demagnetization

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WordNet

1. the process of removing magnetization (同)demagnetisation
2. erase (a magnetic storage device) (同)demagnetise
3. make nonmagnetic; take away the magnetic properties (of); "demagnetize the iron shavings"; "they degaussed the ship" (同)demagnetise, degauss

PrepTutorEJDIC

1. 磁気を除くこと,消磁

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/12/17 17:33:04」(JST)

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This article is about magnetization as it appears in Maxwell's equations of classical electrodynamics. For a microscopic description of how magnetic materials react to a magnetic field, see magnetism. For mathematical description of fields surrounding magnets and currents, see magnetic field.

Electromagnetism
Electricity Magnetism
Electrostatics Electric charge Static electricity Electric field Conductor Insulator Triboelectricity Electrostatic discharge Induction Coulomb's law Gauss's law Electric flux / potential energy Electric dipole moment Polarization density
Magnetostatics Ampère's law Magnetic field Magnetization Magnetic flux Biot–Savart law Magnetic dipole moment Gauss's law for magnetism
Electrodynamics Lorentz force law Electromagnetic induction Faraday's law Lenz's law Displacement current Magnetic potential Maxwell's equations Electromagnetic field Electromagnetic radiation Maxwell tensor Poynting vector Liénard–Wiechert potential Jefimenko's equations Eddy current London equations
Electrical network Electric current Electric potential Voltage Resistance Ohm's law Series circuit Parallel circuit Direct current Alternating current Charged current Neutral current Electromotive force Capacitance Inductance Impedance Resonant cavities Waveguides
Covariant formulation Electromagnetic tensor (stress–energy tensor) Four-current Electromagnetic four-potential
Scientists Ampère Coulomb Faraday Gauss Heaviside Henry Hertz Lorentz Maxwell Tesla Volta Weber Ørsted

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 uniform 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.

Definition

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.

The M-field is measured in amperes per meter (A/m) in SI units.

Physics Application

The magnetization is often not listed as a material parameter for commercially available ferromagnets. Instead the parameter that is listed is residual flux density, denoted Br. Physicists often need the magnetization to calculate the moment of a ferromagnet.

To calculate the dipole moment using the formula: , we have that

m = dipole moment (in A m²) = Br V / μo, where:

Br is Br max, the Residual Flux Density, expressed in Tesla.

V is the volume of the magnet, expressed in cubic meters.

μo is the permeability of a vacuum, or 4 π x 10^-7 N/A².

Magnetization in Maxwell's equations

The behavior of magnetic fields (B, H), electric fields (E, D), charge density (ρ), and current density (J) is described by Maxwell's equations. The role of the magnetization is described below.

Relations between B, H, and M

Main article: Magnetic field

The magnetization defines the auxiliary magnetic field H as

(SI units)

(Gaussian units)

which is convenient for various calculations. The vacuum permeability μ₀ is, by definition, 6993400000000000000♠4π×10⁻⁷ V·s/(A·m).

A relation between M and H exists in many materials. In diamagnets and paramagnets, the relation is usually linear:

where χ_m is called the volume magnetic susceptibility.

In ferromagnets there is no one-to-one correspondence between M and H because of Magnetic hysteresis.

Magnetization current

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 J_f 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.

Magnetostatics

Main article: Magnetostatics

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.

Magnetization dynamics

Main article: Magnetization dynamics

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.

Reversal

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:

1. an applied magnetic field
2. spin injection via a beam of particles with spin
3. magnetization reversal by circularly polarized light; i.e., incident electromagnetic radiation that is circularly polarized

Demagnetization

Main article: Degaussing

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.

See also

• Permeability (electromagnetism)
• Magnetic susceptibility
• Earth's magnetic field
• Geomagnetic reversal
• Geomagnetic excursion
• Magnetometer
• Orbital magnetization

The dictionary definition of magnetization at Wiktionary

References

English Journal

• Experiments Optimized for Magic Angle Spinning and Oriented Sample Solid-State NMR of Proteins.

