Enhanced conduction and ferromagnetic order at (100)-type twin walls in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films

Enhanced conduction and ferromagnetic order at (100)-type twin walls in $L a_{0.7} S r_{0.3} Mn O_{3}$ thin films

Lluís Balcells, Markos Paradinas, Núria Baguès, Neus Domingo, Roberto Moreno, Regina Galceran, Michael Walls, José Santiso, Zorica Konstantinovic, Alberto Pomar, Marie-Jo Casanove, Carmen Ocal, Benjamín Martínez, and Felip Sandiumenge

Phys. Rev. B 92, 075111 – Published 7 August 2015

Abstract

There is increasing evidence supporting the strong potential of twin walls in ferroic materials as distinct, spatially tunable, functional elements in future electronic devices. Here, we report an increase of about one order of magnitude in conductivity and more robust magnetic interactions at (100)-type twin walls in $L a_{0.7} S r_{0.3} Mn O_{3}$ thin films. The nature and microscopic origin of such distinctive behavior is investigated by combining conductive, magnetic, and force modulation scanning force microscopies with transmission electron microscopy techniques. Our analyses indicate that the observed behavior is due to a severe compressive strained state within an $\sim 1 nm$ slab of material centered at the twin walls, promoting stronger Mn $3 d - O 2 p$ orbital overlapping leading to a broader bandwidth and enhanced magnetic interactions.

Received 23 January 2015
Revised 27 May 2015

DOI:https://doi.org/10.1103/PhysRevB.92.075111

Authors & Affiliations

Lluís Balcells¹, Markos Paradinas¹, Núria Baguès^1,2, Neus Domingo², Roberto Moreno², Regina Galceran¹, Michael Walls³, José Santiso², Zorica Konstantinovic¹, Alberto Pomar¹, Marie-Jo Casanove⁴, Carmen Ocal¹, Benjamín Martínez¹, and Felip Sandiumenge^1,*

¹Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
²Centre for Nanoscience and Nanotechnology, CIN2 (CSIC-ICN), Campus de la Universitat Autònoma de Barcelona, 08193 Bellaterra, Catalonia, Spain
³Laboratoire de Physique des Solides, Bureau 242-2ème étage Est–Bâtiment 510–Rue André, Rivière, 91400 Orsay, France
⁴CNRS, CEMES, Universite de Toulouse UPS, 29 Rue J. Marvig, Toulouse 31055, France

^*Corresponding author: felip@icmab.es

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Vol. 92, Iss. 7 — 15 August 2015

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Figure 1
(a) OC-SEM image exhibiting the twin pattern, (b) SFM topographic image (profile along the red line shown as inset), and (c) current map acquired at $V = - 1.4 V$ , of a $40 nm$ thick LSMO film. Note that negative (dark) current values are measured for $V_{tip} < 0$ . (a), (b), and (c) are presented with the same scale as indicated in (c). In (c), enhanced current running along the [100] and [010] directions corresponds to the TW locations. (d) Averaged current profile along the red line in (c). (e) $I - V$ characteristics obtained at the TWs (black) and on the TD (gray). Each $I - V$ corresponds to the average of five different measurements on the same position. In spite of the low current detected at low voltages both $I - V$ curves present a linear behavior around $V = 0]$ . Inset is a zoom at the linear regime.
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Figure 2
MFM images of an as-grown 100 nm thick LSMO film (a) and in the remanent state (b). Bottom images show the corresponding topographic images. In (b) the TWs appear brighter indicating a stronger tip-sample interaction. Color scale bars correspond to a full range of $2 . 5^{\circ}]$ and $1.2 nm$ for MFM and topographic images, respectively.
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Figure 3
(a) Topography image over an area of $2 mm \times 2 mm$ of the surface of a LSMO thin film. The color scale bar corresponds to a full range of $3 nm$ . (b) to (d) show the MFM phase contrast over the same area as in (a) under different conditions: (d) $h = 15$ nm and $A_{MFM} / A_{sp} = 0.9$ , (c) $h = - 11$ nm and $A_{MFM} / A_{sp} = 0.5$ , and (d) $h = - 22$ nm and $A_{MFM} / A_{sp} = 0.12$ . The color scale bar for all MFM images is shown in (d) and corresponds to a full range scale of $2 . 5^{\circ}$ . In (b) and (c) the tip coercive field is smaller than the coercive field of the TWs, which accordingly appear darker than the TDs. In (d) the tip coercive field becomes larger than the coercive of the TWs, causing a contrast reversal (TWs appear brighter than the TDs).
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Figure 4
(a) Cross section HAADF image of a (100)-TW (indicated with vertical arrows) viewed along the [010] axis, of a 40 nm thick LSMO film. The position of the TW is only recognized by the tilt of the (001) planes by twin angle $Ω$ . The contrast of atomic columns (proportional to their atomic number, $Z$ ) is not altered at the TW. (b) Right panel: planar view HRTEM image of a (100)-TW (indicated with vertical arrows) viewed along the [001] axis, of a $40 nm$ thick film. Left panel: same image but laterally compressed to emphasize the increase of $Ω$ in the TW. Inset is a schematic drawing illustrating the distortion of the structure in the TW according to the GPA analysis presented below. (c) and (d) show the corresponding $Ω$ and $E_{x x}^{TW / TD}$ maps, along with their averaged profiles, as obtained by GPA analysis. $E_{x x}^{TW / TD}$ is the strain component perpendicular to the TW, relative to the TD. (e) Distortion mechanisms of the octahedral framework: octahedral tilting distorts the $〈 Mn - O - Mn 〉$ bond angle by $δ Θ$ and distortion of the $Mn O_{6}$ octahedra results in the elastic stretching or compression $(δ d)$ of the Mn-O bond.
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Enhanced conduction and ferromagnetic order at (100)-type twin walls in $L a_{0.7} S r_{0.3} Mn O_{3}$ thin films

Lluís Balcells, Markos Paradinas, Núria Baguès, Neus Domingo, Roberto Moreno, Regina Galceran, Michael Walls, José Santiso, Zorica Konstantinovic, Alberto Pomar, Marie-Jo Casanove, Carmen Ocal, Benjamín Martínez, and Felip Sandiumenge

Phys. Rev. B 92, 075111 – Published 7 August 2015

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Enhanced conduction and ferromagnetic order at (100)-type twin walls in La0.7Sr0.3MnO3 thin films

Lluís Balcells, Markos Paradinas, Núria Baguès, Neus Domingo, Roberto Moreno, Regina Galceran, Michael Walls, José Santiso, Zorica Konstantinovic, Alberto Pomar, Marie-Jo Casanove, Carmen Ocal, Benjamín Martínez, and Felip Sandiumenge

Phys. Rev. B 92, 075111 – Published 7 August 2015

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Enhanced conduction and ferromagnetic order at (100)-type twin walls in $L a_{0.7} S r_{0.3} Mn O_{3}$ thin films