PRL 112, 059701 (2014)
week ending 7 FEBRUARY 2014
PHYSICAL REVIEW LETTERS
Comment on “Robust Formation of Skyrmions and Topological Hall Effect Anomaly in Epitaxial Thin Films of MnSi” In Ref. [1], Li et al. report the formation of Skyrmions over a wide range of out-of-plane magnetic fields and temperatures in epitaxial MnSi nanolayers grown on Si(111). Here, we demonstrate that the data do not support this claim since there is no evidence that the contrast observed in the Lorentz TEM in Fig. 1 of Ref. [1] has a magnetic origin. We obtained very similar images at room temperature where the contrast is due to moiré fringes caused by the lattice mismatch between the MnSi film and the Si substrate. Furthermore, a comparison with our previous results demonstrates that the observed anomalous Hall effect anomalies (Fig. 2 of Ref. [1]) do not reflect the existence of Skyrmions. Easy-axis anisotropy [2] and/or finite-size effects [3] stabilize helicoids (helical modulations) propagating in the film plane and (111)-oriented Skyrmion lattices in freestanding chiral magnetic nanolayers [4] and epitaxial FeGe=Sið111Þ films [5]. In contrast, polarized neutron reflectometry establishes that the hard-axis uniaxial magnetocrystalline anisotropy in MnSi=Sið111Þ epilayers suppresses helicoids propagating in plane and stabilizes the helix with the propagation direction along the film normal with a wavelength LD ¼ 13.9 nm [6,7]. SQUID magnetometry consistent with Fig. 2 of Ref. [1] independently confirms the orientation and the wavelength of the helix and demonstrates that a (111)-oriented helix is the only ground state in all of the investigated MnSi=Sið111Þ nanolayers with a thickness from 7 to 39 nm [6–8]. This experimental work on MnSi=Sið111Þ and the theoretical calculations in Refs. [2,6,9] exclude the formation of inplane helicoids and (111)-oriented Skyrmion lattices claimed by Li et al. [1]. We also performed Lorentz microscopy on a 26.7 nm thick MnSi sample at room temperature and at T ¼ 10 K, below the Curie temperature of 44 K. Our images show patterns very similar to those of Ref. [1], but these were obtained in focus, and such patterns could be seen at both 10 K and at room temperature so that the contrast was not magnetic in origin. These are moiré fringes that arise from the lattice mismatch between the film and the substrate. The fringes in Figs. 1(a) and 1(b) have a spacing that ranges from 6.5 to 9.5 nm, consistent with the 8.5 nm fringe spacing in Fig. 1(d) of Ref. [1]. A calculation of the strain in the 10 nm MnSi film from the reported 8.5 nm stripe spacing gives a residual in-plane tensile strain of 0.8% [10], as expected from our strain measurements [Fig. 6(a) of Ref. [6]). Figure 1(c) was taken from a different region of the specimen, and Fourier filtering [Fig. 1(d)] produced the speckled pattern seen in Ref. [1], Figs. 1(g) and 1(i). The contrast from the moiré fringes can be easily removed by tilting the specimen by as little as 0.7°. A tilt of this magnitude could account for the dramatic change in the 0031-9007=14=112(5)=059701(2)
FIG. 1 (color online). Lorentz TEM mages similar to Fig. 1 of Ref. [1] acquired in focus and at room temperature with power spectra shown as insets. (a) and (c) are raw images acquired from two different regions of the specimen and images (b) and (d) were obtained from these by applying Fourier filters indicated by yellow circles.
appearance of the images in Fig. 1 of Ref. [1] on heating from T ¼ 6 to 18 K. Furthermore, in some regions of the sample, moiré fringes and surface defects can be made to disappear in the in-focus condition in a fashion that mimics the behavior of magnetic features. Anomalous Hall effect anomalies (Fig. 2 of Ref. [1]) were observed in a broad range of temperatures and out-ofplane magnetic fields in MnSi=Sið111Þ. However, Lorentz microscopy and magnetometry measurements have not revealed any nontrivial chiral modulations in this region. T. L. Monchesky Department of Physics and Atmospheric Science Dalhousie University Halifax, Nova Scotia B3H 3J5, Canada
J. C. Loudon Department of Materials Science and Metallurgy University of Cambridge 27 Charles Babbage Road Cambridge CB3 0FS, United Kingdom
M. D. Robertson Department of Physics Acadia University Wolfville, Nova Scotia B4P 2R6, Canada
A. N. Bogdanov IFW Dresden Postfach 270116, D-01171 Dresden, Germany Received 6 August 2013; published 4 February 2014 DOI: 10.1103/PhysRevLett.112.059701 PACS numbers: 75.30.Kz, 72.25.Ba, 73.61.At, 75.70.Ak [1] Y. Li, N. Kanazawa,, X. Z. Yu, A. Tsukazaki, M. Kawasaki, M. Ichikawa, X. F. Jin, F. Kagawa, and Y. Tokura, Phys. Rev. Lett. 110, 117202 (2013). [2] A. B. Butenko, A. A. Leonov, U. K. Rößler, and A. N. Bogdanov, Phys. Rev. B 82, 052403 (2010).
