Low temperature superspin glass behavior in a Co/Ag multilayer

We report on the low temperature magnetic behavior of an epitaxially grown multilayer formed by 32 repetitions of a nominal period corresponding to 1 monolayer (ML) Co and 16 ML Ag. The study of the magnetic properties was based on the measurement of the temperature dependencies of the dc magnetization upon field cooling (FC) and zero field cooling (ZFC) and of the ac field real and imaginary parts of the susceptibility. From our results we conclude about the occurrence of i) a well-defined bilayers stacking sequence matching the nominal one, ii) a discontinuous growth in the Co layers resulting on close-to-monodisperse, spherical Co nanoparticles having an average diameter of 1.6 nm, iii) a frequency dependent peak in the temperature dependence of the real part of the ac susceptibility exhibiting a per decade relative temperature variation of 4.5 x 10−2, iv) an applied dc field, Hdc, variation of the temperature at which the irreversibility is detected in the FC/ZFC curves corresponding to the Almeida-Thouless prediction, and v) a critical behavior characterized by a glass-transition temperature slightly below the peak temperatures observed at low frequency in the temperature dependence of the ac susceptibility and a dynamic scaling exponent in the range of the values usually obtained for spin glass systems. From our results we conclude that i) our sample experiences a superspin-glass/paramagnetic phase transition, ii) the interactions mediating the spin glass freezing process are the dipolar ones taking place among the Co particles (creating fields at the average interparticle distance of the order of 8 x 105 A/m) which provide competitiveness that combined with the reduced amount of disorder built-in the Co layers results on frustration.