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September 15, 2004

Orbital and spin correlations in a manganite probed with soft x-ray resonant diffraction

K.J. Thomas¹, J.P. Hill¹, S. Grenier^1,2, Y.-J. Kim¹, P. Abbamonte³, L. Venema⁴, A. Rusydi³, Y. Tomioka⁵, Y. Tokura^5,6,7, D.F. McMarrow^8,9, G. Sawatzky¹⁰, and M. van Veenendaal^11,12
1Department of Physics, Brookhaven National Laboratory, Upton, NY; ²Department of Physics and Astronomy, Rutgers University, New Brunswick, NJ; ³National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY; ⁴Materials Science Centre, University of Groningen, Netherlands; ⁵Correlated Electron Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; ⁶Department of Applied Physics, University of Tokyo, Japan; ⁷Spin Superstructure Project, ERATO, Japan Science and Technology Corporation, Tsukuma, Japan; ⁸London Centre for Nanotechnology, London, UK; ⁹Department of Physics and Astronomy, University College London, London, UK; ¹⁰University of British Colombia, Vancouver, Canada; ¹¹Northern Illinois University, De Kalb, IL; ¹²Argonne National Laboratory, Argonne, IL

Soft x-ray resonant diffraction was used to directly probe spin and orbital correlations in a near-half doped manganite. The diffraction was performed at the manganese (Mn) L_II and L_III absorption edges, providing a sensitive spectroscopy of the Mn 3d states in the spin and orbitally ordered phases. These measurements suggest that the established "checkerboard" model for charge ordering of the Mn³⁺ and Mn⁴⁺ ions is too simplistic, and reveal a surprising discrepancy between the orbital and magnetic correlation lengths.

In doped manganites of the form RE_1-xA_xMnO₃ (where RE is a rare earth and A is a divalent element), the magnetic coupling between Mn sites depends on the overlap of the Mn 3d electron orbitals. Therefore, a complex behavior arises in these materials when the direction of the highest occupied Mn 3d orbital is itself a degree of freedom. For example, the Mn 3d orbitals can undergo long-range order, usually in association with cooperative distortions of the oxygen octahedra that surround the Mn sites. Understanding how the orbital physics drives the overall ground state necessitates a direct probe of both orbital and magnetic order.

We used resonant x-ray diffraction to directly probe the magnetic and orbital order in the near half-doped manganite Pr_0.6Ca_0.4MnO₃, for which the proposed spin, charge, and orbital ground state is shown in Figure 1. The incident energy at a magnetic, (½ 0 0), or orbital, (0 ½ 0), Bragg peak was tuned through the Mn L_II and L_III atomic absorption edges (~ 650 eV). At the L-edges, core Mn p_1/2 and p_3/2 electrons are resonantly excited into unoccupied 3d levels, which enhances the Mn sites’ contribution to the diffracted intensity. The strength of the 2p → 3d resonance at an Mn site depends on the local charge distribution of the occupied 3d orbitals. This leads to a large contrast between the resonant scattering factors on sites 1 and 2 (Figure 1) and a large enhancement of the orbital (0 ½ 0) Bragg peak. The resonance matrix element also depends on the direction of the spin in the 3d levels, resulting in magnetic resonant scattering at the antiferromagnetic Bragg peak, (½ 0 0). The magnetic enhancement at the L-edges is truly enormous – off resonance, the magnetic scattering is too weak to be observed in these materials – and thus provides a unique opportunity to directly compare the orbital and magnetic correlations in a manganite.

Figure 2(a) shows the orbital and magnetic resonant line shapes (energy scans at fixed Q). The features in the line shapes are the different excited states in the 3d band, which are probed with increasing incident energy. The 3 eV shift in spectral weight and the large difference in intensity between the orbital and magnetic spectra suggests that the checkerboard charge-ordered picture in Figure 1 is too simple [see publication]. Furthermore, longitudinal scans through the orbital and magnetic Bragg peaks show that the orbital peak is approximately two times wider than the magnetic peak (Figure 2(b)). This suggests that the orbital correlations are shorter ranged than the magnetic correlations, a result that appears at odds with orbitally driven magnetic order.

BEAMLINE
X1B

FUNDING
Department of Energy, Division of Materials Science
The Netherlands Organization for Scientific Research (NWO) Spinoza program

PUBLICATION
K. J. Thomas, J. P. Hill, S. Grenier, Y-J. Kim, P. Abbamonte, L. Venema, A. Rusydi, Y. Tomioka, Y. Tokura, D. F. McMorrow, G. Sawatzky and M. van Veendendaal. Soft x-ray resonant diffraction study of magnetic and orbital correlations in a manganite near half-doping. Physical Review Letters, 92, 237204 (2004).

FOR MORE INFORMATION
Jessica Thomas
Department of Physics, X-ray Scattering Group
Brookhaven National Laboratory
Upton, New York
Email: jessica@bnl.gov