Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure

Er-Jia Guo, Ryan Desautels, Dongkyu Lee, Manuel A. Roldan, Zhaoliang Liao, Timothy Charlton, Haile Ambaye, Jamie Molaison, Reinhard Boehler, David Keavney, Andreas Herklotz, T. Zac Ward, Ho Nyung Lee, and Michael R. Fitzsimmons

Phys. Rev. Lett. 122, 187202 – Published 8 May 2019

Abstract

In the bulk, ${LaCoO}_{3}$ (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film’s interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of $- 20.4 %$ per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the ${CoO}_{6}$ octahedra in combination with the ferroelastic order parameter.

Received 30 November 2018

DOI:https://doi.org/10.1103/PhysRevLett.122.187202

Physics Subject Headings (PhySH)

Crystal symmetry Elastic deformation Epitaxial strain Ferroelasticity Jahn-Teller effect Magnetism Magnetization switching Magnetoelastic effect Spin state transition

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Er-Jia Guo^1,2,3,*, Ryan Desautels¹, Dongkyu Lee¹, Manuel A. Roldan⁴, Zhaoliang Liao¹, Timothy Charlton¹, Haile Ambaye¹, Jamie Molaison¹, Reinhard Boehler^1,5, David Keavney⁶, Andreas Herklotz^1,7, T. Zac Ward¹, Ho Nyung Lee¹, and Michael R. Fitzsimmons^1,8,†

¹Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
²Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
⁴Eyring Materials Center, Arizona State University, Arizona 85287, USA
⁵Geophysical Laboratory, Carnegie Institution for Sciences, Washington, DC 20005, USA
⁶Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
⁷Institute for Physics, Martin-Luther-University Halle-Wittenberg, Halle (Saale) 06120, Germany
⁸Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA

^*Corresponding author. ejguo@iphy.ac.cn
^†Corresponding author. fitzsimmonsm@ornl.gov

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Vol. 122, Iss. 18 — 10 May 2019

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Images

Figure 1
Structural property of the STO/LCO heterostructure. (a) XRR of the STO/LCO heterostructure normalized to the asymptotic value of the Fresnel reflectivity. The solid curve is the best fit to the data (open circle). The inset shows the sample geometry and measuring schematic. (b) X-ray SLD depth profile as a function of the distance from the STO substrate. The black dashed line in the inset is the x-ray SLD without taking chemical roughness into account. The error bars of the LCO SLDs are $\sim 0.3 \times 10^{- 6} Å^{- 2}$ . The dashed curve represents the x-ray SLD of the stoichiometric LCO bulk ( $5.23 \times 10^{- 5} Å^{- 2}$ ). (c) Cross-sectional high-resolution high-angle annular dark-field STEM images of the STO/LCO heterostructure grown on a STO substrate.
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Figure 2
PNR probing of chemical and magnetization depth profiles. (a) Schematic of the PNR experimental setup for the STO/LCO heterostructure. (b) Measured (symbols) and fitted (solid lines) neutron reflectivity curves for the spin-up ( $R^{+}$ ) and spin-down ( $R^{-}$ ) polarized neutron beams, with respect to the external magnetic field $H$ . (c) Fit to the SA ratio, defined as the difference between $R^{+}$ and $R^{-}$ divided by the sum, obtained from the experimental and fitted reflectivity in (b). Error bars represent one standard deviation. (d) Nuclear and (e) magnetic SLD depth profiles at 10 K. The error bar of the nuclear SLD within LCO is $0.031 \times 10^{- 6} Å^{- 2}$ . The dashed curve in (d) represents the neutron SLD of the stoichiometric LCO bulk ( $5.026 \times 10^{- 6} Å^{- 2}$ ). The scale on the top of (e) shows the magnetization $M$ of the LCO film.
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Figure 3
Hydrostatic pressure-dependent spin asymmetries and their corresponding fits. The SA ratios (symbols) and fitted (solid lines). The PNR measurements were performed under hydrostatic pressures of (a) 0, (b) 0.45, and (c) 1.63 GPa at 25 K with a magnetic field of 0.5 T. The magenta square symbols in (a) represent the data measured after the hydrostatic pressure is reduced to 0 GPa.
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Figure 4
Suppression of magnetization in the LCO film under hydrostatic pressure. (a) Nuclear (solid lines) and magnetic (shadow areas) SLD depth profiles as a function of the distance from the STO substrate when the hydrostatic pressure increases from 0 to 1.63 GPa. (b) Pressure-dependent magnetization of the LCO film. The square symbols represent the magnetization of the LCO film bulk, and the circle symbols indicate the magnetization of the LCO interface layer in proximity to the STO substrate or capping layer. The dashed lines are linear fits to the magnetization data. (c)–(e) Schematic energy-level diagrams of a ${Co}^{3 +}$ ion with LS ( $t_{2 g}^{6} e_{g}^{0}$ , $S = 0$ ), IS ( $t_{2 g}^{5} e_{g}^{1}$ , $S = 1$ ), and HS ( $t_{2 g}^{4} e_{g}^{2}$ , $S = 2$ ) configurations, respectively.
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