059701-1
© 2014 American Physical Society
PRL 112, 059701 (2014)
PHYSICAL REVIEW LETTERS
[3] F. N. Rybakov, A. B. Borisov, and A. N. Bogdanov, Phys. Rev. B 87, 094424 (2013). [4] X. Z. Yu, Y. Onose, N. Kanazawa, J. H. Park, J. H. Han, Y. Matsui, N. Nagaosa, and Y. Tokura, Nature (London) 465, 901 (2010). [5] S. X. Huang and C. L. Chien, Phys. Rev. Lett. 108, 267201 (2012). [6] E. A. Karhu, U. K. Rößler, A. N. Bogdanov, S. Kahwaji, B. J. Kirby, H. Fritzsche, M. D. Robertson, C. F. Majkrzak, and T. L. Monchesky, Phys. Rev. B 85, 094429 (2012).
week ending 7 FEBRUARY 2014
[7] E. A. Karhu, S. Kahwaji, M. D. Robertson, H. Fritzsche, B. J. Kirby, C. F. Majkrzak, and T. L. Monchesky, Phys. Rev. B 84, 060404 (2011). [8] E. Karhu, S. Kahwaji, T. L. Monchesky, C. Parsons, M. D. Robertson, and C. Maunders, Phys. Rev. B 82, 184417 (2010). [9] U. K. Rößler, A. A. Leonov, and A. N. Bogdanov, J. Phys. Conf. Ser. 303, 012105 (2011). [10] P. Hirsch, A. Howie, R. Nicholson, D. W. Pashley, and M. J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977), 2nd ed., p. 357.
059701-2
week ending 7 FEBRUARY 2014
PHYSICAL REVIEW LETTERS
Comment on “Robust Formation of Skyrmions and Topological Hall Effect Anomaly in Epitaxial Thin Films of MnSi” In Ref. [1], Li et al. report the formation of Skyrmions over a wide range of out-of-plane magnetic fields and temperatures in epitaxial MnSi nanolayers grown on Si(111). Here, we demonstrate that the data do not support this claim since there is no evidence that the contrast observed in the Lorentz TEM in Fig. 1 of Ref. [1] has a magnetic origin. We obtained very similar images at room temperature where the contrast is due to moiré fringes caused by the lattice mismatch between the MnSi film and the Si substrate. Furthermore, a comparison with our previous results demonstrates that the observed anomalous Hall effect anomalies (Fig. 2 of Ref. [1]) do not reflect the existence of Skyrmions. Easy-axis anisotropy [2] and/or finite-size effects [3] stabilize helicoids (helical modulations) propagating in the film plane and (111)-oriented Skyrmion lattices in freestanding chiral magnetic nanolayers [4] and epitaxial FeGe=Sið111Þ films [5]. In contrast, polarized neutron reflectometry establishes that the hard-axis uniaxial magnetocrystalline anisotropy in MnSi=Sið111Þ epilayers suppresses helicoids propagating in plane and stabilizes the helix with the propagation direction along the film normal with a wavelength LD ¼ 13.9 nm [6,7]. SQUID magnetometry consistent with Fig. 2 of Ref. [1] independently confirms the orientation and the wavelength of the helix and demonstrates that a (111)-oriented helix is the only ground state in all of the investigated MnSi=Sið111Þ nanolayers with a thickness from 7 to 39 nm [6–8]. This experimental work on MnSi=Sið111Þ and the theoretical calculations in Refs. [2,6,9] exclude the formation of inplane helicoids and (111)-oriented Skyrmion lattices claimed by Li et al. [1]. We also performed Lorentz microscopy on a 26.7 nm thick MnSi sample at room temperature and at T ¼ 10 K, below the Curie temperature of 44 K. Our images show patterns very similar to those of Ref. [1], but these were obtained in focus, and such patterns could be seen at both 10 K and at room temperature so that the contrast was not magnetic in origin. These are moiré fringes that arise from the lattice mismatch between the film and the substrate. The fringes in Figs. 1(a) and 1(b) have a spacing that ranges from 6.5 to 9.5 nm, consistent with the 8.5 nm fringe spacing in Fig. 1(d) of Ref. [1]. A calculation of the strain in the 10 nm MnSi film from the reported 8.5 nm stripe spacing gives a residual in-plane tensile strain of 0.8% [10], as expected from our strain measurements [Fig. 6(a) of Ref. [6]). Figure 1(c) was taken from a different region of the specimen, and Fourier filtering [Fig. 1(d)] produced the speckled pattern seen in Ref. [1], Figs. 1(g) and 1(i). The contrast from the moiré fringes can be easily removed by tilting the specimen by as little as 0.7°. A tilt of this magnitude could account for the dramatic change in the 0031-9007=14=112(5)=059701(2)
FIG. 1 (color online). Lorentz TEM mages similar to Fig. 1 of Ref. [1] acquired in focus and at room temperature with power spectra shown as insets. (a) and (c) are raw images acquired from two different regions of the specimen and images (b) and (d) were obtained from these by applying Fourier filters indicated by yellow circles.
