Spin-phonon coupling in melanothallite Cu2OCl2

Article abstract of DOI:10.1063/1.5054928

Since interesting magnetodielectric properties were discovered in terbium-based manganites, the search for new magnetodielectric multiferroic materials with high Curie temperatures and strong magnetodielectric coupling has been intense. Recently, it was reported that synthetic melanothalllite, the oxahalide Cu₂OCl₂, is a spin-driven multiferroic magnetodielectric with Néel temperature around 70 K, which is an exceptionally high critical temperature compared to other spin-driven ferroelectric materials. In this letter, we have probed the spin-phonon coupling in Cu₂OCl₂ by Raman spectroscopy, showing that the incommensurate magnetic structure induces a spin-phonon coupling in accord with the mean field theory and similar to the one observed in several stretching modes of perovskite compounds.

Full text of DOI:10.1063/1.5054928

Spin-phonon coupling in melanothallite Cu OCl
2
2
B. S. Araújo, A. M. Arévalo-López, J. P. Attfield, C. W. A. Paschoal, and A. P. Ayala
Citation: Appl. Phys. Lett. 113, 222901 (2018); doi: 10.1063/1.5054928
View online: https://doi.org/10.1063/1.5054928
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APPLIED PHYSICS LETTERS 113, 222901 (2018)
Spin-phonon coupling in melanothallite Cu OCl
2
2
1
,2
2,3
2
1
1,a)
B. S. Ara uꢀ jo, A. M. Ar eꢀ valo-L oꢀ pez, J. P. Attfield, C. W. A. Paschoal, and A. P. Ayala
1
Departamento de F ꢀı sica, Universidade Federal do Cear aꢀ , Campus do Pici, PO Box 6030, 60.455-900
Fortaleza, Cear aꢀ , Brazil
Centre for Science at Extreme Conditions and School of Chemistry, The University of Edinburgh,
Edinburgh EH9 3JZ, United Kingdom
2
3
University of Lille, CNRS, Centrale Lille, ENSCL, University Artois, UMR 8181 - UCCS - Unit eꢀ de Catalyse
et Chimie du Solide, Lille F-59000, France
(Received 5 September 2018; accepted 4 November 2018; published online 26 November 2018)
Since interesting magnetodielectric properties were discovered in terbium-based manganites, the
search for new magnetodielectric multiferroic materials with high Curie temperatures and strong
magnetodielectric coupling has been intense. Recently, it was reported that synthetic melanothall-
lite, the oxahalide Cu OCl , is a spin-driven multiferroic magnetodielectric with N ꢀe el temperature
2
2
around 70 K, which is an exceptionally high critical temperature compared to other spin-driven fer-
roelectric materials. In this letter, we have probed the spin-phonon coupling in Cu OCl by Raman
2
2
spectroscopy, showing that the incommensurate magnetic structure induces a spin-phonon coupling
in accord with the mean field theory and similar to the one observed in several stretching modes of
perovskite compounds. Published by AIP Publishing. https://doi.org/10.1063/1.5054928
Magnetodielectric multiferroic materials have been
attracting the attention of the scientific community because
of their applications in spintronics, magnetic field sensors,
of CuCl and CuO. The starting materials were ground in an
2
agate mortar and pressed into pellets. Afterward, the pellets
ꢁ
were sintered at 350 C for 24 h. Powder X-ray diffraction
1
–7
memory devices, and photovoltaic solar cells,
after the observation of the photovoltaic effect in
mainly
measurements were performed in a Bruker D8 Advance dif-
fractometer (Cu Ka1). Magnetic properties were analyzed in a
superconducting quantum interference device (SQUID) mag-
netometer (MPMS, Quantum Design). Raman spectroscopy
measurements were performed using a Jobin-Yvon T64000
Triple Spectrometer configured in a backscattering geometry
coupled to an Olympus Microscope model BX41 with a 20ꢂ
long-working distance achromatic objective. The 532 nm emis-
sion line of a solid-state laser was used for spectra excitation.
The laser power was kept below 2 mW to avoid the local heat-
ing of the sample. All slits were set up to achieve a spectral
8
–12
13
BiFeO3
and Bi FeCrO , which focus the attention on the flexoelec-
and other ferric materials such as KBiFe O
2 5
14
2
6
1
tricity of multiferroics, and the recent proposals of multi-
5
16
ferroic clusters.
