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Investigation of Structure & Magnetism of LaMn1-xGaxO3

Explore the lattice parameters, orthorhombic strain, and magnetization of LaMn1-xGaxO3 through theory postulates and assumptions. Understand the implications of introducing Mn4+ and removing Mn3+ in LMGO. Study the effects of Ga-doping on orbital ordering and magnetic behavior. Investigate the role of orbital flipping in the evolution of lattice parameters and magnetization. Simulate spin flipping and percolation effects on magnetization. Discuss the importance of long-range orbital and magnetic order disruption in LaMn1-xGaxO3.

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Investigation of Structure & Magnetism of LaMn1-xGaxO3

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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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