with $C$ a constant. This is known as the _Curie law_, and it shows that at high temperature a paramagnetic material becomes less susceptible to magnetisation.

```python
B=np.linspace(-2,2,1000)
fig,ax=pyplot.subplots()
ax.plot(B,np.tanh(B*3),label="low $T$")
ax.plot(B,np.tanh(B),label="high $T$")
ax.legend()
ax.set_ylabel("$M$")
ax.set_xlabel("$B$")
draw_classic_axes(ax,xlabeloffset=.2)
```
### Paramagnetism: free spin $J$
For an atom with magnetic moments $L$ and $S$ the Hamiltonian will become: