Curie temperature
In analogy to ferromagnetic and paramagnetic materials, the Curie temperature can also be used to describe the phase transition between ferroelectricity and paraelectricity.
A loadstone loses some of its virtue by too great a heat; for its humour is set free, whence its peculiar nature is marred.
This has a randomizing effect on aligned magnetic domains, leading to the disruption of order, and the phenomena of the Curie point.
[6][7] Ferromagnetic, paramagnetic, ferrimagnetic, and antiferromagnetic materials have different intrinsic magnetic moment structures.
Ferromagnetic, paramagnetic, ferrimagnetic, and antiferromagnetic structures are made up of intrinsic magnetic moments.
If all the electrons within the structure are paired, these moments cancel out due to their opposite spins and angular momenta.
Below the Curie temperature, the intrinsic structure has undergone a phase transition,[16] the atoms are ordered, and the material is ferromagnetic.
[17] The Boltzmann factor contributes heavily as it prefers interacting particles to be aligned in the same direction.
[19] Below the Curie temperature, the atoms are aligned and parallel, causing spontaneous magnetism; the material is ferromagnetic.
[20] Below the Curie temperature the atoms of each ion are aligned anti-parallel with different momentums causing a spontaneous magnetism; the material is ferrimagnetic.
[21] It is named after Louis Néel (1904–2000), who received the 1970 Nobel Prize in Physics for his work in the area.
The Curie–Weiss law is a simple model derived from a mean-field approximation, this means it works well for the materials temperature, T, much greater than their corresponding Curie temperature, TC, i.e. T ≫ TC; it however fails to describe the magnetic susceptibility, χ, in the immediate vicinity of the Curie point because of correlations in the fluctuations of neighboring magnetic moments.
[35] The Ising model is mathematically based and can analyse the critical points of phase transitions in ferromagnetic order due to spins of electrons having magnitudes of ±1/2.
The spins interact with their neighbouring dipole electrons in the structure and here the Ising model can predict their behaviour with each other.
For example, the surface and bulk properties depend on the alignment and magnitude of spins and the Ising model can determine the effects of magnetism in this system.
[38] The position of particles can therefore have different orientations around the surface than the main part (bulk) of the material.
[42] For terbium which is a rare-earth metal and has a high orbital angular momentum the magnetic moment is strong enough to affect the order above its bulk temperatures.
For example, a composite which has silver in it can create spaces for oxygen molecules in bonding which decreases the Curie temperature[44] as the crystal lattice will not be as compact.
In more than one dimension the Curie temperature begins to increase as the magnetic moments will need more thermal energy to overcome the ordered structure.
Due to the small size of particles (nanoparticles) the fluctuations of electron spins become more prominent, which results in the Curie temperature drastically decreasing when the size of particles decreases, as the fluctuations cause disorder.
The size of a particle also affects the anisotropy causing alignment to become less stable and thus lead to disorder in magnetic moments.
In this phenomenon, fluctuations are very influential causing magnetic moments to change direction randomly and thus create disorder.
The Curie temperature of nanoparticles is also affected by the crystal lattice structure: body-centred cubic (bcc), face-centred cubic (fcc), and a hexagonal structure (hcp) all have different Curie temperatures due to magnetic moments reacting to their neighbouring electron spins.
fcc and hcp have tighter structures and as a results have higher Curie temperatures than bcc as the magnetic moments have stronger effects when closer together.
Pressure directly affects the kinetic energy in particles as movement increases causing the vibrations to disrupt the order of magnetic moments.
For example, the delocalised electrons can be moved onto the same plane by applied strains within the crystal lattice.
[48] The Curie temperature is seen to increase greatly due to electrons being packed together in the same plane, they are forced to align due to the exchange interaction and thus increases the strength of the magnetic moments which prevents thermal disorder at lower temperatures.
Hence, TC is the temperature where ferroelectric materials lose their spontaneous polarisation as a first or second order phase change occurs.
A heat-induced ferromagnetic-paramagnetic transition is used in magneto-optical storage media for erasing and writing of new data.
Curie point electro-magnets have been proposed and tested for actuation mechanisms in passive safety systems of fast breeder reactors, where control rods are dropped into the reactor core if the actuation mechanism heats up beyond the material's Curie point.
