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Magnetic susceptibility

In electromagnetism the magnetic susceptibility (latin: susceptibilis “receptiveness”) is the degree of magnetization of a material in response to an applied magnetic field.

Definition of volume susceptibility

Conversion between SI and cgs units

Note that these definitions are according to SI conventions. However, many tables of magnetic susceptibility give cgs values (often denoted by "emu" or "e.m.u.", short for electromagnetic unit) that rely on a different definition of the permeability of free space:

mathbf{B}^{text{cgs}} = mathbf{H}^{text{cgs}} + 4pimathbf{M}^{text{cgs}} = (1+4pichi_{v}^{text{cgs}}) mathbf{H}^{text{cgs}}

The dimensionless cgs value of volume susceptibility is multiplied by 4π to give the dimensionless SI volume susceptibility value:

chi_v^{text{SI}}=4pichi_v^{text{cgs}}

For example, the cgs volume magnetic susceptibility of water at 20°C is −7.19×10⁻⁷ which is −9.04×10⁻⁶ using the SI convention.

Mass susceptibility and molar susceptibility

There are two other measures of susceptibility, the mass magnetic susceptibility (χ_mass or χ_g, sometimes χ_m), measured in m³·kg⁻¹ in SI or in cm³·g⁻¹ in cgs and the molar magnetic susceptibility (χ_mol) measured in m³·mol⁻¹ (SI) or cm³·mol⁻¹ (cgs) that are defined below, where ρ is the density in kg·m⁻³ (SI) or g·cm⁻³ (cgs) and M is molar mass in kg·mol⁻¹ (SI) or g·mol⁻¹ (cgs).

chi_{text{mass}}=chi_v/rho

chi_{text{mol}}=Mchi_{text{mass}}=Mchi_v/rho

Sign of susceptibility: diamagnetics and other types of magnetism

If χ is positive, then (1+χ_v) > 1 (or, in cgs units, (1+4πχ_v) > 1) and the material can be paramagnetic, ferromagnetic, ferrimagnetic, or antiferromagnetic. In this case, the magnetic field is strengthened by the presence of the material. Alternatively, if χ is negative, then (1+χ_v) < 1 (or, in cgs units, (1+4πχ_v) < 1), and the material is diamagnetic. As a result, the magnetic field is weakened in the presence of the material.

Experimental methods to determine susceptibility

Volume magnetic susceptibility is measured by the force change felt upon the application of a magnetic field gradient . Early measurements were made using the Gouy balance where a sample is hung between the poles of an electromagnet. The change in weight when the electromagnet is turned on is proportional to the susceptibility. Today, high-end measurement systems use a superconductive magnet. An alternative is to measure the force change on a strong compact magnet upon insertion of the sample. This system, widely used today, is called the Evan's balance. For liquid samples, the susceptibility can be measured from the dependence of the NMR frequency of the sample on its shape or orientation.

Tensor susceptibility

The magnetic susceptibility of most crystals is not a scalar. Magnetic response M is dependent upon the orientation of the sample and can occur in directions other than that of the applied field H. In these cases, volume susceptibility is defined as a tensor

M_i=chi_{ij}H_j

where i and j refer to the directions (e.g., x, y and z in Cartesian coordinates) of the applied field and magnetization, respectively. The tensor is thus rank 2, dimension (3,3) describing the component of magnetization in the i-th direction from the external field applied in the j-th direction.

Differential susceptibility

In ferromagnetic crystals, the relationship between M and H is not linear. To accommodate this, a more general definition of differential susceptibility is used

chi^{d}_{ij} = frac{part M_i}{part H_j}

where $chi^{d}_{ij}$ is a tensor derived from partial derivatives of components of M with respect to components of H. When the coercivity of the material parallel to an applied field is the smaller of the two, the differential susceptibility is a function of the applied field and self interactions, such as the magnetic anisotropy. When the material is not saturated, the effect will be nonlinear and dependent upon the domain wall configuration of the material.

