Low temperature superspin glass behavior in a Co/Ag multilayer

E Navarro,B Martínez,U Urdiroz,F Cebollada,J M González,C Magen,M Alonso,A Ruiz,F J Palomares,Ll Balcells

doi:10.1063/1.5130158

Abstract

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.

Highlights

Our work is framed into the latter research scenario,10 since we aim at identifying, from its low temperature magnetic behavior, the actual occurrence of a spin glass freezing transition in a sample characterized by the restriction of the occurrence of disorder exclusively to well stacked magnetic layers separated by a non-magnetic spacer
The transmission electron microscopy (TEM) imaging of the sample showed the fact that the Co layers did not grow continuously but with a morphology corresponding to a close-to-monodisperse distribution of Co particles having a lognormal diameter distribution with an average diameter of 1.6 ± 1 nm and embedded in a single-crystal Ag(001) matrix
The value obtained from the fit for the average moment of the magnetic entities is 4 x 10−15 Am2, which is much larger than the fcc Co atomic moment (1.6 x 10−17 Am2) and indicates that the paramagnetic behavior is related to the aggregation of ca. 200 individual Co moments having fcc co-ordination

Summary

Introduction

The phenomenology and the theoretical understanding of the spin glass phases have continued to be an active topic of research after the initial works on these phases were published at the beginning of the 1970’s decade.1 This is so due to several reasons among which it is worth to notice both the complexity of the experimental behavior,2,3 which, as of today, still incorporates new phenomena4 and the limited suitability of some of the (both standard and developed) statistical mechanical tools that have been considered in order to theoretically analyze the experimental data (the experiments could, in many cases, be explained in the classical statistical mechanics framework if the difficulties linked to the built-in randomness could be tackled and overcome).and relevantly, from the structurally simple canonical spin glass metallic systems where indirect exchange between localized atomic moments is on the ground of the interactions originating the freezing behavior,1 many different materials involving different crystallinities, morphologies, anisotropies, inter-moments interaction types and, above all, quenched disorder types and magnitudes have been identified and continue to be identified5–9 as scitation.org/journal/adv exhibiting spin glass phenomenologies.

Results

Conclusion