Fluctuations of the order parameter in R 0.55Sr0.45MnO3 manganites near the metal-insulator phase transition

Abstract

The magnetic phase transformations induced by changes of the composition, external magnetic field strength, and temperature in manganites with a nearly half-filled conduction band in the vicinity of the metal-insulator phase transition have been investigated experimentally. It has been found that the substitution of rare-earth ions (Sm) for Nd ions with a larger ionic radius in R 0.55Sr0.45MnO3 manganites leads to a linear decrease in the Curie temperature T C from 270 to 130 K and a transformation of the second-order ferromagnetic (FM) phase transition into a first-order phase transition. The results of measurements of the alternating-current (ac) magnetic susceptibility in the (Nd1 − y Sm y )0.55Sr0.45MnO3 system indicate the existence of a Griffiths-like phase in samples with a samarium concentration y > 0.5 in the temperature range T C 0.5, the magnetization isotherms at temperatures above T C exhibit specific features in the form of reversible metamagnetic phase transitions associated with strong fluctuations of the short-range ferromagnetic order in the system of Mn spins in the high-temperature Griffiths phase consisting of ferromagnetic clusters. According to the results of measurements of the ac magnetic susceptibility in the (Sm1 − y Gd y )0.55Sr0.45MnO3 system for a gadolinium concentration y = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering T N ≊ 48.5 K. An increase in the external static magnetic field at 4.2 K leads to an irreversible induction of the ferromagnetic phase, which is stable in the temperature range 4.2–60 K. In the temperature range 60 K < T < 150 K, there exists a high-temperature Griffiths-like phase consisting of clusters (correlations) with a local charge/orbital ordering. The metastable antiferromagnetic structure is retained in samples with gadolinium concentrations y = 0.6 and 0.7, but it is destroyed with a further increase in the gadolinium concentration upon the transition to the spin-glass state. The magnetization isotherm obtained with variations in the external static magnetic field in the field range ±70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility χ suggest that, in the Gd0.55Sr0.45MnO3 ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of χ(T) near T = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature T G ≊ 42 K. The low-temperature state with quantum fluctuations exists in the (Sm1 − y Gd y )0.55Sr0.45MnO3 system for y ≥ 0.5.