THERMODYNAMIC & NEUTRON DIFFRACTION STUDIES ON MULTIFERROIC NdMn2O5

1 Laboratoire de Physique des Solides (LPS), Université Paris-sud, Orsay, France 2Laboratoire Léon Brillouin (CEA-CNRS), CEA Saclay, France

S. Chattopadhyay1, V. Baledent1, F. Damay2, A. Goukassov2,

P. Auban-Senzier1, C. Pasquier1, C. Doubrovsky1, and P. Foury-Leylekian1

Post-doctoral Supervisor: P. Foury-Leylekian

Different Ferroic Orders: • (Anti)ferromagnetism (A)FM. • Ferroelectricity FE • Ferroelasticity • Ferrotoroidicity

A fascinating class of compounds that exhibit more than one ferroic orders simultaneously.

Our Interest: Multiferroicity = Magnetic Order + Electric Order

M P

Mul

tifer

roic

ity

Types of Multiferroics:

Coexistence of magnetic and electric orders without magneto-electric coupling. Example: BaTiO3, BiFeO3

• Coexistence of magnetic and electric orders with magneto-electric coupling.

• First observed in geometrically frustrated magnetic oxides.

• Magnetic origin of FE.

• Potential candidate for spintronic applications.

Example: RMn2O5 (R: rare earths), Ni3V2O8, CuFeO2, CoCr2O4

Dzyaloshinskii-Moriya (DM) interaction:

• Polarization due to the shift of

O2- ligands (x).

Exchange Striction: (Minimization of spin exchange energy by lattice relaxation

in a magnetically ordered state.)

• Frustration + Superexchange interaction.

• Polarization due to the shift of transition

metal ions.

D S Sij i j

×

P

Mn4+

Mn3+

R3+

Si Sj

O2-

x

S-W Cheong et al., Nat. Mater. 6 , 13 (2007).

Exchange Interactions:

• Centrosymmetric orthorhombic Pbam space group.

• Mn3+: MnO6 octahedra.

• Mn4+: MnO4 pyramid.

• Mn4+ chains along c-direction.

• Zig-zag chain of Mn3+ and Mn4+ along a-axis.

c-direction: Between Mn4+-Mn4+

J1: Via O2- in R3+ layers. J2: Via O2- in Mn3+ layers.

ab-plane: Via O2- ligands J3: Between Mn4+-Mn3+

J4: Between Mn3+-Mn4+

J5: Between Mn3+-Mn3+

Frustrated loop with AFM bonds.

57 La

58 Ce

59 Pr

60 Nd

61 Pm

62 Sm

63 Eu

64 Gd

65 Tb

66 Dy

67 Ho

68 Er

69 Tm

70 Yb

71 Lu

Decreasing size

FERROELECTRIC • Successive magnetic transitions (3 to 4) below 45K • Polarization along b-axis.

• Presence of magneto-electric coupling.

Non-ferroelectric ?

Proposed Model: Displacement of the Mn3+ ions (exchange striction) breaks the inversion symmetry invoking the ferroelectric state.

Heat Capacity (CP):

• 4 anomaly in Cp/T vs. T data.

• T1P~30K: Broad peak.

• T2P~26K: Weak shoulder.

• T3P~18K: Weak hump like.

• T4P~4K: Sharp peak.

0 10 20 30 40 50 601250

1500

1750

2000

2250

2500 T4P

T3P

T2P

C P/T (µ

J/K2 /g

)

T (K)

T1P

NdMn2O5

N. Hur et al., PRL 93, 107207 (2004) .

Magnetization (M):

• 3 anomaly in ZFCH: ~ 36K, ~15K and ~5K.

• Thermomagnetic

irreversibility between ZFCH-FCC .

• |θ/TS| ≈ 4.5 (θ is the Curie T)

• Crystal: Anisotropy along c-axis

• ZFCH: Zero field cooled heating. • FCC: Field cooled cooling. • FCH: Field cooled heating.

0 25 50 75 100

2

4

6

T4P

T3P

ZFCH FCC FCH

M (1

0-2 e

mu/

g)

T (K)

H = 100 Oe

Ts~ T1P

0 10 20 30 400.5

1.0

1.5

2.0

2.5

3.0

3.5

c-axis a-axis b-axis

M (1

0-2 e

mu/

g)

T (K)

Dielectric Permittivity (ε’):

• Sharp peak at T2P. • Signature of

FERROELECTRICITY (FE) • Weak hump around T3P

similar to TbMn2O5 (Electromagnon?) .

