Glossary

Anisotropy

The tendency for a magnetic material to be more easily magnetised in certain directions. It is this anisotropy that stabilises magnetism to allow it to be used.

Antiferromagnet

A magnetically ordered material where the moments on each atomic site cancel with their neighbours so that the overall moment is zero.

Coercivity

The reverse field that must be applied to a magnetised sample to reduce its overall magnetisation to zero.

Curie temperature

The critical temperature above which a ferromagnet becomes disordered and has only paramagnetic properties. In an antiferromagnet this critical temperature is called the NĂ©el temperature.

Diamagnet

A material that develops a magnetic moment opposite to any applied field. All materials show this effect very weakly, but it is masked by stronger effects in the other types of magetic matter.

Domain

A small region within a ferromagnet that is uniformly magnetised in a certain direction. They are typically microns in size but can be very different depending on the material and the shape of the sample.

Domain Wall

Thin regions between the domains where the magnetisation rotates from the direction in one domain to the other.

Electron

Fundamental particle possessing both charge and spin. Electrons are responsible for the bonding that forms solids from free atoms, and it delocalised electrons that give rise to the conducting properties of metals. The fact that the electron possesses spin gives rise to the magnetic properties of materials, along with the orbital motion of the electrons around and between atoms.

Exchange

The interaction that gives rise to magnetic ordering – it comes from the rules determining how indistinguishable particles, such as electrons, can be exchanged between systems in quantum mechanics.

Exchange Bias

This phenomenon results from the interactions between a ferromagnetic and antiferromagnetic layer, and comminly manifests itself as a shift, or bias, in the hysteresis loop of the ferromagnet. This is technologically important in the fabrication of spin-valves and MRAM cells.

Ferromagnet

A magnetically ordered material where all the moments are aligned inthe same direction, giving rise to a large net magnetisation. The name is derived from the latin word for iron, which is the most common example of such a material.

Giant Magnetoresistance

Often abbreviated to GMR, the giant magnetoresistance is the large drop in resistance in a magnetic multilayer.

Hysteresis loop

A plot of magnetisation against applied field for a ferromagnet exhibits hysteresis – it is not the same on forward and reverse sweeps. The open area between the two branches of the curve forms a loop. Hysteresis means that the magnetisation doesn’t just depend on the field and temperature of a sample, but also on what the field and temperature were previously.

MRAM

Magnetic random access memory – computer memory is now being developed that makes use of spin-valves and related devices. The major advantage over traditional semiconductor RAM is the lack of a need to supply power to refresh the data.

Moment

A magnetic dipole. Moments will align with a uniform field due to the torque exerted but will experience a force in a field gradient.

Multilayer

A nanostructure where different materials are built up on top of each other at the atomic scale. At the end of the 1980’s scientists first learnt to make sufficiently perfect structures from magnetic metals to uncover a whole variety of new physics unknown in bulk materials.

Neutron

A fundamental particle with zero charge but the same spin as the electron. Found in all atomic nuclei it is known to the condensed matter scientist as a useful probe of magnetic ordering in solids.

Paramagnet

A material that develops a magnetic moment parallel to any applied field. It contains microscopic permanent moments, like a ferromagnet, but the difference is they are only weakly interacting so that no long-ranged order can develop.

Remanance

The amount of magnetisation remaining after a ferromagnet has been magnetised in an applied field.

Spin

The intrinsic angular momentum possessed by fundamental particles – giving the appearance of them ‘spinning’. This angular momentum leads to a magnetic moment of fixed size for each particle.

Spin-Valve

A multilayer structure exhibiting a substantial giant magnetoresistance in a small field. The basic design features two magnetic layers one of which is ‘free’ to respond to fields whilst the other is ‘pinned’ using exchange bias. These are technologically very important, particularly as read-heads in the hard disk storage.

Stray field

The magnetic field found outside a magnetised body. Nature will try to minimise ths by forming domains within the sample – but for permanent magnets to be useful we need to ensure that as much field as possible is in the free space where we can use it.