Permeability (electromagnetism)
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In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter μ. The term was coined in September, 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.
In SI units, permeability is measured in henries per meter (H·m^{−1}), or newtons per ampere squared (N·A^{−2}). The permeability constant (μ_{0}), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined)^{[1]} value µ_{0} = 4π×10^{−7} H·m^{−1}≈ 1.2566370614…×10^{−6} H·m^{−1} or N·A^{−2}).
A closely related property of materials is magnetic susceptibility, which is a measure of the magnetization of a material in addition to the magnetization of the space occupied by the material.
Contents
Explanation[edit]
In electromagnetism, the auxiliary magnetic field H represents how a magnetic field B influences the organization of magnetic dipoles in a given medium, including dipole migration and magnetic dipole reorientation. Its relation to permeability is
where the permeability, μ, is a scalar if the medium is isotropic or a second rank tensor for an anisotropic medium.
In general, permeability is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a nonlinear medium, the permeability can depend on the strength of the magnetic field. Permeability as a function of frequency can take on real or complex values. In ferromagnetic materials, the relationship between B and H exhibits both nonlinearity and hysteresis: B is not a singlevalued function of H,^{[2]} but depends also on the history of the material. For these materials it is sometimes useful to consider the incremental permeability defined as
This definition is useful in local linearizations of nonlinear material behavior, for example in a Newton–Raphson iterative solution scheme that computes the changing saturation of a magnetic circuit.
Permeability is the inductance per unit length. In SI units, permeability is measured in henrys per metre (H·m^{−1} = J/(A^{2}·m) = N A^{−2}). The auxiliary magnetic field H has dimensions current per unit length and is measured in units of amperes per metre (A m^{−1}). The product μH thus has dimensions inductance times current per unit area (H·A/m^{2}). But inductance is magnetic flux per unit current, so the product has dimensions magnetic flux per unit area. This is just the magnetic field B, which is measured in webers (voltseconds) per squaremetre (V·s/m^{2}), or teslas (T).
B is related to the Lorentz force on a moving charge q:
The charge q is given in coulombs (C), the velocity v in meters per second (m/s), so that the force F is in newtons (N):
H is related to the magnetic dipole density. A magnetic dipole is a closed circulation of electric current. The dipole moment has dimensions current times area, units ampere squaremetre (A·m^{2}), and magnitude equal to the current around the loop times the area of the loop.^{[3]} The H field at a distance from a dipole has magnitude proportional to the dipole moment divided by distance cubed,^{[4]} which has dimensions current per unit length.
Relative permeability and magnetic susceptibility[edit]
Relative permeability, sometimes denoted by the symbol μ_{r}, is the ratio of the permeability of a specific medium to the permeability of free space, μ_{0}:
where μ_{0} = 4π × 10^{−7} N A^{−2}. In terms of relative permeability, the magnetic susceptibility is
χ_{m}, a dimensionless quantity, is sometimes called volumetric or bulk susceptibility, to distinguish it from χ_{p} (magnetic mass or specific susceptibility) and χ_{M} (molar or molar mass susceptibility).
Diamagnetism[edit]
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition of an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment in the direction opposing the external field. Diamagnets are materials with a magnetic permeability less than μ_{0} (a relative permeability less than 1).
Consequently, diamagnetism is a form of magnetism that a substance exhibits only in the presence of an externally applied magnetic field. It is generally a quite weak effect in most materials, although superconductors exhibit a strong effect.
Paramagnetism[edit]
Paramagnetism is a form of magnetism which occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields, hence have a relative magnetic permeability greater than one (or, equivalently, a positive magnetic susceptibility). The magnetic moment induced by the applied field is linear in the field strength and rather weak. It typically requires a sensitive analytical balance to detect the effect. Unlike ferromagnets, paramagnets do not retain any magnetization in the absence of an externally applied magnetic field, because thermal motion causes the spins to become randomly oriented without it. Thus the total magnetization will drop to zero when the applied field is removed. Even in the presence of the field there is only a small induced magnetization because only a small fraction of the spins will be oriented by the field. This fraction is proportional to the field strength and this explains the linear dependency. The attraction experienced by ferromagnets is nonlinear and much stronger, so that it is easily observed, for instance, in magnets on one's refrigerator.
