Magnetic coupling in metallic granular systems

Abstract

The coupling between magnetic clusters in granular metals is investigated and the relative importance of the Ruderman-Kittel-Kasuya-Yosida and classical dipolar interactions is discussed. It is shown that beyond a certain critical cluster size, the coupling is entirely dominated by the dipolar term, leading to an antiferromagnetic bias. Numerical results for Cu 1Ϫx Co x alloys indicate that such a critical value corresponds to clusters of about Nϭ100 Co atoms. The importance of those results to the understanding of the giant-magnetoresistance effect in granular materials is pointed out. ͓S0163-1829͑96͒50134-7͔

Memory effect and spin-glass-like behavior in Co-Ag granular films

“…We point out that the dipolar interaction plays a dominant role in granular films when the Co concentration is above a critical concentration as theoretically predicted. 44 However, the positive T 0 's indicate that ferromagneticlike interactions are dominant, which favors the superferromagnetic phase rather than spin-glass phase, as observed by De Toro et al 30 Based on the magnetic phase diagram of a granular system with an insulating matrix proposed by Petracic et al, 45 at sufficiently high concentrations, granular systems with dipolar interactions do not exhibit a superspin glass transition, but a "superferromagnetic transition." It is natural to believe that in the metallic, magnetic granular systems, at a high enough concentration, ferromagneticlike interactions dominate over antiferromagneticlike ones, and thus the condition of competing interactions leading to spin-glass behavior is not fulfilled.…”

Interaction effects in dilute cluster-assembled magnetic nanostructures

“…In a disordered structure, the average coupling strength is estimated to be even lower. 29 Consequently, it is not enough to simply estimate pairwise interactions in order to derive blocking temperatures for dilute systems with small clusters. Another way of taking into account interactions between superparamagnetic clusters in metallic matrices has been introduced by Allia et al ͓interacting superparamagnet ͑ISP͒ model͔.…”

RKKY interactions between nanomagnets of arbitrary shape

“…-The question arises to what extent other interactions, such as magnetostatic interactions, are able to suppress RKKY interactions on a macroscopic scale [4][5][6]. This refers, in particular, to the question of spin-glass ordering in nanocomposites exhibiting giant magnetoresistance [7].…”

“…This refers, in particular, to the magnetoresistance of granular nanostructures such as Co/Ag and Co/Cu [1][2][3], where there is a competition between interparticle and Zeeman interactions. A related problem is the nature of spin-glass interactions between nanoclusters [4][5][6][7].…”

“…This refers, in particular, to the magnetoresistance of granular nanostructures such as Co/Ag and Co/Cu [1-3], where there is a competition between interparticle and Zeeman interactions. A related problem is the nature of spin-glass interactions between nanoclusters [4][5][6][7].The RKKY mechanism describes the interaction of two local magnetic moments (spins) in a sea of free electrons. Due to exchange, itinerant electrons are subject to a spin-dependent local potential, and in second-order perturbation theory the energy of the electron gas depends on whether the two localised spins are parallel or antiparallel.…”