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Structural and magnetic properties of co-sputtered epitaxial Fe-Sn kagome thin films

Callum Brennan-Rich, Sean M. Collins, Stuart Micklethwaite, Zabeada Aslam, Trevor Almeida, Stephen McVitie, Rik M. Drummond-Brydson, Christopher H. Marrows

TL;DR

This work demonstrates the growth of epitaxial Fe–Sn thin films on sapphire with a Pt seed to access Fe3Sn2, FeSn, and mixed-phase compositions. Using XRD and high-resolution 4D-STEM, the authors quantify phase content at the microstructure level and correlate it with magnetization measurements from SQUID-VSM, finding that higher Fe3Sn2 content yields larger saturation magnetization. Notably, a mixed-phase film shows a nonmonotonic coercivity versus temperature near $T\approx 375$ K, indicative of exchange-bias-like FM–AFM interactions at phase boundaries, highlighting the impact of phase purity on magnetic behavior and motivating further exploration of kagome-structured Fe–Sn systems for spintronic applications.

Abstract

In recent years the intermetallic alloys of Fe and Sn have gained significant interest due to a rich variety of magnetic properties present in these materials. The crystal Fe3Sn2 is a frustrated ferromagnet, while the crystallographically similar FeSn, which differs only by the stacking sequence of its Fe-containining kagome and stanene layers, is an antiferromagnet. Thin-film growth techniques such as magnetron sputtering allow for these different stoichiometric compositions to be grown through adjustments of the rate of deposition of the individual Fe and Sn sources, while all other conditions remain constant. Here, we report the production of high quality epitaxial thin films of Fe$_3$Sn$_2$ and FeSn on sapphire with a Pt seed layer, as well as a mixed phase containing intergrowths of both crystals, all of which we have characterized using both X-ray and four-dimensional scanning transmission electron microscopy (4D-STEM) methods. The resulting crystallographic phase content is compared to the results of magnetization measurements, with correspondence between the predicted ferromagnetic phase content and the resulting magnetization. Further magnetic properties of these films can then also be compared, leading to the discovery of a unique behavior in the temperature dependent coercivity within highly mixed phase alloys, a feature that is absent in either pure Fe3Sn2 and FeSn.

Structural and magnetic properties of co-sputtered epitaxial Fe-Sn kagome thin films

TL;DR

This work demonstrates the growth of epitaxial Fe–Sn thin films on sapphire with a Pt seed to access Fe3Sn2, FeSn, and mixed-phase compositions. Using XRD and high-resolution 4D-STEM, the authors quantify phase content at the microstructure level and correlate it with magnetization measurements from SQUID-VSM, finding that higher Fe3Sn2 content yields larger saturation magnetization. Notably, a mixed-phase film shows a nonmonotonic coercivity versus temperature near K, indicative of exchange-bias-like FM–AFM interactions at phase boundaries, highlighting the impact of phase purity on magnetic behavior and motivating further exploration of kagome-structured Fe–Sn systems for spintronic applications.

Abstract

In recent years the intermetallic alloys of Fe and Sn have gained significant interest due to a rich variety of magnetic properties present in these materials. The crystal Fe3Sn2 is a frustrated ferromagnet, while the crystallographically similar FeSn, which differs only by the stacking sequence of its Fe-containining kagome and stanene layers, is an antiferromagnet. Thin-film growth techniques such as magnetron sputtering allow for these different stoichiometric compositions to be grown through adjustments of the rate of deposition of the individual Fe and Sn sources, while all other conditions remain constant. Here, we report the production of high quality epitaxial thin films of FeSn and FeSn on sapphire with a Pt seed layer, as well as a mixed phase containing intergrowths of both crystals, all of which we have characterized using both X-ray and four-dimensional scanning transmission electron microscopy (4D-STEM) methods. The resulting crystallographic phase content is compared to the results of magnetization measurements, with correspondence between the predicted ferromagnetic phase content and the resulting magnetization. Further magnetic properties of these films can then also be compared, leading to the discovery of a unique behavior in the temperature dependent coercivity within highly mixed phase alloys, a feature that is absent in either pure Fe3Sn2 and FeSn.
Paper Structure (8 sections, 5 figures, 1 table)

This paper contains 8 sections, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Materials and Methods
  3. Results and Discussion
  4. Film crystallography
  5. 4D-STEM phase identification
  6. Magnetic characterization
  7. Conclusions
  8. Acknowledgments

Figures (5)

  • Figure 1: Crystal structures of FeSn and Fe$_3$Sn$_2$. (a) Unit cells of the antiferromagnet FeSn and the frustrated ferromagnet Fe$_3$Sn$_2$ crystal. (b) Schematic of the Sn$_2$ stanene layer and the Fe$_3$Sn kagome layer. (c) 2$\theta-\omega$ XRD pattern of three co-sputtered thin films and one clean sapphire substrate. The sapphire (0006) peak seen at 41.68$^{\circ}$ on all patterns, as well as system peaks from the XRD equipment at 20.8$^{\circ}$ and 37.7$^{\circ}$. Films were grown with different growth power of the Fe gun, 40 W, 36 W, and 30 W. Dotted lines indicate the calculated angles for the labeled reflections simulated using the VESTA software VESTA from the unit cells provided by Giefers and Nicol Giefers.
  • Figure 2: STEM results: (a) HAADF image of cross section of the mixed film, with an expanded region explored with (b) high resolution aberration corrected dark field STEM revealing the individual atomic layers. The contrast between the low atomic number ($Z$) sapphire (Al$_2$O$_3$) substrate and high $Z$ Pt of the seed and capping layer are clear. Also shown (c-h) are the nanobeam electron diffraction (NBED) patterns as well as simulated NBED patterns used for orientation and indexing. These correspond to (c,d) Fe$_3$Sn$_2$, (e,f) FeSn, and (g,h) for the sapphire substrate.
  • Figure 3: Phase maps derived from 4D-STEM data for cross sections of: (a) the FeSn film, (b) the Mixed phase film, and (c) the Fe$_3$Sn$_2$ film. Also shown below in each phase map are the associated confidence maps for each phase assignment with bright and dark representing high and low confidence respectively Rauch. (d) shows the NBED pattern from a low confidence region of film, showing the blurred patterns for Fe$_3$Sn$_2$ and FeSn superimposed.
  • Figure 4: In-plane hysteresis loops for the three films from 10 K to 400 K: (a) the Fe$_3$Sn$_2$ film; (b) the Mixed film; and (c) the FeSn film. Note the different field axis scales for the different panels. The inset in all cases shows the loops to full saturation at 600 mT.
  • Figure 5: Coercivity vs. temperature curves between 5 and 600 K for: (a) the Fe$_3$Sn$_2$ film; (b) the Mixed film; and (c) the FeSn film. The measurements taken with the SQUID-VSM in regular and oven mode are indicated. Error bars indicate individual uncertainties associated with measuring a particular coercive field from the hysteresis loop. The Fe$_3$Sn$_2$ film and FeSn film show an expected decrease in coercivity with increasing temperature, but the mixed film demonstrates a minimum in coercivity at 375 K.