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C. Ulrich1, M. Reehuis1,2, G. Khaliullin1, V. Damljanovic1, Ch. Niedermayer3, A. Ivanov4, K. Schmalzl4, K. Hradil5, A. Schneidewind5, A. Maljuk1, and B. Keimer1
1Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany2Hahn-Meitner-Institut, Berlin, Germany
3Paul-Scherrer-Institut, Villigen, Switzerland4Institut Laue-Langevin, Grenoble, France
5FRM II, Munich, Germany
Spin wave dispersion in the helical spin ordered systemSrFeO3 and CaFeO3
Sydney, 26. 9. 2007
LaMnO3: Mn3+, 3d4, t2g3eg
1
• insulator• cooperative Jahn-Teller distortion at 800K• commensurate , collinear spin structure
SrFeO3: Fe4+, 3d4, t2g3eg
1
• metal• cubic, no structural transition• incommensurate, helical spin structure
Metallic SrFeO3-δ
metallic conductivityhelical spin arrangement
eg- Orbital
t2g–Orbital
x2-y2 3z2-r2
x2-y2
3z2-r23z2-r2
yz xz xy
yz
xz
xyxy
Jahn-TellerAufspaltung
splittingcubic splitting tetragonal
Crystal Structures of SrFeO3-δ
Oxygen Vacancy Ordered Phases
SrFeO2.75
orthorhombicCmmm
SrFeO2.875
tetragonalI4mmm
Hodges et al., J. Sol. State Chem. 2000.
cubic SrFeO3.00
ideal cubic perovskite:Pm3m (a = 3.85 Å)no distortion, no rotationof the FeO6 octahedra
Annealing of CaFeO3
4 GPa hydrostatic pressure
2 hr at 10000 C
Annealing of SrFeO3
5 kbar O2-pressure
24 hr at 9500 C
High pressure single crystal annealing
Magnetic Phase Transitions in SrFeO3-δ
0 50 100 150 200 250 3000,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
χ (em
u/m
ol)
SrFeO2.77
SrFeO2.81+0.01
SrFeO2.85+0.02
SrFeO3.00+0.04
Temperature (K)
SrFeO2.95+0.03
A. Lebon et al., PRL 92, 37202 (2004).
0 50 100 150 200 250 3000
5
10
15
20
25
30
orthorhombic T
N = 230 K
cubicT
N = 130 K
tetragonalT
N = 75 K
Asy
mm
etry
Temperature (K)
SrFeOx x=3.00 x=2.85 x=2.81 x=2.75
µSR
magnetic phase transitions:
TN1 = 130 K cubicTN2 = 75 K tetragonalTN3 = 230 K orthorhombic
Helical Magnetic Order
-single crystals (floating zone techique)-annealed under 5 kbar oxygen
cubic
tetragonal
orthorhombic
independentup independentindependentupup
positive MRpositive MR
0T9T0T9T
Magnetoresistance Effects
upward shiftlarge negative MR
downward shiftgiant negative MR
positive MR
A. Lebon et al., PRL 92, 37202 (2004).
SrFeO3-δ Mössbauer Spectra
SrFeO2.87 110K
30K
SrFeO2.87 110K
30K
SrFeO2.87 110K
30K
cubic SrFeO3.00 tetragonal SrFeO2.875
only Fe4+ present at all temperatures
pure spin rearrangement TN1 = 130 K, TN2 = 65 K ?
magnetic phase transition at 75 K is associated with charge ordering 2 Fe3.5+ => Fe3+ + Fe4+
A. Lebon et al., PRL 92, 37202 (2004).
Fe3+/Fe4+ Charge Order in SrFeO2.875
Park et al., PRB 1999
different fromFe3+/Fe5+ charge orderin La1/3Sr2/3FeO3, CaFeO3
magnetoresistance around CO transition: similar to Verwey transition in Fe3O4
Gridin et al., PRB 1996
-0.2 -0.1 0.0 0.1 0.2
15 K
Log.
