Magnetoelastic properties of amorphous and nanocrystalline Co-Fe-Nb-B alloys

The saturation magnetostriction constant, λs, and ΔE of amorphous and nanocrystalline CoxFe93-x-yNb7By alloys (17 ≤ x ≤ 33.9 ≤ y ≤ 15) were studied. Different stages of nanocrystallisation were obtained by field- and conventional annealing at different temperatures. On nanocrystallisation αs slightly decreases but the minimum αs always exceeds 18 ppm. The maximum ΔE is observed for annealing temperatures 400 - 450°C, i.e. before or at the early stage of nanocrystallisation. With the development of nanocrystalline phase ΔE decreases. In the field-annealed samples quite a large in-plane anisotropy transversal to the ribbon axis was observed. On the other hand, some conventionally annealed ribbons exhibit magnetic anisotropy perpendicular to the ribbon surface. The results are discussed in terms of the phenomenological model developed by Squire

Domain structures in "non-magnetostrictive" Co67Fe4Cr7Si8B14 wide ribbons

The domain structure of as-cast 60 mm wide ribbons of a "non-magnetostrictive" metallic glass was studied by means of SEM and computer-aided magnetooptic Kerr effect. Various types of domain structure were observed along the ribbon width. The comparison of the structure on the shiny and wheel sides of the ribbon showed that the wide stripe domains extend through the whole thickness of the ribbon. The changes of the surface domain structure with the applied magnetic field made it possible to conclude that this structure is rather connected to the magnetization of the interior domains than to their wall-associated fields. The nonhomogeneous heat transfer during the production of wide ribbons and the associated temperature gradients across the ribbon width lead to a distribution of directions and magnitudes of local axes of effective anisotropy. The observations of the domain structures proved to be a useful tool for mapping these distributions and domain topography even in a material with rather low magnetostriction

Sr-hexaferrite/maghemite composite nanoparticles—possible new mediators for magnetic hyperthermia

Composite nanoparticles with variable ratios of M-type Sr-hexaferrite and maghemite phases were prepared via the sol–gel method employing polyvinylalcohol as the stabilizing agent, followed by thermal treatment at 600 °C for 32–190 min. The measurements in static magnetic field revealed considerable variation of the coercivity and remanence depending on the relative content of the highly magnetically anisotropic Sr-hexaferrite phase. Calorimetric heating experiments were carried out on aqueous gel suspensions under an alternating magnetic field of maximum amplitude Hmax = 15.1–68.4 kA m-1 and frequency ν = 108 kHz. They showed a strong dependence of the heating efficiency on the coercivity and remanence of the composites, with a specific absorption rate (SAR) value ranging from units to tens of W/g(Feferrimagnetic)

Magnetic heating by cobalt ferrite nanoparticles

In the quest for suitable materials for hyperthermia we explored the preparation and properties of nanoparticles of Co ferrite. The material was produced by coprecipitation from water solution of Co and Fe chlorides and afterwards annealed at 400, 600 and 800 °C. The resulting particles were characterized by XRD, TEM, Mössbauer spectroscopy, and dc and ac magnetometry. The heating experiments in ac magnetic fields of various amplitudes were performed with diluted systems of particles suspended in agarose gel and the results were interpreted on the basis of the ac magnetic losses measured at various temperatures. The increase of magnetic losses and consequently of the heating efficiency with increasing temperature is explained by the strong dependence of the constant of magnetocrystalline anisotropy of Co ferrite on temperature