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The dynamic response in a large experimental time window ([10.sup.-6]-[10.sup.2] s) of Co and CoPt nanoparticle systems is reported. In order to reveal the dynamics effects, along with classic experiments as a.c. susceptibility and static magnetic measurements, a rf temperature dependent transverse susceptibility experiment is used. To study the effect of interaction between particles, samples of Co and CoPt nanoparticles dispersed in an insulated matrix in different volume fractions were investigated. Index Terms--Dynamic properties, interparticle interactions, magnetic nanoparticles, transverse susceptibility.

The magnetic properties of the ultrafine cobalt ferrite particles with diameter d~3.3 nm are discussed. The particles show the presence of the spin-glasslike surface layer with a thickness of 1 angstrom, which leads to a (small) loop shift in the FC process.

A Preisach-Neel-type able to represent complex magnetization processes for magnetic nanoparticle systems is discussed. Future study, based on the same indentification scheme, will test the ability of the model to predict the behavior of other nanoparticulate samples with stronger interactions.

Exchange coupling bias between Cr and ferromagnetic (FM) bilayers is discussed. Despite the fact that Cr is an itinerant antiferromagnet with spin density wave ordering, exchange bias has been established in a number of Py/Cr and Co/Cr bilayers.

An La2Co1.7 crystals was investigated by single-crystal neutron and X-ray diffraction. The neutron measurement was performed with a Laue white-beam technique at 15 K and room temperature, using a large position-sensitive detector. The X-ray measurements were obtained at room temperature from a CCD detector. The average structure of La2Co1.7 is hexagonal with cell parameters a = 4.885 (1), c = 4.273 (2) Å and space group P63/mmc. The satellites are located at the vertices of small hexagons perpendicular to the c axis. The modulated crystal was indexed assuming a sixfold twinned 3 + 1 dimensional structure with q = (α, 0, γ). The structure was solved in the pseudoorthorhombic cell, with a = 8.461 (1), b = 4.885 (1), c = 4.273 (2) Å, in the superspace group C2/m(α, 0, γ). Owing to space requirements, the Co atoms cannot fit precisely into the octahedral sites of the La h.c.p. (hexagonal close packing). Instead, the Co atoms adopt a different periodicity, which is not commensurate with the periodicity of the La atoms. Two structure models have been refined in order to describe this behavior, one using the sawtooth function for the positional modulation of cobalt and the other describing the structure as a composite system. The chemical composition calculated from the composite model is La2O1.8 (1) with the estimated standard deviation arising from the variation of q for different samples. In both models lanthanum is incommensurately modulated, while the position of cobalt seems not to be affected by any relative periodic displacement. [ABSTRACT FROM AUTHOR]