$A$-Site Ordering versus Electronic Inhomogeneity in Colossally Magnetoresistive Manganite Films

$A$ -Site Ordering versus Electronic Inhomogeneity in Colossally Magnetoresistive Manganite Films

V. Moshnyaga, L. Sudheendra, O. I. Lebedev, S. A. Köster, K. Gehrke, O. Shapoval, A. Belenchuk, B. Damaschke, G. van Tendeloo, and K. Samwer

Phys. Rev. Lett. 97, 107205 – Published 8 September 2006

Abstract

Epitaxial ${La}_{3 / 4} {Ca}_{1 / 4} {MnO}_{3} / MgO (100)$ (LCMO) thin film shows an unusual rhombohedral ( $R − 3 c$ ) structure with a new perovskite superstructure at room temperature due to the CE-type ordering of La and Ca with modulation vector $q = 1 / 4 [011]$ . $A$ -site ordered film was found to be electronically homogeneous down to the 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and $A$ -site disordered LCMO demonstrate a mesoscopic phase separation far below the Curie temperature ( $T_{C}$ ). Unique $La / Ca$ ordering compensates the cation mismatch stress within one supercell, $a_{S} \approx 1.55 nm$ , and enhances the electronic homogeneity. The phase separation does not seem to be a unique mechanism for the colossal magnetoresistance (CMR) as very large $CMR \approx 500 %$ was also observed in $A$ -site ordered films.

Received 15 December 2005

DOI:https://doi.org/10.1103/PhysRevLett.97.107205

Authors & Affiliations

V. Moshnyaga¹, L. Sudheendra¹, O. I. Lebedev², S. A. Köster¹, K. Gehrke¹, O. Shapoval³, A. Belenchuk³, B. Damaschke¹, G. van Tendeloo², and K. Samwer¹

¹Erstes Physikalisches Institut, Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany
²EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
³Institute of Applied Physics, Strada Academiei 5, MD-2028, Chisinau, Moldova

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 97, Iss. 10 — 8 September 2006

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Plan-view (a) and cross-section (c) electron diffraction patterns of the cation ordered ${La}_{3 / 4} {Ca}_{1 / 4} {MnO}_{3}$ film with modulated structure. The ED pattern obtained from the MgO substrate indicated by white square in (c). The white rectangles in (a) correspond to the twinned LCMO structure. Plan-view (b) and cross-section (d) HREM images of the same film. The Fourier transforms from single domain of the modulated area [insets in (b) and (d)] exhibit a pattern similar to the corresponding ED patterns in (a) and (c).Reuse & Permissions
Figure 2
Structural model of the La-Ca ordered ${La}_{3 / 4} {Ca}_{1 / 4} {MnO}_{3}$ phase, taking into account the experimental evidence: (a) represents the projection along the $[100]_{C}$ direction and (b) represents the projection along the $[100]_{C}$ direction. The corresponding calculated ED patterns and simulated HREM images obtained at a defocus value (Af), $Af = - 30 nm$ and thickness $t = 18 nm$ based on the proposed model (given as inset in HREM images) show a good agreement with the experimental ones. In (c) a fragment of a HREM image (b) is shown with cations superimposed (red—Ca and blue—La) and Mn (white). The distribution of local stress is shown in (d) with compressivelike (red stripes) and tensilelike (blue stripes). The overall stress is compensated within one supercell, $a_{S} = 4 a_{p} = 1.55 nm$ .Reuse & Permissions
Figure 3
Scanning tunneling microscopy (STM) morphology image ( $500 \times 500 {nm}^{2}$ ) of a pseudocubic LCMO ( $O$ film) (a) and $250 \times 250 {nm}^{2}$ STM morphology scan of the orthorhombic $D$ film (b). The scale bar refers to nm. The tunneling conductivity map of $O$ film (c)- and $D$ films (d), obtained from the same regions as in Fig. 3a, 3b, respectively. The electronically homogeneous behavior in the $O$ film and metal-like (red) and insulatorlike (black) coexistence in the $D$ film are evident. The scale represents the conductance in $nA / V$ .Reuse & Permissions
Figure 4
(a) Temperature dependence of the resistivity, $ρ (T, 0)$ , $B = 0$ for cation ordered film (solid blue curve) and for an orthorhombic film (solid red curve) of the same ${La}_{3 / 4} {Ca}_{1 / 4} {MnO}_{3}$ composition. The broken lines correspond to the $ρ (T, 5 T)$ . The right bottom inset relates to the evaluated temperature dependence of $CMR = 100 % [R (0) - R (5 T)] / R (5 T)$ . The top left inset shows temperature dependence of $α ρ = (d ρ / ρ) / (d T / T)$ ; (b) Field cooled magnetization as a function of temperature for the same films as in (a), measured in external field $H = 100 Oe$ . Inset: $α_{M} = (d M / M) / (d T / T)$ function vs temperature. (c) Correlation between the metal-insulator transition temperatures, $T_{MI}$ , for $LCMO / MgO$ films with two compositions and disorder, quantified by the function $α_{ρ \max } = (d ρ / ρ) / (d T / T)$ . The connecting lines are drawn as guide to the eye.Reuse & Permissions

Physical Review Letters