Paramagnetic resonance and non-resonant microwave absorption in iron niobate

An electron paramagnetic resonance (EPR) study of FeNbO₄ 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 Fe³⁺ ions. Changes in the lineshape of the EPR spectrum, which can be attributed to Fe²⁺ ions, are detected at low temperatures. This behavior can be ascribed to a strong magnetic dipolar interaction between Fe²⁺ and Fe³⁺ 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 Fe²⁺ 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.