Gyromagnetism[edit]
For gyromagnetic media (see Faraday rotation) the magnetic permeability response to an alternating electromagnetic field in the microwave frequency domain is treated as a nondiagonal tensor expressed by:^{[5]}
Values for some common materials[edit]
The following table should be used with caution as the permeability of ferromagnetic materials varies greatly with field strength. For example 4% Si steel has an initial relative permeability (at or near 0T) of 2,000 and a maximum of 35,000^{[6]} and, indeed, the relative permeability of any material at a sufficiently high field strength tends to 1.
Medium  Susceptibility χ_{m} (volumetric SI) 
Permeability μ [H/m]  Relative Permeability μ/μ_{0}  Magnetic field  Frequency max. 

Metglas  1.25  1,000,000^{[7]}  at 0.5 T  100 kHz  
Nanoperm  10×10^{−2}  80,000^{[8]}  at 0.5 T  10 kHz  
Mumetal  2.5×10^{−2}  20,000^{[9]}  at 0.002 T  
Mumetal  50,000^{[10]}  
CobaltIron (high permeability strip material)  18,000^{[11]}  
Permalloy  8,000  1.0×10^{−2}  8,000^{[9]}  at 0.002 T  
Electrical steel  5.0×10^{−3}  4,000^{[9]}  at 0.002 T  
Ferritic Stainless Steel (annealed)  1000  1800^{[12]}  
Martensitic Stainless Steel (annealed)  750  950^{[12]}  
Ferrite (manganese zinc)  >8.0×10^{−4}  640 (or more)  100 kHz ~ 1 MHz  
Ferrite (nickel zinc)  2.0×10^{−5} – 8.0×10^{−4}  16–640  100 kHz ~ 1 MHz^{[citation needed]}  
Carbon Steel  8.75×10^{−4}  100^{[9]}  at 0.002 T  
Nickel  1.25×10^{−4}  100^{[9]} – 600  at 0.002 T  
Martensitic Stainless Steel (hardened)  40  95^{[12]}  
Austenitic Stainless Steel  1.003  7 ^{[13]}^{[12]} ^{[note 1]}  
Neodymium magnet  1.05^{[14]}  
Platinum  1.2569701×10^{−6}  1.000265  
Aluminum  2.22×10^{−5}^{[15]}  1.2566650×10^{−6}  1.000022  
Wood  1.00000043^{[15]}  
Air  1.2566375×10^{−6}  1.00000037 ^{[16]}  
Concrete (dry)  1^{[17]}  
Vacuum  0  π4×10^{−7} (μ_{0})  1^{[18]}  
Hydrogen  −2.2×10^{−9}^{[15]}  1.2566371×10^{−6}  1.0000000  
Teflon  1.2567×10^{−6}^{[9]}  1.0000  
Sapphire  −2.1×10^{−7}  1.2566368×10^{−6}  0.99999976  
Copper  −6.4×10^{−6} or −9.2×10^{−6}^{[15]} 
1.2566290×10^{−6}  0.999994  
Water  −8.0×10^{−6}  1.2566270×10^{−6}  0.999992  
Bismuth  −1.66×10^{−4}  0.999834  
Superconductors  −1  0  0 
A good magnetic core material must have high permeability.^{[19]}
For passive magnetic levitation a relative permeability below 1 is needed (corresponding to a negative susceptibility).
Permeability varies with magnetic field. Values shown above are approximate and valid only at the magnetic fields shown. They are given for a zero frequency; in practice, the permeability is generally a function of the frequency. When frequency is considered the permeability can be complex, corresponding to the in phase and out of phase response.
Note that the magnetic constant μ_{0} has an exact value in SI units (that is, there is no uncertainty in its value), because the definition of the ampere fixes its value to 4π × 10^{−7} H/m exactly.
