Electrons

Atoms are made up of a nucleus in the center and electrons outside. The nucleus is made up of protons (positively charged) and neutrons (not charged) and it is the number of these that governs the element and isotope of the atom.

A schematic of a chlorine atom, which has 17 protons, 18 or 20 neutrons (depending on the isotope) and 17 electrons. The nucleus is shown in green with the core electrons in pink in shells shown in yellow. The outer or valence electrons are shown in red - these are the important ones for magnetism and conductivity.

A neutral atom has the same number of electrons (negatively charged) as protons, but an ion (i.e. a charged atom) may have a few more or less electrons. The electrons are in shells, but in magnetism only the electrons in the outer (or valence) shell are of interest.

Examples... |
Oxygen | 8 protons There are 3 naturally occurring isotopes, one with 8 neutrons (99.76% abundance), one with 9 neutrons (0.04% abundance) and one with 10 neutrons (0.2% abundance).
Calcium and Fluorine | Neutral Calcium has 20 electrons and 20 protons Calcium 2+ has 18 electron and 20 protons (lost two electrons to make it 2+) Neutral Fluorine has 9 electrons and 9 protons Fluorine 1- has 10 electrons and 9 protons (gained one electron to make it 1-) The compound calcium fluoride has the formula CaF2, so that the compound is neutral over all.
Iron | Iron has:
2 electrons in the first shell (1s shell) 8 electrons in the next shell (2s/2p shell) 8 electrons in the next shell (3s/3p shell) | - These are known as the core electrons as they they are not very easy to remove.
8 electrons in the last shell (3d/4s shell) | - These are known as valence electrons, as they are the ones involved in bonding and are the easiest to remove.

Each shell has a certain number of orbitals and each orbital can hold two electrons which have a spin. In chemistry the spin is referred to as "up" or "down", but this does not mean an electron that is "spin up" is spinning in an upward direction, merely that it is opposite to one with that is "spin down". For ease, electrons are drawn as up and down arrows. An orbital can hold up to two electron, but the "Pauli Exclusion Principle" says that they must have opposite spin.

| Here, Iron has had three of it's eight valence electrons removed to give it a 3+ charge. The remaining electrons are can be arranged in two ways - either all pointing "up", or in pairs with one "unpaired" electron.

Links: What is Magnetism? and Moving Electrons

Magnetism

There are several different types of magnetism, including paramagnetism, diamagnetism, ferromagnetism, ferrimagnetism and antiferromagnetism. What all these have in common is that they are an effect caused by electrons acting alone or together. Electrons can be "paired" (two of opposite spin together) or "unpaired" (one alone). Unpaired electrons on one atom can communicate with unpaired electrons on another atom - they can line up parallel, that is all in the same direction, or anti-parallel, which is in the opposite direction. This is the origin of magnetism.

Regions in a material where all the spins on all the atoms line up are called domains or grains. Most materials are made up of many domains and in between the domains there are domain walls where the direction if the spin changes. Since the direction of spins cannot change suddenly as that requires a lot of energy, they usually gradually change direction, making the domain walls quite thick.

Gradual changes in the direction that the spins align at a domain wall. The spin direction changes in a helical fashion. |

Links: Nasa - Magnetism, Introduction to Magnetism, Molecule-based Magnets and Magnetism Snacks!

Magnetic Susceptibility

Magnetic fields produce lines of magnetic force around an material. The density of these lines of force is called the "Magnetic Flux Density" and is given the symbol B. For a vacuum this is related to the magnetic field (H) by a constant called "The Permittivity of Free Space", which is given the symbol, m 0.

B = m 0 H
| The easiest way to see these lines is the school science experiment where a magnet is covered with a piece of paper and iron filings are sprinkled on top. The filings line themselves up along the lines of force.

Magnetic materials when placed in a field however, may increase or decrease the magnetic flux density, which has the effect of increasing or decreasing the field by a factor M, the "Magnetisation".

B = m 0 (H+M)

The magnetisation is usually discussed in relative terms using the "Magnetic Susceptibility", C (The Greek letter chi).

C = M/H

Links: Comparison of the Different Magnetisms and Magnetic Susceptibility

Diamagnetism

Since this is the interaction between paired electrons, to some degree, all materials are diamagnetic. However, as it is a very week effect, in many cases there are other more powerful forms of magnetism that dominate. It is caused by the movement of electrons within the atoms.

Diamagnetic materials reduce the density of force lines compared with a vacuum i.e. there are fewer force lines through the material than if the material wasn't there. This means they have negative values of M (the magnetisation) and negative values of C (the magnetic susceptibility). In practice this means that the density of the field lines within a diamagnetic material is lower than that for a vacuum.

The magnetic field is expelled from the diamagnetic sample, decreasing the density of the field lines. The field (H) is shown in red.

Links: Comparison of the Different Magnetisms, The Wikipedia and Diamagnetic Snack!

Paramagnetism

Paramagnetism is caused by the interactions between unpaired electrons within an individual atom. In contrast to diamagnetism, M and C are positive and large so that diamagnetic effects are negligible compared with paramagnetism.

Since paramagnetic materials have a positive magnetisation, the field lines within the material are more dense than that of a vacuum.

The magnetic field is drawn into the paramagnetic sample, increasing the density of the field lines. The field (H) is shown in red.

Paramagnetic materials are common, but perhaps one of the best known examples is oxygen. Liquid oxygen is pale blue and therefore easily visible. Since it has two unpaired electrons, oxygen is paramagnetic and a common demonstration of this involves "picking up" liquid oxygen by dipping a magnet into it. The oxygen is attracted to the magnet and is drawn away from the surface of the liquid. Although the oxygen evaporates very quickly the effect is clearly visible.

