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Magnetism

Fluctuations of the order parameter in $R_{0.55}$Sr$_{0.45}$MnO$_3$ manganites near the metal-insulator phase transition

F. N. Bukhan'ko, A. F. Bukhanko

Galkin Donetsk Institute for Physics and Engineering, Donetsk

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 $R_{0.55}$Sr$_{0.45}$MnO$_3$ 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 (Nd$_{1-y}$Sm$_y$)$_{0.55}$Sr$_{0.45}$MnO$_3$ system indicate the existence of a Griffiths-like phase in samples with a samarium concentration $y >$ 0.5 in the temperature range $T_C<T<T^*$ (where $T^*\sim$ 220 K). For samples with $y >$ 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 (Sm$_{1-y}$Gd$_y$)$_{0.55}$Sr$_{0.45}$MnO$_3$ system for a gadolinium concentration $y$ = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering $T_N\cong$ 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 $\pm$ 70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility $\chi$ suggest that, in the Gd$_{0.55}$Sr$_{0.45}$MnO$_3$ ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of $\chi(T)$ near $T$ = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature $T_G\cong$ 42 K. The low-temperature state with quantum fluctuations exists in the (Sm$_{1-y}$Gd$_y$)$_{0.55}$Sr$_{0.45}$MnO$_3$ system for $y\ge$ 0.5.

Received: 13.08.2012
Accepted: 28.12.2012

 English version: Physics of the Solid State, 2013, 55:6, 1180–1190

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