Critical Magnetic Behavior in [Ag8/Co0.5]x64, [Ag16/Co1]x32 and [Ag8/Co1]x32 Epitaxial Multilayers

Author / Creator

MMM 2020 (2020)

Conferences

MMM 2020 D1: Exchange Bias and Other Novel Exchange in Thin Films (2020)

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Online

Summary

We analyze in this work the low temperature magnetic behavior of a series of Co/Ag multilayers formed by samples having reduced Co content per period (either one monolayer or half a monolayer) sepa...

We analyze in this work the low temperature magnetic behavior of a series of Co/Ag multilayers formed by samples having reduced Co content per period (either one monolayer or half a monolayer) separated by several Ag monolayers (either 8 or 16). The multilayers were grown by molecular beam epitaxy on a clean MgO (001) surface, alternating Ag and Co deposition up to complete either 32 or 64 periods (in order to maintain constant in the series the nominal number of Co atoms per unit surface of the sample) and protected by a 3nm Ag cap layer. The samples were studied by X-ray reflectivity and diffraction, TEM, SQUID magnetometry, and ac susceptometry. From the results of our measurements we identified the following facts: 1) the occurrence in all the samples of stacking sequences corresponding to the nominal ones 2) the spatial discontinuity of the Co layers that corresponded to a quasi-monodisperse in-plane distribution of Co particles embedded in a Ag(001) matrix; 3) the observation of hysteresis at temperatures (always below 20 K) that depended on both the number of Co and Ag monolayers per period; 4) an ac field frequency dependence of the real part of the susceptibility, χ', maxima temperatures corresponding to a Vogel-Fulcher behavior with a frequency shift parameter of the order in all the samples of 4 x 10-2; 5) the collapse (Figure 1 ), for each sample, of the experimental data corresponding to the ac field frequency, f, and temperature, T, variations of the imaginary part of the ac susceptibility, χ'', in a single curve, G(x) with x = f[(T/Tg) − 1]−Ζν (where Tg and Ζν are the critical temperature and exponent, respectively), and verifying T χ'' (f, T) = [(T/Tg) - 1]β G(x) [1]. Taken together, these results allow us to conclude that our three multilayers experience a phase transition (of the paramagnetic to superspin glass type) at the corresponding critical temperature and that the exponent β in the universal dynamic scaling law takes values dependent on the number of Co monolayers per period.References: [1] P. Beauvillain, C. Dupas, J.P. Renard, and P. Veillet, Physical Review B 29 (7), 4086 (1984).

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