Effect of substitution of Mn by Cr on the Mn-55 NMR and magnetoresistance in La0.6Sr0.2Mn1.2-xCrxO3 (O <= x <= 0.2)

The La0.6Sr0.2Mn1.2-xCrxO3 (0 less than or equal to x less than or equal to 0.2) ceramic samples with perovskite-like structure (R (3) over bar c) hav e been studied using the X-ray diffraction, NMR, magnetic and resistance me thods. The Mn-55 NMR spectrum was shown to depend strongly on the Cr/Mn rat io. A single symmetric line centered at 379 MHz was Found for La0.6Sr0.2Mn1 .2O3 (x = 0). For La0.6Sr0.2Mn1.1Cr0.1O3, the Mn-55 NMR spectrum is slightl y expanded and shifted to lower frequencies. An analysis of the Mn-55 NMR s pectrum for the La0.6Sr0.2MnCr0.2O3 compound has shown that a wide asymmetr ical line can be uniquely expanded into four components with different reso nance frequencies. These components arise due to both different ionization states of the manganese ions and a non-equivalence of their crystallographi c surrounding. There are manganese ions, whose ionization states are close to (3 + ) and (4 +), as well as Mn ions with intermediate valence which res ulted from the high-frequency electron exchange. The Curie temperature and the temperatures of both resistance maximum and magnetoresistance peak were found to decrease, while the value of magnetoresistance increases with inc reasing Cr content. (C) 2000 Elsevier Science B.V. All rights reserved.