Atomic origin of magnetic coupling of antiphase boundaries in magnetite thin films

doi:10.1016/j.jmst.2021.08.017

Abstract

Abstract:

Revealing the magnetic coupling nature of boundary defects is crucial for in-depth understanding of the behavior and properties of magnetic materials and devices. Here, magnetite (i.e., Fe₃O₄) thin films were grown epitaxially on (100) SrTiO₃ single-crystal substrates by pulsed laser deposition. Atomic-scale scanning transmission electron microscopy characterizations reveal that three types of antiphase boundaries (APBs) are formed in the Fe₃O₄ thin film. They are the (100) APB that is formed on the (100) plane with a crystal translation of (1/4)a[01$\bar{1}$], the type I and type II (110) APBs that are both formed on the (110) plane with the same crystal translation of (1/4)a[101] but different terminated atomic planes. The type I (110) APB is terminated at the atomic plane with mixed tetrahedral- and octahedral-sites Fe atoms, the type II (110) APB is terminated at the octahedral-site Fe plane. First-principles calculations reveal that the (100) APB and the type I (110) APB tend to form the ferromagnetic coupling that will not decrease the spin polarization of Fe₃O₄ films, while the type II (110) APB prefers to form the antiferromagnetic coupling that will degrade the magnetic properties. The magnetic coupling modes of the APBs are closely related to the Fe-O-Fe bond angles across the boundaries.

Key words: Fe₃O₄, Transmission electron microscopy, Antiphase boundary, Magnetic coupling, First-principles calculations

Chunyang Gao, Yixiao Jiang, Tingting Yao, Ang Tao, Xuexi Yan, Xiang Li, Chunlin Chen, Xiu-Liang Ma, Hengqiang Ye. Atomic origin of magnetic coupling of antiphase boundaries in magnetite thin films[J]. J. Mater. Sci. Technol., 2022, 107: 92-99.

Figures/Tables 8

Fig. 1. (a) Out-of-plane X-ray diffraction pattern of the 50 nm-thick Fe3O4 thin film on the (100) SrTiO3 substrate, (b) XPS spectra of Fe 2p from the surface of the Fe3O4 film. The XRD pattern includes only 400 and 800 peaks of Fe3O4, no precipitates can be detected. The XPS spectra indicate the typical signatures of Fe3O4 thin films. No satellite peak is detected between Fe 2p3/2 and 2p1/2. (c) Resistivity as a function of temperature for the Fe3O4 films. The resistivity rises abruptly at TV. A broad transition is observed. (d) ZFC temperature-dependent magnetization curve of the Fe3O4 film, in an applied in-plane field of 200 Oe. The change of magnetization at TV can be observed, while the variation is also broad.

Fig. 2. TEM characterization of the Fe3O4(100)/SrTiO3(100) film. (a) bright-field TEM image taken along the [001] zone axis, (b) corresponding composite SAED pattern of the Fe3O4 film and the substrate. The non-uniform contrast in Fe3O4 is usually attributed to the formation of APBs and/or local strain variations. The Fe3O4 thin film grows epitaxially on the (100) SrTiO3 substrate with a cubic-to-cubic orientation relationship.

Fig. 3. Atomic structure of the (100) APBs in Fe3O4. (a) Atom-resolved HAADF STEM image of the APBs along the [001] zone axis. The red arrows denote the boundary. (b) Simulated HAADF image. (c) Atomic structure model of the APBs along the [001] zone axis. (d-f) Three possible atomic models of the APBs viewed along the <011> directions. The crystal translations are (1/4)a[01$\bar{1}$], (1/4)a[011], and (1/4)a[101], respectively. Small red spheres represent oxygen atoms, large blue spheres represent tetrahedral Fe atoms and yellow spheres represent octahedral Fe atoms.

Fig. 4. HAADF images and atomic models of two types of Fe3O4 APBs formed on the (110) crystal plane. (a-c) HAADF image, simulated image, and atomic model of the type I (110) APB along the [001] zone axis. The terminated atomic planes of the type I (110) APB include both tetrahedral- and octahedral-sites Fe atoms. (d-f) HAADF image, simulated image, and atomic model of the type II (110) APB along the [001] zone axis. The terminated atomic planes of the type II (110) APB include octahedral-sites Fe atoms. Both types of (110) APBs have the same crystal translation of (1/4)a[101].

Fig. 5. Spin-polarized local density of states (LDOS) across the (100) APB. The spin-polarized LDOS of the APB are characterized by the consistent minority spin polarization across the boundary, indicating the ferromagnetic coupling nature between the adjacent domains of the APB. The LDOS plots of the regions far away from the boundary (red areas) are in agreement with that of the bulk. Near the boundary within 6 atomic layers (blue areas), there are localized energy shifts of the states in the LDOS plots.

Fig. 6. Spin-polarized local density of states (LDOS) across the type I (110) APB. The minority spin polarization is consistent across the boundary. The spin polarization retains the same direction across the boundary, indicating the ferromagnetic coupling nature of the APB.

Fig. 7. Spin-polarized local density of states (LDOS) across the type II (110) APB. The spin polarization reverses direction at the two sides of the APB, resulting in its antiferromagnetic coupling nature. The decrease of occupied states near the Fermi level is observed in the LDOS plots of two atomic layers near the boundary (blue areas).

Fig. 8. Schematic diagram showing the angles of Fe-O-Fe bonds across the boundary of APBs. There are three type of Fe-O-Fe bonds across the boundaries as FeB-O-FeB (i), FeB-O-FeA (ii) and FeA-O-FeA (iii). (a) (100) APB, (b) Type I (110) APB, (c) Type II (110) APB.

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