Spin wave dispersion in the helical spin ordered system...

transcript

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

• insulator• cooperative Jahn-Teller distortion at 800K• commensurate , collinear spin structure

SrFeO3: Fe4+, 3d4, t2g3eg

• metal• cubic, no structural transition• incommensurate, helical spin structure

Metallic SrFeO3-δ

metallic conductivityhelical spin arrangement

eg- Orbital

t2g–Orbital

x2-y2 3z2-r2

3z2-r23z2-r2

yz xz xy

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

χ (em

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

orthorhombic T

N = 230 K

cubicT

N = 130 K

tetragonalT

N = 75 K

Temperature (K)

SrFeOx x=3.00 x=2.85 x=2.81 x=2.75

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

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

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

Qhkl (0,0,1)

cubic SrFeO3.00

0 20 40 60 80 100 120 140 160 1800

Temperature (K)

0 20 40 60 80 100 120 140 160 1800.095

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

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

T = 4 K

SrFeO2.875

tetragonal

(Qh, Q

K, 1+Q

Tetragonal SrFeO2.875

magnetic Bragg Peaks at δ = 0.2

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

CaFeO3

(QH, Q

K, 1+Q

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))

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

Γ Χ Μ R Γ

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

helical AFMT = 125 Kcharge orderbelow 290 K

A-type AFMT = 140 Korbital order below 780 K

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

J(q) = [cos(q a) + cos(q a) + cos(q a)] -

- [cos(q a)cos(q a) +

x y cos(q a)cos(q a) + cos(q a)cos(q a)]y z z x

This exchange interaction has a minimum when:

a = cos ( + y + z)Q -1 X J1

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

J = Superexchange Fe-O AFM-J = Double Exchange Fe-Fe FM

P.-G. De Gennes,PR 118, 141 (1960)

⇒ Competition between Double Exchange and Superexchange

results in a helical spin structureFe

Double-exchange / Super-exchange

J = J + J (U, )J = J ( ) (2t/ )2J = J /2

1 DE SE pd

4 1 pd pd

∆∆ ∆

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

(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

(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

(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

(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

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

0 20 40 60 80 100 120 140 160 180 2000

Temperature (K)

Int1 Int3

0 20 40 60 80 100 120 140 160 180 2000.7

Temperature (K)

0.6 0.8 1.0 1.2 1.40

1 234 56 78910111213141516171819202122232425262728293031323334353637

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464748495051525354555657585960616263646566676869707172737475767778798081ABCDEFGH I JKLMNOPQRSTUVWXYZAAABACADAEAFAGAHAIAJAK

AOAPAQ

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nuclear

magnetic

magneticSr3Fe2O7

(QH, 0, 0 )

Sr3Fe2O6.88

Elastic Neutron Scattering along the a-axis

δ = 0.25

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

Magnetoresistance

Mössbauer Spectroscopy

Ellipsometry

P. Adler

A. Boris, C. Bernhard, A.V. Pimenov

Universität Karlsruhe, 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

FRM II K. Hradil, A. SchneidewindFRM II, Munich, Germany

10x104

11x104

12x104

13x104

14x104

0 50 100 150 200 2500

47x104

48x104

49x104

50x104

51x104

52x104

53x104

54x104

0 50 100 150 200 250 3000

SrFeO2.84 SrFeO2.95

(0, 0, 2)

(0.13, 0.13, 1.13)

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

100000

120000

140000

160000

pure cubic ?

) (112

2 Theta

y.cubic y.tetra y.ortho 2 K 200 K

0 10 20 30 40 50 600

2 Theta

y.cubic y.tetra y.ortho 2 K 200 K

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

Takeda et al. (JSSC, 1996)

J = 1.2 meVJ = - 0.2 meVJ = - 0.3 meV

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

Qhkl (0,0,1)

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

T = 15 K T = 175 K

(0, 0, Q l)

0 10 20 30 40 50 60 70 80 90 100 110 120 1300

Temperature (K)

(0 0 -0.75)

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

Hund‘s Rules: S = 2

eg- Orbital

t2g–Orbital

x2-y2 3z2-r2

3z2-r23z2-r2

yz xz xy

Jahn -TellerAufspaltung

splittingcubic splitting tetragonal

Spin wave dispersion in the helical spin ordered system...

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