5 Experimental approach

Many techniques are available in our days to measure the magnetic properties of nanomagnets. The challenge lays in measurements of not only macroscopic properties which are generally achieved by the magnetometry measurements but in measurements of local properties at nanoscale. In the past decade several techniques have provided important measurements with the aim to understand surface effects in magnetic films and nanoparticles:

X-ray absorption magnetic circular dichroism (XMCD) This technique was pioneered by Schütz and co-workers [50]. It is based on the changes in the absorption cross section of a magnetic material and uses circularly polarized photons [51]. Through the well-known sum rules it allows to determine the relation between the orbital and the spin magnetic moments $ \mu_{L}/\mu_{s}$. A big advantage of this technique is that it is element specific and it is able to identify moment orientations in ultrathin films and monolayer magnetic materials. The X-ray microscopy also allows imaging with 100 nm. resolution.

(Micro-)Superconducting Quantum Interface Device $ \mathbf{(\mu)-SQUID}$ is a powerful technique to measure the net magnetization on a nanometer-size sample [52]. In 2001 Jamet et. al. have done the first magnetization reversal study on individual Co nanoparticle ($ D=20$ nm.) using a new $ \mu-SQUID$ setup. From those measurements they deduced the magnetic anisotropy magnitude [20] and that the behavior of individual nanoparticles follows the Néel-Arrhenius law [53].

Rocio Yanes