first-principles calculations with perturbed angular correlation experiments in mnas and bamno 3...

First-principles calculations with perturbed angular

correlation experiments in MnAs and BaMnO3

J . N. Gonçalves *, V. S. Am aral, D. L. Rocco

Universidade de Aveiro, Physic s Departm ent and C ICEC O, Aveiro 3810-193, Portugal

J . G. Correia, H. Haas

ISOLDE CERN, CH-1211 Genève 23, Sw itz er land.

A. M. L. Lopes

CFNUL, Av. Prof . Gam a Pinto 2, P- 1699 L isboa Codex, Portugal.

P. B. Tavares

Universidade de Trás-os- Montes e A lto Douro, V ila Real 5001-911, Portugal.

*j oaonsg@ua. pt

Workshop, November 2008

1

Experiment: IS390

BaMnO3

Spintronics

Magnetocaloric effect

MnAs

Family of the manganites with Colossal Magnetoresistance:

Competition of:

SpinOrbital

StructuralCharge

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)

Vzz(1021 V/m2)

η=|Vxx-Vyy|/Vzz

+Magnetic dipole moment with

Magnetic Hyperfine Field

(HFF) (T)

MATERIAL SPECIFIC

EXPERIMENTAL OUTPUT

Experimental MethodPerturbed Angular Correlations

hQeVzzQ /

Interaction Frequencies:

/.nucL Bg

3

4

MnAs

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 temperatureHexagonal 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.

Orthorhombic structure (MnP-type), Paramagnetic (?)

PAC- MnAsImplanted probe: 77Br 77Se

77Br (t1/2) = 57 hoursDecays 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)

6

Intermediate state:

t1/2 = 9.56 ns I=5/2Q=1.1(5) bμ= 1.12(3)μN

Very high anisotropy coefficient A22 = -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 fieldE 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 codeP. 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), ferromagneticFerromagnetism due to Mn atoms at the hexagonal phase

10

Hexagonal Phase – 2x2x2 Supercells with Se probe SeMn:MnAs

SeAs:MnAs Compound

EFG (1021 V/m2) η HFF (T)

SeMn:MnAs 17.4 0 23.4

SeAs:MnAs -1 0 60

Hyperfine parameters at the Se probe atom

Mn0.9275AsSe0.0625

11

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.

Atom EFG (1021 V/m2) HFF (T)Mn -3.7 4.7As 1.6 32.4

Atom EFG (1021 V/m2) HFF (T)Mn -3.9 5.1As 1.4 30.1

Low temperature lattice constantsa=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

Atom Vzz (1021 V/m2) η

SeAs 0.8 0.93

SeMn 15.3 0.03

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?

15

BaMnO3

16

PAC- BaMnO3Implanted probe: 111CdMetastable111Cd t1/2 = 48.6 min.

111Cd Coincidences from γ- γ cascade measured.

Intermediate state:

t1/2 = 84 ns I = 5/2Q = +0.83(13) bμ = -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.

6H Structure

19

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 .

Simulations – Supercells with Cd

20

Cd subst. Ba 1 site

Cd subst. Ba 2 site

Cd subst. Ba 3 site

Cd subst. Ba 4 site

There is no simulated Vzz that accounts for the experimental f2 fraction (BIG

Vzz).

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 – BaMnO3

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 111In and structural and magnetic characterization of the implanted samples will provide more information.

Extra Slides

23

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)

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).

J.J. Adkin, M.A.Hayward, Chem Mat 19 (2007) 755-762

2H, 4H, 6H,8H,10H,15H can be obtained.

26

Hyperfine parameters of both distributions