Study of the Verwey transition in magnetite by low field and magnetically modulated non-resonant microwave absorption

We have investigated the Verwey phase transition (VPT) by two novel non-resonant microwave absorption techniques: low-field absorption (LFA) and magnetically modulated microwave absorption spectroscopy (MAMMAS). Measurements were carried out on sintered polycrystalline samples of Fe₃O₄, in the 77-300 K temperature range. LFA refers to the microwave absorption around the zero DC field range (-1000<H_DC<+1000 G). LFA measurements showed an hysteretic behavior; a minimum in the hysteresis loop width was observed at 126 K, which can be associated with a minimum in magnetic anisotropy, as a consequence of charge localization below the VPT. MAMMAS experiments are based on the variations in microwave absorption (at constant H_DC), as a function of temperature, and seem particularly well adapted to detect a wide range of phase transitions. In the magnetite case, a continuous increase in the microwave power absorption level was observed as temperature decreased, reaching a strong maximum at 130 K and a minimum at 100 K. An inflection point at 126 K was found, in very good agreement with LFA measurements. These results are discussed in detail.