spin structure and dynamics in the half-doped cobaltate la 1.5 sr 0.5 coo 4
DESCRIPTION
Spin structure and dynamics in the half-doped cobaltate La 1.5 Sr 0.5 CoO 4. Igor A. Zaliznyak Brookhaven National Laboratory. Collaboration J. Tranquada BNL G. Gu BNL R. Erwin NIST CNR S.-H. Lee NIST CNR Y. Moritomo CIRSE Nagoya Univ. Outline. - PowerPoint PPT PresentationTRANSCRIPT
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ACNS 2004
Spin structure and dynamics in the half-Spin structure and dynamics in the half-doped cobaltate Ladoped cobaltate La1.51.5SrSr0.50.5CoOCoO44
CollaborationCollaboration
• J. Tranquada BNL• G. Gu BNL• R. Erwin NIST CNR• S.-H. Lee NIST CNR• Y. Moritomo CIRSE Nagoya Univ.
Igor A. ZaliznyakIgor A. Zaliznyak
Brookhaven National LaboratoryBrookhaven National Laboratory
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ACNS 2004
OutlineOutline
• Crystal structure of La1.5Sr0.5CoO4 and electronic properties of Co2+/Co3+ ions in it
• Charge and spin order at half-doping– neutron-scattering signatures of charge and spin order– sample dependence of the short-range order
• Spin-freezing transition: critical slowing down of the spin dynamics
• Low-energy excitations in La1.5Sr0.5CoO4 – magnons– RIP:optic phonon, magnetic continuum?
• Summary
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ACNS 2004
Crystal structure of the layered perovskite Crystal structure of the layered perovskite cobaltate around half-dopingcobaltate around half-doping
LaLa1.51.5SrSr0.50.5CoOCoO44 always (at all T)remains in “high-temperature tetragonal” (HTT) phase
Space group I4/mmm, lattice spacings aa≈≈3.833.83 Ǻ, cc≈≈12.512.5 Ǻ
Perfect “checkerboard” superstructure corresponds to a twice larger unit cell
2aax 2aaxcc, with space group F4/mmm.
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ACNS 2004
LaLa1.51.5SrSr0.50.5CoOCoO44: bulk properties.: bulk properties.
Resistivity: activation behavior,Ea ~ 6000 K
Susceptibilivity: anisotropic, spin-glass-like behavior
T~30 KT~30 K
Moritomo et al (1997)
J~250-450 KJ~250-450 KD~400-900 KD~400-900 K
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ACNS 2004
Charge and orbital order at half-dopingCharge and orbital order at half-doping
Possible checkerboard fillings of the eegg levels on a square lattice
In-plane “zig-zag” (3x2-r2) / (3y2-r2)
Out of plane (3z2-r2)
In-plane (x2-y2)
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ACNS 2004
Electronic structure of CoElectronic structure of Co2+2+/Co/Co3+3+ ions in ions in LaLa1.51.5SrSr0.50.5CoOCoO44
CoCo2+2+ (3d7)
S=3/2
eg
t2g
CoCo3+3+ (3d6)
S=0 S=1 S=2
eg
t2g
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ACNS 2004
Charge order in Charge order in LaLa1.51.5SrSr0.50.5CoOCoO44: neutron : neutron
diffuse elastic scatteringdiffuse elastic scattering
Short-range “charge glass” order, I. Zaliznyak, et. al., PRL (2000), PRB (2001)
cc = 0.62(6)cc abab= 3.5(3)a a 2
Al(1
11)
Al(2
00)
c
c
c
c
c
c
c
c
c c
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ACNS 2004
Spin-entropy driven melting of the charge order in Spin-entropy driven melting of the charge order in LaLa1.51.5SrSr0.50.5CoOCoO44: neutron diffuse elastic scattering: neutron diffuse elastic scattering
Melting of the short-range “charge glass” order, I. Zaliznyak, et. al., PRB (2001)
CoCo2+2+ CoCo3+3+
z
x
x=0.011(1) lu, x=0.011(1) lu, z=0.0068(4) luz=0.0068(4) lu
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ACNS 2004
Charge order and a spin systemCharge order and a spin system
CoCo2+2+ form a square-lattice AFM with almost critical frustration, JJ11~2J~2J22
JJ11
JJ22
CoCo2+2+
S=3/2 2D2D
CoCo3+3+
S=1or
S=2 S z = 0
S z = ±1DD
Strong single-ion anisotropy D~500 KD~500 K quenches CoCo3+3+ spin at low T
S z = ±1/2
S z = ±3/2
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ACNS 2004
-2 0 2 4 6l (rlu)
0
200
400
600
800
1000
Neu
tron
cou
nts
mon
itor
=5.
0e+05
Spin order in Spin order in LaLa1.51.5SrSr0.50.5CoOCoO44: magnetic elastic : magnetic elastic
neutron scattering neutron scattering
-0.5 0.0 0.5 1.0 1.5 2.0h (r lu )
0
200
400
600
800
1000
Neu
tron
cou
nts
mon
itor
=5.
0e+
05
1122
33
44
55
66CoCo2+2+
““CoCo3+3+””
88
77
Q=(0.258(1),0,1)Q=(0.258(1),0,1), in I4/mmm abab=14.5(5)a a 2
cc=0.85(5)cc
mm
mm
mm
mm
Q = (h,h,1)
T=10K
Q = (0.258,0.258,l)
T=6K
E
μ
m
μ
mrN
dEdΩ
Ed
B
Bm
el
,1
12
1
12
022
coscosh
sinh
2
coscosh
sinh
2
,
aQq
q
aq
Q
Lattice-Lorentzian scattering functionI. A. Zaliznyak and S.-H. Lee in “Modern Techniques for Characterizing Magnetic Materials”, ed. Y. Zhu (Kluwer)
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ACNS 2004
Magnetic elastic scattering from the frozen Magnetic elastic scattering from the frozen spin structure in spin structure in LaLa1.51.5SrSr0.50.5CoOCoO44..
