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Neutron and X-ray Scattering Studies of Spin, Charge and Orbital Order in TM Oxides. Andrew Boothroyd Department of Physics, Oxford University. resistivity. La 5/3 Sr 1/3 NiO 4. magnetization. Transition Metal Oxide Research in Oxford Physics Department. Characterization Magnetization - PowerPoint PPT Presentation
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Neutron and X-ray Scattering Studies ofSpin, Charge and Orbital Order in TM Oxides
Andrew BoothroydDepartment of Physics, Oxford University
magnetization
resistivity
100
102
104
106
108
1010
0 50 100 150 200 250 300
La5/3Sr1/3NiO4
Res
ista
nce
(Ohm
)
Temperature (K)
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0 50 100 150 200 250 300
Mag
netiz
atio
n (e
mu/
g)
Temperature (K)
H = 500 OeLa1.67Sr0.33NiO4
0 5 10 15 20
Mag
netiz
atio
n (e
mu/
g)
Temperature (K)
FC
ZFC
H = 50 Oe
× 10–3
0.50
0.49
0.48
La5/3Sr1/3NiO4
Transition Metal Oxide Research in Oxford Physics Department
Sample Preparation lab.
Single CrystalsDr Prabhakaran
Neutron ScatteringDr BoothroydDr ColdeaProf Cowley
Muon-spin RotationDr Blundell
X-ray scatteringDr Hatton (Durham)Prof Cowley
Characterization•Magnetization•Transport etc
Crystal growth — floating-zone method
Image furnace (Clarendon Laboratory)
Oxford single crystals of TM oxides
La5/3Sr1/3NiO4
La0.7Sr0.3MnO3
CoNb2O6
La3/2Sr1/2CoO4
Na0.7CoO2
Neutron scattering studies of stripe-ordered nickelates
D. Prabhakaran Oxford University
Paul Freeman
Mechthild Enderle Institut Laue-Langevin
Jiri Kulda
Arno Hiess
Louis-Pierre Regnault CEA, Grenoble,
Felix Altorfer Paul-Scherrer Institut, Switzerland
Christof Niedermayer
Chris Frost ISIS
Hyungje Woo Brookhaven National Lab/ISIS
Kenji Nakajima University of Tokyo
John Tranquada Brookhaven National Lab
collaborators
Outline
Overview of stripe phenomena in La2–xSrxNiO4
What can be learned by neutron diffraction ?
Interesting aspects of magnetic ordering
Interesting features in magnetic excitation spectra
Stripe order in La2–xSrxNiO4
L a (S r)
O
N i/C u
T (K
)
1 0 0
3 0 0
T C O
0
2 0 0
0 .10 0 .2 0 .3 0 .4 0 .5
S r c on ten t, x
L a S r N iO2 – 4x x
T N
‘g lassy’
T S
antiferro-magnetic
insula tor
charge stripeordered
spin-chargestripe ordered
insula tor
T S
x = 0
x = 1/4 x = 1/3 x = 1/2
0
0.1
0.2
0.3
0.4
0.5
0 0.1 0.2 0.3 0.4 0.5
Hatton et al
Yoshizawa et alInco
mm
ensu
rabi
lity,
Sr doping, x
La2–xSrxNiO4
= x
(Tranquada et al, Cheong et al, Yoshizawa et al)
ideal stripe structures
Neutron Diffraction
Q k i
k f
Braggn = 2d sin
LaueQ =
(1) Scattering from nuclei
Q
Q = ki - kf
Applications:
• Crystal structure refinement
• Structural distortions (e.g. charge order)
Neutron diffraction
(2) Scattering from magnetic moments
V(r) = –n.B(r)
Neutrons scatter from component of magnetization perpendicular to Q
Q
m
m
No!
Yes!
Applications:
• Spin arrangements in ordered phases
• Magnetic form factors
Polarized neutron scattering
x
y
z
QP
1. P || Q
SF: Myy + Mzz
NSF: N
x
y
z
PQ
x
y
z
Q
P
2. P Q (in plane)
3. P Q (vertical)
SF: Mzz
NSF: N + Myy
SF: Myy
NSF: N + Mzz
Magnetic field defines polarization axis, P
Incident neutrons Scattered
neutrons
Spin-flip (SF)
Non-spin-flip (NSF)
Advantages of polarized neutron scatteringfor studying complex order
0
500
1000
1500
2000
2500
3000
0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37
SFNSF
neut
ron
coun
ts /
7 se
c
h (r.l.u.)
