# 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:

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

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 $x \to +\infty$ and -1 as $x \to -\infty$.

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 $M$ on the applied magnetic field $B$ and the total angular momentum quantum number J of the microscopic magnetic moments of the material. The magnetization is given by:[1]

$M = N g \mu_B J \cdot B_J(x)$

where

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

Note that in the SI system of units $B$ given in Tesla stands for the magnetic field, $B=\mu_0 H$, where $H$ is the auxiliary magnetic field given in A/m and $\mu_0$ is the permeability of vacuum.

## Langevin function

Langevin function (red line), compared with $\tanh(x/3)$ (blue line).

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

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

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

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

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

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

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 can be approximated to within 5% accuracy by the formula[3]

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

valid on the whole interval (-1, 1). For small values of x, better approximations are the Padé approximant

$L^{-1}(x) = 3x \frac{35-12x^2}{35-33x^2} + O(x^7)$

and the Taylor series[4]

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

## High-temperature limit

When $x \ll 1$ i.e. when $\mu_B B / k_B T$ is small, the expression of the magnetization can be approximated by the Curie's law:

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

where $C = \frac{N g^2 J(J+1) \mu_B^2}{3k_B}$ is a constant. One can note that $g\sqrt{J(J+1)}$ is the effective number of Bohr magnetons.

## High-field limit

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

$M = N g \mu_B J$

## 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. ^ Cohen, A. (1991). "A Padé approximant to the inverse Langevin function". Rheologica Acta 30 (3): 270–273. doi:10.1007/BF00366640.
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.