In a magnetic nanoparticles system the thermal evolution of the dynamics depends on the particle concentration and the nature of the inter-particle interactions. In sufficiently concentrated nanoparticle systems, strong dipolar interaction combined with random orientation of anisotropy axes determine a competition between different moment alignments leading to a collective freezing of particle moments in a disordered magnetic state, known as superspin glass (SSG), below a characteristic glass temperature (Tg). SSG exhibits slow dynamics which is qualitatively indistinguishable from that observed in atomic spin glasses, and typical properties of an ordinary SG, such as aging, memory and rejuvenation are observed. In this talk, the non-equilibrium dynamics of two different SSG systems will be discussed: concentrated MnFe2O4 nanoparticles (<D> @2 nm), a model of SSG, and a dilute system, a peculiar one, of Co particles (<D> @2 nm) embedded in an antiferromagnetic Mn matrix with a volume fraction of 4.7 % (Co@Mn 4.7%)..
For MnFe2O4, the transition temperaturedetected by Mössbauer spectroscopy(TM)is close to Tmax(TM/Tmax@1), despite the significantly different time scales, as expected for a collective magnetic state. Non-equilibrium dynamics of the system has been studied by memory and rejuvenation effects using different protocols (TRM; ZFC, IRM), showing the validity of the principle of superpositon for a SSG system (figure).
In Co@Mn 4.7%non-equilibrium dynamics typical of SSG is observed. This is due to a strong interface exchange coupling betwen Co nanoparticle and Mn matrix, which transmits an effective long range interparticle correlation giving rise to a SSG freezing at low temperature. By memory and rejuvenation experiments and Monte Carlo simulations we showed that the memory effects increase with the concentration and that both the interface exchange coupling and the dipolar interparticle interactions contribute to the observed dynamical behavior. .
 G. Margaris, M. Vasilakaki, D. Peddis, K. N. Trohidou, S. Laureti, C. Binns, E. Agostinelli, D. Rinaldi, R. Mathieu, D. Fiorani, Nanotechnolgy 2017, 28, 35701.
 M. Vasilakaki, K. N. Trohidou, D. Peddis, D. Fiorani, R. Mathieu, M. Hudl, P. Nordblad, C. Binns, S. Baker, Phys. Rev. B 2013, 88, 1.