In magnetodielectric applications, a large magnetodi-
electric coupling is the key feature for devices with enhanced
efficiency. In type-II multiferroics, in which the ferroelec-
tricity is due to the magnetic ordering, this kind of coupling
is natural. However, in contrast to the high magnetodielectric
coupling, usually, type-II multiferroics exhibits low critical
ꢃ1
resolution lower than 1 cm . Temperature-dependent Raman
1
7–20
temperatures,
Recently, Zhao et al. showed that the melanothallite
which have limited their applications.
2
measurements from 40 up to 300 K were carried out by using a
closed-cycle He cryostat in which the temperature was con-
trolled by a Lakeshore temperature controller model 330 with
precision of 0.1 K. The Raman spectra were deconvoluted with
1
2
oxahalide Cu OCl is a spin-driven magnetodielectric mul-
2
2
2
tiferroic, which has a quite high critical temperature when
compared to other type-II multiferroics and whose ferroelec-
33
Voigt functions by means of Fityk software (version 0.9.8).
Cu OCl crystallizes in an orthorhombic structure belong-
ing to the space group Fddd and has lattice parameters
ð Þ ˚ ð Þ ˚
tricity emerges at T ꢀ 70 K. It was also observed, using
2
2
N
powder neutron diffraction measurements, that the magnetic
structure is incommensurate below TN, which is in disagree-
ment with the previously reported pyrochlore-like all-in-all-
a ¼ 7:469ð2Þ A; b ¼ 9:597 2 A; and c ¼ 9:700 2 A with
˚
34
8 molecules per unit cell. In this structure, the Cu ion is
coordinated by two oxygen and four chlorine ions. CuO Cl
2
23
out spin structure. This spin structure shows that Cu OCl2
2
2
is, in fact, a spin-induced multiferroic material.
squares form chains running along ½110ꢄ and equivalent
Since the magnetodielectric coupling in multiferroic
materials is usually driven by the spin-phonon coupling,
Raman spectroscopy has been frequently applied to investi-
directions. Such squares are the basis of strongly distorted
octahedra with two Cl ions located at the apical
OCl can also
CuO
positions. The structural configuration of Cu
Cl
4
2
2
2
2
4–30
31,32
gate this phenomenon
In this letter, we report a spin-phonon coupling in Cu OCl2
even in type II multiferroics.
be described as O ions tretrahedrally coordinated by Cu ions
forming a corner-sharing 3D network, while chlorine ions
2
at the antiferromagnetic transition undergone by this material
probed by Raman spectroscopy.
Cu OCl polycrystalline samples were synthesized by
are located in the free space between these OCu
[Fig. 1(a)].
tetrahedra
4
The X-ray powder diffraction (XRPD) pattern shown in
Fig. 1(c) confirms that polycrystalline Cu OCl was synthe-
2
2
the solid-state reaction method using an equimolar mixture
2
2
tized. A small amount of a CuO secondary phase was
detected, which is usual for such a synthesis procedure. CuO
a)
Author to whom correspondence should be addressed: ayala@fisica.ufc.br
0003-6951/2018/113(22)/222901/5/$30.00
113, 222901-1
Published by AIP Publishing.

222901-2
Ara uꢀ jo et al.
Appl. Phys. Lett. 113, 222901 (2018)
FIG. 2. Temperature dependence of selected bands of the Raman spectra of
2 2
Cu OCl .
In the absence of structural phase transitions, the
temperature-dependent behavior of a phonon mode of fre-
2
FIG. 1. (a) Crystalline structure of Cu OCl2. (b) Normalized magnetization.
3
c) X-ray powder pattern compared with the reported structure (ICSD-
4
(
quency x is mainly defined by the phonon anharmonicity
according to Balkanski’s model, in which the temperature-
43
96610) and CuO (ICSD-64699) as a minor secondary phase peaks. (d)
Raman spectrum of Cu
processing.
