The effect of the substitution for Mn with Al in the magnetoresistive perovskite La 2/3 Ca 1/3 MnO 3 has been studied by preparing the series La 2/3 Ca 1/3 Mn 1Ϫx Al x O 3 (xр0.2). A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques. Up to xϭ0.05 the Curie temperature ͑and the associated metal-insulator transition͒ decreases drastically with Al doping and the magnetoresistive properties do not change very much. For xу0.1 the lattice spontaneously begins to lose oxygen atoms and for xϭ0.2, 3% of oxygen vacancies are present. This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view. However, the magnetoresistance is enhanced, reaching colossal values of 10 7 % at Hϭ12 T at low temperatures for xϭ0.2.
has been studied by means of a.c. magnetic susceptibility, x-ray and neutron diffraction from 1.5 up to 330 K. This compound shows a structural transition at 299.7 K. Below this temperature a charge ordering state was observed by both x-ray and neutron diffraction measurements. The structure is orthorhombic at temperatures higher than 300 K, but at lower temperatures the patterns show new reflection peaks that can be accounted for in a monoclinic unit cell with stripes of and in the ac-plane. Moreover, the system orders antiferromagnetically at around 120 K giving rise to a magnetic structure of CE type. The magnetic susceptibility measurement clearly shows an anomalous behaviour at both transition temperatures. The charge ordering temperature for the compound is the highest reported in the series (RE = rare earth). This is argued to be a consequence of the larger orthorhombic distortion produced in the unit cell by the lower size of ion.
La,Nd,Sm͒ oxides with the perovskite structure have been studied. All of these compounds show a spin-glass behavior at low temperatures. Isostructural compounds in which Ni or Co are replaced by a nonmagnetic element do not show spin-glass behavior. Therefore, simultaneous presence of Ni and Co is needed to develop competitive ferro-and antiferromagnetic interactions in the ͑RE͒Ni 0.3 Co 0.7 O 3 compounds. Moreover, large negative magnetoresistance below freezing temperature is observed in these samples. We conclude the existence of an electronic transfer between Ni 31 and Co 31 ions in these compounds which gives rise to ferromagnetic interactions.[S0031-9007 (98)05513-6] PACS numbers: 71.30. + h, 75.10.Nr, 75.70.PaThe ͑RE͒MO 3 oxides with perovskite structure, RE being a trivalent rare-earth ion and M being a 3d transition metal, have generated a great deal of interest due to the variety of the magnetic and electrical behavior [1,2]. This interest has grown due to the high temperature superconductivity in related copper perovskites and the recent discovery of the colossal magnetoresistance in manganese perovskites [3][4][5]. The electronic ͑RE͒NiO 3 phase diagram is composed of a paramagnetic insulator, an antiferromagnetic insulator, and paramagnetic metal phases depending on the temperature and the RE ionic radius [6,7]. The perovskite lattice distortions, in particular the Ni -O-Ni angle, are related to the RE 31 ionic size. Therefore, a systematic change of the transfer integral between Ni-3d and O-2p orbitals (p-d mixing) is observed in ͑RE͒NiO 3 perovskites. When the Ni-O-Ni angle is close to 180 ± , a metallic phase is favored while the insulator phase is stabilized when this angle decreases [8]. Then, LaNiO 3 is a metallic compound while NdNiO 3 and SmNiO 3 show a metal-insulator transition at 220 and at 400 K, respectively. In addition, NdNiO 3 and SmNiO 3 develop an antiferromagnetic ordering at 220 K. On the other hand, ͑RE͒CoO 3 perovskites [9-14] show a semiconductor behavior below room temperature. Their ground state is a nonmagnetic Co 31 ͑S 0, t 6 2g ͒. This low spin state changes to a higher spin configuration with increasing temperature but the exact transformation is still an open question; some authors [10] claim a change from the low spin to the high spin state ͑S 2, t 4 2g e 2 g ͒ while other authors [13] suggest the formation of the intermediate spin configuration ͑S 1, t 5 2g , e 1 g ͒. The magnetic and electrical behavior of ͑RE͒Ni 12x -Co x O 3 mixed perovskites have been studied by several authors [14 -18]. LaNi 12x Co x O 3 shows metallic behavior below room temperature up to x 0.6 and semiconductor behavior for higher Co content. In the Nd and Sm compounds, the main effect of Ni replacement by Co is the suppression of the temperature-driven metal-insulator transition, stabilizing the metallic phase for x # 0.2. Higher Co content leads to semiconducting samples as occurs in the La compounds. Earlier magnetic measurements have shown the presence of ferromagnetic interactions in the se...
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