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Abstract:

We report a study of the magnetic and electrical properties of the ferromagnetic insulator La1.5Sr0.5MnRhO6. A significant magnetoresistance is found in this system which is largest at the Curie temperature (Tc) even though there is no metal-insulator transition. The electrical transport is found to be activated above Tc and described by a variable range hopping law below Tc. Above Tc the carriers are magnetic polarons with a size which increases as the temperature approaches the magnetic transition. Rh substitution preserves ferromagnetic ordering, in contrast with the effect of other dopants on the B sites, but modifies the electrostatic potential leading to carrier localization. We attribute the peak in the magnetoresistance at Tc to the field-induced suppression of critical spin fluctuations which modulate the energetic barriers seen by the carriers.

Abstract:

Colossal magnetoresistance manganites have been synthesised by the microwave dielectric heating of metal nitrates, which act both as effective microwave susceptors, and a ready source of oxidizing NO2 to induce mixed valency.

Abstract:

We study effects on magnetotransport produced by changing the size of the trivalent ions R3+ in the layered polycrystalline materials Sr2RMn2O7 (R = Pr, Nd, Ho, Y) and by varying the mixed-valence ratio Mn3+/Mn4+ in the series Sr2-xNd1+xMn2O7 (x = 0, 0.1, 0.2, 0.5). The results of our magnetic and electrical measurements are related to the measured structural characteristics of each compound. For the Sr2RMn2O7 series, colossal magnetoresistance (CMR) is observed below 150 K for R = Pr and R = Nd but not for samples containing R = Ho or R = Y ions, which have smaller ionic radii. For R = Ho and R = Y, the absence of CMR is correlated with the paramagnetic and insulating properties of the samples. Within the series Sr2-xNd1+xMn2O7, the CMR effect is also observed for temperatures below 150 K and increases as the temperature decreases. In this series the samples show a mixture of spin-glass and antiferromagnetic behaviour (x = 0, 0.1) or no long-range magnetic order (x = 0.2, 0.5). The largest CMR at high temperatures (above 100 K) and high magnetic fields (14 T) is observed for the x = 0.2 sample, which also shows the largest magnetization per formula unit. The observed CMR in the compounds studied here is attributed to magnetic cluster formation.

Abstract:

The non-metallic, magnetically-dilute, cation-disordered perovskite La1.5Sr0.5RhMnO6 is shown to be a soft ferromagnet with T(c) = 105 K with an ordered magnetic moment of 3.56(8) μ(B) per Mn.

Abstract:

The room-temperature crystal structures of the n = 3 Ruddlesden-Popper phases Ca4Mn2FeO9.75 and Sr4Mn2FeO9.80 have been refined from neutron and X-ray powder diffraction data. Both adopt space group 14/mmm with (a,c) = (Ca, 3.73683(1), 27.0860(1) ̊), (Sr, 3.83393(1), 27.8148(1) ̊). In both compounds the cation site at the center of the perovskite blocks is preferentially occupied by Fe (Ca, Mn:Fe = 0.424:0.576(4)), and the anion vacancies are found around this site. The occupied anion sites show static disorder in Ca4Mn2FeO9.75 but not in Sr4Mn2FeO9.80. Both compounds are electrical insulators which order antiferromagnetically at TN = 75 K (Ca) or 90 K (Sr). Susceptibility and M(H) data suggest that not all the Mn and Fe cations take part in the long-range magnetic ordering, and there is evidence of a spin glass transition in both compounds at ̃11 K. The magnetic structure of Ca4Mn2FeO9.75 at 5 K has been determined by neutron diffraction. No ordered moment was detected on the Mn/Fe site at the center of the perovskite blocks; 0.74(1) μB per transition metal cation was measured at the sites on the block edges. Possible causes of magnetic frustration in this crystal structure are considered. Ca4Mn2FeO9.75 has a magnetotresistance of -4% at 137 K in a 14 T field. © 1999 American Chemical Society.