THz magneto-electric excitations in the chiral compound Ba NbFe Si

THz magneto-electric excitations in the chiral compound Ba NbFe Si

THz magneto‐electric excitations
in the chiral compound Ba
in the chiral
compound Ba3NbFe3Si2O14
2
S. de Brion, L. Chaix, F. Levy‐Bertrand, V. Simonet, R. Ballou, B. Canals, P. Lejay
Institut Néel, CNRS et Université Joseph Fourier, Grenoble , France G. Creff, F. Willaert, J‐B. Brubach
,
,
and P. Royy
SOLEIL / AILES Lorme des Merisiers, France
A. Cano
ESRF, Grenoble, France
1.
2
2.
3.
4.
5
5.
6.
THz spectroscopy
Dynamical magneto‐electric
magneto electric effects: electromagnons
effects: electromagnons
The langasite Ba3NbFe3Si2O14
THz spectroscopy on Ba3NbFe3Si2O14
C
Comparison
i
with
ith neutron inelastic
t
i l ti measurements
t
Model for the magneto‐electric excitation in Ba3NbFe3Si2O14
07/01/2013
1
THz spectroscopy for condensed matter
magnetic excitations
optical phonons
electronic excitations superconducting gaps
Carrier scattering rates
THz
1/7/2013
FIR IR
Sophie deBrion
Vis
2
THz spectroscopy for condensed matter
magnetic excitations
optical phonons
magnetic field
electric field
1/7/2013
Sophie deBrion
3
Dynamical magneto electric effects : electromagnons
Pimenov et al: possible evidence for electromagnons in multiferroic
manganites
it
Nature 2006
TbMnO3: excitation at
: excitation at 25 cm
25 cm‐1
for e//a in the cycloidal
phase.
c
b
+60 cm‐1 for e//a in the cycloidal
phase
+ … quite
it complex!?!
l !?!
Pimenov 2006
Valdes Aguilar 2009
MAGNONS DRESSED WITH ELECTRIC CHARGES EXCITABLE BY A DYNAMICAL ELECTRIC FIELD
07/01/2013
4
DYNAMICAL
ELECTRI FIELD
different
coupling mecanisms
(static/dynamical) …
Dynamical magneto electric effects
MAGNONS DRESSED WITH ELECTRIC CHARGES EXCITABLE BY A DYNAMICAL ELECTRIC FIELD
Pimenov et al: possible evidence for electromagnons
in multiferroic manganites
Nature 2006
c
b
REVERSE EXCITATIONS : ATOMIC VIBRATIONS DRESSED WITH ELECTRICAL CURRENT HENCE MAGNETO ACTIVE
In the chiral compound Ba3NbFe3Si2O14
07/01/2013
5
Fe langasites : structure N
Non centro
t symetric
t i Space group P321 S
P321
Triangular network of Fe3+ triangles (S = 5/2 L =0) stacked along c (D3
(D3 symetry) )
Crystal with
y
right / left
handed
chirality
07/01/2013
6
Fe langasites : magnetic structure below TN=27 K ILL (
ILL (neutrons diffraction)
diff
i )
c
Helices propagating i
along c
120° moments on triangles in the (a, b) plane  magnetic i
frustration
b
a
One single chirality within the triangles
One single chirality within the helices 07/01/2013
K. Marty et al, PRL 2008
7
NbFe langasite
Magnetic properties
Magnetic properties
Ba3NbFe3Si2O14 single crystal
0 020
0.020
M/H (emu/m
mol Fe)
Dielectric properties
Dielectric properties
H c
H  c
0.015
0.010
0.005
0.000
0
50
100
150
200
250
300
T ((K))
AFM order: TN =27 K Weak satic
magneto‐
electric coupling at
coupling at TN
Curie‐Weiss
Curie
Weiss behavior behavior : :
CW = ‐ 170 K 07/01/2013
S. deBrion
8
Fe langasites : Dynamical properties in the THz
range on the AILES beamline at SOLEIL
range on the AILES beamline
Synchrotron Intensity: I0
Synchrotron Intensity: I
Transmitted Intensity: I
Transmitted Intensity: I
sample
electric field e
magnetic field h
d
White beam + Fourier Transform
Transmission : T = I / I0 ≈ exp  d
Transmission : T = I / I0 ≈ exp 
d )) White beam + Fourier Transform Absorbance : Abs= ‐ Ln (T) ≈ d
spectrometer : Abs (frequency)
‐1
q
y
8 cm‐1 < frequency < 100 cm
07/01/2013
9
THz measurements
100 cm-1
10 cm-1
1 meV = 8 cm‐1 = 0.24 THz The AILES beam line at SOLEIL + IFS 125 FT spectrometer + He pumped bolometer + cryostat . ‐1
‐1
07/01/2013 An intense broadband THz source, 8 – 80 cm , res 0.5 cm
10
98 % vertical Polarisation
Fe langasite : THz absorbtion
Larger overall absorption for e // c
Larger
overall absorption for e // c
that always increases with temperature
OK with low frequency dielectric response
OK with low frequency dielectric response
07/01/2013
11
THz relative absorbtion with 6 m Si beam splitter :
07/01/2013
12
THz AILES measurements with 6 m Si beam splitter
07/01/2013
13
THz AILES measurements with 125 m Si beam
splitter
litt
07/01/2013
14
THz / neutron measurements
07/01/2013
15
THz excitations in Ba3NbFe3Si2O14
• Excitations at 13 cm‐1 clearly observed below TN, width 0.8 cm‐1, with THz (h//c) and neutron probes (spin correlation along c) : MAGNONS
(spin correlation
c) : MAGNONS
• excitation at 23 and 29 cm‐1, width 10 and 5 cm‐1,
• observed up to T
p
probe only: y
p ≈4 TN,, with THz p
PHONONS appearing below Tp ≈ 110 K: structural changes.
S. deBrion
07/01/2013
• With different selection rules : one excited for e // c, one for h  c !!!
16
origin of the THz phonon modes D3 symetry: rotations R
rotations
Rz about the z‐axis (c about the z‐axis (c
axis) may produce an electric polarization along c
Pz =(p2)Rz
: coupling constant p : frequency of the phonon frequency of the phonon
associated to Pz
That can be excited by e // c :
ℒe= ‐
= Pz ez
Coupling to the magnetic field: (Dzialoshinskii and Mills 2009)
Coupling to the magnetic field: (Dzialoshinskii
and Mills 2009)
ℒH = gh . (P x dP/dt) = -g (p2)(dh  /dt x
P  )z R z
P  non zero: lost of D3 symetry → helicoidal polarisation.
07/01/2013
17
Hints for the lost of D3 symmetry
Forbidden in perfect
triangular
arrangement of the spins (0,0,0)
(0,0,1)
(0,0,1‐)
c
(0,0,1+)
b
a
07/01/2013
Lost of 3 fold axis
D3 → C2
18
Dispersion law and selection rules
• excitation at 23 cm‐1
excited for e //c
//
•Excitation at 29 cm‐1, excited for for for h 
h  c c
07/01/2013
19
suggested phase diagram
Ginzburg Landau free energy:
Landau free energy:
07/01/2013
20
CONCLUSION
New dual excitation in the chirl langaiste: rotation mode of the lattice whose
magneto‐electric
magneto
electric activity reveals a transition towards
a transition towards a state of helical
a state of helical polarisation
07/01/2013
07/01/2013
S. DE BRION
21
07/01/2013
22
Hints for the lost of D3 symmetry
NbFe lame Y
e//c
8
Abssorbance
6
300 K
4
200 K
100 K
2
55 K
0
10
width (cm-1)
60
40
60
20
30
40
50
60
Freq enc (cm-1)
Frequency
(cm 1)
30
20
40
20
200
Area
a
ffrequency (cm-1
1)
65
freq (cm-1)
TN
exp (T/89)
100
0
55
50
100
150
200
250
300
T (K)
10
100
Temperature ( K )
THz measurements : two localised
phonon modes split below
≈200 K
07/01/2013
23
Manip SOLEIL : polarisation du faisceau
Polarisation:
•verticale (95 – 98 %) dans le THz
•faible (15% horizontale) au
dessus de 700 cm-1
•décroit linéairement entre 100 et
700 cm-1 de 2% tous les 10 cm-1
environ
Lumière polarisée à 98 % dans le THz, champ électrique excatement vertical,
perpendiculaire à l’orbite des électrons dans l’anneau:
magnetic field
k
x
e
electric field
07/01/2013
24
THz absolute / relative absorbtion :
Beamspli