2 2
OCl showing the fitting model used in the data
dependence of the phonon position is given by
ꢁ
ꢀ
2
xðTÞ ¼ x0 þ C 1 þ
ð ð ꢁh x0=2KBTÞ
ꢃ 1
Þ
e
35–37
exhibits two magnetic transitions around 230 and 210 K,
"
#
3
3
but no anomalies that could be associated with this impurity
were observed in magnetic measurements, as shown in the
Fig. 1(b). The CuO impurity was also not detected in the
Raman spectrum [Fig. 1(d)], since this compound has three
þ D 1 þ
þ
;
ð ð ꢁh x0=3KBTÞ
Þ
2
e
ꢃ 1
ðe
ð ꢁh x =3K TÞ
0
B
ꢃ 1Þ
(
1)
ꢃ1
where x , C, and D are the fitting parameters. In this model,
phonons at 290, 350, and 630 cm (Refs. 38–40), which
0
the second term at the right side is related to two-phonon
processes, while the third one is related to three phonons;
higher-order phonon interactions were omitted. A detailed
analysis of the temperature dependence of some phonon
were not observed in our measurements. The magnetic order-
ing temperature (T ) of Cu OCl , determined by the maxi-
mum of the magnetization-temperature slope [Fig. 1(b)], is
N
2
2
21,41
around 68 K in good agreement with previous studies.
Figure 1(d) shows the Raman spectrum of Cu OCl ,
where 8 vibrational modes can be observed, with the most
positions observed in Cu OCl is shown in Figs. 3(a)–3(c).
2 2
2
2
We clearly observe a deviation of the measured phonon
behavior [indicated by circles in Figs. 3(a)–3(c)] from the
one expected in the case of a process only determined by
anharmonic contributions [indicated by solid lines in Figs.
3(a)–3(c)]. The starting point of this effect coincides with the
ꢃ1
intense one located around 550 cm . Based on group theory
analysis, the distribution of the vibrational modes of
Cu OCl in terms of the irreducible representations of the
2
2
D2h group factor at the C point of the Brillouin zone is acous-
tic : B1uꢀB2uꢀB3u, silent: 4A1u, infrared: 4B1uꢀ5B2uꢀ5B3u,
N ꢀe el temperature of Cu
OCl , suggesting a spin-phonon cou-
2 2
4
2
and Raman: Agꢀ2B1gꢀ3B2gꢀ3B3g. Thus, up to 9 modes
of mixed symmetries are expected in the Raman spectra of
the polycrystalline sample in good accordance with our
observation. Figure 2 shows the temperature dependence of
selected bands of the Raman spectra of Cu OCl . As
pling between the magnetic ordering and lattice phonons.
The deviation of a given phonon mode from the anhar-
monic model can be described by taking into account the
44,45
spin-spin correlation function
2
2
Dx ꢅ x ꢃ x0 ¼ khSi ꢆ Sji;
(2)
expected, since Cu OCl does not undergo any structural
2
2
phase transition in the temperature range investigated, the
Raman spectra did not show any remarkable change.
where x is the renormalized phonon frequency due to a spin-
phonon coupling at a fixed temperature, x is the phonon
0

222901-3
Ara uꢀ jo et al.
Appl. Phys. Lett. 113, 222901 (2018)
43
other authors. A departure from the expected behavior
was also observed in the FWHM of the phonons, which are
also plotted in Figs. 3(a)–3(c). FWHM is related to the pho-
51
non lifetime, and when it shows anomalies at the magnetic
transition, the observed spin-phonon coupled is usually
52
direct and not mediated by magnetostriction. Clearly, we
observe anomalies in all analyzed phonons, suggesting that
the observed effects are not mediated by magnetostriction,
53
orbital ordering or other lattice effects.
53
Granado et al. have proposed a mechanism to describe
the spin-phonon coupling by considering a molecular-field
approximation and obtaining a phonon renormalization which
2
scales with M (Ref. 53). This model has been effective to
describe the spin-phonon coupling in several perovskites, e.g.,
5
3
24
LaMnO3, Y NiMnO , Gd(Co Mn )O , La NiMnO ,
25
27
2
6
1/2
1/2
3
2
6
28
30
Nd NiMnO , Gd NiMnO , and Y CoMnO , as well as for
2
6
2
6
2
6
50
54
non-perovskite compounds such as NiO and ZnCr O
2
In
4.
this context, the authors describe the phonon renormalization
due to a spin-phonon coupling as
!