Complex permeability[edit]
A useful tool for dealing with high frequency magnetic effects is the complex permeability. While at low frequencies in a linear material the magnetic field and the auxiliary magnetic field are simply proportional to each other through some scalar permeability, at high frequencies these quantities will react to each other with some lag time.^{[20]} These fields can be written as phasors, such that
where is the phase delay of from . Understanding permeability as the ratio of the magnetic flux density to the magnetic field, the ratio of the phasors can be written and simplified as
so that the permeability becomes a complex number. By Euler's formula, the complex permeability can be translated from polar to rectangular form,
The ratio of the imaginary to the real part of the complex permeability is called the loss tangent,
which provides a measure of how much power is lost in a material versus how much is stored.
See also[edit]
 Antiferromagnetism
 Diamagnetism
 Electromagnet
 Ferromagnetism
 Figure of merit
 Magnetic reluctance
 Paramagnetism
 Permittivity
 SI electromagnetism units
Notes[edit]
 ^ The permeability of Austenitic Stainless Steel strongly depends on the history of mechanical stress applied to it, such as cold working
References[edit]
 ^ "The NIST reference on fundamental physical constants". Physics.nist.gov. Retrieved 20111108.
 ^ Jackson (1975), p. 190
 ^ Jackson, John David (1975). Classical Electrodynamics (2nd ed. ed.). New York: Wiley. ISBN 047143132X. p. 182 eqn. (5.57)
 ^ Jackson (1975) p. 182 eqn. (5.56)
 ^ Kales, M. L. (1953). "Modes in Wave Guides Containing Ferrites". Journal of Applied Physics 24 (5): 604–601. Bibcode:1953JAP....24..604K. doi:10.1063/1.1721335.
 ^ G.W.C. Kaye & T.H. Laby, Table of Physical and Chemical Constants, 14th ed, Longman
 ^ ""Metglas Magnetic Alloy 2714A", ''Metglas''". Metglas.com. Retrieved 20111108.
 ^ ""Typical material properties of NANOPERM", ''Magnetec''" (PDF). Retrieved 20111108.
 ^ ^{a} ^{b} ^{c} ^{d} ^{e} ^{f} ""Relative Permeability", ''Hyperphysics''". Hyperphysics.phyastr.gsu.edu. Retrieved 20111108.
 ^ "Nickel AlloysStainless Steels, Nickel Copper Alloys, Nickel Chromium Alloys, Low Expansion Alloys". Nickelalloys.net. Retrieved 20111108.
 ^ ""Soft Magnetic CobaltIron Alloys", ''Vacuumschmeltze''". www.vacuumschmeltze.com. Retrieved 2013803.
 ^ ^{a} ^{b} ^{c} ^{d} Carpenter Technology Corporation (2013). "Magnetic Properties of Stainless Steels". Carpenter Technology Corporation.
 ^ British Stainless Steel Association (2000). "Magnetic Properties of Stainless Steel". Stainless Steel Advisory Service.
 ^ Juha Pyrhönen, Tapani Jokinen, Valéria Hrabovcová (2009). Design of Rotating Electrical Machines. John Wiley and Sons. p. 232. ISBN 0470695161.
 ^ ^{a} ^{b} ^{c} ^{d} Richard A. Clarke. "Clarke, R. ''Magnetic properties of materials'', surrey.ac.uk". Ee.surrey.ac.uk. Retrieved 20111108.
 ^ B. D. Cullity and C. D. Graham (2008), Introduction to Magnetic Materials, 2nd edition, 568 pp., p.16
 ^ NDT.net. "Determination of dielectric properties of insitu concrete at radar frequencies". Ndt.net. Retrieved 20111108.
 ^ exactly, by definition
 ^ Dixon, L H (2001). "Magnetics Design 2  Magnetic Core Characteristics". Texas Instruments.
 ^ M. Getzlaff, Fundamentals of magnetism, Berlin: SpringerVerlag, 2008.
External links[edit]
 Electromagnetism  a chapter from an online textbook
 Relative Permeability
 Soil Permeability Test
 Magnetic Properties of Materials