(h, h, 5)T = 14 K
kf = 2.662 Å–1
P || Q
static: charge vs magnetic order
dynamic: spin fluctuations vs phonons
1. Distinguishing magnetic and non-magnetic scattering
2. Separating different magnetic components
static: determining moment directions
dynamic: identifying anisotropy gaps and anisotropic fluctuations
La5/3Sr1/3NiO4
real space
Spin & charge order in La3/2Sr1/2NiO4
600
700
800
900
1000
1100
1200
1300
1400
1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7
Inte
nsity
(cou
nts/
M1=
2000
0)
h (r.l.u.)
T 190 K
(1.5, 1.5, 1.15)
0
100
200
300
400
1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7
SFNSF
Inte
nsity
(cou
nts/
M1=
2500
0)
h (r.l.u.)
(1.5, 1.5, 1.15)
T = 10 K
P || Q
0
500
1000
1500
2000
0.15 0.2 0.25 0.3 0.35
SFNSF
Inte
nsity
(cou
nts/
M1=
2500
0)
h (r.l.u.)
(h, h, 3)T = 10 K
P || Q
170 K < T < 460 K
‘checkerboard’ charge ordering
N i 2 +
N i 3 +
charge order magnetic orderT < 170 K spin & charge ordering
Not simple checkerboard pattern
Freeman et al, Phys. Rev. B 66 (2002) 212405
Kajimoto et al, Phys. Rev. B 67 (2003) 14511Half-doping:
• Correlation length 35 Å
T (K
)
1 0 0
3 0 0
T C O
0
2 0 0
0 .10 0 .2 0 .3 0 .4 0 .5
S r con ten t, x
L a S r N iO2 – 4x x
T N
‘glassy’
T S
antiferro-ma gnetic
insula tor
charge stripeordered
sp in-chargestripe ordere d
insula tor
T S
Models for spin-charge order in La3/2Sr1/2NiO4
Possible diagonal stripe pattern:
• Charge peaks at Qco = (½, ½) and ), 4/9
• Magnetic peaks at Qm = (½, ½) ± )
x x x x x x x x x x x x x x x x x x x x
x
N i sp in2+
O 2 – ion
h o le
ch e ck e rb o a rd
in co m m en su ra tes trip e
x x x x x x x x x x x x x x x x xx x x
Spin reorientations
1.1
1.15
1.2
1.25
1.3
1.35
1.4
0 20 40 60 80 100
Mag
netiz
atio
n (E
MU
per
mol
)
Temperature (K)
H = 500 OeH ll ab
FC
ZFC
TSR
TSO
30
40
50
60
70
80
90
0 20 40 60 80
Ang
le
from
stri
pe d
irect
ion
(deg
)
Temperature (K)
La2–xSrxNiO4
x = 0.37
x = 0.5
La3/2Sr1/2NiO4
La1.63Sr0.37NiO4
0.9
1
1.1
1.2
1.3
1.4
1.5
0 50 100 150 200 250 300 350
Mag
netiz
atio
n (E
MU
/ mol
)
Temperature (K)
H = 500 OeH ll ab
TSR
TSO TCO
FC
ZFC1.25
1.30
1.35
1.40
0 10 20 30 40 50
Mag
netiz
atio
n (e
mu/
mol
)
Temperature (K)
H = 500 OeH ll ab
TSR
FC
ZFC
Similar results for La5/3Sr1/3NiO4 reported by Lee et al, Phys. Rev. B 63 (2001) 60405(R)[TSR = 50 K, = 13 deg]
Overview of spin dynamics in La2–xSrxNiO4
0
20
40
60
80
100
0 0.2 0.4 0.6 0.8 1
Dispersion || (h, h, 0)
Ene
rgy
(meV
)
h (r.l.u.)
real space
e.g. La5/3Sr1/3NiO4
Spin waves Gap-like features 2 components
Low energy quasi-1D spin fluctuation in La5/3Sr1/3NiO4
Diffuse inelastic scatteringScans along line A
Ni3+ ions (probably) carry spin S = ½ Consistent with quasi-1D AFM chains
0 0.1 0.2 0.3 0.4
neut
ron
coun
ts /m
onito
r=80
0,00
0
(r.l.u.)
