Videos of frogs and other diamagnets levitated in a strong magnetic field
• Video of levitating pyrolytic graphite
• Video of Meissner-Ochsenfeld effect involving liquid nitrogen
• Video of a piece of neodymium magnet levitating between blocks of bismuth.
  • Website about this device, with images (in Finnish).