1. Neutron reflectometry
Introduction & application to oxide interfaces
B. Keimer
Max-Planck-Institute for Solid State Research
motivation • neutron reflectometry: part of “interface toolbox”
• state-of-the-art instrument available
for Max Planck users & collaborators
outline • self-contained introduction: neutron scattering & reflection
• small selection of applications to (oxide) interfaces
7. Elastic nuclear neutron scattering
scattering length b ~ size of nucleus ~ 10-15 m
depends on isotope
Bragg peaks
at reciprocal lattice vectors K
nuclear structure factor
8. Scattering cross section: x-rays versus neutrons
N.B. b for deuterium is negative
http://www.ncnr.nist.gov/AnnualReport/FY2003_html/RH2/fig2.png
9. Neutron radiography
two metallic cylinders attached by an adhesive
only the adhesive is seen on the neutron radiograph
http://einrichtungen.physik.tu-muenchen.de/antares/
12. Elastic magnetic neutron scattering
one electron
“classical electron radius”
magnitude comparable to b
non-spin-flip average for
unpolarized beam
σz → σx , σy spin-flip (not possible for nuclear scattering)
13. Elastic magnetic neutron scattering
one atom
approximated as magnetized sphere, magnetization density M(r)
14. Elastic magnetic neutron scattering
generalization for collinear magnets
Bragg peaks
polarization factor magnetic structure factor
magnetic reciprocal lattice vectors
from here on, assume collinear magnetism, one atom per unit cell for simplicity
18. Reflection from interfaces
conveniently discussed in terms of classical ray optics
index of refraction for neutron wave inside material
example natural Ni
similar to x-rays but δ can be negative for neutrons
example natural Ti
example isotopically pure 62Ni
can drastically change scattering power without changing chemistry & physics
perspectives not yet explored for hard materials
25. Neutron guides
neutron guide hall @ FRM-II
http://www2.fz-juelich.de/iff/datapool/iffnews/news_28-04-2009_bild1.jpg
26. NREX reflectometer
Thomas Keller
+49-89-289-12164
Thomas.Keller@frm2.tum.de
state of the art instrument
owned and operated by Max Planck Society
privileged access to beamtime
27. Nonuniform density distribution
“kinematic” approximation ignore multiple reflections
contribution to R whenever density changes
analog of magnetic form factor in diffraction
example film on substrate
29. Multiple reflections
kinematic approximation recovered
0
waveguide effect resonant enhancement of neutron wavefunction inside layer
can use this effect to enhance contribution of single buried layer to reflectivity
30. Multiple reflections
multilayers
image adapted from
Hoppler et al.,
Nature Materials 2009
numerical calculations: Parratt formalism
32. Reflection from graded interfaces
quality of surfaces, buried interfaces can be determined by reflectometry
example Nb film
Fresnel
70 Å surface roughness
Felcher et al.
PRL 1984
33. Reflection from ferromagnets
M || H
η H || z
magnetic scattering amplitude
neutron spin operator electronic magnetic moment
determined by magnetic field component perp.to Q-vector
ordinary ferromagnet
no neutron spin flip
34. Reflection from ferromagnets
M
η H || z
magnetization components H, Q
e.g. spin canting at interface, strong anisotropy
neutron spin flip
35. Spin-polarized neutron reflectometry
nuclear-magnetic interference effect
total scattering amplitude
four different reflectivities for single interface: R++, R--, R+-, R-+
reflection, transmission amplitudes in Parratt calculations become matrices
polarizing mirror
38. SrRuO3 – La0.7Sr0.3MnO3 Heterostructures
SrRuO3 TC = 140 K, M SL
La0.7Sr0.3MnO3 TC = 320 K, M || SL
Ziese, Vrejoiu et al. (Halle group)
PRL 2010
antiferromagnetic coupling through Mn-O-Ru bond
competing interactions at interfaces
39. SrRuO3 – La0.7Sr0.3MnO3 Heterostructures
M || Q inside SRO layer
invisible to neutrons
M Q at interface
through Ru-O-Mn coupling
J.H. Kim et al. (MPI-FKF)
44. Superconductor – Ferromagnet Heterostructures
inverse proximity effect
at interface between superconductor and ferromagnet
Bergeret et al.
PRB 2004
45. Superconductor – Ferromagnet Heterostructures
engineered waveguide structure to observe inverse proximity effect
amplitude of waveguide resonance
suggestive of inverse proximity effect
Khaydukov et al. (Dubna group)
arXiv:1005.0685
46. YBCO-LCMO interface
YBa2Cu3O7 (YBCO): high-Tc superconducor
La0.7Ca0.3MnO3 (LCMO): double-exchange ferromagnet
SrTiO3 (001) substrate
CuO2 layers || interface
coherence length interface very small
SC proximity effects not expected
Zhang et al.
APL 2009
48. YBCO-LCMO interface
suppression of superconductivity suppression of metallicity
for YBCO layers thinner than ~ 5 nm
Sefrioui et al., PRB 2003 Holden et al.
PRB 2004
50. YBCO-LCMO magnetic reconstruction
neutron reflectometry
two interface models yield equivalent fits:
- antiferromagnetically polarized layer
- magnetically “dead” layer
Stahn et al.
PRB 2005
model 1 model 2
J.H. Kim
NREX @ FRM-II
51. YBCO-LCMO magnetic reconstruction
additional information from XMCD
Chakhalian et al., Nature Phys. 2006
• superexchange coupling
• ferromagnetic polarization through Cu-O-Mn bond
of Cu in YBCO
• direction antiparallel to Mn Chakhalian et al.
Nature Phys. 2006
53. In-plane domain structure
FePd films YBCO-LCMO superlattice
Qx
T > 100 K
Qz
T < 100K
Chakhalian et al.
Fermon et al.
Nature Phys. 2006
magnetic stripe domains new magneto-structural domain state
periodicity ~ 1µm
54. In-plane domain structure
YBCO-LCMO superlattice on SrTiO3
origin: structural phase transition novel superconductivity-induced
in STO substrate magnetic domain structure
J. Hoppler, C. Bernhard et al.
Nature Mat. 2009
55. In-plane domain structure
LaNiO3-LaAlO3 superlattice on SrLaAlO4
simpler structure of superlattice
no structural transitions in substrate
full crystallographic description of lattice structure, A. Frano
strain-induced domains
56. Magnetic depth profiling by soft x-rays
639 eV
This image cannot currently be displayed.
620 eV
resonant reflectometry fit
intensity (arb. units)
with circularly polarized x-rays
element-specific magnetization profile
example CaRuO3 — CaMnO3 superlattices
experiment
dichroic difference
model
Freeland et al.
PRB 2010
momentum transfer (nm-1)
57. Neutron versus resonant x-ray reflectometry
neutron reflectometry advantages
• yields total magnetization, independent of electronic structure
• cross section completely understood, no calculation required
• no beam heating can reach mK temperatures
• isotopic labeling, sensitivity to hydrogen
• Larmor phase manipulation of neutron spin, spin-echo experiments
resonant x-ray reflectometry advantages
• element specific
• yields valence state, orbital occupation, magnetization in one shot
(software available soon) S. Macke
• higher intensity, dynamic range
