v. pomjakushin i.m.frank laboratory of neutron physics, jinr, dubna in collaboration with: a....
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V. PomjakushinI.M.Frank Laboratory of Neutron Physics, JINR, Dubna
In collaboration with:
A. BalagurovFrank Laboratory of Neutron Physics, JINR, DubnaP. Fischer, D. SheptyakovLaboratory for Neutron Scattering, ETH Zurich and PSI, VilligenD. Khomskii Solid State Physics Laboratory, Materials Science Centre, University of GroningenV. YushankhaiBogolyubov Laboratory of Theoretical Physics, JINR, DubnaA. Abakumov, E. Antipov, M. Lobanov, M. RozovaChemistry Department, Moscow State University, Moscow
Magnetic structure, disorder effects and unconventional superexchange interactions in
A2MnGaO5+ (A=Sr,Ca) layered oxides
SINQ user’s meting 2002, Bonn 2
Why A2MnGaO5+x (A=Sr, Ca)? Manganese oxides with possible CMR
LaMn3+O3/ SrMn4+O3
La2Sr1Mn3+2O7/
La1Sr2Mn4+2O7
LaSrMn3+O4/ Sr2Mn4+O4
(AO)
MnO2
MnO2
…(MnO2)n –(AO) (AO) – (MnO2)n…
3D Mn-O network
SINQ user’s meting 2002, Bonn 3
Crystal structures of A2MnGaO5+x (A=Sr, Ca)
x=0, Mn3+
Pnma, Ima2, Imcm P4/mmm, …x=0.5, Mn4+
MnO2
(AO)
(AO)
MnO2
GaO1+x8Å
4Å
Brownmillerite type of crystal structure
There is space for extra oxygen in GaO1+x-layers
SINQ user’s meting 2002, Bonn 4
Neutron diffraction. Crystal structure
HRPT/SINQ, =1.5 Å, Sr2MnGaO5+x, T=10K
0 20 40 60 80 100 120 140
2
-5000
0
5000
10000
15000
x=0•Ima2•Imcm with disorder in GaO4 tetrahedra
0 20 40 60 80 100 120 140
2
-10000
0
10000
20000
30000
40000
x=0.5P4/mmm with partially filled GaO6 octahedra
SINQ user’s meting 2002, Bonn 5
Neutron diffraction. Magnetic structureDMC/SINQ, =2.56 Å, T=2K G-structure
h=2n+1
C-structureh=2n
x
y z0
40000
80000
10 20 30 402
0
20000
40000
S r2G aM nO 5.0
(110
)(1
01)
(200
)
(100
)
(301
)(3
10)
S r2G aM nO 5.5
(010
)/(0
01)
(200
)
(210
)/(2
01)
0
2000
4000
6000
8000
Inte
grat
ed in
tens
ity
S r2G aM nO 5.0(110)
(101)
(301)
0 50 100 150 200T , K
0
1000
2000
3000
Inte
grat
ed in
tens
ity
S r2G aM nO 5.5(010)/(001)
(210)/(201)
(110)/(101)
Short-range G-type
G-type
C-type
SINQ user’s meting 2002, Bonn 6
Short range ordering effects
G
40Å5 layers
10 spins
C
C
C
2 0 2 5 3 0 3 5 4 02
2000
2200
2400
2600
2800
3000
S r2G aM nO 5.5=2.56Å
(010
)/(0
01)
(210
)/(2
01)
100K
220K
(110
)/(1
01)
cos)2/(L
Size effect peak broadening
2
40Å
G-type short-ranged phase inside predominant C-type matrix
Short-range G-type
C-type
Sr2MnGaO5.5
Short-range G-type
C-type
0 50 100 150 200T , K
0
1000
2000
3000
Inte
grat
ed in
tens
ity
S r2G aM nO 5.5(010)/(001)
(210)/(201)
(110)/(101)
SINQ user’s meting 2002, Bonn 7
Magnetic and crystal structures
G-type
GaO4
Mn3+
C-type
GaO6
Mn4+
AF F
Sr2MnGaO5Sr2MnGaO5.5
AF
SINQ user’s meting 2002, Bonn 8
Mn3+/ Mn4+ in octahedral site
x
y
z
Mn3+
t2g
xy, yz, zx
Mn4+
SINQ user’s meting 2002, Bonn 9
AF in-plane superexchange (SE)
Similar to CaMnO3
Similar to RP phase (n=1) (SrCa)MnO4
Mn3+O2 plane
2.4Å
1.9Å
Mn3+
2zd
O2-
zp
Mn3+
++
