Superspin glass behavior of Co nanocrystal arrays periodically stacked in a Ag(001) matrix

Abstract

Superspin glass (SSG) magnetic order is often observed in granular metallic alloys, where a high degree of disorder in the magnetic particles positions constitutes a key ingredient of the collective behavior. Here we present a study of the structure and magnetic properties of granular Co/Ag multilayers, a system with reduced disorder in comparison to granular solids due to the stacking periodicity. Multilayers were grown by molecular beam epitaxy on MgO(001) substrates. The nominal thickness of the Co layers was 0.5 and 1 monolayers (ML) and that of Ag was 8 ML and 16 ML. Samples were studied by X-ray reflectivity and diffraction, transmission electron microscopy (TEM), magnetometry (VSM and SQUID) and ac-susceptibility. From these results we concluded that: i) a clear superlattice periodicity was built in the samples, see Fig.1; ii) the Co layers were discontinuous, presenting a highly monodisperse Co particle distribution (the mean diameter of the Co nanocrystals embedded in the Ag(001) matrix was 1.6 nm, as derived from TEM images); iii) samples exhibited hysteresis below temperatures in the range 5 - 20 K (depending on sample composition) and magnetization not saturable at fields of up to 6 T; iv) samples evidenced sharp peaks in the zero-field cooling dc- and ac- susceptibility curves at low temperatures (Tp), in the same range as the hysteresis onset; v) the frequency dependence of these peak temperatures in the ac- curves can be explained according to the Vogel-Fulcher law (see Fig. 2) and yielded values of the frequency shift parameter between Γ = 0.040 and Γ =0.05; vi) Henkel plots obtained from the isothermal and demagnetization remanences indicated demagnetizing interactions at the whole field range of the magnetization process. Hence, we conclude that, below the critical temperatures identified from the ac- susceptibility curves, our samples behave as a SSG, originated by the interparticle dipolar interactions. The degree of disorder within the Co layers of these samples seems thus to be sufficient to cause this magnetic collective behavior.