Polarized inelastic neutron scattering in the CMR manganite La0'70Ca0'30MnO3

1
Polarized inelastic neutron scattering in the CMR
manganite La0.70Ca0.30MnO3
J. A. Fernandez-Baca, Mark Hagen, Jiri Kulda

• Center for Neutron Scattering, Oak Ridge
  National LaboratorySpallation Neutron Source,
  Oak Ridge National laboratoryInstitut Laue
  Langevin, Grenoble, France

PINS workshop, BNL April 6-7, 2006
2
Polarized inelastic neutron scattering in the CMR
manganite La0.70Ca0.30MnO3

• OUTLINE
• Motivation LCMO30 Softening and damping of spin
  waves near the zone boundary.
• The magnon-phonon interaction
• Experimental
• polarized neutron setup (half polarized and full
  polarization)
• magnons and phonons in LCMO
• Unpolarized measurements
• Results and discussion
• Summary

3
COLOSSAL MAGNETORESISTANCE MANGANITES
La1-xCaxMnO3

• Cubic perovskite
• La/Ca in A site
• Substitution of Ca2 for La3
• leads to mixed Mn3 /Mn4.

Double exchange
4
Spin-wave excitations Double exchange (DE)
t electron hoping
JH Hund coupling between localized t2g
electrons (S3/2) and eg electrons.

• In the strong coupling limit (JHgtgtt) the spin
  wave spectrum
• is approximately the same as that for a
  Heisenberg FM with nn interactions.

Furukawa J. Phys. Soc. Jpn. (1996)
D µ J µ TC
Heisenberg model
5
Spin waves in CMR manganites Ln1-xAxMnO3 near
x0.30

• Pr0.63Sr0.37MnO3
• Hwang et al. PRL (1998)
• Nd0.30Sr0.30MnO3
• Fernandez-Baca et al, PRL (1998)
• La0.70Ca0.30MnO3
• Dai et al., PRB (2000)
•  
• Similar dispersion throughout the Brillouin zone
• SW softening broadening at the Brillouin
  zone boundary

6
Spin waves in CMR manganites Ln1-xAxMnO3 near
x0.30
Proposed explanations Peculiar ground state (not
globally FM) (Zang et al J. Phys. Cond Matt
(1999) Purely magnetic DE (Solovyev and Terakura,
PRL 82, 2959 (1999)) Charge and orbital
fluctuations (Khaliullin, PRB 71, 3494
(2000)) Disorder (Motome and Furukawa) Magnon-pho
non interaction (Dai, PRB 61, 9553(2000))
7
Spin waves in CMR manganites Ln1-xAxMnO3 near
x0.30
Proposed explanations Peculiar ground state (not
globally FM) (Zang et al J. Phys. Cond Matt
(1999) Purely magnetic DE (Solovyev and Terakura,
PRL 82, 2959 (1999)) Charge and orbital
fluctuations (Khaliullin, PRB 71, 3494
(2000)) Disorder (Motome and Furukawa) Magnon-pho
non interaction (Dai, PRB 61, 9553(2000))
Predicts broadening and softening
8
Magnon- Phonon Interactions(S. Lovesey, Theory
of Neutron Scattering from Condensed Matter
systems, sect. 9.8)

• Linked to significant magnetoelastic interactions
• Magnon-phonon hybridization Lattice vibrations
  may modulate the orbital properties. The
  modulation is transmitted to the spin by the
  spin-orbit interaction, which is not strong in 3d
  ions.
• More common in 4f ions but observed in FeF2
  (Rainford) and FeCl2(Ziebeck 1976).

9
Magnon- Phonon Interactions(S. Lovesey, Theory
of Neutron Scattering from Condensed Matter
systems, sect. 9.8)

• Linked to significant magnetoelastic interactions
• Magnon-phonon hybridization Lattice vibrations
  may modulate the orbital properties. The
  modulation is transmitted to the spin by the
  spin-orbit interaction, which is not strong in 3d
  ions.
• More common in 4f ions but observed in FeF2
  (Rainford) and FeCl2(Ziebeck 1976).

10
Magnon-phonon hybridization

• Tb-10 Ho. Möller (1968)
• When magnon and phonon
• branches cross, excitations
• are mixed.
• An energy gap appears.
• No magnon broadening expected

11
Magnon- Phonon Interactions(S. Lovesey, Theory
of Neutron Scattering from Condensed Matter
systems, sect. 9.8)

• Linked to significant magnetoelastic interactions
• Modulation of the exchange interaction Lattice
  vibrations modulate can modulate J.
• Two-ion coupling of lattice vibrations and spins.
  Magnon damping. (Lovesey)

12
Magnon- Phonon Interactions(S. Lovesey, Theory
of Neutron Scattering from Condensed Matter
systems, sect. 9.8)

• Linked to significant magnetoelastic interactions
• Modulation of the exchange interaction Lattice
  vibrations modulate can modulate J.
• Two-ion coupling of lattice vibrations and spins.
  Magnon damping. (Lovesey)

Furukawa (J. Phys. Soc. Japan (1999))

• A magnon with k and wq decays into
• A phonon with q and wq and
• A magnon with k-q and wk-q

k-q
k
q
13
Magnon- Phonon Interactions(S. Lovesey, Theory
of Neutron Scattering from Condensed Matter
systems, sect. 9.8)