2
2
2
@ J MsublattðTÞ
ðDxÞsꢃph ꢇ ꢃ
;
(3)
mx @u2
4lB
where MsblattðTÞ is the sublattice magnetization, while J is
the superexchange integral and u is the atomic displacement
vector due to the phonon.
In the case of incommensurate magnetic ordering, it is
necessary to extend this model by including nearest-neighbor
(
NN) and next-nearest-neighbor (NNN) interactions, as sug-
55
gested by Kimura et al. Based on this proposal, phonon
renormalization should be rewritten as follows:
X
2
1
@ JðrÞ
Dx ¼ 2
hSi ꢆ Siþri;
(4)
2
mx
@u
r
FIG. 3. Panels (a)–(c): temperature dependence of the phonon energy and
full width at half maximum (FWHM) of selected Raman-active modes of
where x is the phonon frequency, m is the oxygen mass, and
u is the oxygen displacement. For an incommensurate order-
ing with propagation vector k, Laverdi e` re et al. proposed
the spin correlation function as
4
3
2 2
Cu OCl . Solid lines are the fit based on the Balkanski’s model.
26
frequency in the absence of this coupling (due only to anhar-
monic effects), k is the coupling constant, and hSi ꢆ Sji
denotes a statistical-mechanical average for adjacent spins
on opposite sublattices. Hence, in the paramagnetic phase,
there is no spin ordering and the hS ꢆ S i term vanishes,
hSi ꢆ Siþri ¼ KðTÞcosð2pk ꢆ rÞ;
(5)
where KðTÞ is a temperature-dependent prefactor [KðTÞ
sublatt
i
j
2
while in magnetically ordered phases, it is expected that the
spin-phonon coupling generates an additional contribution to
the phonon frequency. In general, k depends not only on the
/ M
].
21
Powder neutron diffraction measurements have shown
that Cu OCl magnetic order rising below T can be described
2
2
N
26
magnetic order but also on the character of the phonon.
A
as incommensurate with propagation vector k ¼ ðk 0 0Þ with
x
systematic trend on the renormalized phonon frequency was
observed in a large number of compounds on which a spin-
phonon coupling was confirmed. Usually, in ferromagnetic
materials, a softening effect on phonon frequencies is
k ¼ 0:827ð7Þ. The magnetic moment is aligned with the Cu-
x
O-Cu chains formed by the OCu4 tetrahedra along the
½61 0 1ꢄ directions. Taking as reference the Cu ion placed at
the lattice origin, two sets of two Cu could be considered as
NN, because their bonds distances are similar: one located at
2
4,27
observed,
a hardening effect.
ter of the k coupling constant, the finding of compounds
whereas in antiferromagnetic systems, there is
ꢂ
ꢃ
2
5,46
1 1
4
ꢁ
˚
However, due to the complex charac-
6 0 4 (Cu-Cu: 3.0403 A; Cu-O-Cu: 102.9 ) being associated
ꢂ
ꢃ
1
1
with the D interaction and the other located at 6
(Cu-Cu: 3.0593 A; Cu-O-Cu: 103.8 ) assigned as the D inter-
1
0
4
4
25,47–50
ꢁ
˚
which are exceptions to this rule is not surprising.
the case of the spin-phonon coupling of Cu OCl , we
In
2
ꢂ
ꢃ
1
1
action. On the other hand, two Cu placed at 6 0 can be
2
2
4
classified as NNN (Cu-Cu: 3.4121 A; Cu-O-Cu: 122.7 ) and
4
˚
ꢁ
observed a phonon hardening due to the coupling, which is
compatible with an antiferromagnetic material as verified by
our magnetic susceptibility measurements and observed by
are associated with D interactions. Thus, the resulting phonon
3
renormalization is

222901-4
Ara uꢀ jo et al.
