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