• Linked to significant magnetoelastic interactions
• Modulation of the exchange interaction Lattice
  vibrations modulate can modulate J.
• Two-ion coupling of lattice vibrations and spins.
  Magnon damping. (Lovesey)

Furukawa (J. Phys. Soc. Japan (1999))

• A magnon with k and wq decays into
• A phonon with q and wq and
• A magnon with k-q and wk-q
• Damping when phonon and magnon branches cross
• Softening occurs

k-q
k
q
14
Institut Laue Langevin High Flux Reactor neutron
source
15
Magnons and phonons in LCMO

• To separate magnons and phonons need polarized
  neutrons
• Need to understand the phonon behavior
• Polarized and unpolarized inelastic scattering
  experiments
• Polarized neutrons
• IN20 at ILL
• Half polarization and full polarization
• Unpolarized neutrons HB3 at HFIR

16
Full polarization analysis

• Triple Axis Spectrometer
• Define incident wavevector/energy
• Define final wavevector/energy
• Measure wavevector/energy change
• Neutron wavevector/energy change sample
  (phonon/magnon)
  wavevector/energy
• Polarized neutrons
• Define incident/final neutron spin
  (polarization)
• Measure change in neutron spin change in
  sample ang. momentum
• Magnon ang. mom. 1 Phonon ang. mom. 0

17
Half polarization Holden-Stirling Method

• Holden-Stirling method (J. Phys. F 7, 1901,
  (1977))
• Ferromagnet - saturate the magnetisation
• Measure with Q parallel M and P both parallel
  and antiparallel
• P-parallel 0 x magnetic P-antiparallel 4
  x magnetic
• Nuclear (phonon) scattering is independent of P
• Take the difference to separate magnetic and
  nuclear
• Advantage --- dont need a polarising analyser,
  use HOPG and have higher count rate

18
Half polarization Holden-Stirling Method

• Holden-Stirling method (J. Phys. F 7, 1901,
  (1977))
• Ferromagnet - saturate the magnetisation
• Measure with Q parallel M and P both parallel
  and antiparallel
• P-parallel 0 x magnetic P-antiparallel 4
  x magnetic
• Nuclear (phonon) scattering is independent of P
• Take the difference to separate magnetic and
  nuclear
• Advantage --- dont need a polarising analyser,
  use HOPG and have higher count rate

P-parallel Nuclear P-antiparallel Nuclear 4x
Magnetic P-antiparallel - P-parallel 4x
Magnetic
19

• Does the subtraction work??
• Ferromagnet isnt saturated
• Beam isnt fully polarized
• Is it the same as fully polarized?
• Corrections for stray field in Mezei flipper?

Holden-Stirling (squares) Fully polarized
(circles) Scale factor of 7.5 for analyzers
background estimate from (-,)
Fully polarized Circles (, -), Squares (-,
)
20
Polarized Neutron Measurements
21
Spin waves in LCMO30
Magnetic
Nuclear
22
Spin waves in LCMO30
23
Spin waves in LCMO30
24
Spin waves in LCMO30
Linewidths
25
Measuring Longitudinal and Transverse Phonons
Eigenvectors Longintudinal eL parallel to
q Transverse eT perpendicular to q
26
Transverse phonon measurements TA TO1 HB3
triple axis spectrometer at HFIR reactor,
ORNLHOPG(002) HOPG(002), fixed EF 35meV
27
TO1 Optic Mode
(3,3,0) Zone TA modes have weakstructure
factor TO1 is broad and dispersionless
overwhole zone
28
(No Transcript)
29
Phonons in LCMO30

• Recent experiment at HFIR
• Flat TO mode at E20meV
• Strongly damped
• (FWHM 13meV)

• External mode not a MnO6 mode but external
  vibration of La/Ca against MnO6 octahedra

30
Phonons in LCMO30

• Recent experiment at HFIR
• Flat TO mode at E20meV
• Strongly damped
• (FWHM 13meV)

LSMO Reichardt and Braden, Physica B (1999)
31
Spin waves in LCMO30

• This TO is a external mode not a MnO6 mode but
  external vibration of La/Ca against MnO6
  octahedra

32
Spin waves in LCMO30
Broadening occurs when SW branch crosses the
phonon band
33
Magnon dispersion along 110
34
Spin waves in LCMO30
Significant difference between the polarized and
unpolarized measurements Unpolarized
measurements do not separate both contributions
(PSMO)
35
Spin waves in LCMO30

• Magnon-phonon interaction expected to produce
  softening
• Magnitude of softening difficult to predict
• Fit to SW dispersion needs several neighbor
  coupling
• Fits not satisfactory up to order four (J1 and J4)

36
Summary

• Half-polarized, full-polarized and unpolarized
  experiments to study the role of the phonons in
  the SW dispersion of LCMO30.
• Half-polarized measurements essential to separate
  magnon and phonon contributions.
• Holden Stirling method (horizontal H) very useful
  provided magnetic saturation is achieved.
• SW broadening near zone boundary likely due to
  magnon-phonon interaction.
• Magnitude of SW softening due to magnon-phonon
  interaction difficult to quantify.
• Observed softening likely due to other origins.

37
Recent results on Sm0.55Sr0.45MnO3

• Zone boundary magnons below the acustic phonon
• Fit with J1 and J4
• J4 attributed to (3z2-r2) orbital fluctuations

Endoh et al. Phys. Rev. Lett 94, 17206 (2005)
38
New experimental results

• In all cases there is a remarkable similarity of
  low energy spin waves
• Softening at zone boundary increases with doping