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First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO 3

Experiment: IS390. First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO 3. Workshop, November 2008. MnAs. BaMnO 3. Family of the manganites with Colossal Magnetoresistance: Competition of: Spin Orbital Structural Charge degrees of freedom.

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First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO 3

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  1. Experiment: IS390 First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO3 Workshop, November 2008

  2. MnAs BaMnO3 Family of the manganites with Colossal Magnetoresistance: Competition of: Spin Orbital Structural Charge degrees of freedom. Spintronics Magnetocaloric effect Both are magnetic compounds with theoretical challenges and possibility of applications.

  3. Experimental MethodPerturbed Angular Correlations Hyperfine interactions: Quadrupole electric moment interacts with Electric Field Gradient (EFG) Vzz(1021 V/m2) η=|Vxx-Vyy|/Vzz + Magnetic dipole moment with Magnetic Hyperfine Field (HFF)(T) MATERIAL SPECIFIC Interaction Frequencies: EXPERIMENTAL OUTPUT

  4. MnAs

  5. MnAs – Properties of different phases Orthorhombic structure (MnP-type), Paramagnetic (?) Low temperature Hexagonal structure (NiAs-type) Ferromagnetic metal 1st order phase transition at 45 C Increasing temperature: • 2% volume loss • Hexagonal-Orthorhombic • Loss of Ferromagnetism • Increase in resistivity Between 45 C and 120 C the orthorhombic distortions disappear and the structure becomes again hexagonal of NiAs-type, paramagnetic. 5

  6. PAC- MnAs Intermediate state: t1/2 = 9.56 ns I=5/2 Q=1.1(5) b μ= 1.12(3)μN Implanted probe: 77Br 77Se 77Br (t1/2) = 57 hours Decays to 77Se (e- capture) 77Se Coincidences from γ- γ cascade measured γ1=755.4 KeV (start) (M1+9%E2) γ2=249.8 KeV (stop) (E2) Very high anisotropy coefficient A22 = -0.45

  7. MnAs – PAC spectra Measurements around the 1st phase transition at 45 C: raising and lowering the temperature Spectra are path-dependent: Hysteresis time(ns)‏ time (ns)‏

  8. Fraction of the main distribution (%)‏ • First measure: defect • 2nd and 3rd measures: H 100%. • Orthorhombic phase consists of a main EFG distribuition. • Irreversible transition: Magnetic phase appears at a lower T when cooling. H  magnetic field E  Electric Field Gradient

  9. Hyperfine parameters of the main distribution • EFG frequencies very low: no exp. resolution to discern the asymmetry parameter η. η fixed at zero in all fits.

  10. Simulations First-principles calculations - Density Functional Theory Wien 2k code P. Blaha et al., TUVienna Basis APW+lo Full potential: Augmented Plane Waves + local orbitals Periodic - Use of supercells to include the probe in small concentrations Relaxation of structural parameters, by minimization of total energy or calculated forces, when necessary Generalized Gradient Approximation (PBE) to theexchange-correlation potential LDA gives poor results for MnAs (Zhao et al., Phys. Rev. B 35, 113202) Spin-polarized calculations (collinear), ferromagnetic Ferromagnetism due to Mn atoms at the hexagonal phase

  11. Hexagonal Phase – 2x2x2 Supercells with Se probe Hyperfine parameters at the Se probe atom SeMn:MnAs SeAs:MnAs Mn0.9275AsSe0.0625 MnAs0.9275Se0.0625 • HFF 2x the experimental value. HFF is very sensitive and undergoes big changes in the phase transition. Disagreement factor of 2 does not seem unreasonable. EFG=17!! • Can we include a small EFG in the fits to the data? Vzz<1 is also a good fit.

  12. Hyperfine parameters at Mn and As Hexagonal Structure from α-phase Room temperature lattice constants a=3.722 Å, c=5.702 Å Low temperature lattice constants a=3,732 Å, c=5.678 Å

  13. Orthorhombic Phase – EFG • PAC – very small quadrupolar electric frequencies With Se impurities, Se should be at the As site, of lesser EFG, as before.

  14. Conclusions for MnAs • Se occupies the As site. • The temperature irreversibility of the 1st order phase trasition is seen locally by the hysteresis of the hyperfine field, similar to the hysteresis found in the magnetization. • The small EFG at temperatures where the Hyperfine field is the main fraction shows coexistence of phases in the hysteresis region. • Improved simulations for magnetic field?

  15. BaMnO3

  16. PAC- BaMnO3 Implanted probe: 111Cd Metastable 111Cd t1/2 = 48.6 min. 111Cd Coincidences from γ- γ cascade measured. Intermediate state: t1/2 = 84 ns I = 5/2 Q = +0.83(13) b μ = -0.766(13) μN

  17. BaMnO3- PAC results Only different quadrupolar fields are observed for all temperatures. (paramagnetic phase)

  18. PAC spectra can be fitted satisfactorily with 2 quadrupolar frequencies. One higher well defined Vzz and one lower frequency with higher atenuation. Assymetry values were fixed to zero.

  19. 6H Structure Space group P63mmc 4 equivalency classes for Ba sites! Ba 1 (0,0,0) Ba 2 (1/3,2/3,1/2) Ba 3(1/3,2/3,1/6) Ba 4(2/3,1/3,0.3365) Assuming the Cd probes will go to the cation Ba sites, as is the case for all the manganites measured: 4 simulations are required to study the situations in the 4 inequivalent sites independently, with (possibly) different EFGs .

  20. Simulations – Supercells with Cd Cd subst. Ba 1 site Cd subst. Ba 2 site There is no simulated Vzz that accounts for the experimental f2 fraction (BIG Vzz). Is the Cd concentration too high? Cd subst. Ba 3 site Cd subst. Ba 4 site

  21. Is the Cd concentration too high in the previous simulations? Simulation of a larger supercell with 4 times less Cd concentration, at the Cd1 (0,0,0) site. Electric field gradient (sensitive quantity) remains similar at all the atoms with both a conventional 6H and a 2x2x1 supercell. It appears the smaller cells give already semi-quantitavely converged results .

  22. Conclusion – BaMnO3 • The simulations give values with the same order of magnitude, but different. Do the implanted samples keep the same structure? • New PAC measurements with 111In and structural and magnetic characterization of the implanted samples will provide more information. Thank you for your attention

  23. Extra Slides

  24. ABX3 – Structures Structures of divalent manganites Ca/Sr/Ba Polytypes corresponding of different layer stacking of ABX3 octahedra • Cubic perovskite - repetition of (abc) forming a fcc structure (apex shared octahedra). • Ideal Hexagonal 2-layered (2H) (ab) – infinite stacking of face- shared octahedra. 4H(abac)‏ 6H(abcacb)‏ 9H (ababcbcac)‏ BaMnO3 Ideal Hexagonal structure (2H) at low temperature or atmospheric pressure.

  25. BaMnO3 - Structure Synthesized sample Structure sensitive to: Thermodynamic conditions, temperature and oxygen partial pressure, in the preparation and cooling steps (slow cool or quenching) J.J. Adkin, M.A.Hayward, Chem Mat 19 (2007) 755-762 2H, 4H, 6H,8H,10H,15H can be obtained. BaMnO3 Synthesized samples were single phase, 6H. Ba is divalent in the manganite, as the Cd probe, in principle no effects due to charge differences need to be accounted in the calculations and no Jhan-Teller/other complicated effects introduced by one extra electron at manganese (e. g. LaMnO3).

  26. Hyperfine parameters of both distributions

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