Effect of oxygen deficiency on the magnetic, electrical, magnetoelectric, and magnetoelastic properties of La1-x Sr x MnO3-δ manganites

This paper reports on a study of the effect of oxygen deficiency on the magnetic and electrical properties of the La1 - x Sr (x) MnO3 - delta manganites (x = 0, 0.2, 0.4; delta = 0, 0.13, 0.2). In compositions with x = 0 and delta = 0.13 and 0.2, the temperature dependence of the magnetization M (T) can be approximated by the Langevin function with the moment of superparamagnetic clusters mu = 77 mu(B) (delta = 0.13) and 86 mu(B) (delta = 0.2) at temperatures of 25 K a parts per thousand currency sign T a parts per thousand currency sign 250 K, with the exception of a small temperature range near 132 K in which a maximum characteristic of the LaMnO3 antiferromagnet is seen. These compositions, as well as Sr-doped compositions with delta not equal 0, reveal a difference between the magnetizations of the sample cooled in a weak magnetic field and in zero field. At T = 6 K in a magnetic field of 16 kOe, the compositions with x = 0.2 and 0.4, delta = 0.13 possess a magnetic moment per formula unit lower than that with delta = 0, as well as reveal an anomalous relation between the Curie temperature T (C) and the Curie paramagnetic point gJ, namely, T (C) > gJ. The magnetization of compositions with x = 0.2 and 0.4, delta = 0.2 follows the Langevin function with mu = 40 mu(B) and 130 mu(B), respectively. The electrical resistivity of samples with x = 0.4 and delta = 0.13 and 0.2 is one to two orders of magnitude larger than that of the composition with x = 0.4, delta = 0, which evidences partial or complete compensation of acceptor defects (Sr2+ ions) by donor defects (doubly charged O2- vacancies). The above properties of oxygen-deficient compositions suggest that they contain an insulating ferro-antiferromagnetic magnetically double-phase state. The magnetoresistance and volume magnetostriction in samples with x = 0.4 and delta = 0, 0.13, and 0.2 are small; indeed, in a magnetic field of 8 kOe, they do not exceed 1.4% and 6 x 10(-6), respectively. On this basis, it is concluded that the unbalanced doubly charged donors (O vacancies) are in the state with antiparallel spins and, thus, do not initiate the formation of ferron-type ferromagnetic clusters. The significance of both compensated and unbalanced doubly charged donors consists in that they give rise to the formation of fractured Mn-O-Mn bonds, which bring about lowering of the magnetic moment per formula unit in compositions with x = 0.2 and 0.4 and delta = 0.13 and the transition to superparamagnetism in compositions with x = 0.2 and 0.4, delta = 0.2.