SUPERSPIN GLASS: A USELESS PIECE OF MATERIALS

AUTHOR(S) / AUTOR(I): Davide Peddis 

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DOI: 10.46793/Phys.Chem24I.023P

ABSTRACT / SAŽETAK:

In a magnetic nanoparticles the thermal evolution of the magnetization dynamics depends on the particle concentration and the nature of the inter-particle interactions¹. 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).² 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. This talk focus on the non-equilibrium magnetization dynamics of two different SSG systems: concentrated MnFe2O4 nanoparticles ( @ 2 nm), a model of SSG, and a dilute, really peculiar system, of Co particles ( @ 2 nm) embedded in an antiferromagnetic Mn matrix with a volume fraction of 4.7 % (Co@Mn 4.7%). Non-equilibrium dynamics of these 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 Co@Mn 4.7% represent a very interesting example of SSG behaviour in a dilute system. Generally, for such particle dilution in diamagnetic matrix (e.g. Co embedded in Ag matrix), dipolar interactions are weak and an ordinary blocking process is observed. In Co@Mn 4.7% non-equilibrium dynamics typical of SSG is observed. This is due to a strong interface exchange coupling between Co nanoparticle and Mn matrix, which transmits an effective long range interparticle correlation giving rise to a SSG freezing at low temperature.^3–5 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. In particular, the interface exchange interaction provides an additive source for the frustration of the system resulting in an enhancement of the memory effect. In 2001 J.P. Bouchand⁶ claim that lthough (super) spin glasses are totally useless pieces of material, they constitute an exceptionally convenient laboratory frame for theoretical and experimental investigations define. In this perspective the importance of these materials has been clearly demonstrated buy the physics Nobel Prize in 2021.

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REFERENCES / LITERATURA:

• New Trends in Nanoparticle Magnetism. vol. 308 (Springer International Publishing, Cham, 2021).
• Peddis, et al, Scientific Report 11, (2021).
• Fiorani, & D. Peddis, Journal of Physics: Conference Series 521 (2014) 12006.
• Peddis, M. Hudl, C. Binns, D. Fiorani, & P. Nordblad. J. Phys.: Conf. Ser. 200 (2010) 072074.
• Margaris, G. et al. Nanotechnolgy 28(2017). 035701.

1. P. Bouchaud, V. Dupuis, J. Hammann, J. & E. Vincent, Phys. Rev. B 65 (2001) 024439.