Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of ${\mathrm{SrTiO}}_{3}$

Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of ${SrTiO}_{3}$

Aaron J. Rosenberg, Ferhat Katmis, John R. Kirtley, Nuh Gedik, Jagadeesh S. Moodera, and Kathryn A. Moler

Phys. Rev. Materials 1, 074406 – Published 15 December 2017

Abstract

The combination of ferromagnets with topological superconductors or insulators allows for new phases of matter that support excitations such as chiral edge modes and Majorana fermions. EuS, a wide-bandgap ferromagnetic insulator with a Curie temperature around 16 K, and ${SrTiO}_{3}$ (STO), an important substrate for engineering heterostructures, may support these phases. We present scanning superconducting quantum interference device measurements of EuS grown epitaxially on STO that reveal micron-scale variations in ferromagnetism and paramagnetism. These variations are oriented along the STO crystal axes and only change their configuration upon thermal cycling above the STO cubic-to-tetragonal structural transition temperature at 105 K, indicating that the observed magnetic features are due to coupling between EuS and the STO tetragonal structure. We speculate that the STO tetragonal distortions may strain the EuS, altering the magnetic anisotropy on a micron scale. This result demonstrates that local variation in the induced magnetic order from EuS grown on STO needs to be considered when engineering new phases of matter that require spatially homogeneous exchange.

Received 20 June 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.1.074406

Physics Subject Headings (PhySH)

Magnetic anisotropy Magnetic domains Magnetic susceptibility Magnetism Magnetoelastic effect

Magnetic techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Aaron J. Rosenberg^1,*, Ferhat Katmis^2,3,4, John R. Kirtley¹, Nuh Gedik³, Jagadeesh S. Moodera^2,3, and Kathryn A. Moler^1,5,6

¹Department of Applied Physics, Stanford University, Stanford, California 94305, USA
²Fracsis Bitter Magnetic Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
³Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
⁴Department of Physics, Middle East Technical University, 06800, Ankara, Turkey
⁵Department of Physics, Stanford University, Stanford, California 94305, USA
⁶Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

^*rosenberg.aaronj@gmail.com

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Vol. 1, Iss. 7 — December 2017

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Figure 1
Captured RHEED patterns of EuS after growth and annealing. RHEED snapshot taken with a 15-keV electron beam along (a) the ${[110]}_{p}$ azimuth and (b) the ${[001]}_{p}$ azimuth (c) after growth and (d) after annealing, respectively. (e) X-ray diffraction of the EuS/STO heterostructure (black line) and STO substrate without a EuS layer (red line). Besides the characteristic intense substrate reflection on the low-angle side, another reflection is visible around $≃ 29 . 5^{\circ}$ , which belongs to the ${(002)}_{p}$ reflection of EuS. The representative data also show pronounced Laue oscillations (oscillations in the vicinity of the EuS Bragg peak), which indicate coherency of the top and the bottom surface parallelism of the epitaxial EuS layer.
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Figure 2
Representative magnetometry images of EuS grown on STO showing modulated magnetic features that point primarily along the (a) ${[110]}_{p}$ , (b) ${[010]}_{p}$ , and (c) ${[100]}_{p}$ STO crystallographic axes. Black and white arrows show the direction of the training field. (d) A line cut of image (c), shown as the red line. (e) The edge of the sample, which allows us to determine the STO pseudocubic axes labeled in (a).
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Figure 3
Magnetometry dependence on the in-plane field training angle. Before acquiring each image, we thermal cycled the sample to 30 K, well above the Curie temperature of EuS, and cooled it with a 13-G field in the direction specified by the arrow. With respect to the ${[100]}_{p}$ direction, the angle of the training field is (a) $180^{\circ}$ , (b) $150^{\circ}$ , (c) $120^{\circ}$ , (d) $90^{\circ}$ , (e) $60^{\circ}$ , and (f) $30^{\circ}$ .
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Figure 4
Temperature dependence of the ferromagnetism and the susceptibility to determine the Curie temperature $T_{C}$ . (a) Magnetometry image reproduced from Fig. 2, plotted with (b) the corresponding susceptibility image at 4.2 K. We performed susceptibility touchdowns as a function of the temperature at the location marked with the red X in (b). (c) Representative touchdowns showing how the susceptibility changes with the temperature close to and away from $T_{C}$ . The height $z$ is defined as the separation between the SQUID substrate and the sample surface. (d) Plot of the fitted susceptibility at $z = 0$ with the temperature, including error bars as determined by bootstrapping. The susceptibility diverges, indicative of a paramagnetic-to-ferromagnetic phase transition. The paramagnetism above the Curie temperature is fit to a Curie-Weiss law with $T_{C} = 15.4$ K. (e–k) Magnetometry and (l–r) susceptibility images as a function of the temperature. The paramagnetism above the fitted Curie temperature is spatially inhomogeneous, similar to the large ferromagnetic features below the Curie temperature at 4.2 K.
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Figure 5
Ferromagnetic spatial variations upon thermal cycling reveal a relationship between the magnetic structure and the STO structural phase transition. (a) Thermal history of one region; all images were taken at 4.2 K. (b) The sample was cooled from room temperature without field training, showing resolution-limited domains. (c) The sample was warmed above the Curie temperature of the EuS (30 K) and field trained with 1 G, revealing a stripelike magnetic configuration. The sample was then thermal cycled to (d) 30 K, (e) 120 K, (f) 80 K, and (g) 120 K, each cooled with a 1-Gauss. From (d) to (e) and from (f) to (g), there are changes in the magnetic configuration corresponding to thermal cycling above the STO cubic-to-tetragonal phase transition, but the magnetic configuration does not change otherwise, suggesting that the observed magnetic configuration is related to the STO structural phase transition.
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Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO3

Aaron J. Rosenberg, Ferhat Katmis, John R. Kirtley, Nuh Gedik, Jagadeesh S. Moodera, and Kathryn A. Moler

Phys. Rev. Materials 1, 074406 – Published 15 December 2017

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Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of ${SrTiO}_{3}$