DYNAMICS OF PHASE-STABILITY AND MAGNETIC ORDER IN MAGNETORESISTIVE (LA0.83SR0.17MN0.98FE0.02O3)-FE-57

Two samples with the same nominal composition (La0.83Sr0.17Mn0.98Fe0.0 2O3)-Fe-57 were prepared in air and argon. Thermograviametric, structu ral, resistive, and magnetic measurements show that the air-made sampl e has slightly higher Mn4+/Mn3+ ratio; i.e., it is slightly more highl y doped by similar to -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 bounda ry. Dramatic differences are found in transport properties for small d ifferences in Mn4+ concentration near a structural and magnetic phase boundary. The dynamics of charge hopping and exchange via the effect o f relaxation of the magnetic hyperfine field distribution and its aver age value are monitored by temperature-dependent Mossbauer effect meas urements. Ferromagnetic clustering is observed above T-c at Fe-57 site s. The compound undergoes complex temperature dependent magnetic and p hase behaviors that are monitored locally by the Fe-57 Mossbauer effec t. The iron probe has a stable 3+ valance configuration whose hyperfin e interaction reflects the structural phase changes, the Jahn-Teller d istortion and the time-dependent Mn3+-Mn4+ charge fluctuations. It is shown that regions of magnetic order exist over similar to 100 K range beginning well above T-c. Moreover, the low-temperature structural ph ase. reported to be orthorhombic at this composition, shows dynamic ma gnetic fluctuation behavior that motionally narrows at low temperature . Any changes in hyperfine interaction due to the structural transitio n are masked by the fluctuating magnetic behavior, but clearly the two structures coexist in the temperature region near T-c.