The principle of ferromagnetic are two quantum mechanics phenomenon: spin and Pauli exclusion principle.
Electron spin and the orbital angular momentum in a dipole magnetic moment and the formation of a magnetic field.
In most of the material of all electronic total dipole magnetic moment is zero.
Only the shell of the atoms (
May be in the absence of external magnetic field a net magnetic moment.
There are many such electronic ferromagnetic material.
If they are arranged together they can get together to produce an observable macroscopic field.
The dipole tend to be pointing in the direction of external magnetic field.
This phenomenon is called the paramagnetism.
Dipole of ferromagnetic materials tend to be in the absence of external magnetic field is pointing to the same direction.
This is a phenomenon of quantum mechanics.
According to the classical electromagnetism two neighboring magnetic dipole tend to be pointing in opposite directions (
Cause antiferromagnetic material)
But they tend to point to the same direction in the ferromagnetic material.
The reason is that the Pauli exclusion principle: two spin the same electron cannot occupy the same position, so that they can feel additional repelling force, reduce the electric potential energy.
The energy difference is called can exchange, it leads to a nearby electronic arranged in direction.
On the long distance,
Thousands of ions)
Can exchange the role of relative arranged by classic dipole gradually trend, it was in the balance,
Cases of ferromagnetic material dipole in general don't line up.
In the absence of magnetism of the ferromagnetic material in the magnetic dipole is divided outside the domain.
Each domain, the internal short geomagnetic dipole array point to the same direction, but in long distance on different domain does not agree the arrangement of the magnetic dipole.
Different domain, the boundary between known as domain wall, the atoms within the domain wall to change gradually.
Therefore a piece of iron without magnetic or its magnetic is very weak.
But in a strong enough in the external magnetic field, all outside the domain will line up along the magnetic field, outside the external magnetic field disappeared after the si domain will continue to keep the same point.
The magnetic field and the relationship between the external magnetic field by a hysteresis curve.
Although the neat rows of outside the domain of energy is not the lowest, but it is very stable.
At the bottom of the sea of magnetite millions of years to pointing in the direction it formed when the earth's magnetic field.
By heating and then cooling in the absence of external magnetic field of the magnetic field of the magnets will disappear.
Temperature after heat oscillation (
Compete with ferromagnetism of dipole array.
Temperature is higher than the Curie point after the secondary phase change occurs in the crystal, the entire system cannot be magnetized, with the help of external magnetic field when the ferromagnetic material shows paramagnetism.
Under the Curie point symmetry broken, outside the domain.
Curie point itself is a threshold, theoretically the magnetic susceptibility to infinity, although there is no magnetization, but outside of any length range similar spin fluctuation in the domain.
Especially the use of simplified ising spin model to study the ferromagnetic phase transition plays a huge role to the development of statistical physics.
Here mean field theory obviously unable to correctly predict the Curie point phenomenon, need to be replaced by renormalization group theory.