Dynamics of phase stability and magnetic order in magnetoresistive La0.83Sr0.17Mn0.9857Fe0.02O3

Abstract

Two samples with the same nominal composition ${\mathrm{La}}_{0.83}{\mathrm{Sr}}_{0.17}{\mathrm{Mn}}_{0.98}^{57}{\mathrm{Fe}}_{0.02}{\mathrm{O}}_3$ were prepared in air and argon. Thermograviametric, structural, resistive, and magnetic measurements show that the air-made sample has slightly higher ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio; i.e., it is slightly more highly doped by ∼0.15% cation vacancies. Substitution of Fe for Mn depresses the ferromagnetic transition temperature $T_c$ but has very small effect on the rhombohedral/orthorhombic structural phase boundary. Dramatic differences are found in transport properties for small differences in ${\mathrm{Mn}}^{4+}$ concentration near a structural and magnetic phase boundary. The dynamics of charge hopping and exchange via the effect of relaxation of the magnetic hyperfine field distribution and its average value are monitored by temperature-dependent Mossbauer effect measurements. Ferromagnetic clustering is observed above $T_c$ at ${}^{57}\mathrm{Fe}$ sites. The compound undergoes complex temperature dependent magnetic and phase behaviors that are monitored locally by the ${}^{57}\mathrm{Fe}$ Mossbauer effect. The iron probe has a stable 3+ valance configuration whose hyperfine interaction reflects the structural phase changes, the Jahn-Teller distortion and the time-dependent ${\mathrm{Mn}}^{3+}{-\mathrm{M}\mathrm{n}}^{4+}$ charge fluctuations. It is shown that regions of magnetic order exist over ∼100 K range beginning well above $T_c.$ Moreover, the low-temperature structural phase, reported to be orthorhombic at this composition, shows dynamic magnetic fluctuation behavior that motionally narrows at low temperature. Any changes in hyperfine interaction due to the structural transition are masked by the fluctuating magnetic behavior, but clearly the two structures coexist in the temperature region near $T_c.$