Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles

Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes $$ $\approx$ 2 to 4 nm. The blocking temperatures $T_B^m$ extracted from magnetization data increased from $\approx 10$ to 20 K for increasing particle size. Low-temperature \MOS measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the \FH phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility \chi_{ac}(\emph{f},T) measurements showed the dependence of the effective magnetic anisotropy energies $E_{a}$ with particle volume, and yielded an effective anisotropy value of $K_{eff} = 312\pm10$ kJ/m$^3$.Comment: 8 pages, 10 figures. Nanotechnology, v17 (Nov. 2006) In pres

Evidence of random magnetic anisotropy in ferrihydrite nanoparticles based on analysis of statistical distributions

We show that the magnetic anisotropy energy of antiferromagnetic ferrihydrite depends on the square root of the nanoparticles volume, using a method based on the analysis of statistical distributions. The size distribution was obtained by transmission electron microscopy, and the anisotropy energy distributions were obtained from ac magnetic susceptibility and magnetic relaxation. The square root dependence corresponds to random local anisotropy, whose average is given by its variance, and can be understood in terms of the recently proposed single phase homogeneous structure of ferrihydrite.Comment: 6 pages, 2 figure

Social roles and aging from a life-span perspective

Este trabalho investigou os papéis sociais e as tarefas evolutivas desempenhados por adultos. O local escolhido para investigação foi um assentamento de famílias de baixa renda do Distrito Federal criado em 1989. Utilizou-se um questionário contendo 17 questões abertas e 15 questões fechadas, preenchido pela primeira autora durante uma visita domiciliar. Participaram 98 respondentes (73 F e 25 M), sendo 51 entre 50 e 59 anos e 47 a partir de 60 anos. Os resultados apontaram que este grupo é heterogêneo e que seus papéis sociais são influenciados pelas variáveis demográficas (idade, sexo, escolaridade, ocupação, naturalidade e estado civil) e também pelas variáveis relativas à moradia atual. Concluiu-se também que as expectativas sociais, o suporte social e a escolarização são fatores de suma importância para oferecer recursos para a otimização e compensação necessárias a um envelhecimento bem sucedido. __________________________________________________________________________________________________________ ABSTRACTThis study investigated the social roles and developmental tasks of adults. The study took place in a settlement of low-income families, created in The Federal District in 1989. Data were collected through a questionnaire composed of 17 open and 15 closed questions, administered by the first author during a home visit. There were 98 respondents, 73 female and 25 male, being 51 between 50 to 59 years old and 47 elders above the age of 60. The result indicated that this group is heterogeneous and that its social roles are influenced by the demographic variables - age, sex, educational level, work, place of the birth and marital status, as well for the relative variables to current residence. The data allow the conclusion that social expectations, social support and the educational level are important resources for the optimization and necessary compensation to successful aging