P. Didukh

Surface effects in CoFe2O4 magnetic fluids studied by Mössbauer spectrometry

Abstract Magnetic properties of CoFe2O4 particles were investigated by means of zero-field and in-field Mossbauer spectrometry. The studies include superparamagnetic (SP) relaxation as well as the surface phenomena of samples with different particle sizes both in the form of ferrofluids and powders. A core–shell model was applied to estimate the thickness of the surface “spin-canted” shell around particles.

Magnetocaloric effect in slightly crystallised Co–Nb–Cu–Si–B alloy

Abstract Temperature dependence of the entropy variation Δ S M was calculated from the magnetisation data. A considerable magnetic entropy change was observed in nanocrystalline and amorphous samples even for relatively small field changes. The maximum of Δ S M was found to appear in the vicinity of the cluster glass–superparamagnetic transition which occurs around the Curie temperature of the amorphous phase.

The micromagnetic simulations of CoNbCuSiB nanocrystalline material

The micromagnetic formalism was used to study the magnetic behaviour of an assembly of single-domain ferromagnetic particles with random anisotropy axes directions, embedded in a diamagnetic or weakly magnetic matrix. The calculated hysteresis loops present strong similarities with the experimental magnetisation curves of multiphase Co-based alloy.

Transition from the collective magnetism towards superparamagnetism in Co-Nb-Cu-Si-B nanostructure

Abstract Amorphous and slightly crystallized CoNbCuSiB alloys were studied by the magnetisation measurements. Precipitation of Co-based crystallites destroys the ferromagnetic ordering of the amorphous alloy and leads to a cluster-glass state. At elevated temperatures, the grains embedded in the paramagnetic matrix exhibit the superparamagnetic behaviour.

Magnetic Properties of Ferrofluid with Cobalt Ferrite Particles

A ferrofluid is a stable colloidal suspension of nano-sized magnetic particles dispersed in a carrier liquid. At sufficiently low temperatures, frozen ferrofluids exhibit a spin-glass type disordered structure with the frustration caused by the competing dipole—dipole interactions of randomly oriented grains [1]. At elevated temperatures the magnetic behaviour of ferrofluids is dominated by fast superparamagnetic fluctuations of the particles magnetic moments [2-4]. It is known that the magnetic properties of small particles differ from those of the corresponding bulk materials. Besides the relaxation effects, the most important factors responsible for these differences are: (i) the symmetry restriction at the particle surface, and (ii) high chemical reactivity of the surface atoms. In a ferrofluid the magnetic properties of an assembly of ultrafine particles can be changed and controlled by the surfactant used to coat the particles; the surfactant can prevent intergrain interactions but can also considerably enhance the non-collinear spin structures in the surface layer [2]. The present work was motivated by the desire to study these various surface effects. The cobalt ferrite particles were chosen since they are available in a powder form either as particles covered by the surfactant molecules or in a form of ferrofluid in which the particles are in a non-magnetic liquid. Moreover, the structure and magnetic properties of the bulk CοFe2O4 crystals are well known