Charge ordering (CO) of Fe ions with different charge states in bulk magnetite has been suggested to be responsible for the metal-insulator phase transition at low temperature, below the so-called Verwey transition temperature Tv= 122 K. Despite the intensive efforts more than 65 years, the origin of this CO has been unresolved so far. CO phenomenon has been reported to exist at single crystal surfaces even at room temperature, as observed by scanning tunneling microscopy (STM)[1,2]. We have also reported STM results with similar CO on epitaxially grown films on MgO[3]. The important role of structural defects on this surface CO has also been observed. The surface CO might be related to the CO in the bulk, and should have significant consequences for conductance and magnetic properties at the surface that are decisively in device performances such as magnetic tunnel junction. However, the physical origin of this surface CO is also a matter of debate.
In this work, we report STM and scanning tunneling spectroscopy(STS) results on the role of antiphase domain boundaries (APBs) in modifying the surface CO in magnetite films. The magnetite films grown on MgO are known to exhibit a high density of APBs, which are the result of the intrinsic growth properties. The APBs have been considered to play an important role on the anomalies properties of magnetite films, for instance, the high magnetic saturation and superparamagnetism for thin films[4]. Since the APBs are kind of structural defects, their existence at the surface could modify the CO and the related surface electronic properties. Indeed, we have observed several patterns of CO as a result of modification by the APBs. For examples, ordered and disordered domains are observed on adjacently formed domains separated by an APB. Moreover, we have observed CO on two adjacent domains, but the STS data showed different characteristics of local density of states. This difference suggests the presence of CO with different charge disproportionation between the two domains.
[1] R. Wiesendanger et al., Science 255 (1992) 583. [2] G. Mariotto et al., Phys. Rev. B 66 (2002) 245426. [3] A. Subagyo et al., Jpn. J. Apll. Phys. 44 (2005) 5447; ibid 45 (2006) 2255. [4] D. T. Margulies et al., Phys. Rev. Lett. 79 (1997) 5162.
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