Lattice-Lorentzian scattering from a damped spin spiral in
the a-ba-b plane gives perfect fit to the measured intensity
Intensity map, calculatedfrom the fit
Al(
111)
Al(
200)
Al(
111)
Al(
200)
T=6 KT=6 K
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ACNS 2004
Universal or sample-dependent?Universal or sample-dependent?
Sample #1, by
Y. Moritomo, m≈0.5g
Sample #2, by
G. Gu, m≈6g
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ACNS 2004
Charge-order scattering from big new Charge-order scattering from big new sample #2sample #2
abab= 3.4(6)a a 2cc = 0.2cc(fixed)
Al(1
11)A
l(200)
x=0.011(1) lu, x=0.011(1) lu, z=0.0068(4) lu, z=0.0068(4) lu,
zzLa/SrLa/Sr=0.0010(1) lu=0.0010(1) lu
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ACNS 2004
2500
2000
1500
1000
500
0
Inte
nsity
(co
unts
in 2
8 s)
0.0 0 .5 1 .0 1 .5h(rlu)
400
300
200
100
0
sam ple #2 = 0.255(1)
Al (
111)
Al (
200)
(a)
Al (
220)
sam ple #1 = 0.258(1)
(b)
,Q = ( ,3)
Magnetic scattering from two samplesMagnetic scattering from two samplesT=8K
T (K)
Q = (0.256,0.256,1)
abab=14.5(5)a a 2
abab=23.0(5)a a 2
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ACNS 2004
Melting of the frozen spin order.Melting of the frozen spin order.
abab15 a a 2
cc0.9cc
abab8 a a 2
cc0.5cc
abab4 a a 2
cc0.2cc
6K6K
38K38K
50K50K
BT2&BT4, Ef=14.7 meV, 60’-20’-20’-100’.
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ACNS 2004
Temperature evolution of the magnetic Temperature evolution of the magnetic scattering: raw data.scattering: raw data.
BT2&BT4, Ef=14.7 meV, 60’-20’-20’-100’. SPINS, Ef=3.7 meV, 40’-60’-60’-240’.
~40K?~40K?
~30K?~30K?
~40K?~40K?
~30K?~30K?
Where is the spin-ordering transition?
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ACNS 2004
Slowing down of the spin fluctuations: is Slowing down of the spin fluctuations: is there a criticality?there a criticality?
EE~(T-T~(T-Tcc))
EE~T~T EE~ ~ 00+T+T
EE~ ~ 00+(T-T+(T-Tcc))
Although the critical behavior EE~(T-T~(T-Tcc)), =3.0(3)=3.0(3) is not ruled out, log(log(EE)) is surprisingly linear in log(T):log(T): EE~T~T with ~8 ~8 (!?).
log(log(EE)~log(T) )~log(T) ?? log(log(EE)~log(T) )~log(T) ??
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ACNS 2004
Spin dynamics: acoustic magnonsSpin dynamics: acoustic magnons
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RIP: scattering at higher energy: phonon, RIP: scattering at higher energy: phonon, magnetic continuum?magnetic continuum?
E (m eV )
0
5
10
15
20
Inte
nsit
y (c
ts/m
in)
0 10 20 30
(0.5,0.5,5)
(1.5,1.5,1)
phonon magnetic continuum?
0 100 200 300
5
10
15
Inte
nsi
ty (c
ts/m
in)
0
T (K )
E = 25 m eVQ = (0 ,0 ,5 )
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ACNS 2004
RIP, dynamics in RIP, dynamics in LaLa1.51.5SrSr0.50.5CoOCoO44: acoustic : acoustic
magnons, optic phonon, magnetic continuum?magnons, optic phonon, magnetic continuum?
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.00
5
10
15
20
25
30
h (rlu)
T = 5 K = (h,h,3)Q
v s in(2 (h - ) = 21.2(5), v = 0.244(1)
E (
meV
)
m agnetic scattering
phonon
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ACNS 2004
SummarySummary
• A short-range checkerboard charge order yields a peculiar spin system in La1.5Sr0.5CoO4
• A short-range, incommensurate spin order results from the frustration and the lattice distortion– the incommensurability and the correlation length are slightly
sample dependent
• Static spin ordering: a spin-freezing transition at Ts ≈ 30 K– relaxation rate vanishes– correlation length saturates
• Dynamics at low E is dominated by a well-defined, strong band of acoustic magnons– crosses an optic phonon at 15 meV – interaction?
• Continuum magnetic scattering at 20 meV < E < 30 meV?
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ACNS 2004
Exchange modulation by superlattice Exchange modulation by superlattice distortiondistortion
Heisenberg spin Hamiltonian
Superlattice distortion
Modulated-exchange Hamiltonian
(eg)
++
==
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ACNS 2004
Spin-spiral ground state better adapts to Spin-spiral ground state better adapts to distortiondistortion
Harmonics at nQc are generated in spin distribution,
As a result, the MF ground state energy of a spin spiral is lowered
To the leading order,
In the presence of a superlattice distortion in the crystal antiferromagnetism may loose to a competing near-by spiral state
I. A. Zaliznyak, Phys. Rev. B (2003).