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A Search for Defect Related Ferromagnetism in SrTiO$_3$

Room temperature ferromagnetic hysteresis is observed in commercial SrTiO$_3$ substrates purchased from a variety of suppliers. It is shown that the ferromagnetic signal comes from the unpolished surfaces. Surface impurity phases cannot be detected using either x-ray diffraction or energy dispersive x-ray spectra on the unpolished surfaces. However, a possible correlation between surface disorder (xray diffraction peak linewidth) and ferromagnetism is observed. Ar ion bombardment (10keV-90 keV) can be used to produce surface layer disorder but is not found to induce ferromagnetism. Annealing of the substrates at temperatures ranging from 600 to 1100 $^\circ$C is found to alter the hysteresis curves differently depending on whether the annealing is performed in air or in vacuum. Identical annealing behaviour is observed if the substrates are artificially spiked with iron. This suggests that the ferromagnetic hysteresis of as purchased SrTiO$_3$ could be due to Fe contamination of the unpolished surfaces. In addition, it is shown that no ferromagnetism is observed in samples that contain $10^{19}-10^{21}$ cm$^{-3}$ of oxygen vacancies if all the faces are polished.