In classical electromagnetism, magnetization (magnetisation in British English) or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. The origin of the magnetic moments responsible for magnetization can be either microscopic electric currents resulting from the motion of electrons in atoms, or the spin of the electrons or the nuclei. Net magnetization results from the response of a material to an external magnetic field, together with any unbalanced magnetic dipole moments that may be inherent in the material itself; for example, in ferromagnets. Magnetization is not always homogeneous within a body, but rather varies between different points. Magnetization also describes how a material responds to an applied magnetic field as well as the way the material changes the magnetic field, and can be used to calculate the forces that result from those interactions. It can be compared to electric polarization, which is the measure of the corresponding response of a material to an electric field in electrostatics. Physicists and engineers define magnetization as the quantity of magnetic moment per unit volume. It is represented by a vector M.
The magnetization field or M-field can be defined according to the following equation:
Where dm is the elementary magnetic moment and dV is the volume element; in other words, the M-field is the distribution of magnetic moments in the region or manifold concerned. This is better illustrated through the following relation:
where m is an ordinary magnetic moment and the triple integral denotes integration over a volume. This makes the M-field completely analogous to the electric polarisation field, or P-field, used to determine the electric dipole moment p generated by a similar region or manifold with such a polarization:
Where dp is the elementary electric dipole moment.
Magnetization in Maxwell's equations
Relations between B, H, and M
The magnetization defines the auxiliary magnetic field H as
- (SI units)
where χm is called the volume magnetic susceptibility.
The magnetization M makes a contribution to the current density J, known as the magnetization current or bound (volumetric) current:
and for the bound surface current:
so that the total current density that enters Maxwell's equations is given by
where Jf is the electric current density of free charges (also called the free current), the second term is the contribution from the magnetization, and the last term is related to the electric polarization P.
In the absence of free electric currents and time-dependent effects, Maxwell's equations describing the magnetic quantities reduce to
These equations can be solved in analogy with electrostatic problems where
In this sense −∇⋅M plays the role of a fictitious "magnetic charge density" analogous to the electric charge density ρ; (see also demagnetizing field).
It is important to note that there is no such thing as a "magnetic charge," but that issue was still debated through the whole 19th century. Other concepts, that went along with it, such as the auxiliary field H, also have no real physical meaning in their own right. However, they are convenient mathematical tools, and are therefore still used today for applications such as modeling the magnetic field of the Earth.
The time-dependent behavior of magnetization becomes important when considering nanoscale and nanosecond timescale magnetization. Rather than simply aligning with an applied field, the individual magnetic moments in a material begin to precess around the applied field and come into alignment through relaxation as energy is transferred into the lattice.
Magnetization reversal, also known as switching, refers to the process that leads to a 180° (arc) re-orientation of the magnetization vector with respect to its initial direction, from one stable orientation to the opposite one. Technologically, this is one of the most important processes in magnetism that is linked to the magnetic data storage process such as used in modern hard disk drives. As it is known today, there are only a few possible ways to reverse the magnetization of a metallic magnet:
- an applied magnetic field
- spin injection via a beam of particles with spin
- magnetization reversal by circularly polarized light; i.e., incident electromagnetic radiation that is circularly polarized
Demagnetization is the reduction or elimination of magnetization. One way to do this is to heat the object above its Curie temperature, where thermal fluctuations have enough energy to overcome exchange interactions, the source of ferromagnetic order, and destroy that order. Another way is to pull it out of an electric coil with alternating current running through it, giving rise to fields that oppose the magnetization.
One application of demagnetization is to eliminate unwanted magnetic fields. For example, magnetic fields can interfere with electronic devices such as cell phones or computers, and with machining by making cuttings cling to their parent.
- Permeability (electromagnetism)
- Magnetic susceptibility
- Earth's magnetic field
- Geomagnetic reversal
- Geomagnetic excursion
- Orbital magnetization
The dictionary definition of magnetization at Wiktionary
- "Units for Magnetic Properties" (PDF). Lake Shore Cryotronics, Inc. Retrieved 2009-10-24.
- Stohr, J.; Siegmann, H. C. (2006), Magnetism: From fundamentals to Nanoscale Dynamics, Springer-Verlag
- Stanciu, C. D. et al. (2007), Physical Review Letters 99, 217204
- "Magnetic Component Engineering". Magnetic Component Engineering. Retrieved April 18, 2011.
- "Demagnetization". Introduction to Magnetic Particle Inspection. NDT Resource Center. Retrieved April 18, 2011.