From Finite Size and Surface Effects to Glassy Behaviour in Ferrimagnetic Nanoparticles

Abstract

This chapter is aimed at studying the anomalous magnetic properties (glassy behaviour) observed at low temperatures in nanoparticles of ferrimagnetic oxides. This topic is discussed both from numerical results and experimental data. Ferrimagnetic fine particles show most of the features of glassy systems due to the random distribution of anisotropy axis, interparticle interactions and surface effects. Experiments have shown that the hysteresis loops display high closure fields with high values of the differential susceptibility. Low magnetisation as compared to bulk, shifted loops after field-cooling, highfield irreversibilities between zero-field and field cooling processes and ageing phenomena in the time-dependence of the magnetisation, are also observed. This phenomenology indicates the existence of some kind of freezing phenomenon arising from a complex hierarchy of the energy levels, whose origin is currently under discussion. Two models have been proposed to account for it: i) the existence of a spin-glass state at the surface of the particle which is coupled to the particle core through an exchange field; and ii) the collective behaviour induced by interparticle interactions. In real systems, both contributions simultaneously occur, being difficult to distinguish their effects. In contrast, numerical simulations allow us to build a model just containing the essential ingredients to study solely one of two phenomena. Glassy behaviour in ferrimagnetic nanoparticlesThe assemblies of fine magnetic particles with large packing fractions and/or nanometric sizes show most of the features which are characteristic of glassy systems (for a recent review see Ref. [12]). This glassy behaviour results from a complex interplay between surface and finite-size effects, interparticle interactions and the random distribution of anisotropy axis throughout the system. In many cases, these contributions are mixed and in competition, Sample characterisationThe phenomenology of the glassy state in strong interacting fine particles is illustrated through the study of the magnetic properties of nanocrystalline BaFe 10.4 Co 0.8 Ti 0.8 O 19 [11]. M -type barium ferrites have been studied for a long time because of their technological applications [33,34,35,36], such as microwave devices, permanent magnets, and high-density magnetic and magneto-optic recording media, as well as their large pure research interest [37,38]. The compounds obtained by cationic substitution of the pure BaFe 12 O 19 ferrite display a large variety of magnetic properties and structures, which go from collinear ferrimagnetism to spin-glass-like behaviour [37,39,38], depending on the degree of frustration introduced by cationic substitution. In particular, the BaFe 10.4 Co 0.8 Ti 0.8 O 19 compound seems to be ideal for perpendicular magnetic recording [40], since the Co 2+ -Ti 4+ doping scheme reduces sharply the high values of the coercive field of the pure compound, which precludes their technological applications. For this composition the magnetic structur...

SIZE-DEPENDENT MAGNETIZATION FLUCTUATIONS IN NiO NANOPARTICLES

“…The broken bonds and defects at the surface layer gives rise to high magnetic response of the disordered surface spins than the core spins. 22,23,24,25 This surface effect can be incorporated in two different ways; viz. an ordered surface with surface spins ordering owing to internal field due to core spins, and a disordered surface with spins oriented randomly at the surface which cancel out thus giving zero contribution to the total magnetic moment.…”

“…Recently it has been pointed out that the roughness at the surface layer gives rise to higher magnetic response for the surface spins than the core spins. 22,23,24,25 In view of these studies, we investigate the large magnetic moment in NiO nanoparticle by invoking a different ordering for surface spins than the bulk Néel state ordering for core spins.…”

Modeling of isochronal complex magnetic susceptibility of polymer–magnetic nanocomposites using fractional calculus

“…Deviations of normal ferrimagnetic ordering at spinel ferrite nanoparticles have been attributed to incomplete coordination on surface cations or to broken bonds with its oxygen neighbors. [4][5][6][7] This situation leads to no colineal arrangement between surface spins and ferrimagnetically ordered spins at nanoparticles core, which has been suggested to be randomly canted. 8,9 Moreover, in systems with a high concentration of nanoparticles, interactions among particles can be significant, due to dipole-dipole interactions and exchange interactions among particles in close contact; both kinds of interactions are important sources of changes on the spins response.…”

“…These interactions can favor either parallel or antiparallel orientation of spins, depending on particles morphology, magnetic anisotropy and its sizes. 4,10 Therefore, magnetic nanoparticles systems can depict analogous behavior to that showed by glassy magnetic materials (e.g., spin-glasses and diluted magnets), where surface-to-core interactions and interactions among particles can impose frozen magnetic states. This situation leads to deviations from the picture of single-domain particles as superspins with stable magnetic ordering.…”

“…1,2 In addition to the aforementioned characteristics, magnetic nanoparticles are capable to show deviations from ordered magnetic arrangement, due to the presence of a large amount of superficial atoms or ions. 4 Specifically, the studies of magnetic properties of ferrimagnetic nanostructures, such as spinel ferrites, have suggested the existence of a layer of disordered spins on the particles surface. Deviations of normal ferrimagnetic ordering at spinel ferrite nanoparticles have been attributed to incomplete coordination on surface cations or to broken bonds with its oxygen neighbors.…”