Int
ensi
ty
(cnt
s / 3
3 se
c)
Qhkl (0,0,1)
140 K
130 K
100 K
60 K
cubic SrFeO3.00
(110)
(001)
0 20 40 60 80 100 120 140 160 1800
5
10
15
20
25
30
Inte
grat
ed In
tens
ity
(cnt
s / 3
3 se
c)
Temperature (K)
0 20 40 60 80 100 120 140 160 1800.095
0.100
0.105
0.110
0.115
0.120
0.125
0.130
Del
ta
δ
Temperature (K)
Elastic Neutron Scattering in cubic SrFeO3.00
helical spinarrangement
TN1 = 130 K magnetic „satellite“ peaks around structural Bragg reflections propagation vector along the [111]-direction µ = 2.48 µB/Fe4+-ionTN2 = 65 K but change in the magnetic correlation length at 65 K weak additional magnetic Bragg peaks at (0, 0, 1/4 )
0 25 50 75 100 125 150 1750
50
100
150
200
250
corr
elat
ion
leng
th
( Å
)
Temperature (K)
Inelastic Neutron Scattering in cubic SrFeO3.00
cubic SrFeO3.0
Metalnegative magnetoresistance 65 Khelicale spin order TN = 130 Kno charge order
Helix: δ = 0.131.2 – 3.8 meV
Inelastic Neutron Scattering
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
500
1000
1500
2000
2500
T = 4 K
7 meV
6 meV
5 meV
4 meV
3 meV
SrFeO2.875
tetragonal
Inte
nsity
(c
nts
/ 100
sec
)
(Qh, Q
K, 1+Q
L)
Tetragonal SrFeO2.875
magnetic Bragg Peaks at δ = 0.2
Inelastic Neutron Scattering
Cubic CaFeO3.0
magnetic Bragg Peaks at δ = 0.16
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4
500
1000
1500
2000
2500
3000
CaFeO3
3 meV
4 meV
5 meV
6 meV
7 meV
8 meV
9 meV
2 meV
Inte
nsity
(c
nts
/ 80
sec)
(QH, Q
K, 1+Q
L)
Inelastic Neutron Scattering
cubic SrFeO3.0
Metalnegative magnetoresistance 65 Khelicale spin order TN = 130 Kno charge order
Insulatornegative magnetoresistancehelical spin order TN = 75 Kcharge order at TN = 75 K
CaFeO3.0tetragonal SrFeO2.875
Metal-Insulator Transitionno magnetoresistance effecthelical spin order TN = 125 Kcharge disprop. at TN = 290 K
Helix: δ = 0.131.2 – 3.8 meV
Helix: δ = 0.202.2 – 7.8 meV
Helix: δ = 0.162.0 – 6.0 meV
Strong hybridization of the Fe-eg and O-σ orbitals
M. Mostovoy (PRL 94, 137205 (2005))
t2g
eg
t2g
eg
3d4 3d L5 oxygen p
∆ pd < 0 (- 3 eV)photoemissionBocquet (1992)
charge transfer energy
large negative charge transfer energy ∆pd
for the eg hole, both spin directions are possible
mixing of both states results in a further lowering of the ground state
helical spin arrangement is preferred- 6
- 4
- 2
0
2
Γ Χ Μ R Γ
Ener
gy
(eV
)
Bands for a fictitious FM state
- spin-down dp-holes- spin-up p-holes
xx x x(U/t)cr U/t
SrFeO3 CaFeO3 LaMnO3
helical AFMT = 130 Kno orbital order
N
helical AFMT = 125 Kcharge orderbelow 290 K
N
A-type AFMT = 140 Korbital order below 780 K
N
InsulatorFMDE
Metal»SE
AFMorbital order
critical
Double Exchange versus Superexchange
cubic SrFeO3.00: - no Jahn-Teller distortion - no orbital order - metallic conductivity of the 3d-Fe4+ electrons enhances the Double Exchange interaction - Superexchange in LaMnO3 and SrFeO3
is comparable
Model for helical spin arrangement
Let’s consider a cubic crystal with:
J = ij J AFM for the 6 nearest neighbors- J FM for the 12 next nearest neighbors
1
2{
J(q) = [cos(q a) + cos(q a) + cos(q a)] -
- [cos(q a)cos(q a) +
x y z
x y cos(q a)cos(q a) + cos(q a)cos(q a)]y z z x
2J1
N4J2
N
This exchange interaction has a minimum when:
a = cos ( + y + z)Q -1 X J1
4J2
^ ^^
If J < 4J the spin configuration has an incommensurate wavelength with the lattice spacing.
