Presentation on theme: "First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO 3 Workshop, November 2008 1 Experiment: IS390."— Presentation transcript:
First-principles calculations with perturbed angular correlation experiments in MnAs and BaMnO 3 Workshop, November 2008 1 Experiment: IS390
BaMnO 3 Spintronics Magnetocaloric effect MnAs Family of the manganites with Colossal Magnetoresistance: Competition of: Spin Orbital Structural Charge degrees of freedom. Both are magnetic compounds with theoretical challenges and possibility of applications. 2
Hyperfine interactions: Quadrupole electric moment interacts with Electric Field Gradient (EFG) V zz (10 21 V/m 2 ) η=|V xx -V yy |/V zz + Magnetic dipole moment with Magnetic Hyperfine Field (HFF) (T) MATERIAL SPECIFIC EXPERIMENTAL OUTPUT Experimental Method Perturbed Angular Correlations Interaction Frequencies: 3
1st order phase transition at 45 C Increasing temperature: 2% volume loss Hexagonal-Orthorhombic Loss of Ferromagnetism Increase in resistivity 5 MnAs – Properties of different phases Low temperature Hexagonal structure (NiAs-type) Ferromagnetic metal Between 45 C and 120 C the orthorhombic distortions disappear and the structure becomes again hexagonal of NiAs-type, paramagnetic. 5 Orthorhombic structure (MnP-type), Paramagnetic (?)
PAC- MnAs Implanted probe: 77 Br 77 Se 77 Br (t 1/2 ) = 57 hours Decays to 77 Se (e - capture) 77 Se Coincidences from γ- γ cascade measured γ 1 =755.4 KeV (start) (M1+9%E2) γ 2 =249.8 KeV (stop) (E2) 6 Intermediate state: t 1/2 = 9.56 ns I=5/2 Q=1.1(5) b μ= 1.12(3)μ N Very high anisotropy coefficient A 22 = -0.45
7 time (ns) MnAs – PAC spectra time(ns) Measurements around the 1st phase transition at 45 C: raising and lowering the temperature Spectra are path-dependent: Hysteresis
Fraction of the main distribution (%) 8 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
Hyperfine parameters of the main distribution 9 EFG frequencies very low: no exp. resolution to discern the asymmetry parameter η. η fixed at zero in all fits.
Simulations First-principles calculations - Density Functional Theory Wien 2k 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 MnAs (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 – 2x2x2 Supercells with Se probe Se Mn :MnAs Se As :MnAs CompoundEFG (10 21 V/m 2 )ηHFF (T) Se Mn :MnAs17.4023.4 Se As :MnAs060 Hyperfine parameters at the Se probe atom Mn 0.9275 AsSe 0.0625 11 MnAs 0.9275 Se 0.0625 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? V zz <1 is also a good fit.
AtomEFG (10 21 V/m 2 )HFF (T) Mn-3.74.7 As1.632.4 AtomEFG (10 21 V/m 2 )HFF (T) Mn-3.95.1 As1.430.1 Low temperature lattice constants a=3,732 Å, c=5.678 Å Hyperfine parameters at Mn and As 12 Room temperature lattice constants a=3.722 Å, c=5.702 Å Hexagonal Structure from α-phase
13 Orthorhombic Phase – EFG AtomV zz (10 21 V/m 2 )η Se As 0.80.93 Se Mn 15.30.03 With Se impurities, Se should be at the As site, of lesser EFG, as before. PAC – very small quadrupolar electric frequencies
Conclusions for MnAs 14 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?
16 PAC- BaMnO 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) μ N
17 BaMnO 3 - 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 V zz and one lower frequency with higher atenuation. Assymetry values were fixed to zero.
6H Structure 19 Space group P6 3 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.
Simulations – Supercells with Cd 20 Cd subst. Ba 1 siteCd subst. Ba 2 site Cd subst. Ba 3 siteCd subst. Ba 4 site There is no simulated V zz that accounts for the experimental f2 fraction (BIG V zz ). Is the Cd concentration too high?
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.
Conclusion – BaMnO 3 22 Thank you for your attention 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.
24 ABX 3 – 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) BaMnO 3 Ideal Hexagonal structure (2H) at low temperature or atmospheric pressure.
25 BaMnO 3 - Structure Synthesized sample Structure sensitive to: Thermodynamic conditions, temperature and oxygen partial pressure, in the preparation and cooling steps (slow cool or quenching) BaMnO 3 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. LaMnO 3 ). J.J. Adkin, M.A.Hayward, Chem Mat 19 (2007) 755-762 2H, 4H, 6H,8H,10H,15H can be obtained.