• Das BB, Lin EC, Opella SJ.AbstractStructure determination by solid-state NMR of proteins is rapidly advancing as result of recent developments of samples, experimental methods, and calculations. There are a number of different solid-state NMR approaches that utilize stationary, aligned samples or magic angle spinning of unoriented 'powder' samples, and depending on the sample and the experimental method can emphasize the measurement of distances or angles, ideally both, as sources of structural constraints. Multi-dimensional correlation spectroscopy of low-gamma nuclei such as 15N and 13C is an important step for making resonance assignments and measurements of angular restraints in membrane proteins. However, the efficiency of coherence transfer predominantly depends upon the strength of dipole-dipole interaction, and this can vary from site to site and between sample alignments, for example, during the mixing of 13C and 15N magnetization in stationary aligned and in magic angle spinning samples. Here, we demonstrate that the efficiency of polarization transfer can be improved by using adiabatic demagnetization and remagnetization techniques on stationary aligned samples; and proton assisted insensitive nuclei cross-polarization in magic angle sample spinning samples. Adiabatic cross-polarization technique provides an alternative mechanism for spin-diffusion experiments correlating 15N/15N and 15N/13C chemical shifts over large distances. Improved efficiency in cross-polarization with 40% - 100% sensitivity enhancements are observed in proteins and single crystals, respectively. We describe solid-state NMR experimental techniques that are optimal for membrane proteins in liquid crystalline phospholipid bilayers under physiological conditions. The techniques are illustrated with data from both single crystals of peptides and of membrane proteins in phospholipid bilayers.
• The journal of physical chemistry. B.J Phys Chem B.2013 Sep 17. [Epub ahead of print]
• Structure determination by solid-state NMR of proteins is rapidly advancing as result of recent developments of samples, experimental methods, and calculations. There are a number of different solid-state NMR approaches that utilize stationary, aligned samples or magic angle spinning of unoriented '
• PMID 24044695

• Demagnetization treatment of remanent composite microspheres studied by alternating current susceptibility measurements.

• van Berkum S, Erné BH.SourceVan't Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands. b.h.erne@uu.nl.
• International journal of molecular sciences.Int J Mol Sci.2013 Sep 4;14(9):18093-109. doi: 10.3390/ijms140918093.
• The magnetic remanence of silica microspheres with a low concentration of embedded cobalt ferrite nanoparticles is studied after demagnetization and remagnetization treatments. When the microspheres are dispersed in a liquid, alternating current (AC) magnetic susceptibility spectra reveal a constant
• PMID 24009021

• Thermally assisted all-optical helicity dependent magnetic switching in amorphous Fe(100-x)Tb(x) alloy films.

• Hassdenteufel A, Hebler B, Schubert C, Liebig A, Teich M, Helm M, Aeschlimann M, Albrecht M, Bratschitsch R.SourceInstitute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany.
• Advanced materials (Deerfield Beach, Fla.).Adv Mater.2013 Jun 11;25(22):3122-8. doi: 10.1002/adma.201300176. Epub 2013 Apr 25.
• All-optical switching (AOS) in ferrimagnetic Fe(100-x)Tb(x) alloys is presented. AOS is witnessed below, above, and in samples without a magnetic compensation point. It is found that AOS is associated with laser heating up to the Curie temperature and intimately linked to a low remanent sample magne
• PMID 23616209

Japanese Journal

• モータ用永久磁石の3次元減磁分布測定手法の開発(第1報) (回転機 リニアドライブ 家電・民生合同研究会・小形モータ一般・永久磁石モータ一般/リニアドライブ一般/家電・民生一般)

• 荒木 辰太郎,浅野 能成,山際 昭雄
• 電気学会研究会資料. HCA 2015(24-40), 65-70, 2015-08-06
• NAID 40020572796

• 減磁を考慮した磁石内側配置HEFSMの設計検討 (モータドライブ 回転機 自動車合同研究会 自動車用など用途指向形モータドライブ : 小型軽量・高効率・高機能化の実現へ向けた次世代モータドライブ)

• 東松 拓実,小坂 卓,松井 信行
• 電気学会研究会資料. VT 2015(1-17), 75-80, 2015-07-22
• NAID 40020550133

• 減磁を考慮した磁石内側配置HEFSMの設計検討 (モータドライブ 回転機 自動車合同研究会・自動車用など用途指向形モータドライブ : 小型軽量・高効率・高機能化の実現へ向けた次世代モータドライブ)

• 東松 拓実,小坂 卓,松井 信行
• 電気学会研究会資料. MD 2015(73-89), 75-80, 2015-07-22
• NAID 40020550093

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