Helical spin arrangement can arise if we have two competing interactions: short range versus long range
1 2
J = Superexchange Fe-O AFM-J = Double Exchange Fe-Fe FM
1
2
P.-G. De Gennes,PR 118, 141 (1960)
J
⇒ Competition between Double Exchange and Superexchange
results in a helical spin structureFe
O
J4J4
J22
J1J1
Fe
O
Double-exchange / Super-exchange
J = J + J (U, )J = J ( ) (2t/ )2J = J /2
1 DE SE pd
4 1 pd pd
2 4
∆∆ ∆
Fe
O
J4J4
J22
J1J1
Fe
O
Charge fluctuations:
Double-exchange: JDE
Super-exchange: JSE
resulting magnonen-dispersion.Fit of J2 and θ to the experimental data.
excellent aggrement for allother directions in the Brillouin zone.
P.-G. De Gennes, PR 118, 141 (1960)improved model G. Khaliullin (2006)
0.00 0.05 0.10 0.15 0.20 0.25 0.300
2
4
6
8
10
12
Ener
gy
(meV
)
(Qh+1, QK, QL)
cubic SrFeO3 Θ = 47 J2 = 0.34 meV tetra. SrFeO
2.875 Θ = 72 J2 = 0.34 meV
CaFeO3 Θ = 58 J2 = 0.45 meV
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.500
5
10
15
20
25
30
35
40
45
50
55
En
ergy
(m
eV)
(Qh+1, QK, QL)
cubic SrFeO3 Θ = 47 J2 = 0.34 meV tetra. SrFeO
2.875 Θ = 72 J2 = 0.34 meV
CaFeO3 Θ = 58 J2 = 0.45 meV
Double-exchange / Super-exchange
0.00 0.05 0.10 0.15 0.20 0.25 0.300
2
4
6
8
10
12
Ene
rgy
(m
eV)
(Qh+1, QK, QL)
cubic SrFeO3 Θ = 47 J2 = 0.34 meV
tetra. SrFeO2.875
Θ = 72 J2 = 0.34 meV CaFeO
3 Θ = 58 J2 = 0.45 meV
0.0 0.1 0.2 0.3 0.4 0.50
5
10
15
20
25
30
35
40
45
50
55
Ene
rgy
(m
eV)
(Qh+1, QK, QL)
cubic SrFeO3 Θ = 47 J2 = 0.34 meV
tetra. SrFeO2.875 Θ = 72 J2 = 0.34 meV CaFeO3 Θ = 58 J2 = 0.45 meV
Fit to the data of tetragonal SrFeO2.875 and CaFeO3
- extrapolation for cubic SrFeO3
- found high energy branch of tetragonal SrFeO2.875
Conclusions: SrFeO3-δ
rich electronic phase diagram:
cubic SrFeO3.00
tetragonal SrFeO2.875
-TN1 = 130 K helical spin arrangement no CO, no OO-TN2 = 65 K comensurate spin structure
large magnetoresistance effect no CO, no OO
-TN = 75 K helical spin arrangement charge order 2 Fe3.5+ => Fe3+ + Fe4+
huge negative MR effect
metallic
insulating
tetragonal CaFeO3.0 metal-insulator transition-TN = 125 K helical spin arrangement-TCO = 290 K charge order 2 Fe4+ => Fe3+ + Fe5+
no MR effect
Conclusions: Ferrates
Charge fluctuations at the borderline of the metal - insulator transition are the reason for the different electronic properties.
