A Search for Defect Related Ferromagnetism in SrTiO$_3$

TL;DR

This study investigates the origin of room temperature ferromagnetism in SrTiO$_3$ substrates, suggesting it is likely due to surface Fe contamination rather than intrinsic defects or surface disorder.

Contribution

It demonstrates that ferromagnetism in SrTiO$_3$ is linked to surface Fe contamination and not caused by oxygen vacancies or surface disorder alone.

Findings

Ferromagnetic hysteresis originates from unpolished surface regions.

Surface impurity phases are not detectable by x-ray diffraction or EDX.

Annealing effects differ based on atmosphere and surface treatment.

Abstract

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.