A Study of Ferromagnetic Signals in SrTiO_3 Nanoparticles

摘要

It has been suggested that ferromagnetism may be a universal feature of nanoparticles related to particle size. We study this claim for the case of commercially produced SrTiO_3 nanoparticles purchased from Alfa-Aesar. Both loosely-packed nanoparticle samples and pellets formed using uniaxial pressure were studied. Both loose and pressed samples were annealed in either air or in vacuum of 5×10~(-6) Torr at 600, 800 and 1000?C. Then x-ray diffraction and SQUID measurements were made on the resulting samples. It was found that annealed loose powder samples always had a linear diamagnetic magnetization versus field response, while their pressed pellet counterparts exhibit a ferromagnetic hysteresis component in addition to the linear diamagnetic signal. Williamson-Hall analysis reveals that the particle size in pressed pellet samples increases with annealing temperature but does not change significantly in loose powder samples. The main conclusion is that the act of pressing pellets in a die introduces a spurious ferromagnetic signal into SQUID measurements.