Nanocrystalline soft magnetic materials: intergrain coupling and spin freezing effects

Abstract

The static and dynamic magnetic properties of the nanocrystalline soft magnetic FeNbB alloys consisting of Fe-nanograins embedded in an amorphous matrix were investigated by means of dc and ac susceptibility, magnetization and hysteresis loop measurements in a wide temperature range. We show that these alloys exhibit particular complex magnetic behaviour versus temperature. The spin-glass like behaviour at cryogenic temperatures is followed by a very soft magnetic behaviour at intermediate temperatures, and finally, a marked magnetic hardening due to decoupling between the nanograins is observed when temperature approaches the Curie temperature of the amorphous matrix. Upon further increase of temperature, the thermal relaxation of magnetization plays a dominant role. The results are discussed within a model that takes into account the presence of weakly magnetic Nb-rich shells around the nanocrystalline grains.Introduction The reduction of the grain sizes to the nanometer range may vary drastically the physical properties of materials, including the magnetic behaviour. Typical examples of such systems are nanocrystalline soft magnetic alloys prepared by a devitrification of the melt-spun amorphous precursors. The structure of these materials consists of the nanocrystalline grains dispersed in the surrounding amorphous matrix. An early report on the materials that exhibited significant reduction of coercivity after the grain sizes decreased to 10-20 nm was given by O'Handley et al.[1] for a devitrified Co-based meltspun alloy. The discovery of the excellent soft magnetic properties in the nanocrystalline alloys based on FeCuNbSiB, also called FINEMET

The huge influence of nanograins on the magnetic properties of iron-based Fe–Cu–Nb–B nanocrystalline alloys

“…The interface of the nanoparticles due to the complex magnetic structure seems to be able to tilt the magnetic moment of the nanoparticles from the parallel direction offered by the external magnetic field. Such influences were reported, observing the increase of coercivity at low temperatures, by Skorvanek et al [15,16]. The authors relate this behaviour and also the observed creep to magnetic moments of the nanocrystalline grains and highly distorted interfaces frozen in their random anisotropy orientations.…”

Domain imaging in FINEMET ribbons

“…As a result, a nanocrystalline structure of ferromagnetic nanocrystals embedded in a ferromagnetic amorphous matrix with magnetic correlation length larger than mean grain size is obtained. The random anisotropy model extended by Herzer [13] explained that when grain dimensions are kept below the ferromagnetic exchange length, the anisotropic of the homogenously distributed nanocrystals continues to average out [14]. In addition, this composite structure leads to the compensation of l s between the amorphous (positive contribution) and the crystalline (negative contribution) phases, decreasing the magnetoelastic energy.…”