Figure 2.

Single magnetic nanoparticle. The magnetic field $\overrightarrow{H}$ is applied along the z-axis. $\theta$ defines the angle between $\overrightarrow{m}$ and the unit vector along z, ${\overrightarrow{u}}_z$, and $\alpha$ the angle between the projection of $\overrightarrow{m}$ in the $xy$ plane and the unit vector along x, ${\overrightarrow{u}}_x$.