F ( cm )
12
5K
(c)
2
10 K
1
0.2
14 K
TN
(d)
0.3
18 K
24 K
0.0
10
12
14
16
-1
Frequency ( cm )
-1
16
-1
18
5
10
15
Temperature ( K )
Ab (T) Abs (45 K )
Abs (T) –
Ab (45 K )
20 25
0.0
-1
14
W ( cm )
12
ec
h // c
(a)
9K
Extrapolate
ed absorptio
on (mm )
14
0.4
A ((arb. units)
-1
Relative absorption (mm )
16
(b)
-1
(a)
10 K
6K
F ((cm )
Beamsplitter 125 m
1
e  c
hc
16 K
10
20
30
40
F (cm-1)
31 K
55 K
90 K
110 K
0
0.3
ec
hc
(e)
5K
16 K
32 K
100 K
0.0
0.3
ec
(f)
h // c
5K
32 K
16 K
110 K
0.0
10
20
30
40
Frequency (cm-1)
07/01/2013
Abs (T0) –
Abs
(T0) Abs (T) + FFT band block filter→ Abs (T) + FFT band block filter→
extrapolated Abs(T0)
25
Fe langasites : spin wave modeling
Spin correlations along a or b
07/01/2013
S i
Spin correlations
l ti
along
l
c
26
NbFe langasite : inelastic neutron measurements
07/01/2013
M. Loire et al, PRL 2011
27
The langasite Ba3NbFe3Si2O14
Dynamical properties
SOLEIL FIR measurements
500
AbsFe300K
AbsFe200K
AbsFe150K
AbsFe100K
AbsFe60K
AbsFe30K
AbsFe25K
AbsFe20K
absFe13K
F langasite
Fe
l
it
Abs
sorbance (cm-1)
400
300
200
100
0
100
200
300
400
500
Wave number (cm-1)
Multiple phononic bands …
07/01/2013
28
NbFe langasite
Magnetic properties
Magnetic properties
Ba3NbFe3Si2O14 single crystal
0 020
0.020
M/H (emu/m
mol Fe)
Dielectric properties
Dielectric properties
H c
H  c
0.015
0.010
0.005
0.000
0
50
100
150
200
250
300
T ((K))
AFM order: TN =27 K Weak satic
magneto‐
electric coupling at
coupling at TN
Curie‐Weiss
Curie
Weiss behavior behavior : :
CW = ‐ 170 K 07/01/2013
S. deBrion
29
07/01/2013
30