Influence of chemical composition and crystallographic orientation on the interfacial magnetism in $\mathrm{BiFe}{\mathrm{O}}_{3}$/$\mathrm{L}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ superlattices

Influence of chemical composition and crystallographic orientation on the interfacial magnetism in $BiFe O_{3}$ / $L a_{1 − x} S r_{x} Mn O_{3}$ superlattices

Er-Jia Guo, Manuel A. Roldan, Xiahan Sang, Satoshi Okamoto, Timothy Charlton, Haile Ambaye, Ho Nyung Lee, and Michael R. Fitzsimmons

Phys. Rev. Materials 2, 114404 – Published 13 November 2018

Abstract

The emergence of magnetism unique to the interface between the multiferroic $BiFe O_{3}$ (BFO) and ferromagnetic $L a_{1 - x} S r_{x} Mn O_{3}$ (LSMO) offers an opportunity to control magnetism in nanoscale heterostructures with electric fields. In this paper, we investigate the influence of chemical composition and crystallographic orientation on the interfacial magnetism of BFO/LSMO superlattices. Our results reveal that the induced net magnetic moment in the BFO layers increases monotonically with increasing saturation magnetization of the LSMO layers. For the (100)-BFO/LSMO ( $x = 0.2$ ) superlattice, the induced moment reaches a record high value of $\sim 2.8 μ_{B} / Fe$ . No interfacial magnetization is observed at the (100)-BFO/LSMO interface when LSMO is an antiferromagnet. In contrast to (100)-oriented superlattices, no induced moment is observed in (111)-BFO layers. Our results suggest the interfacial structural reconstruction may not be a sufficient condition for the enhanced net moment in BFO layer. Instead, spin canting induced by interfacial exchange coupling is proposed in the (100)- but not in the (111)-BFO, leading to the large net magnetization at the (100)-oriented interface. This work further demonstrates the importance of exchange coupling across heterointerfaces for spin canting in nominally antiferromagnets, providing a pathway to control the magnetic properties of artificial oxide heterostructures.

Received 5 September 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.114404

Physics Subject Headings (PhySH)

Antiferromagnetism Exchange interaction Ferroelectricity Ferromagnetism Magnetic coupling Magnetic interactions

Antiferromagnets Multiferroics Multilayer thin films

Magnetization measurements Neutron reflectometry

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Er-Jia Guo^1,2,3,4,*, Manuel A. Roldan⁵, Xiahan Sang⁶, Satoshi Okamoto², Timothy Charlton¹, Haile Ambaye¹, Ho Nyung Lee², and Michael R. Fitzsimmons^1,7,*

¹Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
²Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
³Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
⁵Eyring Materials Center, Arizona State University, Tempe, Arizona 85287, USA
⁶Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
⁷Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, USA

^*Corresponding authors: ejguo@iphy.ac.cn; fitzsimmonsm@ornl.gov

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Vol. 2, Iss. 11 — November 2018

