*Article* **AC Susceptibility Studies of Magnetic Relaxation in Mn12-Stearate SMMs on the Spherical Silica Surface**

**Oleksandr Pastukh , Piotr Konieczny , Magdalena Laskowska \* and Łukasz Laskowski**

Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 Krakow, Poland; oleksandr.pastukh@ifj.edu.pl (O.P.); piotr.konieczny@ifj.edu.pl (P.K.); lukasz.laskowski@ifj.edu.pl (Ł.L.)

**\*** Correspondence: magdalena.laskowska@ifj.edu.pl

**Abstract:** The study of magnetic relaxations in Mn12-stearate single-molecule magnets deposited on the surface of spherical silica nanoparticles was performed. For such a purpose, the investigation of AC magnetic susceptibility dependence on the frequency and temperature was performed. Based on the Argand plots obtained for different temperatures and temperature dependencies of susceptibility, obtained for different frequencies of AC field, the corresponding relaxation times were derived. Fitting to the Arrhenius law revealed the values of an effective energy barrier and a mean relaxation time, which were consistent for both measuring techniques (*Ue f f* /*k<sup>B</sup>* <sup>∼</sup> 50 K and *<sup>τ</sup>*<sup>0</sup> <sup>∼</sup> <sup>10</sup>−<sup>7</sup> s) and similar to the corresponding values for the analogous bulk compounds. Additionally, the obtained relaxation parameters for the Mn12-stearate molecules on the spherical silica surface were compared with corresponding values for the Mn12-based single-molecule magnets deposited upon other types of nanostructured silica surface.

**Keywords:** nanostructured silica; single-molecule magnets; nanocomposites; magnetic relaxation
