Twinning control of magnetic properties of multidomain magnetite below the Verwey transition revealed by measurements on individual particles

To elucidate mechanisms controlling the magnetic properties of multidomain magnetite below the Verwey transition, we have conducted magnetization measurements between 5 and 300 K as a function of temperature and of pre-treatment magnetic field strength, ranging from zero to 5 T, on two individual magnetite grains of similar to 75 mu m (grain 1 thereafter) and similar to 120 x 80 x 40 mu m (grain 2) in size. Both grains show a well-defined Verwey transition, with the transition temperatures (T-V) of (123.33 +/- 0.02) K on cooling versus (123.53 +/- 0.02) K on heating for grain 1 and (116.33 +/- 0.06) K versus (116.69 +/- 0.08) K for grain 2, indicative of nearly stoichiometric magnetite. The principal experimental observations can be summarized as follows: (1) At 10 K both the coercive force and saturation remanence (SIRM) are higher after cooling through the Verwey transition temperature in zero magnetic field (ZFC), compared to cooling in a strong field (FC). This indicates that during ZFC-as opposed to FC-the grains are subdivided into regions with different easy magnetization axis directions (transformation twins). (2) Cooling in fields not exceeding certain critical value H-l (similar to 60 mT for grain 1 and similar to 20 mT for grain 2) leads to ZFC-like LT SIRM with a SIRM maximum around H-l. In contrast, cooling in higher fields produces FC-like SIRM. The transition into FC regime is quite sharp in grain 1, but gradual in grain 2. (3) Below T-V more numerous and larger jumps occur in magnetization versus temperature curves than above T-V. This suggests that the pinning sites are formed on cooling through the transition and must, therefore, be caused by twinning. The observed diminishing of jumps with increasing cooling field also fits into this picture. (4) The fully stable FC state only forms after cooling in a strong field down to 60 K in grain 1, and down to 90 K in grain 2. Switching off the field at higher temperatures (but below T-V) apparently still leads to twinning, probably of magnetostrictive origin. The above experimental evidence strongly suggests that two different mechanisms of twin formation act in the low-temperature phase of magnetite: (i) transformation twinning immediately related to cooling through T-V and (ii) magnetostrictive deformation twinning below T-V.