__
Antiferromagnetic MnO2 planes both for Mn3+ and Mn4+ in accord with standard SE.180o SE of half-filled both t2g- t2g and dz2- dz2 orbitals is always AFM
x
y
z
xzd
Mn4+ Mn4+
_
++ _
O2-
SINQ user’s meting 2002, Bonn 10
Interplane Mn-O-O-(O)-Mn superexchange
Mn3+ and GaO4
2zd
zp
zp
AFGa
p
180o-AFM superexchange
+
_
M n A
3+
O A
O B
+
_
Vertical SE MnA –MnB
weak FM
weak AFM
p
p
U
J
2
2~
2)(~ tt
p-d
J
ttt
ttt
p
zx
ppx
pdx
transfer charge
constant s Hund'
2/)(
/
Diagonal SE MnA –MnC
weak FM
AFM
22
2)(~ tt
U
J
p
p
)(~ 12
2)2(2
4
dp UUt
Unconventional “diagonal” superexchange
M n C
4+
O L O R
O B
O A
M n A
4+
+
_O C
x
y
z
++
__
++
__
+ _+ _
+_px
Mn4+ and GaO6
ppzt
ppxt
AF
pdt
SINQ user’s meting 2002, Bonn 11
M n C
O L O R
O B
O A
M n A
+
_
O C
x
y
z
++
__
Mn-O-O-Mn diagonal 180o superexchange
180o-AFM superexchange
90o weak FM superexchange
++
__
+ _+ _
+_
px, pz orbitals rotated by 45o
SINQ user’s meting 2002, Bonn 12
Meff accessed by neutron diffraction
Effective magnetic moment vMM eff 0
effective occupancy
real moment
volume fraction
Spin vacancy
Spin flip M/M=-2c
SINQ user’s meting 2002, Bonn 13
Magnetic moments
Magnetic moment seen by neutron diffraction is appreciably reduced -- local disorder, hybridization?
S r2G aM nO 5.0
0 5 0 1 0 0 1 5 0 2 0 0T , K
0 .0
1 .0
2 .0
3 .0
4 .0
Mag
netic
mom
ent,
B
C a 2G aM nO 5.0
S r2G aM nO 5.5
Mn3+
Mn4+
Spin-only values
Mn3+
Mn4+
))(1()( 0 NTTMTM
DMC/SINQ
SINQ user’s meting 2002, Bonn 14
Muon spin polarization P(t) below TN
~
~
~2
)sin/(cos3
2
3
1)(
),,()(
2/12
22/)(
2
1
2
a
B
Bf
tttetG
ftGatP
loc
loc
t
iiii
B x,y,z
Spe
ctra
l den
sity
of B
x,y,
z
2/12 locB
locB
Local magnetic field distribution seen by SR
0 0.04 0.08 0.12 0.16 0.2t, s
-0 .3
-0.2
-0.1
0.0
P(t
)
S r2G aM nO 5.54K
90K
-0.3
-0.2
-0.1
0.0
P(t)
C a2G aM nO 5.0
S r2G aM nO 5.0
4K
Coherent precession – long range ordering of Mn-spins
Muon spin relaxation – disorder of Mn-spin configuration/value/direction
ordered fraction
SINQ user’s meting 2002, Bonn 16
Local magnetic disorder
+B
-B
Ca2MnGaO5.0
Sr2MnGaO5.0 (G-type)
fSr=fCa, while M/M10%(/(2f)) 12%
Spin flips (concentration<<1) can explain ND-SR “contradiction”
Sr2MnGaO5.5 (C-type)
Local spin fluctuations (spin-flips)
0 50 100 150 200T , K
0
1000
2000
3000
Inte
grat
ed in
tens
ity
S r2G aM nO 5.5(010)/(001)
(210)/(201)
(110)/(101)
Volume fraction decreased – second phase develops spatially separated
S r2G aM nO 5.0
0 5 0 1 0 0 1 5 0 2 0 0T , K
0 .0
1 .0
2 .0
3 .0
4 .0
Mag
netic
mom
ent,
B
C a 2G aM nO 5.0
S r2G aM nO 5.5
Mn3+
Mn4+
Moment from neutron diffraction
0
10
20
f, M
Hz
0
20
40
f, MH
z
0
20
40
60
80
s-1
S r2G aM nO 5.0
C a 2G aM nO 5.0
S r2G aM nO 5.5
S r2G aM nO 5.5
(S r,C a)2G aM nO 5.0
T N
0 50 100 150 200T , K
0
0.05
0.1
0.15
0.2
Asy
mm
etr
y
(S r,C a)2G aM nO 5.0S r2G aM nO 5.5
T N
(a )
(b)
(c)
SINQ user’s meting 2002, Bonn 17
Summary
Novel manganese layered oxides A2MnGaO5+ (A=Sr,Ca) with adjustable Mn3+/Mn4+-valence: synthesis and structure.