Susceptibility in the frequency domain

When the magnetic susceptibility is measured in response to an AC magnetic field (i.e. a magnetic field that varies sinusoidally), this is called AC susceptibility. AC susceptibility (and the closely-related "AC permeability") are complex quantities, and various phenomena (such as resonances) can be seen in AC susceptibility that cannot in constant-field (DC) susceptibility. In particular, when an ac-field is applied perpendicular to the detection direction (called the "transverse susceptibility" regardless of the frequency), the effect has a peak at the ferromagnetic resonance frequency of the material with a given static applied field. Currently, this effect is called the microwave permeability or network ferromagnetic resonance in the literature. These results are sensitive to the domain wall configuration of the material and eddy currents.

In terms of ferromagnetic resonance, the effect of an ac-field applied along the direction of the magnetization is called parallel pumping.

For a tutorial with more information on AC susceptibility measurements, see here (external link)

Examples

Magnetic susceptibility of some materials
Material	Temperature	Pressure	$chi_{text{mol}}$ (molar susc.)		$chi_{text{mass}}$ (mass susc.)		$chi_{v}$ (volume susc.)		M (molar mass)	$rho$ (density)
Units	(°C)	(atm)	SI (m³·mol⁻¹)	cgs (cm³·mol⁻¹)	SI (m³·kg⁻¹)	cgs (cm³·g⁻¹)	SI	cgs (emu)	(10^-3 kg/mol) or (g/mol)	(10³ kg/m³) or (g/cm³)
vacuum	Any	0	0	0	0	0	0	0	–	0
water	20	1	−1.631×10⁻¹⁰	−1.298×10⁻⁵	−9.051×10⁻⁹	−7.203×10⁻⁷	−9.035×10⁻⁶	−7.190×10⁻⁷	18.015	0.9982
bismuth	20	1	−3.55×10⁻⁹	−2.82×10⁻⁴	−1.70×10⁻⁸	−1.35×10⁻⁶	−1.66×10⁻⁴	−1.32×10⁻⁵	208.98	9.78
Diamond	r.t.	1	−6.9×10⁻¹¹	−5.5×10⁻⁶	−5.8×10⁻⁹	−4.6×10⁻⁷	−2.0×10⁻⁵	−1.6×10⁻⁶	12.01	3.513
He	20	1	−2.38×10⁻¹¹	−1.89×10⁻⁶	−5.93×10⁻⁹	−4.72×10⁻⁷	−9.85×10⁻¹⁰	−7.84×10⁻¹¹	4.0026	0.000166
Xe	20	1	−5.71×10⁻¹⁰	−4.54×10⁻⁵	−4.35×10^-9	−3.46×10⁻⁷	−2.37×10⁻⁸	−1.89×10⁻⁹	131.29	0.00546
O₂	20	0.209	4.3×10⁻⁸	3.42×10⁻³	1.34×10⁻⁶	1.07×10⁻⁴	3.73×10⁻⁷	2.97×10⁻⁸	31.99	0.000278
N₂	20	0.781	−1.56×10⁻¹⁰	−1.24×10⁻⁵	−5.56×10⁻⁹	−4.43×10⁻⁷	−5.06×10⁻⁹	−4.03×10⁻¹⁰	28.01	0.000910
Al		1	2.2×10⁻¹⁰	1.7×10⁻⁵	7.9×10⁻⁹	6.3×10⁻⁷	2.2×10⁻⁵	1.75×10⁻⁶	26.98	2.70
Ag	961	1					−2.31×10⁻⁵	−1.84×10⁻⁶	107.87

Sources of confusion in published data

There are tables of magnetic susceptibility values published on-line that seem to have been uploaded from a substandard source, which itself has probably borrowed heavily from the CRC Handbook of Chemistry and Physics. Some of the data (e.g. for Al, Bi, and diamond) are apparently in cgs Molar Susceptibility units, whereas that for water is in Mass Susceptibility units (see discussion above). The susceptibility table in the CRC Handbook is known to suffer from similar errors, and even to contain sign errors. Effort should be made to trace the data in such tables to the original sources, and to double-check the proper usage of units. Use them at your own risk!