Effect of substitution of Mn by Cr on the 55Mn NMR and magnetoresistance in La 0.6Sr 0.2Mn 1.2− xCr xO 3 (0⩽ x⩽0.2)

Abstract

The La 0.6Sr 0.2Mn 1.2− x Cr x O 3 (0⩽ x⩽0.2) ceramic samples with perovskite-like structure (R 3 ̄ c) have been studied using the X-ray diffraction, NMR, magnetic and resistance methods. The 55Mn NMR spectrum was shown to depend strongly on the Cr/Mn ratio. A single symmetric line centered at 379 MHz was found for La 0.6Sr 0.2Mn 1.2O 3 ( x=0). For La 0.6Sr 0.2Mn 1.1Cr 0.1O 3, the 55Mn NMR spectrum is slightly expanded and shifted to lower frequencies. An analysis of the 55Mn NMR spectrum for the La 0.6Sr 0.2MnCr 0.2O 3 compound has shown that a wide asymmetrical line can be uniquely expanded into four components with different resonance frequencies. These components arise due to both different ionization states of the manganese ions and a non-equivalence of their crystallographic surrounding. There are manganese ions, whose ionization states are close to (3+) and (4+), as well as Mn ions with intermediate valence which resulted 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 increasing Cr content.