The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1222͒ have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ϳ40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization ͑TRM͒ measurements at Tϭ60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.

We present an ab initio density functional theory study of the magnetic moments that arise in graphite by creating single carbon vacancies in a three-dimensional ͑3D͒ graphite network using full potential, all electron, spin polarized electronic structure calculations. In previous reports, the appearance of magnetic moments was explained in a two-dimensional graphene sheet just through the existence of the vacancies itself ͓Carbon-Based Magnetism, edited by F. Palacio and T. Makarova ͑Elsevier, Amsterdam, 2005͒; D. C. Mattis, Phys. Rev. B 71, 144424 ͑2005͒; Y. Kobayashi et al., ibid. 73, 125415 ͑2006͒; R. Yoshikawa Oeiras et al., ibid. ͑to be pub-lished͒; P. O. Lehtinen et al., Phys. Rev. Lett. 93, 187202 ͑2004͔͒.The dependence of the arising magnetic moment on the nature and geometry of the vacancies for different supercells is reported. We found that the highest value of magnetic moment is obtained for a 3 ϫ 3 ϫ 1 supercell and that the highly diluted 5 ϫ 5 ϫ 1 supercell shows no magnetic ordering. The results obtained in this paper are indicative of the importance of interlayer interactions present in a 3D stacking. We conclude that this should not be underestimated when vacancy-based studies on magnetism in graphitic systems are carried out.

In this paper we report on the magnetic properties of pure bulk ferromagnetic graphite, obtained by a chemical route previously described. This magnetic graphite has been obtained by a vapor reaction consisting of a controlled etching on the graphite structure. By magnetic force microscopy we have verified that its magnetic properties are related to the topographic defects introduced in the pristine material. Also, the magnetic properties have been verified through magnetization measurements as a function of temperature and applied magnetic field. At low temperatures ͑2 K͒ the saturation magnetization reaches a value of 0.58 emu/ g, leading to a defect concentration of 1250 ppm. The system is highly irreversible due to the inhomogeneity of the distribution of defects in the material. Two transition temperatures are detected, T c1 = 115͑5͒ K and T c2 = 315͑5͒ K. These transitions could be associated to the weak coupling between ferromagnetic regions related to defects and to the ferromagnetism inside the defect regions.

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The present study attempts to better identify the neurophysiological changes occurring during flow experience and how this can be related to the mobilization of attentional resources. Self-reports of flow (using a flow feelings scale) and attention (using thought probes), autonomic activity (heart rate, heart rate variability, and breathing rate), and cerebral oxygenation (using near-infrared spectroscopy) in two regions of the frontoparietal attention network (right lateral frontal cortex and right inferior parietal lobe) were measured during the practice of two simple video games (Tetris and Pong) played at different difficulty conditions (easy, optimal, hard, or self-selected). Our results indicated that an optimal level of difficulty, compared with an easy or hard level of difficulty led to greater flow feelings and a higher concentration of oxygenated hemoglobin in the regions of the frontoparietal network. The self-selected, named autonomy condition did not lead to more flow feelings than the optimal condition; however, the autonomy condition led to greater sympathetic activity (reduced heart rate variability and greater breathing rate) and higher activation of the frontoparietal regions. Our study suggests that flow feelings are highly connected to the mobilization of attentional resources, and all the more in a condition that promotes individuals' choice and autonomy.

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