Spin structures and magnon dispersion relationsare almost identical in the Ferrates
Interplay between
Double-Exchange and Super-Exchange
Consequence: helicale spin-order
Outlook: bilayer Sr3Fe2O7
As-grown Sr3Fe2O7-x boule – grown by A. Maljuk
0 50 100 150 200 250 3000.00
0.01
0.02
0.03
0.04
0.05
0.06
mag
netic
sus
cept
ibili
ty, e
mu/
mol
.
Temperature, K.
as-grown Sr3Fe2O7-x crystal, 10 Oe.
T1=78 K
T2=115 K T3=148 K.
annealed Sr3Fe2O7-x crystal, 10 Oe.
Magnetic susceptibility of the Sr3Fe2O7-x crystal.
Ruddlesden-Popper SeriesSrFeO3-x, Sr3Fe2O7-x and Sr2FeO4-x
Sr3Fe2O6.88
Outlook: bilayer Sr3Fe2O7
0 20 40 60 80 100 120 140 160 180 2000
200
400
600
800
1000
1200
1400
1600
1800
Inte
nsity
(c
nts
/ 12
sec)
Temperature (K)
Int1 Int3
0 20 40 60 80 100 120 140 160 180 2000.7
0.8
0.9
1.0
1.1
1.2
1.3
Pos
ition
(Q
H, 0
, 0 )
Temperature (K)
0.6 0.8 1.0 1.2 1.40
200
400
600
800
1000
1200
1400
1600
1800
2000
1 234 56 78910111213141516171819202122232425262728293031323334353637
3839
404142
43
4445
464748495051525354555657585960616263646566676869707172737475767778798081ABCDEFGH I JKLMNOPQRSTUVWXYZAAABACADAEAFAGAHAIAJAK
ALAM
AN
AOAPAQ
AR
ASAT
AUAVAWAXAYAZBABBBCBDBEBFBGBHBIBJBKBLBMBNBOBPBQBRBSBTBUBVBWBXBYBZCACBCCa b cd e f gh i j k l mn opq r s t u vwx y zaaabacadaeafagahaiaj
akalam
anaoap
aq
ar
asatauavawaxayazbabbbcbdbebfbgbhbibjbkblbmbnbobpbqbrbsbtbubvbwbxbybzcacbcc
nuclear
magnetic
magneticSr3Fe2O7
Inte
nsity
(c
nts/
12 s
ec)
(QH, 0, 0 )
Sr3Fe2O6.88
Elastic Neutron Scattering along the a-axis
δ = 0.25
Outlook: bilayer Sr3Fe2O7
Group: Prof. B. Keimer
A. Lebon
G. Khaliullin, M. Mostovoy,
A. Maljuk, P. Balock, C.T. Lin
Max-Planck-Institut FKF, Stuttgart, Germany.
Max-Planck-Institut FKF, Stuttgart, Germany.
Max-Planck-Institut FKF, Stuttgart, Germany
Max-Planck-Institut FKF, Stuttgart, Germany.
Magnetoresistance
Mössbauer Spectroscopy
Ellipsometry
P. Adler
A. Boris, C. Bernhard, A.V. Pimenov
Universität Karlsruhe, Germany.
Max-Planck-Institut FKF, Stuttgart, Germany.
.
Theory:
Crystal Growth:
M. Rheinstaedter, W. Schmidt,
D. Reznik,
Institut Laue-Langevin, Grenoble, France.
Laboratoire Léon Brillouin, Saclay, France.
M. Reehuis,
B. Ouladdiaf
Hahn-Meitner-Institut, Berlin, Germany.
Institut Laue-Langevin, Grenoble, France.
Inelastic neutron scattering:
Neutron diffraction:
Ch. Niedermayer, N. Cavadini, Paul-Scherrer Institut, Villigen Switzerland.
Ch. Niedermayer, C. BernhardPaul-Scherrer Institut, Villigen Switzerland.Max-Planck-Institut FKF, Stuttgart, Germany.