Importently, NMR cannot be used to characterise paramagnetic materials as the magnetism is much larger in magnitude than the mgnetism caused by the nuclear spin.

A major use of ferromagnetic and ferrimagnetic materials is as transformer cores and are used in motors. Infact most modern cars benefit from more than twenty magnets.

Links: Comparison of the Different Magnetisms, Nitrogen and Oxygen and Paramagnetic Compounds

Ferromagnetism

Like paramagnetism, the origin of ferromagnetism is in unpaired electrons. Whereas paramagnetism is caused by interactions between unpaired electrons within one atom, ferromagnetism is caused by interactions between electrons within different atoms (and this is true for antiferromagnetism and ferrimagnetism as well).

Ferromagnetism is caused by the spins of unpaired electrons align making areas or domains where the spins are in the same direction. In some cases these domains can be temporarily aligned by placing the material in a magnetic field. The classic example of this is magnetising a needle by stroking it with a magnet. This needle can then (with care!) be floated on the surface of water and the direction it points to can be controlled by moving the magnet near it. This is in essence how a compass works, except that the compass needle aligns within the magnetic field created by the movement of the earth's iron core.

In some cases a magnet might be "locked" into the aligned state. These are called permanent magnets and the magnetism can only be removed by heating it. A permamnent magnet can be made by slowly cooling certain melted ferromagnetic materials within a magnetic field. This causes the domains to align and once aligned they can't escape the field caused by their neighbours.

A domain...

Ferromagnetic materials have been used for a long time to make permanent magnets. CrO2 is a ferromagnetic material that has been widely used in the manufacture of audio tapes. Chromium dioxide powder on the tape is magnetised by the tape head. This is a piece of iron with a wire wrapped round it which is connected to the diaphragm of the microphone. The diaphragm vibrates when sound waves hit it and passes this vibration on to the wire of the tape head. This in turn, also vibrates in and out of a magnetic field causeing a fluctuating electric current in the coil (dependent on the frequency of the vibrations). These cause a fluctuating magnetic field in the iron of the tape head, magnetising the tape. This process is reversed on play back.

Links: Comparison of the Different Magnetisms and Permanent Magnets

Antiferromagnetism

Like ferromagnetism, the origin of antiferromagnetism is also in unpaired electrons. In ferromagnetism however, the spins line up in the same direction, but in antiferromagnetism, the spins alternate so that overall there is no net direction i.e. the spins cancel out.

A domain...

Nickel oxide (NiO) is the most commonly quited example of an antiferromagnetic structure. However, while MnO, FeO, CoO and NiO are all antiferromagnetic, they all have a transtions and become paramagnetic too. This transition occurs at the Néel temperature.

There are also occasions where the spins don't line up properly, but the overall magnetic moment is zero. These are called imperfect antiferromagnets and are quite common in nature.

A domain...

Links: Comparison of the Different Magnetisms, Louis Néel and The Secrets of Antiferromagnetism (Berkeley Lab)

Ferrimagnetism

Ferrimagnetism is almost like an intermediate between ferromagnetism and antiferromagnetism. In the case of ferrimagnetism, like antiferromagnetism the spins alternate, however like ferromagnetism there is an overall net direction because the spins don't cancel out completely - i.e. the size of the contribution in one direction is larger than in the other.

A domain...

Early computers used ferrimagnetic materials as data storage devices. Some ferrimagnetic materials have a very square hysteresis curve i.e. the transition from ferrimagnetic to paramagnetic is very abrupt. Though superseded now, this made ideal memory for computers where a clean, reliable transition between the two states, "0" and "1" is vital.

Links: Comparison of the Different Magnetisms and Making Magnetite and Heamatite

A Comparison of Different Magnetisms

All these types of magnetism can seem confusing, but they all have subtle differences leading to important variations in their behaviour.

| Sign of Magnetic Susceptibility (C) | Cause
Diamagnetism | Negative | Interactions between paired electrons within atoms
Paramagnetism | Positive | Interactions between unpaired electrons within atoms
Ferromagnetism | Positive | Interaction between unpaired electrons between different atoms which line up parallel
Antiferromagnetism | Positive | Interaction between unpaired electrons between different atoms which line up anti-parallel, so there is no overall spin "direction"
Ferrimagnetism | Positive | Interaction between unpaired electrons between different atoms which line up anti-parallel, but the spins don't cancel out leaving a net spin

Practically, the different types of magnetism can be distinguished by the way the Magnetic Susceptibility (C) varies with temperature.

Diamagnetism (C is independent of T) Paramagnetism C = C/T Ferromagnetism (Below TC) C = C/(T - TC) Antiferromagnetism (Below TN) C = C/(T +TN) |

This variation with temperature is described by the "Curie Law".

For a paramagnetic material the Magnetic susceptibility is inversely proportional to temperature. C is material dependent and is known as the "Curie Constant". | C = C/T
For a ferromagnetic material the Curie Law is modified to take in to account the change over from cooperative behavior (ferromagnetic) to none cooperative behaviour (paramagnetic). TC is the "Curie Temperature" for the material. | C = C/(T - TC)
For an antiferromagnetic material the "Curie Law" is again modified. In this case the transition is known as the "NéelTemperaure", TN. | C = C/(T + TN)

Links: Magnetics and More Magnetics!

Last updated 29 October, 2002