appearance of the images in Fig. 1 of Ref. [1] on heating from T ¼ 6 to 18 K. Furthermore, in some regions of the sample, moiré fringes and surface defects can be made to disappear in the in-focus condition in a fashion that mimics the behavior of magnetic features. Anomalous Hall effect anomalies (Fig. 2 of Ref. [1]) were observed in a broad range of temperatures and out-ofplane magnetic fields in MnSi=Sið111Þ. However, Lorentz microscopy and magnetometry measurements have not revealed any nontrivial chiral modulations in this region. T. L. Monchesky Department of Physics and Atmospheric Science Dalhousie University Halifax, Nova Scotia B3H 3J5, Canada
J. C. Loudon Department of Materials Science and Metallurgy University of Cambridge 27 Charles Babbage Road Cambridge CB3 0FS, United Kingdom
M. D. Robertson Department of Physics Acadia University Wolfville, Nova Scotia B4P 2R6, Canada
A. N. Bogdanov IFW Dresden Postfach 270116, D-01171 Dresden, Germany Received 6 August 2013; published 4 February 2014 DOI: 10.1103/PhysRevLett.112.059701 PACS numbers: 75.30.Kz, 72.25.Ba, 73.61.At, 75.70.Ak [1] Y. Li, N. Kanazawa,, X. Z. Yu, A. Tsukazaki, M. Kawasaki, M. Ichikawa, X. F. Jin, F. Kagawa, and Y. Tokura, Phys. Rev. Lett. 110, 117202 (2013). [2] A. B. Butenko, A. A. Leonov, U. K. Rößler, and A. N. Bogdanov, Phys. Rev. B 82, 052403 (2010).
059701-1
© 2014 American Physical Society
PRL 112, 059701 (2014)
PHYSICAL REVIEW LETTERS
[3] F. N. Rybakov, A. B. Borisov, and A. N. Bogdanov, Phys. Rev. B 87, 094424 (2013). [4] X. Z. Yu, Y. Onose, N. Kanazawa, J. H. Park, J. H. Han, Y. Matsui, N. Nagaosa, and Y. Tokura, Nature (London) 465, 901 (2010). [5] S. X. Huang and C. L. Chien, Phys. Rev. Lett. 108, 267201 (2012). [6] E. A. Karhu, U. K. Rößler, A. N. Bogdanov, S. Kahwaji, B. J. Kirby, H. Fritzsche, M. D. Robertson, C. F. Majkrzak, and T. L. Monchesky, Phys. Rev. B 85, 094429 (2012).
week ending 7 FEBRUARY 2014
[7] E. A. Karhu, S. Kahwaji, M. D. Robertson, H. Fritzsche, B. J. Kirby, C. F. Majkrzak, and T. L. Monchesky, Phys. Rev. B 84, 060404 (2011). [8] E. Karhu, S. Kahwaji, T. L. Monchesky, C. Parsons, M. D. Robertson, and C. Maunders, Phys. Rev. B 82, 184417 (2010). [9] U. K. Rößler, A. A. Leonov, and A. N. Bogdanov, J. Phys. Conf. Ser. 303, 012105 (2011). [10] P. Hirsch, A. Howie, R. Nicholson, D. W. Pashley, and M. J. Whelan, Electron Microscopy of Thin Crystals (Krieger, Malabar, FL, 1977), 2nd ed., p. 357.
059701-2