Transversal Field µSR
ILL
HMI
LLB
PSI
ILL
PSI
FRM II K. Hradil, A. SchneidewindFRM II, Munich, Germany
7x104
8x104
9x104
10x104
11x104
12x104
13x104
14x104
0 50 100 150 200 2500
50
100
150
200
250
47x104
48x104
49x104
50x104
51x104
52x104
53x104
54x104
0 50 100 150 200 250 3000
300
600
900
1200
1500
SrFeO2.84 SrFeO2.95
(0, 0, 2)
Inte
nsity
(c
nts
/ 10
sec)
(0.13, 0.13, 1.13)
Inte
nsity
(c
nts
/ 10
sec)
Temperature (K)
(0, 0, 2)
(0.13, 1.13, 0.13)
Temperature (K)
Elastic Neutron Scattering: SrFeO3-δ
- phase mixture: tetragonal/cubic
- magnetic moment: 2.48(2) µB / Fe4+-ion
- evidence for a structural phase transition in the tetragonal phase below 75 K
0 20 40 60 80 100 120 140
0
20000
40000
60000
80000
100000
120000
140000
160000
pure cubic ?
(111
)
(002
)
(012
) (112
)
(022
)
(011
)
(003
)
(001
)
S3
Inte
nsity
(
cnts
/ 3
.3 s
ec)
2 Theta
y.cubic y.tetra y.ortho 2 K 200 K
0 10 20 30 40 50 600
20
40
60
80
100
120
(0, 0
, 0.7
5)
(0, 0
, 0.2
5)
Inte
nsity
(c
nts
/ 3.3
sec
)
2 Theta
y.cubic y.tetra y.ortho 2 K 200 K
Elastic Neutron Scattering in cubic SrFeO3.00
ratiotetragonal : cubic
sample 1 30 : 70 %sample 2 50 : 50 %sample 3 (cubic) 90 : 10 %
in agreement to the volume fractionsobtained from zero field µSR
tetragonal : cubic
J = Superexchange Fe-O AFM-J = Double Exchange Fe-Fe FM
1
2
Takeda et al. (JSSC, 1996)
J = 1.2 meVJ = - 0.2 meVJ = - 0.3 meV
1
2
4
{
J4
J2
J1
Fe
O
Model for helical spin arrangement
⇒ Competition between Double Exchange and Superexchange
J1 = Superexchange Fe-O AFM-J2 = Double Exchange Fe-Fe FM
-0.2 -0.1 0.0 0.1 0.2
15 K
Log.
Int
ensi
ty
(cnt
s / 3
3 se
c)
Qhkl (0,0,1)
140 K
130 K
100 K
60 K
cubic SrFeO3.00
(110)
(001)
Elastic Neutron Scattering in cubic SrFeO3.00
helical spinarrangement
TN1 = 130 K magnetic „satellite“ peaks around structural Bragg reflections propagation vector along the [111]-direction µ = 2.48 µB/Fe4+-ionTN2 = 65 K additional Bragg reflections appear at (0, 0, ¼) doubling of the structural unit cell + antiferromagnetic order ?
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
10
20
30
40
50
60
70
T = 15 K T = 175 K
Inte
nsity
(c
nts
/ 22
sec)
(0, 0, Q l)
0 10 20 30 40 50 60 70 80 90 100 110 120 1300
2
4
6
8
10
12
14
16
18
20
22
24
26
Inte
nsity
(
cnts
/ 3
3 se
c)
Temperature (K)
(0 0 -0.75)
(0,0
, ¼)
(0,0
, ¾)
The orbital degeneracy in ground state is lifted by: - Jahn-Teller coupling - Superexchange interaction - Spin-orbit coupling
Splitting of the 3d levels
LaMnO3 Mn3+
3d4, t2g3 eg
1
Hund‘s Rules: S = 2
eg- Orbital
t2g–Orbital
x2-y2 3z2-r2
x2-y2
3z2-r23z2-r2
yz xz xy
yz
xz
xyxy
Jahn -TellerAufspaltung
splittingcubic splitting tetragonal
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