Browsing by Author "Restrepo, J."

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    Effect of surface anisotropy on the magnetic properties of magnetite nanoparticles
    (2008) Mazo-Zuluaga, J.; Restrepo, J.; Mejia-Lopez, J.
    In this study, we analyze the effect of surface anisotropy on the magnetic properties of magnetite Fe3O4 nanoparticles on the basis of a core-shell model. Magnetization, magnetic susceptibility, and specific heat are computed over a wide range of temperatures. In our model, we stress on magnetite nanoparticles of 5 nm in diameter which consist of 6335 ions. Our theoretical framework is based on a three-dimensional classical Heisenberg Hamiltonian with the nearest magnetic neighbor interactions between iron ions involving tetrahedral (A) and octahedral (B) sites. Terms dealing with cubic magnetocrystalline anisotropy for core ions, a single-ion site surface anisotropy for those Fe ions belonging to the shell, and the interaction with a uniform external magnetic field are considered. To compute the equilibrium averages, a single-spin movement Monte Carlo-Metropolis dynamics was used. Results reveal the occurrence of low-temperature spin configurations different from those expected for a collinear single-domain ferrimagnetic state, depending on the magnitude and sign of the surface anisotropy constant. A transition to a spike state, with magnetization close to zero, is obtained beyond a certain critical positive surface anisotropy value. Such a transition is not observed for negative values. Moreover, a two-pole magnetic state is developed at sufficiently high negative values. Such differences are explained in terms of the interplay between the superexchange couplings and the easy directions imposed by the surface anisotropy vectors. Our results are summarized in a proposal of phase diagram for the different spin structures as a function of the surface-to-core anisotropy ratio. Lastly, hysteretic behavior is evaluated. Nanoparticles become magnetically harder as the surface anisotropy increases in magnitude, and the way in wich the coercive field changes with this quantity is explicitly shown. (C) 2008 American Institute of Physics.
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    Latin American Registry of Pediatric Renal Transplantation 2004-2008
    (2010) Goulart, P.; Koch, P.; Medina-Pestana, J.; Garcia, C.; Bittencourt, V.; Medeiros, M.; Munoz, R.; Delucchi, A.; Lillo, A. M.; Ariza, M.; Bosque, M.; Carvalho, D.; Matuck, T.; Meneses, R.; Fontes, J.; Monteiro, D. C.; Neto, E. Davi; Pinto, V.; Salas, P.; Prates, L.; Belanguero, V.; Pereira, L.; Lima, E.; Penido, J. M.; Benini, V.; Laranjo, S.; Silva, J. M.; Orta, N.; Coronel, V.; Cisneros, A.; Arriaga, J.; Sebastian, M. J.; Abbud-Filho, M.; Fernandez, I.; Gastelbondo, R.; Medjia, N.; Rosati, P.; Hevia, P.; Baptista, M. A.; Ramalho, H.; Diaz, M.; Monteverde, M.; Ferraris, J.; Repetto, H.; Exeni, R.; Florentin, L.; Florin, J.; Cazorla, N.; Casadei, D.; Melendez, K.; Calderon, R.; Silva, V.; Patino, J.; Palacio, D.; Madrigal, G.; Sandoval, M.; Urbina, C.; Loza, R.; Cavagnaro, F.; Vogel, A.; Jimenez, W.; Lou, R.; Rodriguez, C.; Aguilar, C.; Galvez, H.; Rodriguez, L.; Paladini, J.; Agusti, J.; Puelma, F.; Troche, A.; Martinez-Pico, M.; Higueras, W.; Liendo, C.; Restrepo, J.; Caicedo, L. A.; Socorro, F.; Semprum, P.
    The Latin American Pediatric Nephrology Association (ALANEPE) reports the first regional kidney transplant registry in Latin America. A mean range of 75%, 30% to 100% renal transplant recipients under 21 yr old were included. Out of 20 countries invited to participate, 14 performed renal transplant, 11 had universal financial support from their governments. The centers included: Brazil (9), Argentina (4), Chile (4), Venezuela (3), Mexico (2), and one in: Cuba, Colombia, Costa Rica, Nicaragua, Guatemala, Ecuador, Honduras, Paraguay and Peru. The registry included 1458 patients, average of 291 per year, 55% male. Mean follow-up 23.4 +/- 17 months; mean age was 11.7 +/- 4.3 yr (1-21), 11% under five yr of age. Etiology: uropathy/ reflux nephropathy 27%, glomerulopathies 24% (included 12% FSGS), hypo/dysplasia (11%), vascular (6%), congenital/hereditary (5%), unknown (19%). Induction therapy: 71% anti-IL2RAb, 13% ATG/TIMO, 14% non-induction. Maintenance therapy: Tacrolimus 64%, Cyclosporine 32%, MMF 54%, MPS 20%, noTORi 96%, steroids 90%, withdrawal or steroid avoidance 10%. Loss of graft 155/1458 (11%), death with functioning graft (3.4%), vascular thrombosis (2.8%), acute rejection (2.8%), recurrence of disease (1%). Forty-eight patients died (3.3%); infection was the main cause 23 (2.1%). Global patient survival rate at one, three, and five yr was 97%, 96%, and 96%. Graft survival rate at one, three, and five yr LRD was 96%, 93% and 89%; for DD 92%, 86% and 76% respectively. Both survival rates were higher in LRD (p < 0.008 and p < 0.001). A pediatric renal transplant study has started, making information available to be shared between the centers and the world.
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    Magnetic Behavior and Stoichiometry on Magnetite Nanoparticles
    (2009) Restrepo, J.; Mejía López, José Félix