2.5 meV
4 meV
6 meV
8 meV
2
4
6
8
× 103
(a)
0.1 0.15 0.2 0.25 0.3 0.35 0.40
2
4
6
8
10
12
(r.l.u.)
Energy (m
eV)
(0.08+, 1.08–, 0)
(b)
Scans along line B
Soft X-ray scattering studies of manganates
Peter Hatton University of Durham
Philip Spencer
Stuart Wilkins Institute of Transuranium Elements,Karlsruhe, and
European Synchrotron Radiation Facility, Grenoble
D. Prabhakaran Oxford University
Steve Collins Daresbury Laboratory
Mark Roper
collaborators
X-ray Scattering
Non-resonant scattering
• X-rays can probe charge density (Thomson) – very strong!
• X-rays also scatter from spin and orbital moments – very weak!
Resonant scattering
Strong enhancements at atomic absorption edges
• Resonant scattering from spin and orbital moments
• Element specific
• Higher-order ‘anomalous’ scattering processes observable
Examples of X-ray resonant magnetic scattering
Antiferromagnetic order in PrBa2Cu3O6+x
J.P. Hill et al, Phys. Rev. B 61 (2000) 1251
Cu K edge Pr LII edge
J.P. Hill et al, Phys. Rev. B 58 (1998) 11211
X-ray resonant scattering from Mn
1s
4p
EA = 6.55 keV
= 1.9 Å
K edge resonance
2p
3dEA = 0.65 keV = 19 Å
L edge resonance
Soft X-rays
Long period structures only
• Probes 3d orbitals directly• Very large resonant enhancements• Long wavelengths
Indirect probe of 3d magnetism
Daresbury Laboratory
• 2 GeV Machine
• 5U1 – Soft X-ray Undulator
• Good Sample Environment
• Awful Food
Soft X-ray scattering at 5U1, SRS
Air absorption at 650 eV is severe!
Soft X-ray resonant scattering from La2–2xSr1+2xMn2O7 (x = 0.45)
630 635 640 645 650 655 6600.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Nor
mal
ised
Inte
nsity
[arb
.uni
ts]
Photon Energy [eV]
LIII
LII
T = 83 K
• Charge order between 120 K and 220 K• Antiferromagnetic order below 170 K
(001) AFM Bragg peak
80 100 120 140 160 180 200 2200
5
10
15
20
25
30
35
40
(0, 0, 1) AFM
Inte
grat
edIn
tens
ity[a
rb.u
nits
]
Temperature [K]
0
5
10
15
20
25
30
35
40(1.5, 0.5, 0) CO
Inte
grat
edIn
tens
ity[a
rb.u
nits
]
S.B. Wilkins et al Phys. Rev. Lett. 90 (2003) 187201
Orbital ordering in La0.5Sr1.5MnO4
Spin, charge and orbital order below 240 K;Jahn-Teller distortion very small
AFM order
Soft X-ray scattering & theory
(Castelton & Altarelli, Phys. Rev. B 62 (2000) 1033)
Theoretical predictions
No Jahn-Tellerdistortion
Strong JTdistortion
(¼, ¼, 0)
Soft X-ray resonant scatteringAt the orbital ordering Bragg peak
(S.B. Wilkins et al, to appear in Phys. Rev Lett.)
Conclusion: scattering is due to combined orbital ordering and cooperative Jahn-Teller distortions
Conclusions
• Spin and charge order – La3/2Sr1/2NiO4 not checkerboard!• Spin reorientations occur• Evidence for coupling of spin excitations to charge stripes• AFM spin correlations on the charge stripes
Neutron scattering studies of stripe-ordered nickelates
Soft X-ray studies of spin-charge-orbital ordered manganates
• Large resonant enhancements at L edge• Can probe ordering of 3d orbitals directly• Limited to ordering phenomena with period d > 10 Å• Jahn-Teller mechanism important in driving orbital order