Paramagnetic resonance and non-resonant microwave absorption in iron niobate

摘要

An electron paramagnetic resonance (EPR) study of FeNb04 powder samples in monoclinic phase (wolframite-type) at X-band (8.8-9.8 GHz), in the 90-300 K temperature range, is presented. For all the temperatures, the EPR spectrum shows a single line associated with Fe~(3+) ions. Changes in the lineshape of the EPR spectrum, which can be attributed to Fe~(2+) ions, are detected at low temperatures. This behavior can be ascribed to a strong magnetic dipolar interaction between Fe~(2+) and Fe~(3+) ions. The non-resonant microwave absorption techniques: magnetically-modulated microwave absorption spectroscopy (MAMMAS) and low-field microwave absorption spectroscopy (LFMAS), were used for a further knowledge on this material. MAMMAS response suggests also the presence of Fe~(2+) ions, that originates a change in microwave absorption regime for T< T_p (=140 K), associated with the presence of short-range magnetic correlations. LFMAS spectra showed a linear behavior with positive slope and non-hysteretic traces. The profiles obtained by plotting the slope vs. temperature of the LFMAS line are similar to those detected by the MAMMAS technique, confirming that both types of measurement show the same processes of absorption.