It is shown that the double-exchange Hamiltonian, with weak antiferromagnetic interactions, has a rich variety of first-order transitions between phases with different electronic densities and/or magnetizations. The paramagnetic-ferromagnetic transition moves towards lower temperatures, and becomes discontinuous as the relative strength of the double-exchange mechanism and antiferromagnetic coupling is changed. This trend is consistent with the observed differences between compounds with the same nominal doping, such as La 2/3 Sr 1/3 MnO 3 and La 2/3 Ca 1/3 MnO 3 . Our results suggest that, in the low doping regime, a simple magnetic mechanism suffices to explain the main features of the phase diagram.

The Hybrid Monte Carlo algorithm is adapted to the simulation of a system of classical degrees of freedom coupled to non self-interacting lattices fermions. The diagonalization of the Hamiltonian matrix is avoided by introducing a path-integral formulation of the problem, in d + 1 Euclidean space-time. A perfect action formulation allows to work on the continuum Euclidean time, without need for a Trotter-Suzuki extrapolation. To demonstrate the feasibility of the method we study the Double Exchange Model in three dimensions. The complexity of the algorithm grows only as the system volume, allowing to simulate in lattices as large as 16 3 on a personal computer. We conclude that the second order paramagnetic-ferromagnetic phase transition of Double Exchange Materials close to half-filling belongs to the Universality Class of the three-dimensional classical Heisenberg model. 

The phase diagram of the double perovskites of the type Sr 2Ϫx La x FeMoO 6 is analyzed, with and without disorder due to antisites. In addition to an homogeneous half metallic ferrimagnetic phase in the absence of doping and disorder, we find antiferromagnetic phases at large dopings, and other ferrimagnetic phases with lower saturation magnetization, in the presence of disorder.

The miniaturization of analytical systems for different applications is currently a very active field of research. The inherent advantages of micro total analysis systems (lTASs) are well known. Although the fluidic platforms and the development of suitable miniaturized detection systems have been studied extensively, the integration in a single substrate of the electronics that is needed to manage the whole system using a single technology is still a sizeable challenge. In this overview, we discuss the role of the low-temperature co-fired ceramics (LTCC) as a potential alternative for miniaturizing analytical systems, since a single technology can easily combine fluidics and electronics to produce a number of novel chemical microanalyzers.

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