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Structure & Magnetism of LaMn 1- x Ga x O 3

Structure & Magnetism of LaMn 1- x Ga x O 3. J. Farrell & G. A. Gehring Department of Physics and Astronomy University of Sheffield. Contents. Why LaMn 1- x Ga x O 3 ? Theory: postulates and assumptions Lattice parameters → Orthorhombic strain; cell volume Magnetisation Conclusions.

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Structure & Magnetism of LaMn 1- x Ga x O 3

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  1. Structure & Magnetism of LaMn1-xGaxO3 J. Farrell & G. A. Gehring Department of Physics and Astronomy University of Sheffield

  2. Contents • Why LaMn1-xGaxO3? • Theory: postulates and assumptions • Lattice parameters → Orthorhombic strain; cell volume • Magnetisation • Conclusions

  3. Why LaMn1-xGaxO3? • LaMnO3: parent compound of many CMR manganites. • Typically, the Mn3+ is replaced by Mn4+: → La1-xCaxMnO3, La1-xSrxMnO3 • Electron hopping betweenMn3+ and Mn4+ →Double exchange So, any observed effects may be attributed to: • Introduction of Mn4+ • Removal of Mn3+

  4. LaMn1-xGaxO3 (LMGO) • To investigate only the removal of Mn3+, dope LaMnO3 with “vacancies” • Try Ga3+ • Diamagnetic (unlike Mn3+) • Jahn-Teller inactive (unlike Mn3+) • Any disorder will be negligible → rGa = 76 pm; rMn = 78.5 pm Could also try Sc3+ or Al3+ but there is more data for LMGO (~ 7 experimental papers)

  5. LaMnO3→ LMGO • Long-range, static, Jahn-Teller ordering of the Mn3+egorbitals • Long-range AFM is a direct consequenceof orbital ordering. • GKA predictions: Mn O Mn Mn O Mn Mn O Mn LMGO, x < 0.5: dM/dx > 0 → Orbital flipping; FM evolution along z.

  6. Khomskii D. I. & Kugel K. I. PRB 67, 134401 Orbital Flipping • Random Ga-doping causes the x or y orbitals to flip into the z direction. • Significant elastic energy penalty forbids strong overlap. z

  7. Orbital Flipping z y x x Forbidden scenario FM Coupling

  8. Lattice Parameters • Bond lengths from neutron diffraction: Blasco et al., PRB 66, 174431 • Ga-O = 1.97 Ǻ; Mn-O = 1.92 Ǻ (compression) • JT: Mn-O = 1.90 and 2.18 Ǻ (LaMnO3) • Gallium-doping: long-range, static JT is suppressed but local, static JT persists. • Simulations on L = 10 cubic lattice with periodic boundary conditions.

  9. Lattice Parameters b a • O´→ O stuctural transition at x ≈ 0.55 • Good agreement with experimental results.

  10. Lattice Parameters b Experimental data: Blasco et al., PRB 66, 174431 a

  11. Lattice Parameters b Blasco et al., PRB 66, 174431 a • O´→ O structural transition at x ≈ 0.55

  12. Orthorhombic Strain ε = 2(b – a)/(b + a): Vertruyen et al., Cryst. Eng., 5, 299

  13. Cell Volume V = abc:Vertruyen et al., Cryst. Eng., 5, 299

  14. Magnetisation • Competition between FM and AFM bonds may lead to frustration. Suggestions of: • Spin glass (Zhou et al., PRB 63, 184423) • Spin canting (Blasco et al., PRB 66, 174431) • Spin flipping (this work).

  15. Monte Carlo Simulations • 10000 MCS/S. • JFM = 9. 6 K, JAFM = - 6.7 K. • T = 5 K; B = 5.5 T applied along easy axis. • Spins ↑ or ↓ only; no canting. • At large x, assume that M evolves due to percolation. • Isolated Mn contribute negligibly to M. • nn Mn couple ferromagnetically (4 µB each).

  16. Results • Obvious discrepancy at x = 0 accounts for canting. • Broad plateau is not observed; M peaks at x = 0.5. • At small x, dM/dx is predicted well. • At large x, percolation assumption is only qualitatively correct. Vertruyen et al., Cryst. Eng., 5, 299 Simulation

  17. Spin Flipping • At small x, Gallium may be placed on ↑ or ↓. z + 12 µB + 20 µB → Good estimation of linearity at small x (~ 16 µB/Ga)

  18. Conclusions • LaMn1-xGaxO3 is an ideal system in which the disruption of long-range orbital- and magnetic- order can be investigated. • Orbital flipping (local-JT) correctly describes the evolution of the lattice parameters. • The magnetism depends on the orbital order → Orbital ordering is paramount • Magnetisation successfully described in terms of spin-flipping.

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