Abstract
We investigate the low temperature magnetic behavior of three epitaxial Co/Ag multilayers, grown onto MgO (001) substrates, with a nominal content per period of either half a monolayer or one monolayer of Co, and either 8 or 16 Ag monolayers. The samples were studied by X-ray reflectivity and diffraction, transmission electron microscopy, magnetometry and ac susceptometry. The results indicated a well defined stacking sequence in the growth direction, the number of periods and of Ag monolayers per period being coincident with the nominal values for each sample. The Co layers were found to be discontinuous and corresponded to a quasi-monodisperse in-plane distribution of Co nanoparticles embedded in a Ag(001) matrix. The zero-field cooled and field cooled temperature variations of the low field magnetization indicated the presence of irreversibilities at temperatures below 20 K. The ac field frequency (f) and temperature (T) dependencies of the real part of the susceptibility (χ′) corresponded to a Vogel–Fulcher behavior in the three samples, and indicated a frequency shift parameter (Γ) of the order of 4 x 10-2. For each sample, the experimental data corresponding to the variations of the imaginary part of the ac susceptibility (χ″) with f and T were found to collapse into a single curve according to the dynamic scaling law. Taken together, these results allow us to conclude that the three multilayers experience a phase transition of the paramagnetic to superspin glass type, driven by the dipolar interactions between the Co nanoparticles. Regarding the influence of the multilayer features, we found a clear dependence of the order parameter of the transition on the nominal number of Co monolayers per period.
Highlights
Quenched disorder and competing interactions coexisting in a system constitute the necessary ingredients for the occurrence of the so-called spin glass behavior.1 That behavior is largely ubiquitous and has been identified in many different fields,1 like the analysis of the human brain functionality,2 neural networks,3,4 prebiotic evolution5 or protein folding.6 the paradigm of the spin glass phenomenology and of its theoretical understanding are magnetic moments systems including distributed interactions
The experimental data corresponding to the variations of the imaginary part of the ac susceptibility (χ′′) with f and T were found to collapse into a single curve according to the dynamic scaling law. These results allow us to conclude that the three multilayers experience a phase transition of the paramagnetic to superspin glass type, driven by the dipolar interactions between the Co nanoparticles
The XRD, X-rays reflectivity (XRR), and scanning transmission electron microscopy (STEM) results indicated that the molecular beam epitaxy (MBE) deposited films exhibited good crystallinity, fcc(001) structure and clear superlattice periodicities, the number of periods in each sample being coincident with the nominal ones
Summary
Quenched disorder and competing interactions coexisting in a system constitute the necessary ingredients for the occurrence of the so-called spin glass behavior.1 That behavior is largely ubiquitous and has been identified in many different fields,1 like the analysis of the human brain functionality,2 neural networks,3,4 prebiotic evolution5 or protein folding.6 the paradigm of the spin glass phenomenology and of its theoretical understanding are magnetic moments systems including distributed interactions. Scitation.org/journal/adv on a Co/Ag multilayer with granular morphology at the Co layers, we have shown the occurrence at low temperature of a superspin glass phase transition governed by the dipolar interactions between the freezing Co nanoparticles.
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