02/12/2008

GDR MICO, Autrans, 2008

1

Magnetic Order and Magnetoelectric effect in the frustrated Fe langasites

V. Simonet, M. Loire, R. Ballou

Outline : - Structure- Magnetic properties- Dielectric properties- Conclusion

K. Marty, P. Bordet, P. Lejay, O. Isnard

E. RessoucheINAC/SPSMS/MDN, CEA, Grenoble, France

Institut Néel, CNRS/UJF, Grenoble, France

02/12/2008

GDR MICO, Autrans, 2008

2

b

a

Ba3NbFe3Si2O14 : space group P321

Fe-langasite : structure

c

Geometrical frustrationAcentric structure

Crystal grown by floating zone methodin image furnace

triangular lattice of Fe3+ triangles, S=5/2

⇒Magnetic properties?⇒Magnetoelectric effect?

⇒Multiferroism?

02/12/2008

GDR MICO, Autrans, 2008

3

Fe-langasite : magnetic properties

Magnetic transition → antiferromagnetic order at TN∼28 K

0 50 100 150 200 250 3000.000

0.005

0.010

0.015

0.020M

/H (e

mu/

mol

Fe)

T (K)

Ba3NbFe3Si2O14 single crystal

H ⊥ c H // c

Macroscopic measurementsTN

02/12/2008

GDR MICO, Autrans, 2008

4

20 30 40 50 60 70 80

0

50000

100000

150000

200000

250000 T=30 K T=1.5 K 1.5 K - 30 K Calc 1.5 K

Neu

tron

coun

ts (a

. u.)

2θ (°)

D1Bλ=2.52 Å

Below TN=28 K, AF order withpropagation vector k=(0,0,∼1/7)

Fe-langasite: magnetic structureNeutron diffractionon powder (D1B), and single crystal (D15) at ILL

Powder pattern

02/12/2008

GDR MICO, Autrans, 2008

5

Fe-langasite: magnetic structureHelices propagating along c120° moments on triangles

in the (a, b) plane → signature of frustration

02/12/2008

GDR MICO, Autrans, 2008

6

Chirality

• Structural chirality : acentric structureX-ray (anomalous) ⇒ aenantiomorph

What about Fe-langasite?

• Chirality domains unbalanced by external constrains• Single domain in non-centrosymmetric crystals

e. g. MnSi - helical modulation and sense of rotation- due to Dzyaloshinskii-Moryia interaction

• Triangular chirality [ ]∑ ×=triangle

jiT SSCrrr

332

• Helicity [ ]jiH SSCrrr

×=

02/12/2008

GDR MICO, Autrans, 2008

7

Fe-langasite: magnetic domainsDiffraction on single crystalwith unpolarized neutrons&with polarized neutron andspherical polarization analysis

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0100% single helicity

Cal

cula

ted

PfObserved Pf

CRYOPAD (IN22, ILL)

⇒Monodomain magnetic structure(helicity & triangular chirality)

Calculated vs measured final polarization for several magnetic reflections

02/12/2008

GDR MICO, Autrans, 2008

8

c axis

Mean-field analysis: probe of the microscopic mechanisms

Fe-langasite: magnetic properties

J4

J3

J5

Heisenberg Hamiltonian :super-exchange interactions2 intra-plane3 between adjacent planes

J1

J2

AF J1, J2J3 ≠ J5

02/12/2008

GDR MICO, Autrans, 2008

9

c axis

Mean-field analysis: probe of the microscopic mechanisms

Fe-langasite: magnetic properties

J3

Heisenberg Hamiltonian :super-exchange interactions2 intra-plane3 between adjacent planes

⇒ Describe the magnetic structure⇒ selects two pairs of helicity and triangular chirality (CH, CT)

02/12/2008

GDR MICO, Autrans, 2008

10

Link between the magnetic chiralities (CH, CT) and the structural one

2

13

Fe-langasite: magnetic properties

3’

Structure : Left handedness

Left triangular chirality

1’

2’

k=(0,0,1/6) ⇐ 60°

Right helicity

Marty et al., Phys. Rev. Lett. (in press)

02/12/2008

GDR MICO, Autrans, 2008

11

Chirality and ferroelectricity

Case of RbFe(MoO4)2 : triangular lattice of Fe3+

Kenzelmann et al. PRL (2007)

120° magnetic structurebut centro-symmetric ⇒ 2 domains of magnetic chiralities∃ PE at TN ⇒ symmetry based phenomenological theoryProportional to the chirality unbalance

Conclusion :«trigonal stacked triangular antiferromagnetswith 120° structure are multiferroics.»

