No abstract

Abstract-Powders of undoped ferrihydrite nanoparticles and ferrihydrite nanoparticles doped with cobalt in the ratio of 5 : 1 have been prepared by hydrolysis of 3d-metal salts. It has been shown using Mössbauer spectroscopy that cobalt is uniformly distributed over characteristic crystal-chemical positions of iron ions. The blocking temperatures of ferrihydrite nanoparticles have been determined. The nanoparticle sizes, magnetizations, surface anisotropy constants, and bulk anisotropy constants have been estimated. The doping of ferrihydrite nanoparticles with cobalt leads to a significant increase in the anisotropy constant of a nanoparticle and to the formation of surface rotational anisotropy with the surface anisotropy constant K u = 1.6 × 10 -3 erg/cm 2 .

We report the results of investigations of the effect of cooling in an external magnetic field starting from the temperature over superparamagnetic blocking temperature TB on the shift of magnetic hysteresis loops in systems of ferrihydrite nanoparticles from ~2.5 to ~5 nm in size with different TB values. In virtue of high anisotropy fields of ferrihydrite nanoparticles and open hysteresis loops in the range of experimentally attainable magnetic fields, the shape of hysteresis loops of such objects in the field-cooling mode is influenced by the minor hysteresis loop effect.A technique is proposed for distinguishing the exchange bias effect among the effects related to the minor hysteresis loops caused by high anisotropy fields of ferrihydrite particles. The exchange bias in ferrihydrite is stably observed for particles no less than 3 nm in size or with TB over 40 K, and its characteristic value increases with the particle size.

No abstract

No abstract