# Brillouin and Langevin functions

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The Brillouin and Langevin functions are a pair of special functions that appear when studying an idealized paramagnetic material in statistical mechanics.

## Brillouin function

The Brillouin function[1][2] is a special function defined by the following equation:

${\displaystyle B_{J}(x)={\frac {2J+1}{2J}}\coth \left({\frac {2J+1}{2J}}x\right)-{\frac {1}{2J}}\coth \left({\frac {1}{2J}}x\right)}$

The function is usually applied (see below) in the context where x is a real variable and J is a positive integer or half-integer. In this case, the function varies from -1 to 1, approaching +1 as ${\displaystyle x\to +\infty }$ and -1 as ${\displaystyle x\to -\infty }$.

The function is best known for arising in the calculation of the magnetization of an ideal paramagnet. In particular, it describes the dependency of the magnetization ${\displaystyle M}$ on the applied magnetic field ${\displaystyle B}$ and the total angular momentum quantum number J of the microscopic magnetic moments of the material. The magnetization is given by:[1]

${\displaystyle M=Ng\mu _{B}J\cdot B_{J}(x)}$

where

• ${\displaystyle N}$ is the number of atoms per unit volume,
• ${\displaystyle g}$ the g-factor,
• ${\displaystyle \mu _{B}}$ the Bohr magneton,
• ${\displaystyle x}$ is the ratio of the Zeeman energy of the magnetic moment in the external field to the thermal energy ${\displaystyle k_{B}T}$:
${\displaystyle x={\frac {g\mu _{B}JB}{k_{B}T}}}$
• ${\displaystyle k_{B}}$ is the Boltzmann constant and ${\displaystyle T}$ the temperature.

Note that in the SI system of units ${\displaystyle B}$ given in Tesla stands for the magnetic field, ${\displaystyle B=\mu _{0}H}$, where ${\displaystyle H}$ is the auxiliary magnetic field given in A/m and ${\displaystyle \mu _{0}}$ is the permeability of vacuum.

Takacs[3] proposed the following approximation to the inverse of the Brillouin function:

${\displaystyle B_{J}(x)^{-1}={\frac {axJ^{2}}{1-bx^{2}}}}$

where the constants ${\displaystyle a}$ and ${\displaystyle b}$ are defined to be

${\displaystyle a={\frac {0.5(1+2J)(1-0.055)}{(J-0.27)2J}}+{\frac {0.1}{J^{2}}}}$
${\displaystyle b=0.8}$

## Langevin function

Langevin function (blue line), compared with ${\displaystyle \tanh(x/3)}$ (magenta line).

In the classical limit, the moments can be continuously aligned in the field and ${\displaystyle J}$ can assume all values (${\displaystyle J\to \infty }$). The Brillouin function is then simplified into the Langevin function, named after Paul Langevin:

${\displaystyle L(x)=\coth(x)-{\frac {1}{x}}}$

For small values of x, the Langevin function can be approximated by a truncation of its Taylor series:

${\displaystyle L(x)={\tfrac {1}{3}}x-{\tfrac {1}{45}}x^{3}+{\tfrac {2}{945}}x^{5}-{\tfrac {1}{4725}}x^{7}+\dots }$

An alternative better behaved approximation can be derived from the Lambert's continued fraction expansion of tanh(x):

${\displaystyle L(x)={\frac {x}{3+{\tfrac {x^{2}}{5+{\tfrac {x^{2}}{7+{\tfrac {x^{2}}{9+\ldots }}}}}}}}}$

For small enough x, both approximations are numerically better than a direct evaluation of the actual analytical expression, since the latter suffers from Loss of significance.

