Effect of isotopic composition and microstructure on the crystalline and magnetic phase states in R0.5Sr0.5MnO3

• Published in: Journal of experimental and theoretical physics Vol. 106; no. 3; pp. 528 - 541
• Main Authors: Balagurov, A. M., Bobrikov, I. A., Pomjakushin, V. Yu, Sheptyakov, D. V., Babushkina, N. A., Gorbenko, O. Yu, Kartavtseva, M. S., Kaul, A. R.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 01.03.2008
• Subjects: Classical and Quantum Gravitation; Disorder; Elementary Particles; Order; Particle and Nuclear Physics; Phase Transition in Condensed Systems; Physics; Physics and Astronomy; Quantum Field Theory; Relativity Theory; Solid State Physics; 61.05.fm; 75.30.-m
• ISSN: 1063-7761; 1090-6509
• DOI: 10.1134/S1063776108030126

The results of structural neutron experiments on determining crystal and magnetic phase states of perovskite-like manganites R 0.5 Sr 0.5 MnO 3 (R = 152 Sm, Nd 0.772 Tb 0.228 , and Nd 0.544 Tb 0.456 ) are reported. Experiments are carried out for revealing microscopic factors responsible for the giant oxygen isotope effect that was discovered recently in Sm 1− x Sr x MnO 3 for x ≈ 0.5. It is shown that separation into two crystal phases P 1 and P 2 with the same spatial symmetry but different types of Jahn-Teller distortions in MnO 6 octahedra and magnetic ordering of Mn atoms takes place in all studied compounds at low temperatures. Structural analysis has been carried out successfully owing to exceptionally large differences in the unit cell parameters of the coexisting phases. The P 1 phase is ferromagnetic and MnO 6 octahedra are distorted only slightly. The P 2 phase is antiferromagnetic ( A -type ordering) and MnO 6 octahedra are strongly compressed in the apical direction. The relative volumes occupied by the P 1 and P 2 phases depend on the mean radius of the A cation, and the replacement of 16 O by 18 O results in their redistribution in favor of the P 2 phase. The results unambiguously point to the percolation nature of the metal-insulator transition in a Sm-containing compound upon isotopic substitution of oxygen due to a sharp decrease (from 65 to 13%) in the fraction of ferromagnetic phase P 1 . In all investigated compounds, the ordered magnetic moment of manganese Mn in the P 1 and P 2 phases varies from 1.7μ B to 3.5μ B . The data on the evolution of the miscrostructure parameters during a phase transition to the stratified state indicate that the initial spread in the A cation radii, as well as the internal microstrains, produce a critical effect on the formation of mesoscopic phase separation.