Terahertz magneto-electric
excitations in a chiral
compound
We have determined the terahertz spectrum
of the chiral langasite Ba
3
NbFe
3
Si
2
O
14
using
the AILES beamline. Two excitations are
revealed that are shown to have a different
nature. The first one, purely magnetic,
is observed at low temperature in the
magnetically ordered phase and is assigned
to a magnon. The second one persists far
into the paramagnetic phase and exhibits
both an electric and a magnetic activity
at slightly different energies. This magneto-
electric excitation is interpreted in terms
of atomic rotations and requires a helical
electric polarization.
The electric-field control of spins
and the converse magnetic-field control
of electric dipoles inspire a number
of hybrid technologies and motivate
fundamental research on multiferroics
and magneto-electric materials [1].
These magneto-electric couplings can
also have signatures on the elementary
excitations emerging from the ordered
states that prevail in these compounds.
Magnons or phonons, for instance,
correspond to coherent deviations
of the spins or of the atomic positions
out of their equilibrium. Recently
a novel type of excitations was
discovered in multiferroics, the so-called
electromagnons [2]. It is now perceived
as an electric-charge dressing
of magnons. This dressing enables
the electric-field control of magnons and
is thus foreseen to be used in magnonics.
Here we report experimental evidence
of the dual phenomenon, which are atomic
vibrations dressed with currents, hence
magnetically active, that we observe
in the terahertz (THz) spectrum of the
chiral compound Ba
3
NbFe
3
Si
2
O
14
.
THz spectroscopy is particularly well suited
to investigate magneto-electric excitations
since (i) the THz energy range coincides
with that of magnons and phonons and (ii)
the THz wave delivered by the synchrotron
radiation (98% polarised at SOLEIL)
carries both an electric (e) and a magnetic
field (h), so that rotating the sample with
respect to the wave polarization allows
the unambiguous determination of the
activating field. Probing THz excitations
is not an easy task since no intense
sources exist. This region of the spectrum
has only begun to be explored. One recent
way is the use of synchrotron sources.
The AILES beamline at SOLEIL has been
designed with that purpose.
When chirality meets magnetic order,
unusual magnetic and electric properties
often appear, encouraging the search
for novel multiferroic and magneto-
electric phenomena [3]. Ba
3
NbFe
3
Si2O
14
crystallizes within the P321 space group
thus displaying structural chirality (Figure
➊
).
Its remarkable magnetic properties
are due to the Fe
3+
ions [4] arranged
in triangles. Below the Néel temperature
T
N
=27 K, the system develops a chiral
magnetic order with a 120° spin structure
within the Fe triangles that rotates along
the c axis and forms magnetic helices.
Introduction
THz spectroscopy
Ba
3
NbFe
3
Si
2
O
14
, a chiral magnetic compound
PHYSICS AND CHEMISTRY OF CONDENSED MATTER, EARTH SCIENCES
➊
Suggested phase diagram for the
Fe langasite due to the chiral, tubular
structure of the compound. Below
27 K, a helical magnetic order occurs
and magnons can be excited. Below
120K, a helical polarisation sets
in which is responsible for the E1-E2
magneto-electric excitation.
100
SOLEIL
HIGHLIGHTS
2013
