1,721,031 research outputs found
Relation of double exchange ferromagnetic and charge ordered phases
No full textElectronic-magnetic phase diagrams are proposed for double exchange-superexchange systems which include charge ordering. The results come very close even to the very complex experimental phase diagrams which are observed with the manganites. Also, phase separation phenomena and unusual pressure dependences of the transition temperatures can be expected, both being observed experimentally in manganites, too. The approach also opens up a possibility to describe the large variety of phases as observed in electron-magnon/electron-phonon coupled systems in a comprehensive manner
Bloch wall anomaly at the canted-ferromagnetic order-order transition of mixed double-exchange/superexchange magnetic systems
No full textIn mixed coupled, but still magnetically polarized compounds. order-order transitions occur at which occasion the Bloch wall thickness delta can go to zero. This is demonstrated for a deGennes mixed double-exchange/superexchange situation. Experimentally, such Bloch wall anomalies can be detected via Bloch wall enhanced NMR. We discuss one example, i.e. the intermetallic compound MnAs
Cooling cycles based on the AF-F transition in Fe-Rh induced by magnetic field
No full textBy using the model S-T diagram of the Fe-Rh alloy in the first-order antiferromagnetism-ferromagnetism phase transition region, the cooling cycles are drawn up based both on the latent heat of transition absorption and emission when the transition is induced isothermally and on the change in the temperature of the alloy when the transition is induced adiabatically by applying magnetic field. It is found that the values of the coefficient of performance of similar to38 at DeltaT = 5 K and similar to29 at DeltaT = 10 K near room temperature, where DeltaT is the difference between the temperature of the surroundings and that of the cooled object, can be reached when organizing the cooling cycles based on adiabatic inducement the AF F transition by field within the limits of 2.2 x 10(6) A m(-1). (C) 2003 Elsevier B.V. All rights reserved
Positive and negative magnetoresistance in mixed superexchange-double exchange systems
No full textThe occurence of large positive and negative magnetoresistance (+/-MR) in mixed magnetically coupled and weakly lattice coupled spin systems is discussed and compared with some experimentally observed examples of giant magnetoresistance (GMR) in manganites R1-xExMnO3-delta. We indeed find even some of the more capricious features of the MR reproduced by experiment
Positive and negative magnetoresistance in mixed superexchange-double exchange systems
No full textThe occurence of large positive and negative magnetoresistance (+/-MR) in mixed magnetically coupled and weakly lattice coupled spin systems is discussed and compared with some experimentally observed examples of giant magnetoresistance (GMR) in manganites R1-xExMnO3-delta. We indeed find even some of the more capricious features of the MR reproduced by experiment
Sol-gel deposition and characterization of Mn2+-doped silicate phosphor films
No full textMn2+ doped Zn2SiO4 and Mg2Gd8(SiO4)(6)O-2 phosphor films were deposited on silicon and quartz glass substrates by sol-gel process (dip-coating). The variations of sol viscosity with time and film thickness with the number of layers were investigated in Zn2SiO4: Mn system. The results of XRD and IR showed that the Zn2SiO4: Mn films remained amorphous below 700 degrees C and crystallized completely around 1000 degrees C. From AFM studies, it was observed that the grains with 0.5-0.8 mu m size packed closely in Zn2SiO4: Mn films, which were uniform and crack free. The luminescence properties of Zn2SiO4: Mn films were characterized by absorption, excitation and emission spectra as well as luminescence decay. These properties were discussed in derail by a comparison with those of Mn2+ (and Pb2+)-doped Mg2Gd8(SiO4)(6)O-2 phosphor films. (C) 2000 Elsevier Science S.A. All rights reserved
Electroresistive effect in slightly oxygen deficient Nd0.7Sr0.3MnO3-delta ceramic
No full textWe observe a strong electroresistive (ER) effect in slightly oxygen deficient Nd0.7Sr0.3MnO3-delta ceramic which behaves equivalent to the well-known magnetoresistive effect and which apparently involves both the higher insulating grain boundaries as well as parts of the domain (grain) volume, just like the MR in ceramics. While the magnetic field acts on the magnetic dipoles of the manganites (MR), the nature of the electrical dipole moments - intrinsic or extrinsic - is not fully understood as yet, but as the ER-effect increases dramatically with 6, for these compounds the evidence points to extrinsic electrical dipoles which appear together with a local restructuring when oxygen is formally missing. (c) 2005 Elsevier B.V. All rights reserved
Determination of the trap state density differences in hydrogenated amorphous silicon-germanium alloys
No full textTime-resolved photo- and thermoelectric effects (TTE) were used to determine simultaneously trap levels and trap state density differences in amorphous (a-SiGe:H) samples. In particular, the trap state density differences are obtained from the decay of the ambipolar charge distribution (i.e., stage II of the TTE transients). This type of spectroscopy has been applied for the first time to a-SiGe:H samples, and indeed trap states that seem to relate to concentration fluctuations, that is, Si(Ge) and Ge(Si) clusters, are observed
Phase transitions in the La0.50Ca0.50MnO3-y manganites
No full textThe crystal structure, magnetic and electrical properties of the La0.50Ca0.50MnO3-gamma (0 less than or equal to gamma less than or equal to 0.50) reduced manganites have been experimentally investigated. It is found that the samples of the La0.50Ca0.50MnO3-gamma system display a number of phase transitions in the ground state versus gamma. The antiferromagnetic state of gamma = 0 composition transforms into the inhomogeneous ferromagnetic one at gamma = 0.02 which in turn seems to convert into the cluster spin-glass state at gamma similar to 0.12. At gamma = 0.25 an additional ferromagnetic component appears and at gamma = 0.30 the fraction of this ferromagnetic component is the highest. At gamma greater than or equal to 0.35 the samples seem to be a mixture of the antiferromagnetic phase and ferromagnetic clusters. The strongly reduced sample gamma = 0.25 has shown a large magnetoresistance of about 50%. The realized investigations have allowed to construct a hypothetical magnetic phase diagram of the La0.50Ca0.50MnO3-gamma System. For the explanation of the obtained results the oxygen vacancies are supposed to be non-randomly distributed over the crystal lattice in the concentration range of 0.12 less than or equal to gamma less than or equal to 0.25 and trend to be ordered at gamma greater than or equal to 0.30 in Sr2Fe2O5-type manner
COP of cooling cycles around the AF-F transition in FeRh based on experimental data
No full textBy the heat capacity of the FeRh alloy when heating in the AF-F transition region in zero magnetic field and the change in the temperature of the alloy when the transition is adiabatically induced by applying magnetic field and the reverse transition is adiabatically induced by removing magnetic field measurement data, the entropy-temperature diagram of the alloy when heating and cooling in various magnetic fields up to 2.5 T is constructed. Using this diagram, the cooling cycles based on adiabatic inducement of the transition by magnetic field are built up. The values of the coefficient of performance of the cycles obtained from the diagram have been found to be ranging from 37 to 55 in particular in a 2.5 T field, which is close to the ones calculated by the model entropy-temperature diagram of the alloy drawn up under the assumption that the transition is ideally isothermal. (c) 2005 Elsevier B.V. All rights reserved
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