The complex and interesting properties of ferrimagnetic spinel ferrite thin films are of great fundamental interest, as well as being of practical importance for applications in spintronic devices and ultra-high-density recording media. The presence of antiphase boundary (APB) defects is responsible for reduced spin polarization and magnetism in spinel ferrites^[1,2]. There is also considerable discussion about the relationship between the atomic structures of APBs and their magnetic properties. Whereas atomic structures of many APBs have been determined using high-resolution transmission electron microscopy and high-angle annular dark field (HAADF) imaging^[3], local measurements of magnetic properties at APBs on the nm scale have not yet been achieved.

      Electron magnetic circular dichroism (EMCD) was demonstrated experimentally in 2006 for a specific diffraction geometry^[4]. Since then, the spatial resolution of EMCD has been improved to approximately 1 nm using nanobeam diffraction^[5]. In 2013, we developed a site-specific EMCD method for magnetic structure determination and achieved EMCD spectra with high signal-to-noise ratio^[6]. Here, we combine site-specific EMCD in nanobeam diffraction mode with high-resolution HAADF imaging, in order to simultaneously determine the magnetic circular dichroism and atomic structure of APBs in NiFe₂O₄ thin films.

      We find the atomic structures of APBs that are formed on {111} planes by a crystallographic translation of 1/4a[0-11]  using HAADF imaging. The EMCD signals at such defects were obtained using an electron beam with a diameter of ~1 nm and compared with signals obtained from a perfect single crystalline region under the same illumination and acquisition conditions. We demonstrate experimentally that the strength of the magnetic circular dichroism at APBs is suppressed significantly when compared with that in the perfect area. The capability of EMCD at 1 nm spatial resolution enable us to correlate our experimental magnetic circular dichroism spectra from local defects with corresponding structural and chemical information recorded at the atomic scale, opening the door to experimental investigations of the relationship between atomic structure and magnetic properties of local defects in materials.

References

[1] D.T. Margulies, et al, Phys. Rev. Lett., 79 (25), 5162, 1997

[2] W. Eerenstein, et.al. Phys.Rev.B.68.014428(2003)

[3] K.P. McKenna, et.al, Nat. Commun. 5,5740 (2014).

[4] P. Schattschneider, et al, Nature 441, 486–488 (2006).

[5] J. Salafranca, et al, Nano Lett., 12: 2499, 2012

[6] Z.Q.Wang, X.Y. Zhong, et al, Nat. Commun. 4, 1395 (2013).

Acknowledgements

This work is financially supported by the National Basic Research Program of China (2015CB921700), the National Natural Science Foundation of China (51471096), the Tsinghua University Initiative Scientific Research Program and partly supported by JST under Collaborative Research Based on Industrial Demand ”High Performance Magnets: Towards Innovative Development of Next Generation Magnets”. This work made use of the resources of the National Centre for Electron Microscopy in Beijing and the Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons in Forschungszentrum Jülich. RDB is grateful to the European Research Council for an Advanced Grant. We are grateful to Prof. R. Yu, Prof. J. Zhu, Prof. C. L. Jia, Prof. J. Yuan, Dr. M. Bornhöfft and Mr. D. S. Song for valuable discussions

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