The **Curie constant** is a material-dependent property that relates a material's magnetic susceptibility to its temperature.

The Curie constant, when expressed in SI units, is given by

C
=
μ
0
μ
B
2
3
k
B
N
g
2
J
(
J
+
1
)
where
N
is the number of magnetic atoms (or molecules) per unit volume,
g
is the Landé g-factor,
μ
B
(9.27400915e-24 J/T or A·m^{2}) is the Bohr magneton,
J
is the angular momentum quantum number and
k
B
is Boltzmann's constant. For a two-level system with magnetic moment
μ
, the formula reduces to

C
=
1
k
B
N
μ
0
μ
2
The constant is used in Curie's Law, which states that for a fixed value of a magnetic field, the magnetization of a material is (approximately) inversely proportional to temperature.

M
=
C
T
B
This equation was first derived by Pierre Curie.

Because of the relationship between magnetic susceptibility
χ
, magnetization
M
and applied magnetic field
H
:

χ
=
M
H
this shows that for a paramagnetic system of non-interacting magnetic moments, magnetization
M
is inversely related to temperature
T
(see Curie's Law).