Appl. Phys. Lett. 113, 222901 (2018)
ꢀ
ꢄ
ꢅꢁ
KðTÞ
pkx
Dx ¼
D1 þ ðD2 þ D3Þcos
;
(6)
mx
2
2
2
where D ¼ @ J =@u ðj ¼ 1; 2; 3Þ.
j
j
According to the spin structure of Cu OCl , cations
2
2
associated with D and D belong to two Cu-O-Cu chains
1
3
with magnetic moments aligned along the ½1 0 1ꢄ direction,
whereas D2 represents a chain along ½1 0 1ꢄ which includes
the cation at the lattice origin (see in Fig. 4). In this way, D1
and D have associated similar bridging parameters, but, as
2
they belong to different chains, they have opposite signs
while D is aligned with D . Notice that this arrangement
3
1
could fulfill the condition jD j ꢈ jD þ D j and, as dis-
1
2
3
2
6
cussed by Laverdi e` re et al., Eq. (6) reduces to Eq. (3), as
proposed by Granado et al.
In addition, the magnetic susceptibility of Cu OCl shows
5
3
2
2
a broad maximum at approximately 140K derived from the
low dimensionality of spin-ordering between CuO Cl square
2
2
41
chains. Okabe et al. determined that this magnetic suscepti-
56,57
bility can be described by a mean-field corrected model
until 85K, implying that Cu OCl is a 3D antiferromagnetic
system with strong intrachain interactions, which remains in
2
2
2
3,41
the absence of long-range ordered magnetic states.
In this
way, the magnetization [MðTÞ] in such a material can no lon-
ger be described exclusively as a consequence of the long
range order, but as a combination of this effect with the intra-
and interchain interactions, as follows:
MTðTÞ ¼ ðv þ v Þ ꢆ H;
(7)
L
C
where v is the susceptibility contribution of the long range
L
ordered magnetic state and vC is the susceptibility from the
CuO Cl low-dimensional interactions. Thus, Eq. (3) could not
2
2
be directly applied to describe the magnetic influence on pho-
non positions of this material. Instead, keeping in mind the dis-
cussed models, we investigate the magnetization dependence
1
=2
1
=2
of the phonon renormalization by plotting ðDxÞ vs M ðTÞ,
FIG. 5. Panels (a)–(c): ðDxÞ of selected modes of Cu OCl as a function
T
2
2
0
of M ðTÞ. The dashed red lines are the linear fit of data.
where MTðTÞ is the magnetization described by Eq. (7).
1=2
Figures 5(a)–5(c) show ðDxÞ as a function of MTðTÞ,
confirming the existence of a temperature range where the
spin-phonon effect scales with the square of the magnetiza-
tion, in good agreement with Granado’s model [Eq. (3)].
However, this model is not valid in the full range where the
multiferroic effect is observed. Just below 70 K, the long
range order does not seem to be the dominant contribution to
the susceptibility and, although the phonon energy deviates
from the standard anharmonic behavior, the quadratic mag-
netization dependence of phonon renormalization starts
below 67 K. Another interesting feature is observed at low
temperatures, because there is a departure from the linear
relationship observed in Fig. 5 below ꢀ55 K. This effect is
related to the upturn of the magnetization at low temperature
(
Fig. 1(b)), which could be ascribed to magnetic impurities.
Thus, the analysis of the spin-phonon results based on
1
=2
ðDxÞ vs M ðTÞ plots describes the validity range of the
T
proposed models for the phonon energy renormalization cor-
rection. This approach is especially useful when the magneti-
zation is not only determined by long-range order but also by
low-dimensional or strongly frustrated spin systems.
Summarizing, in this letter, we reported the spin-phonon
coupling in melanothallite Cu OCl . The phonons harden
FIG. 4. Basal view of OCu
planes and the NN and NNN Cu ions.
4
tetrahedra showing the ½011ꢄ and ½011ꢄ Cu chain
2
2

222901-5
Ara uꢀ jo et al.
Appl. Phys. Lett. 113, 222901 (2018)
2
5
below the magnetic transition due to spin-phonon coupling is
in accordance with the antiferromagnetic ordering of the
sample. Besides the incommensurate magnetic structure, the
observed coupling behaviors are similar to those observed in
stretching phonons of perovskites, whose renormalization
follows the mean field behavior. This finding suggests that
the renormalization due to the spin-phonon coupling depends
only on the spin-phonon interactions and not on the phonon
symmetry.
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