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Figure 1
Scanning transmission electron microscopy (STEM) and x-ray reflectivity (XRR) measurements on the BFO/LSMO superlattices (SLs). (a) Low-magnification STEM- high-angle annular dark-field (HAADF) image of a (100)-SL ( $x = 0.3$ ), in which the BFO layer thickness is 5 u.c., LSMO layer thickness is 20 u.c., and BFO/LSMO bilayer repeats ten times. The brightness of a layer is proportional to the atomic number Z of the elements. (b) Atomic-resolution STEM-HAADF image of the same sample in the region, marked with yellow rectangle in (a). Schematic of the (100)-BFO/LSMO( $x$ ) SLs is shown on the right hand of STEM-HAADF image, where $x$ is the Sr doping level. (c) XRR curves of (100)-SLs with various $x$ . Open circles are the experimental data and the solid lines are the best fits to the XRR data. Using GenX, the chemical depth profiles of (100)-SLs can be determined accurately (not shown). The total thickness of all (100)-SLs is ∼100 nm. XRR results are in agreement with STEM measurements.
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Figure 2
Structural characterization of the BFO/LSMO SLs. (a) X-ray diffraction (XRD) θ–2θ scans of (100)-SLs with various $x$ . For the (100)-SLs ( $x \geq 0.3$ ), the superlattice peaks up to eight orders of Kiessig fringes were observed, indicating the epitaxial growth of high-quality superlattice. For the (100)-SLs ( $x < 0.3$ ), the superlattice peak was not obvious due to the low density contrast between the BFO and LSMO. (b)–(f). Reciprocal space maps (RSMs) of (100)-SLs around the substrate's 114 reflections, indicating all superlattices are coherently grown on STO substrates.
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Figure 3
Transport properties of BFO/LSMO SLs and LSMO single layers. (a)–(e) R(T) curves and (f)–(j). M(T) curves of all (100)-SLs with various $x$ . R(T) and M(T) curves of LSMO single layers and (100)-SLs with the same $x$ are shown in the same plot for comparison. R(T) measurements were conducted under zero magnetic field during sample warming. M(T) measurements were performed under an in-plane magnetic field of 1 kOe. Inset of (j) shows the zoom-in illustration of M(T) curves of a SL ( $x = 0.6$ ) and a BFO single layer. The small magnetization from a SL ( $x = 0.6$ ) is comparable to that of a BFO single layer at low temperatures, suggesting the magnetic response in SL ( $x = 0.6$ ) comes from the BFO layers.
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Figure 4
Magnetic hysteresis loops of BFO/LSMO SLs and LSMO single layers. (a)–(e) M(H) loops of LSMO single layers and SLs with the same $x$ are shown in the same plot for comparison. M(H) loops were measured at 10 K after field cooling in 1 kOe. The magnetic field was applied along the in-plane direction. (f) The induced magnetization in BFO layers as a function of the saturation magnetization of LSMO layers. The magnetization of BFO layer is antiparallel to that of the LSMO layer due to the antiferromagnetic exchange coupling.
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Figure 5
Magnetic properties of BFO/LSMO SLs with different crystallographic orientations. (a) M(T) curves and (b). M(H) loops of a (111)-LSMO single layer, a (111)-SL, and a (100)-SL, respectively. M(T) curves were measured after field cooling in 1 kOe. M(H) loops were recorded at 10 K with applied in-plane magnetic field. (c) Schematic of G-type AFM spin ordering in bulk BFO. (d), (e) Schematic of spin alignments across the (100)- and (111)-oriented BFO/LSMO interfaces, respectively. The spins from Fe and Mn ions are AFM coupled across the interfaces. In (111)-SL, the spins of Fe ions cant along the field direction due to the Dyzaloshinskii–Moriya Interaction (DMI), producing a small net moment. This behavior is the same with the bulk BFO.
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Figure 6
Polarized Neutron Reflectometry (PNR) measurements on a (111)-oriented BFO/LSMO SL. Measured (open symbols) and fitted (solid lines) reflectivity curves for spin-up ( $R^{+}$ ) and spin-down ( $R^{-}$ ) polarized neutrons are shown as a function of wave vector q. (a), (b) show the PNR results measured at 10 and 300 K, respectively. PNR measurements were performed after field cooling in 1 T. The insets of (a), (b) show the spin asymmetries (SA) of corresponding PNR data and fits. (c), (d) Depth profiles of neutron nuclear and magnetic scattering length densities (SLD), respectively. (e) PNR-derived magnetization of BFO (blue open squares) and LSMO (blue solid squares) layers as a function of temperature. The saturation magnetization of the same (111)-SL measured from SQUID (red stars) is shown for comparison.
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Physical Review Materials

Influence of chemical composition and crystallographic orientation on the interfacial magnetism in BiFeO3/La1−xSrxMnO3 superlattices

Er-Jia Guo, Manuel A. Roldan, Xiahan Sang, Satoshi Okamoto, Timothy Charlton, Haile Ambaye, Ho Nyung Lee, and Michael R. Fitzsimmons

Phys. Rev. Materials 2, 114404 – Published 13 November 2018

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Influence of chemical composition and crystallographic orientation on the interfacial magnetism in $BiFe O_{3}$ / $L a_{1 − x} S r_{x} Mn O_{3}$ superlattices