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X-ray and neutron-diffraction, dc magnetic susceptibility, magnetization, and specific-heat measurements are reported for FexCo(1-x)Ta2O6 mixed oxides. X-ray refinement indicates homogeneous samples for all the reported concentrations. The neutron-diffraction measurements reveal magnetic structures with double propagation vectors (+/-1/4,1/4,1/4) for CoTa2O6, and (1/2,0,1/2) and (0,1/2,1/2) for FeTa2O6. The latter remain unchanged in the Fe-rich samples, for 0.46< or =x<1.00, while the Co-rich samples show propagation vectors (+/-1/4,1/4,0) for 0.09< or =x<0.46. The temperature vs x phase diagram exhibits a bicritical point at about T=4.9 K and x=0.46. For this concentration, and at low temperatures, the system shows coexistence of both magnetic structures. This novel bicritical behavior is interpreted as induced by competition between the different magnetic and crystallographic structures.

The system Cu2FeO2BO3 is an oxyborate belonging to the family of the ludwigites. In this paper we present AC susceptibility, magnetization measurements and Mössbauer spectroscopy on this material which allows for a complete characterization of its complex magnetic behavior. We find an hierarchy of interactions which clearly defines three regimes with decreasing temperature. These are associated with, the freezing of the Fe moments, the antiferromagnetic ordering of the Cu sub-lattice and finally the coupling between both systems.PACS. 75.30.Cr Saturation moments and magnetic susceptibilities -75.10.Nr Spin-glass and other random models -75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects -76.80.+y Mössbauer effect; other γ-ray spectroscopy

We present an investigation of the magnetic properties of the ludwigite Ni 2 FeO 2 BO 3 . This material is an oxyborate that presents in its crystalline structure subunits in the form of walls where the transition metal ions are located. Our Mössbauer, ac susceptibility, and magnetization measurements show that Fe and Ni ions form two independent subsystems in this material down to T N ϭ15 K. We explain this behavior in terms of a hierarchy of interactions.

We present an investigation of the magnetic properties of a series of warwickites with different transition metals. These materials are characterized by one-dimensional structures called ribbons where the transition metals are randomly located. The present results allow for a systematic investigation of the hierarchy of magnetic interactions in these systems. While the intraribbon interactions are mainly direct d-d interactions between the transition-metal ions, the interribbon interaction is due to superexchange between metals in different octahedra mediated by the oxygen in the common corner. ͓S0163-1829͑97͒00625-5͔

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IntroductionCoronary artery bypass grafting is a safe procedure performed worldwide with low rates of mortality and morbidity in general population.ObjectiveTo investigate risk factors for mortality of patients undergoing coronary artery bypass grafting coronary artery bypass grafting surgery.MethodsA total of 1,628 consecutive patients undergoing on-pump coronary artery bypass grafting were retrospectively studied from December 1999 to February 2012. Data analysis involved paired Student t test, Mann-Whitney test and Fisher’s exact test for the categorical data. Logistic regression, Odds Ratio and 95%CI were used for definition of risk factors for mortality.ResultsOf a total of 1,628 patients undergoing on-pump coronary artery bypass grafting, 141 (8.7%) died. The following risk factors for mortality were identified after logistic regression: dialysis (OR=7.61; 95%CI 3.58-16.20), neurologic dysfunction type I (OR=4.42; 95%CI 2.48-7.81), use of IABP (OR=3.38; 95%CI 1.98-5.79), cardiopulmonary bypass time (OR=3.09; 95%CI 2.04-4.68), serum creatinine on admission and peak values > 0.4mg/dL (OR=2.67; 95%CI 1.79-4.00), age > 65 years (OR=2.31; 95%CI 1.55-3.44), and time between hospital admission and and surgical procedure (OR=1.53; 95%CI 1.03-2.27).ConclusionDialysis, type I neurologic dysfunction, use of IABP, cardiopulmonary bypass time (> 115 minutes), serum creatinine on admission and peak values>0.4mg/dL, age > 65 years and time between hospital admission and surgical procedure were considered as risk factors for mortality in patients undergoing on-pump coronary artery bypass grafting surgery.