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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.