A. Peña

Magnetic and transport properties of Pb perovskites and Fe containing giant magnetoresistance perovskites

The magnetic and magnetoresistive properties of the Ln0.7Pb0.3Mn1−xFexO3 (Ln=La, Nd, and x=0, 0.1) perovskites, prepared by the solgel low temperature method, have been studied. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior with Tc values ranging form 345 K observed for La0.7Pb0.3MnO3 to 75 K for Nd0.7Pb0.3Mn0.9Fe0.1O3. The 10% Fe contribution lowers the Tc by about 130 K in the La compound and 90 K in the Nd one. These results can be interpreted in terms of antiferromagnetic coupling between Mn and Fe. However, the influence of the Nd ions is not well known although it seems not ordered down to 10 K. The magnetoresistence [MR=ΔR/R(0)] measured at 6 T in pellets of pressed powder reaches values of about 80% around the magnetic transition.

Observation of isotropic-dipolar to isotropic-Heisenberg crossover in Co- and Ni-substituted manganites

High-precision ac susceptibility data have been taken on the La0.7Pb0.3Mn1−y(Co, Ni)yO3 (y=0, 0.1, 0.2 and 0.3) manganite system over a wide range of amplitudes and frequencies of the ac driving field in a temperature range that embraces the critical region near the ferromagnetic (FM)–paramagnetic (PM) phase transition (occurring at the Curie point TC). Elaborate data analysis was performed that (i) enabled the first observation of a crossover from a three-dimensional (3D; d=3) isotropic long-range dipolar asymptotic critical behavior to a d=3 isotropic short-range Heisenberg critical regime as the temperature is raised from TC in the compositions y≠0 (no such crossover is observed in the parent compound, y=0) and (ii) brought out clearly the importance of dipole–dipole interactions between the eg electron spins and/or between eg–t2g electron spins in establishing long-range FM order in the insulating state. The final charge and spin states of Co and Ni ions, substituting for the Mn3+ and/or Mn4+ ions, are arrived at by using a scenario of substitution that is consistent not only with the present results but also with the previously published structural, thermo-gravimetric, bulk magnetization, dc magnetic susceptibility and electrical resistivity data on the same system. The marked similarity seen between the magnetic behavior of the manganite system in question and the quenched random-exchange ferromagnets, within and outside the critical region, suggests that the percolation model forms an adequate description of the FM metal-to-PM insulator transition.

Magnetism in La0.7Pb0.3(Mn0.9TM0.1)O3 (TM=Fe, Co, Ni) CMR perovskites

Abstract In this work, we present structural and magnetic properties of La 0.7 Pb 0.3 (Mn 0.9 TM 0.1 )O 3 (TM=Fe, Co, Ni) manganites. Samples were fabricated by the sol-gel low temperature method. At room temperature, the first structural characterization of these phases gave the rhombohedral space group (R-3c). All the compositions show ferromagnetic behaviour and magnetoresistance. The measured magnetic moment (at low temperature) and the Curie temperature continuously decrease, with respect to the undoped composition, as Mn ions are substituted by Ni, Co, or Fe, respectively. The measured magnetoresistance reaches 65% for the Fe-doped composition. From these results, the influence of the TM doping is interpreted in terms of ferro- or antiferromagnetic competing Mn 3+ -Mn 4+ and TM 3+ -Mn 4+ interactions.

Structure and magnetism in Sr2(Fe1-xAlx)MoO6 (0?x?0.3) double perovskite compounds

The effect of Fe substitution by Al in Sr2(Fe1-xAlx)MoO6 (0≤x≤0.3) double perovskites is reported. X-ray diffraction, magnetic measurements and Mossbauer spectroscopy have been used in the investigation. Contrary to other substitutions in similar compounds, Al increases the cationic order up to x = 0.1 and beyond. This ordering effect is accompanied by an increase in the magnetic moment per Fe atom, but not by corresponding higher Curie temperatures.

Magnetic order changes in Al substituted Sr2FeMoO6 double perovskites

Abstract We have studied the effect of Al substituting for Fe in Sr 2 (Fe 1− x Al x )MoO 6 ( x ⩽0.3) double perovskites. X-ray diffraction, magnetic measurements at low and room temperature and Mossbauer spectroscopy have been used to determine the structural and magnetic properties of these compounds. The main observed effect is an increase with respect to the undoped composition, of the cationic order with Al doping levels up to 20%. This ordering effect is accompanied by an increase of the magnetic moment per Fe atom, but not by corresponding higher Curie temperatures.

Magnetic properties and magnetoresistance of Bi and Fe substituted manganites

Abstract In this work structure, magnetic and transport properties of La0.7−x BixCa0.3MnO3 (0.05⩽x⩽0.7), La0.7Pb0.3Mn1−xFexO3 and La0.7Sr0.3Mn1−xFexO3 (0⩽x⩽0.3) manganites have been studied. Competition between ferromagnetic and antiferromagnetic interactions has been observed giving rise to complex magnetic behaviour, including metamagnetism and spin glass features. Such features are interpreted as arising from the weakening of the double exchange mechanism, of ferromagnetic character, as a consequence of the substitutions. In the case of Bi substituted compounds the covalent character of Bi strongly localizes the hopping electron, while the Fe substitution destroys the double exchange mechanism by the change in energy levels of the Fe4+ ions with respect to the Mn4+ ones.

Magnetic and transport properties of La0.7Cd0.3(Mn0.9TM0.1)O3 transition metal doped manganites

Abstract In this work we present structural and magnetic properties of La 0.7 Cd 0.3 Mn 0.9 TM 0.1 O 3 (TM=Fe, Co, Ni) manganites. Samples were prepared by the sol–gel low temperature method. Preliminary room temperature structural characterization of these compounds shows a mixture of rhombohedral (R3c) and orthorhombic (Pnma) phases. The low value of the tolerance factor already indicates a distorted structure of the perovskite cell. Different kinds of magnetic interactions have been observed as the doping TM element is changed. Thus, respect to the undoped La 0.7 Cd 0.3 MnO 3 composition, the Curie temperature decreases for the Fe doped sample, but progressively increases as Mn ions are substituted by Co or Ni, respectively. The measured low temperature magnetic moment values are interpreted in terms of ferro- (for the Co and Ni doped samples) or antiferromagnetic (for the Fe doped sample) TM 3+ -Mn 3+ /Mn 4+ interactions. Despite their ferromagnetic character, all compositions show insulating behaviour in the whole temperature range studied.

Synthesis characterization and magnetic properties of the Ln0.7A0.3Mn0.9Fe0.1O3 (Ln = La, Nd; A = Pb, Cd) manganites

Abstract The Ln0.7A0.3Mn0.9Fe0.1O3 (Ln = La, Nd; A = Pb, Cd) oxides have been synthesised using the sol—gel method. Magnetic measurements show that Tc values decrease considerably when cadmium or iron are present and the ZFC magnetization drops to zero at low temperatures. Values of magnetoresistance around 70% have been obtained. Powder EPR measurements showed that the minimum peak-to-peak width is near to the critical temperature Tc in each sample.

Magnetic properties of Fe66Cr12Al22 crystalline ribbon

Abstract Thermal dependence of both magnetic and structural properties of crystalline ribbons with the nominal composition Fe66Cr12Al22 were investigated by X-ray diffraction, 57Fe Mossbauer spectrometry and magnetic measurements. An improvement of the magnetic properties of the ribbon has been observed, which can be due to both structural relaxation and defect effects, which are created during the thermal treatment.