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Magnetic field melting of the charge-ordered state of La1∕2Ca1∕2MnO3: A local structure perspective

T. A. Tyson, M. Deleon, M. Croft, V. G. Harris, C.-C. Kao, J. Kirkland, and S-W. Cheong
Phys. Rev. B 70, 024410 – Published 15 July 2004
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  • INTRODUCTION
  • EXPERIMENTAL METHODS
  • RESULTS AND DISCUSSION
  • SUMMARY
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    The local structure about the Mn site in the half-doped system La1∕2Ca1∕2MnO3 was measured in magnetic fields up 10 T to probe the melting of the charge-ordered state. Examination of the Mn-O and Mn-Mn correlations reveals three distinct regions in the structure-field diagram. A broad region with weak field dependence (mainly antiferromagnetic phase below 7.5 T), a narrow-mixed phase region near ∼8.5T and a high-field ferromagnetic phase region with strong field-structure coupling are found. At high field the Mn-O radial distribution becomes Gaussian and the Mn-Mn correlations are enhanced—consistent with the dominance of a ferromagnetic phase. Comparison of the structural measurements with transport and magnetization measurements suggests that the exponential changes in resistivity in the first region are dominated by the reordering of the moments on the Mn sites from CE type antiferromagnetic to ferromagnetic order with only a weak change in the local distortions of the MnO6 octahedra.

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    • Received 9 June 2003

    DOI:https://doi.org/10.1103/PhysRevB.70.024410

    ©2004 American Physical Society

    Authors & Affiliations

    T. A. Tyson1, M. Deleon1, M. Croft2, V. G. Harris3,*, C.-C. Kao4, J. Kirkland3, and S-W. Cheong2

    • 1Department of Physics, New Jersey Institute of Technology, Newark, New Jersey 07102, USA
    • 2Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
    • 3Materials Physics Branch, Naval Research Laboratory, Washington DC 20375, USA
    • 4Brookhaven National Laboratory, Upton, Long Island, New York 11973, USA

    • *Present address: Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115, USA.

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    Vol. 70, Iss. 2 — 1 July 2004

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    Images

    • Figure 1
      Figure 1
      (a) Extracted La1∕2Ca1∕2MnO3 XAFS data for two consecutive scans at 115K and zero field are displayed to show the level of the noise in the data. The first and second scan are displayed as thick and thin lines, respectively. (b) Typical fit to Mn-O shell over the range 0.75<R<1.95Å with thin lines corresponding to the data and thick lines corresponding to the fit. The dashed and solid lines correspond to the magnitude and imaginary parts, respectively.Reuse & Permissions
    • Figure 2
      Figure 2
      (a) Fourier transfrom of the field dependent XAFS data (as in Fig. 1) over the range 2.54<k<14.05Å−1 with the Mn-O, Mn-Ca/La, and Mn-Mn/Mn-O-Mn peaks labeled. (b) The expanded Mn-Mn/Mn-O-Mn peak shows that three distinct structural regions exist. The zero field 295 and 115K spectra are shown the dotted line and dashed lines, respectively.Reuse & Permissions
    • Figure 3
      Figure 3
      Temperature-dependent data at zero field (treated in the same manner as the field-dependent measurements) are shown for comparison. In (a) the full spectra are shown. The second peak is expanded in panel (b). Note the collapse of the Mn-Mn/Mn-O-Mn peak occurs near ∼180K (between Tc and TN). Data were transformed over the useful data range 2.54<k<13.18Å−1 limited by the reduction in signal to noise with increased temperature.Reuse & Permissions
    • Figure 4
      Figure 4
      The field-dependent Debye-Waller factor (σ2=⟨(R−⟨R⟩)2⟩, open circles) for the Mn-O bond compared with the magnetization [solid dots taken from Xiao et al. (Ref. 5)].Reuse & Permissions
    • Figure 5
      Figure 5
      The field-dependent fourth cumulant (C4=⟨(R–⟨R⟩)4⟩−3σ4) for Mn-O bond. This quantity measures the flatness (kurtosis) of the radial distribution. At high field C4 vanishes indicating the disappearance of the Jahn Teller distortion [resulting in a narrow Gaussian distribution of distances (see Fig. 4)].Reuse & Permissions
    • Figure 6
      Figure 6
      The average Mn-O bond distance is seen to approach 1.96Å, the value measured in the optimally ferromagnetic region of the La1−xCaxMnO3 phase diagram near x=0.33.Reuse & Permissions
    • Figure 7
      Figure 7
      Debye-Waller factor plotted as a function of magnetization. The JT distortion varies linearly in both the metallic (FMM) and charge-ordered (CO) regions. The rate of change of the JT distortion is 100 times faster after crossing into the ferromagnetic metallic region.Reuse & Permissions
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