T3P T2P

Electric Polarization (P):

• Onset of spontaneous polarization below T2P (~27 K).

• Reaches to maximum around 21 K.

• Changes sign below ~16 K.

• Sign reversal: Not uncommon.

NdMn2O5 is ferroelectric below T2P

~ 27K

0 10 20 30 40 50 60-0.6

-0.4

-0.2

0.0

0.2

0.4

P (µ

C/m

2 )

T (K)

Epole = 7kV/cmHeating @ 5K/min

T3P T2P

NdMn2O5 Powder

DyMn2O5

Z. Y. Zhao et al., Sci. Rep. 4, 3984 (2014).

Synchrotron based X-ray diffraction:

• No additional reflections.

• Retains Pbam space group

at 3K.

• No exchange striction effect.

• Successive transitions are of magnetic /electric origin.

Neutron Diffraction:

Ionic displacements:

Reduction of stretching in Mn4+O6 octahedra at 22K: Possible influence on exchange interactions J3 and J4 .

c a

b

O2

O3

O3

O4

O2

O4

O4

O4

O1

O1

Mn4+

Mn3+

(dO4-O4-d O3-O2 )22K < (dO4-O4-d O3-O2 )35K

Neutron Diffraction

Magnetic Satellites:

• Appears below 30 K (Arrow

marks).

• Incommensurate magnetic (ICM) phase(s).

Neutron

Evolution of magnetic propagation vectors:

• ~15K(~T3P) < T < 28K (~T1P): Two ICM

propagation vectors: qM1 = (0.5, 0, 0.4-δ1) qM2 = (0.5, 0, 0.4-δ2) • 4K (~T1P) < T ≤ 15K:

Only qM2 exists. “Lock-in” transition ~15K.

• T ≤ 4K: qM2 and qM3 = (0.5, 0, 0)

associated with Nd3+ order.

Neutron Diffraction

Neutron Diffraction

Magnetic Structure at 15K: (Using FullProf Suite)

• Refinement is extremely difficult in

other T region: multiple q-vectors.

• Spins are in the ab-plane: Similar to other RMn2O5 multiferroics.

• qM2 = (0.5, 0, 0.399±0.002)

• Partial ordering of Nd3+ ions.

Nd3+

Mn3+

Mn4+

Neutron Diffraction

Comparison: TbMn2O5

b

a

G. R. Blake et al., Phys. Rev. B 71, 214402 (2005)

• TbMn2O5: @ 27K, qM = (0.5, 0, 0.25) HoMn2O5: @ 26K, qM = (0.5, 0, 0.25) DyMn2O5: @ 2K, qM = (0.5, 0, 0) • Commensurate qM for all.

• Spins are in the ab plane.

• Presence of multiple phase transitions at:

~30K (ICM), ~26K±2K (FE), ~15K±2K (Lock-in), and ~4K (Nd3+ ions order).

• FE in ICM state: A new observation in RMn2O5 family.

Magnetic structure @ 15K:

Characteristic(neutrons) 300K 35K 28K 22K 10K 2K

D=dO4-O4-d O3-O2 (octahedra) (Å)

0.050(2) 0.046(2) 0.047(2) 0.030(2) 0.034(2) 0.027(2)

(Sqiri)/e (tetrahedra) (Å) 0.018(2) 0.017(2) 0.0176(20) 0.0167(20) 0.016(2) 0.0146(20)

a (Å) 7.513 7.494 7.494 7.494 7.495 7.495

b (Å) 8.618 8.615 8.615 8.615 8.615 8.615

c (Å) 5.703 5.694 5.694 5.694 5.694 5.693

dMn4+-Mn3+ (J3) (Å) 3.399 3.408 3.403 3.41 3.421 3.423

dMn4+-Mn3+ (J4) (Å) 3.608 3.601 3.613 3.613 3.607 3.612

dMn3+-Mn3+ (J3) (Å) 2.9 2.873 2.873 2.869 2.864 2.83

dMn4+-Mn4+ (J1) (Å) 2.77 2.77 2.78 2.78 2.812 2.79

dNd3+-Nd3+ (Å) 5.703 5.694 5.694 5.694 5.694 5.703

Ionic displacements:

• In the ‘proper’ ferroelectrics, structural instability towards the polar state, associated with the electronic pairing, is the main driving force of the transition. • On the other hand, if polarization is only a part of a more complex lattice distortion or if it appears as an accidental by-product of some other ordering, the ferroelectricity is called ‘improper’