thermodynamic & neutron diffraction studies on
TRANSCRIPT
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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
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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
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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
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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).
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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.
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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.
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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) .
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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)
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Dielectric Permittivity (ε’):
• Sharp peak at T2P. • Signature of
FERROELECTRICITY (FE) • Weak hump around T3P
similar to TbMn2O5 (Electromagnon?) .
T3P T2P
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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).
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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.
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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
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Neutron Diffraction
Magnetic Satellites:
• Appears below 30 K (Arrow
marks).
• Incommensurate magnetic (ICM) phase(s).
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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
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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+
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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.
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• 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.
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Magnetic structure @ 15K:
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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:
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• 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’
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