Journal of Magnetism and Magnetic Materials 310 (2007) 819–821
Orbital ordering of layered manganites from resonant
soft X-ray scattering
K.S. Chao
a,b, D.J. Huang
a,b,c,, J. Okamoto
a, H.-J. Lin
a, C.-H. Hsu
a, Y. Kaneko
d,
R. Mathieu
d, W.B. Wu
b, Y. Tokura
d,e,f, C.T. Chen
aaNational Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan bDepartment of Electrophysics, National Chiao-Tung University, Hsinchu 30010, Taiwan
cDepartment of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan dSpin Superstructure Project, JST, AIST, Central 4, Tsukuba 305-8562, Japan eCorrelated Electron Research Center, AIST Central 4, Tsukuba 305-8562, Japan
fDepartment of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
Available online 15 November 2006
Abstract
We present measurements of Mn L-edge resonant soft X-ray scattering on single-layered manganites with different sizes of cations. Orbital ordering of Pr0:5Ca1:5MnO4exhibits a stronger three-dimensional character and a dramatically enhanced transition temperature, as compared with those of La0:5Sr1:5MnO4. The c-axis correlation length of orbital ordering in Pr0:5Ca1:5MnO4is about half of the in-plane correlation length. Our results indicate that reduction in one-electron bandwidth and quenched disorder strongly enhances the stabilization of charge–orbital ordering.
r2006 Published by Elsevier B.V.
PACS: 60.10.i; 75.47.Lx; 78.70.Ck
Keywords: Manganite; Charge–orbital ordering; Resonant soft X-ray scattering
1. Introduction
Physical phenomena, such as colossal magnetoresistance and metal–insulator transition, of correlated electron compounds typically arise from the interplay among the spin, charge, orbital, and lattice degrees of freedom[1]. For example, the colossal magnetoresistance of manganites are strongly affected by the lattice distortion resulting from the Jahn–Teller effect or the sizes of the A-site cations; the latter leads to the change of the effective one-electron bandwidth and quenched disorder. The smaller the average radius rA of the A-site cations is, the narrower the
bandwidth is. Such a narrowing of bandwidth tends to stabilize the charge–orbital ordering [2]. The mismatch between the ionic radii of divalent and trivalent cations,
i.e., quenched disorder, also affect the stabilization of spin and charge–orbital ordering[3,4].
To observe orbital ordering directly is a difficult task. Experimental results of resonant X-ray scattering (RXS) at the Mn 1s threshold of La0:5Sr1:5MnO4 have been
presented to be direct evidence for orbital ordering [5]. However, RXS detects the 3d charge ordering indirectly via the 1s ! 4p transition and the hybridization between 4p and 3d electronic states. Calculations based on a local-density approximation including on-site Coulomb interac-tions [6,7] and multiple scattering theory [8]indicate that RXS measurements pertain mainly to Jahn–Teller distor-tion, instead of directly observing 3d orbital ordering. In contrast, resonant soft X-ray scattering around the Mn L-edge (2p ! 3d) are dipole-allowed and suitable for probing the Mn 3d charge–orbital orderings directly and with high sensitivity.
Here, we present measurements of Mn L-edge resonant soft X-ray scattering on La0:5Sr1:5MnO4and Pr0:5Ca1:5MnO4
ARTICLE IN PRESS
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0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.707
Corresponding author. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan.
to measure the transition temperature and the correlation length of the orbital ordering with variations of one-electron bandwidth and quenched disorder.
2. Experimental setup
Single crystals of La0:5Sr1:5MnO4 and Pr0:5Ca1:5MnO4
were grown with the floating zone method, and were characterized with X-ray diffraction at room temperature. La0:5Sr1:5MnO4 has a larger rA (1:28 ˚A) than that of
Pr0:5Ca1:5MnO4 (1:18 ˚A); Pr0:5Ca1:5MnO4 also has a
much smaller variance of ionic radii of the A-site ions[9]. We measured resonant soft X-ray scattering on single crystals of La0:5Sr1:5MnO4 and Pr0:5Ca1:5MnO4 at 90 K
with the elliptically polarized-undulator beamline of Na-tional Synchrotron Radiation Research Center (NSRRC), Taiwan. Crystals were cut with [1 1 0] surface normal, and aligned in a two-circle soft X-ray diffractometer with the [1 1 0] and [0 0 1] axes defining the scattering plane. The E vector of photons were perpendicular to the scattering plane, and the energy resolution was 0:14 eV at 640 eV.