02/12/2008

GDR MICO, Autrans, 2008

12

Fe-langasite: dielectric properties

Dielectric constantvia capacity measurementsAnomaly near TN for E ⊥ c

0 10 20 30 40

7.88

7.90

7.92

7.94

Coll. with B. Zawilsky, J. Marcus, Institut Néeland C. Simon, B. Kundys, CRISMAT, ENSICAEN

ε

T (K)

TN=27 Kν=100 kHzE⊥c

02/12/2008

GDR MICO, Autrans, 2008

13

0 20000 40000 60000 80000 100000-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0 10 20 30 400.0

2.0x10-12

4.0x10-12

6.0x10-12

8.0x10-12

Δε/ε

(%)

20 K 27 K 35 K

H (Oe)

B

T (K)

Δε/ε = BH2TNMagnetodielectric

couplingquadratic in HMaximum at TN

Fe-langasite: dielectric properties

02/12/2008

GDR MICO, Autrans, 2008

14

Why?

Pyroelectric current measurement No evidence of spontaneous electrical polarization?

Fe-langasite: dielectric properties

...21

21

0 +−−−−= jiijjiijiiii HHEEHMEPFF χε

Results similar to YMnO3, ferroelectric at TE= 930 Kand 120° antiferromagnetic order at TN=74 K Bellido et al.

Free Energy :

lkjiijklkjiijkkjiijkiiij HHEEHEEHHEHE δγβα21

21

21

−−−−

02/12/2008

GDR MICO, Autrans, 2008

15

ConclusionNon-centrosymmetric Ba3NbFe3Si2O14 langasite :

• Very original magnetic structure below TN=28 Ksingle triangular chirality and helicity linkedby the twist of the magnetic interactions

• Magnetodielectric coupling/ No ferroelectricity (?)

→Further investigation of dielectric properties →Many other possible chemical substitutions→Probe of the excitations

02/12/2008

GDR MICO, Autrans, 2008

16ECOLE THÉMATIQUEApport des symétries en matière condensée

Presqu'île de Giens, 11-17 mai 2009

Organisationthgroupes@ill.frB. Grenier UJF&CEA, GrenobleV. Simonet CNRS, GrenobleH. Schober ILL, Grenoble

Site WEB opérationnel courant juinhttp://www.ill.eu/news-events/workshops-events/ecole-theorie-des-groupes/

Programme- Formalisme de la théorie des groupes et des représentations, théorie de Landau- Applications : groupes d'espace et transitions de phase cristallographiques,

structures magnétiques, excitations (phonons, champ cristallin, …)- Exemples : multiferroïques et supraconductivité

OrateursJ. Villain, D. Fruchart, R. Ballou, B. Canals, B. Grenier, V. Simonet, P. Toledano, J. M. Perez-Mato, M. I. Aroyo, J. Rodriguez-Carvajal, F. Bourée, J. Schweizer,

M. Amara, B. Ouladdiaf, G. Eckold, J. Kreisel, S. Raymond, L. Chapon, M. Houzet

02/12/2008

GDR MICO, Autrans, 2008

17

(-1,-1)(1,1)

Link between the magnetic chiralitiesand the structural one

1’

(-1,1)(1,-1)2

3’

2’

13

Fe-langasite: magnetic properties

Left handedness right handedness

02/12/2008

GDR MICO, Autrans, 2008

18

0 10 20 30 40

8.7

8.8

8.9

9.0

εν=100 kHzE//c

T (K)

TN=27 K