Experiment IS 390 Firstprinciples calculations with perturbed angular

Experiment: IS 390 First-principles calculations with perturbed angular correlation experiments in Mn. As and Ba. Mn. O 3 Workshop, November 2008 1

Mn. As Ba. Mn. O 3 Family of the manganites with Colossal Magnetoresistance: Spintronics Magnetocaloric effect Competition of: Spin Orbital Structural Charge degrees of freedom. Both are magnetic compounds with theoretical challenges and possibility of applications. 2

Experimental Method Perturbed Angular Correlations Hyperfine interactions: Interaction Frequencies: Quadrupole electric moment interacts with Electric Field Gradient (EFG) Vzz(1021 V/m 2) η=|Vxx-Vyy|/Vzz + Magnetic dipole moment with Magnetic Hyperfine Field (HFF) (T) MATERIAL SPECIFIC EXPERIMENTAL OUTPUT 3

Mn. As 4

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

PAC- Mn. As Implanted probe: 77 Br (t 1/2) = 57 hours Decays to 77 Se (e- capture) 77 Se Intermediate state: t 1/2 = 9. 56 ns I=5/2 Q=1. 1(5) b μ= 1. 12(3)μN 77 Se Coincidences from γ- γ cascade measured γ 1=755. 4 Ke. V (start) (M 1+9%E 2) γ 2=249. 8 Ke. V (stop) (E 2) Very high anisotropy coefficient A 22 = -0. 45 6

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

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

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. Ø 9

Simulations First-principles calculations - Density Functional Theory Wien 2 k code P. Blaha et al. , TU Vienna 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 the exchange-correlation potential LDA gives poor results for Mn. As (Zhao et al. , Phys. Rev. B 35, 113202) Spin-polarized calculations (collinear), ferromagnetic Ferromagnetism due to Mn atoms at the hexagonal phase 10

Hexagonal Phase – 2 x 2 x 2 Supercells with Se probe Hyperfine parameters at the Se probe atom Se. As: Mn. As Compound Se. Mn: Mn. As Se. As: Mn. As EFG (1021 V/m 2) 17. 4 -1 Mn. As 0. 9275 Se 0. 0625 • HFF 2 x the experimental value. HFF is very sensitive and undergoes big changes in the phase transition. Disagreement factor of 2 does not seem unreasonable. • Can we include a small EFG in the fits to the data? Vzz<1 is also a good fit. η HFF (T) 0 0 23. 4 60 Se. Mn: Mn. As Mn 0. 9275 As. Se 0. 0625 EFG=17!! 11

Hyperfine parameters at Mn and As Hexagonal Structure from α-phase Room temperature lattice constants a=3. 722 Å, c=5. 702 Å Atom Mn As EFG (1021 V/m 2) -3. 7 1. 6 HFF (T) 4. 7 32. 4 Low temperature lattice constants a=3, 732 Å, c=5. 678 Å Atom Mn As EFG (1021 V/m 2) -3. 9 1. 4 HFF (T) 5. 1 30. 1 12

Orthorhombic Phase – EFG PAC – very small quadrupolar electric frequencies Atom Se. As Se. Mn Vzz (1021 V/m 2) 0. 8 15. 3 η 0. 93 0. 03 With Se impurities, Se should be at the As site, of lesser EFG, as before. 13

Conclusions for Mn. As • Se occupies the As site. • The temperature irreversibility of the 1 st 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? 14

Ba. Mn. O 3 15

PAC- Ba. Mn. O 3 Implanted probe: 111 Cd Metastable 111 Cd t 1/2 = 48. 6 min. 111 Cd Coincidences from γ- γ cascade measured. Intermediate state: t 1/2 = 84 ns I = 5/2 Q = +0. 83(13) b μ = -0. 766(13) μN 16

Ba. Mn. O 3 - PAC results Only different quadrupolar fields are observed for all temperatures. (paramagnetic phase) 17

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. 18

6 H Structure Space group P 63 mmc 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. 19

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

Is the Cd concentration too high in the previous simulations? Simulation of a larger supercell with 4 times less Cd concentration, at the Cd 1 (0, 0, 0) site. Electric field gradient (sensitive quantity) remains similar at all the atoms with both a conventional 6 H and a 2 x 2 x 1 supercell. It appears the smaller cells give already semi-quantitavely converged results. 21

Conclusion – Ba. Mn. O 3 • The simulations give values with the same order of magnitude, but different. Do the implanted samples keep the same structure? • New PAC measurements with 111 In and structural and magnetic characterization of the implanted samples will provide more information. Thank you for your attention 22

Extra Slides 23

ABX 3 – Structures of divalent manganites Ca/Sr/Ba Polytypes corresponding of different layer stacking of ABX 3 octahedra Cubic perovskite - repetition of (abc) forming a fcc structure (apex shared octahedra). Ideal Hexagonal 2 -layered (2 H) (ab) – infinite stacking of faceoctahedra. shared 4 H(abac) 6 H(abcacb) 9 H (ababcbcac) Ba. Mn. O 3 Ideal Hexagonal structure (2 H) at low temperature or atmospheric pressure. 24

Ba. Mn. O 3 - 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 2 H, 4 H, 6 H, 8 H, 10 H, 15 H can be obtained. Ba. Mn. O 3 Synthesized samples were single phase, 6 H. 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. La. Mn. O 3). 25

Hyperfine parameters of both distributions 26