The principal structure difference between the 0 (Mn3+) and 0.5 (Mn4+) is GaO1+ buffer layer, which is formed by tetrahedra or partially filled octahedra
AFM (0) --> FM (0.5) coupling between the AFM ordered MnO2-layers. Unconventional diagonal superexchange Mn4+-O-O-O-Mn4+
Disorder effects in magnetic ordering - spin flops and short range phase separation. The magnetic disorder can be caused by the disorder in oxygen positions in GaO1+ -layer.
SINQ user’s meting 2002, Bonn 18
The end
G-type
GaO4
Mn3+
C-type
GaO6
Mn4+
AF F
Sr2MnGaO5Sr2MnGaO5.5
AF
SINQ user’s meting 2002, Bonn 19
Intermediate Mn-valence in Sr2MnGaO5+.
=0.13, =0.41
0 5 0 1 0 0 1 5 0T em p era tu re , K
0
0 .5
1
1 .5
2
2 .5
3
3 .5
Mag
neti
c m
omen
t (B
ohr
mag
neto
ns)
S r541-12
S r 2G a M n O 5 .4 11
2
1 + 2 ) /2
G
+C
=1
2
0 5 0 1 0 0 1 5 0T , K
0 .0
0 .5
1 .0
1 .5
2 .0
Mag
net
ic m
omen
t, B
S r-541gc
S r 2G a M n O 5 .4 1G
C
SINQ user’s meting 2002, Bonn 20
+
_
M n A
3+
O A
O B
M n C
4+
O L O R
O B
O A
M n A
4+
+
_O C
x
y
z
+
_
++
__
++
__
+ _+ _
+_
SINQ user’s meting 2002, Bonn 21
Configurational disorder
35
)(3
rrff
dip
mrrmB
0mBloc
Disordered component of moment
10 mmm
Bloc
2/1201
0
)(
mm
mm
m
Disorder of spin-configuration:Spin-flips with concentration c<<1
SR frequency f ~ m0
ND magnetic moment M ~ m0
Local field
+B
-B
),(),( rmBrmB fdipfdip
f~ m, while M ~ m(1-2c)
cB
mB
loc
loc
2/12
Disordered field from the flipped spin mf(r)
-r, -mf
r, mfm
BB diploc
12/12 mBloc
SINQ user’s meting 2002, Bonn 22
Why A2MnGaO5+x (A=Sr, Ca)?
Known Mn-oxides with intermediate Mn-valence 3+x possessing CMR effect are from Ruddlesden-Popper (RP) series (LaSr)n+1MnnO3n+1
…(MnO2)n –(AO) (AO) – (MnO2)n…
La1-xSrxMn3+xO3 – 3D Mn-O network (RP, n=)
La2-2xSr1+2xMn3+x2O7 – 2D Mn-O network (RP, n=2)
A2MnGaO5+x are novel manganese oxides with Mn3+/Mn4+—O network
allowing double exchange. New doping possibility through incomplete oxygen sublattice.
…MnO2 –(AO)(GaO1+x)(AO) – MnO2…
SINQ user’s meting 2002, Bonn 23
Experiment
• Sample synthesis , X-ray at Inorg. Chem. Lab., MSU, Moscow
• Crystal and magnetic structures: powder neutron diffraction at HRPT and DMC, SINQ/PSI, Villigen • Local disorder: SR at PSI muon facilities
SINQ user’s meting 2002, Bonn 24
Meff accessed by neutron diffraction
2
20)(
FN
MN
I
I
nucl
mag
nucl
mag
Magnetic structure factor
)(2)()( HrMHHF ji
jjj eP
Effective magnetic moment vMM eff 0
effective occupancy
real moment
volume fraction
Spin vacancy
Spin flip M/M=-2c