3. Results and discussion
With measurements of soft X-ray scattering, we found that orbital ordering in both La0:5Sr1:5MnO4 and
Pr0:5Ca1:5MnO4 exhibits a modulation vector ð14140Þ in
reciprocal lattice units. Fig. 1 shows the photon-energy dependence of the ð1
4 1
40Þ scattering intensities around the
Mn L edges, consistent with previous measurements[10]. The spectra of ð1
4 1
40Þ resonant scattering from these
two half-doped manganites have a similar line shape; detailed analysis of the measured energy dependence of soft X-ray scattering from orbital ordering will be
discussed elsewhere. The transition temperature TCO of
charge–orbital ordering in La0:5Sr1:5MnO4 is 235 K;
whereas Pr0:5Ca1:5MnO4 has a dramatically enhanced
TCO of 326 K. In addition, the charge–orbital ordering of
Pr0:5Ca1:5MnO4has a thermal hysteresis shown in the inset
ofFig. 1, in agreement with measurements of resistivity[9].
We also measured the correlation lengths along the [1 1 0] and the [0 0 1] directions, denoted as x1 1 0 and x001,
respectively.Fig. 2displays momentum-transfer dependence of Mn L-edge resonant ð14 140Þ scattering of La0:5Sr1:5MnO4
and Pr0:5Ca1:5MnO4 with momentum transfer q along these
two directions, i.e., q110 and q001 scans. The correlation
length is defined as the inverse of the width at the half-maxima in the q scan. The measured x110 and x001 are 340
and 75 ˚A, respectively, for La0:5Sr1:5MnO4, and 668 and
370 ˚A for Pr0:5Ca1:5MnO4. The results indicate that the
in-plane correlation x110of Pr0:5Ca1:5MnO4about twice of that
of La0:5Sr1:5MnO4. In addition, the ratio of x001=x110 in the
orbital ordering of Pr0:5Ca1:5MnO4 is 0.55, significantly
larger than that of La0:5Sr1:5MnO4, 0.22.
Based on the correlation lengths measured at the same temperature in the two compounds with different ordering temperatures, we show that the reduction in one-electron bandwidth and quenched disorder strongly enhances the stabilization of orbital ordering. Pr0:5Ca1:5MnO4 also
exhibits a more three-dimensional-like orbital ordering than that of La0:5Sr1:5MnO4.
ARTICLE IN PRESS
Intensity (arb. units)
655 650 645 640 635 Photon energy 350 300 250 200 150 Temperature (K)
Intensity (arb. units)
cooling heating La0.5Sr1.5MnO4 Pr0.5Ca1.5MnO4 PCMO LSMO
Fig. 1. Photon-energy dependence of the Mn L-edge ð1
4140Þ scattering of
La0:5Sr1:5MnO4 and Pr0:5Ca1:5MnO4, denoted as LSMO and PCMO,
respectively. The inset displays the temperature dependence of the ð1 4140Þ
scattering intensities.
Intensity (arb. units)
0.655 0.635 0.615 0.595 0.575 0.555 0.535 0.515 q110 (1/Angstrom) -8x10-2 -6 -4 -2 0 2 4 6 8 q001 (1/Angstrom) q001 scan ξ = 74 Å q110 scan ξ =340 Å
Intensity (arb. units)
0.611 0.601 0.591 0.581 0.571 0.561 0.551 q110 (1/Angstrom) -30x10-3 -20 -10 0 10 20 30 q001 (1/Angstrom) q001 scan ξ=370 Å q 110 scan ξ = 668 Å La0.5Sr1.5MnO4 Pr0.5Ca1.5MnO4
a
b
Fig. 2. Momentum-transfer dependence of Mn L-edge resonant ð1 4140Þ
scattering of La0:5Sr1:5MnO4 and Pr0:5Ca1:5MnO4 along the [1 1 0] and
[0 0 1] directions recorded at 90 K. K.S. Chao et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 819–821 820
Acknowledgments
We thank NSRRC staff, particularly L.L. Lee and H.W. Fu, for their technical support. This work was supported in part by the National Science Council of Taiwan.
References
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