The inverse Langevin function L−1(x) is defined on the open interval (−1, 1). For small values of x, it can be approximated by a truncation of its Taylor series[4]

${\displaystyle L^{-1}(x)=3x+{\tfrac {9}{5}}x^{3}+{\tfrac {297}{175}}x^{5}+{\tfrac {1539}{875}}x^{7}+\dots }$

${\displaystyle L^{-1}(x)=3x{\frac {35-12x^{2}}{35-33x^{2}}}+O(x^{7}).}$
Graphs of relative error for x ∈ [0, 1) for Cohen and Jedynak approximations

Since this function has no closed form, it is useful to have approximations valid for arbitrary values of x. One popular approximation, valid on the whole range (−1, 1), has been published by A. Cohen:[5]

${\displaystyle L^{-1}(x)\approx x{\frac {3-x^{2}}{1-x^{2}}}.}$

This has a maximum relative error of 4.9% at the vicinity of x = ±0.8. Greater accuracy can be achieved by using the formula given by R. Jedynak:[6]

${\displaystyle L^{-1}(x)\approx x{\frac {3.0-2.6x+0.7x^{2}}{(1-x)(1+0.1x)}},}$

valid for x ≥ 0. The maximal relative error for this approximation is 1.5% at the vicinity of x = 0.85. Even greater accuracy can be achieved by using the formula given by M. Kröger:[7]

${\displaystyle L^{-1}(x)\approx {\frac {3x-x(6x^{2}+x^{4}-2x^{6})/5}{1-x^{2}}}}$

The maximal relative error for this approximation is less than 0.28%. More accurate approximation was reported by R. Petrosyan:[8]

${\displaystyle L^{-1}(x)\approx 3x+{\frac {x^{2}}{5}}\sin \left({\frac {7x}{2}}\right)+{\frac {x^{3}}{1-x}},}$

valid for x ≥ 0. The maximal relative error for the above formula is less than 0.18%.[8]

## High-temperature limit

When ${\displaystyle x\ll 1}$ i.e. when ${\displaystyle \mu _{B}B/k_{B}T}$ is small, the expression of the magnetization can be approximated by the Curie's law:

${\displaystyle M=C\cdot {\frac {B}{T}}}$

where ${\displaystyle C={\frac {Ng^{2}J(J+1)\mu _{B}^{2}}{3k_{B}}}}$ is a constant. One can note that ${\displaystyle g{\sqrt {J(J+1)}}}$ is the effective number of Bohr magnetons.

## High-field limit

When ${\displaystyle x\to \infty }$, the Brillouin function goes to 1. The magnetization saturates with the magnetic moments completely aligned with the applied field:

${\displaystyle M=Ng\mu _{B}J}$

## References

1. ^ a b c C. Kittel, Introduction to Solid State Physics (8th ed.), pages 303-4 ISBN 978-0-471-41526-8
2. ^ Darby, M.I. (1967). "Tables of the Brillouin function and of the related function for the spontaneous magnetization". Brit. J. Appl. Phys. 18 (10): 1415–1417. Bibcode:1967BJAP...18.1415D. doi:10.1088/0508-3443/18/10/307.
3. ^ Takacs, Jeno (2016). "Approximations for Brillouin and its reverse function". COMPEL. 35 (6): 2095.
4. ^ Johal, A. S.; Dunstan, D. J. (2007). "Energy functions for rubber from microscopic potentials". Journal of Applied Physics. 101 (8): 084917. Bibcode:2007JAP...101h4917J. doi:10.1063/1.2723870.
5. ^ Cohen, A. (1991). "A Padé approximant to the inverse Langevin function". Rheologica Acta. 30 (3): 270–273. doi:10.1007/BF00366640.
6. ^ Jedynak, R. (2015). "Approximation of the inverse Langevin function revisited". Rheologica Acta. 54 (1): 29–39. doi:10.1007/s00397-014-0802-2.
7. ^ Kröger, M. (2015). "Simple, admissible, and accurate approximants of the inverse Langevin and Brillouin functions, relevant for strong polymer deformations and flows". J Non-Newton Fluid Mech. 223: 77–87. doi:10.1016/j.jnnfm.2015.05.007.
8. ^ a b Petrosyan, R. (2016). "Improved approximations for some polymer extension models". Rheologica Acta. doi:10.1007/s00397-016-0977-9.