Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂
Neutron scattering studies on powder and single crystals have provided new evidences for unconventional magnetism in Cu₂Te₂O₅Cl₂. The compound is built from tetrahedral clusters of S = 1/2 Cu²⁺ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN = 18.2 K, leads to a very comp...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
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| Cite this: | Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ / O. Zaharko, H.M. Ronnow, A. Daoud-Aladine, S. Streule, F. Juranyi, J. Mesot, H. Berger, P.J. Brown // Физика низких температур. — 2005. — Т. 31, № 8-9. — С. 1068-1072. — Бібліогр.: 17 назв. — англ. |
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Zaharko, O. Ronnow, H.M. Daoud-Aladine, A. Streule, S. Juranyi, F. Mesot, J. Berger, H. Brown, P.J. 2017-06-15T13:32:51Z 2017-06-15T13:32:51Z 2005 Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ / O. Zaharko, H.M. Ronnow, A. Daoud-Aladine, S. Streule, F. Juranyi, J. Mesot, H. Berger, P.J. Brown // Физика низких температур. — 2005. — Т. 31, № 8-9. — С. 1068-1072. — Бібліогр.: 17 назв. — англ. 0132-6414 PACS: 75.30.–m, 61.12.Ld, 75.10.Jm https://nasplib.isofts.kiev.ua/handle/123456789/121701 Neutron scattering studies on powder and single crystals have provided new evidences for unconventional magnetism in Cu₂Te₂O₅Cl₂. The compound is built from tetrahedral clusters of S = 1/2 Cu²⁺ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN = 18.2 K, leads to a very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate (ICM) wave vector k ~ [0.15, 0.42, 1/2]. The Cu²⁺ ions carry a magnetic moment of 0.67(1) μB/Cu²⁺ at 1.5 K and form a four helices spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (Hc ≈ 0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical frustration of the Cu²⁺ spins within the tetrahedra, intra- and inter-tetrahedral couplings having similar strengths and strong Dzyaloshinski–Moriya anisotropy. Candidates for the dominant intra- and inter-tetrahedral interactions are proposed. The work was performed at SINQ, Paul Scherrer Institute, Villigen, Switzerland, at ILL reactor, Grenoble, France. We thank Prof. F. Mila, Dr. M. Prester, and Prof. A. Furrer for fruitful discussions and Swiss NCCR research pool MANEP of the Swiss NSF for financial support. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур К семидесятилетию антиферромагнетизма Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ |
| spellingShingle |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ Zaharko, O. Ronnow, H.M. Daoud-Aladine, A. Streule, S. Juranyi, F. Mesot, J. Berger, H. Brown, P.J. К семидесятилетию антиферромагнетизма |
| title_short |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ |
| title_full |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ |
| title_fullStr |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ |
| title_full_unstemmed |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ |
| title_sort |
incommensurate magnetism in the coupled spin tetrahedra system cu₂te₂o₅cl₂ |
| author |
Zaharko, O. Ronnow, H.M. Daoud-Aladine, A. Streule, S. Juranyi, F. Mesot, J. Berger, H. Brown, P.J. |
| author_facet |
Zaharko, O. Ronnow, H.M. Daoud-Aladine, A. Streule, S. Juranyi, F. Mesot, J. Berger, H. Brown, P.J. |
| topic |
К семидесятилетию антиферромагнетизма |
| topic_facet |
К семидесятилетию антиферромагнетизма |
| publishDate |
2005 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
Neutron scattering studies on powder and single crystals have provided new evidences for unconventional
magnetism in Cu₂Te₂O₅Cl₂. The compound is built from tetrahedral clusters of S = 1/2
Cu²⁺ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN = 18.2 K, leads to a
very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate
(ICM) wave vector k ~ [0.15, 0.42, 1/2]. The Cu²⁺ ions carry a magnetic moment
of 0.67(1) μB/Cu²⁺ at 1.5 K and form a four helices spin arrangement with two canted pairs
within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the
tetragonal plane (Hc ≈ 0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized
by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing
to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical
frustration of the Cu²⁺ spins within the tetrahedra, intra- and inter-tetrahedral couplings having
similar strengths and strong Dzyaloshinski–Moriya anisotropy. Candidates for the dominant
intra- and inter-tetrahedral interactions are proposed.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/121701 |
| citation_txt |
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂ / O. Zaharko, H.M. Ronnow, A. Daoud-Aladine, S. Streule, F. Juranyi, J. Mesot, H. Berger, P.J. Brown // Физика низких температур. — 2005. — Т. 31, № 8-9. — С. 1068-1072. — Бібліогр.: 17 назв. — англ. |
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2025-11-24T06:16:40Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9, p. 1068–1072
Incommensurate magnetism in the coupled spin
tetrahedra system Cu2Te2O5Cl2
O. Zaharko1,H.M. Ronnow1, A. Daoud-Aladine1, S. Streule1, F. Juranyi1,
J. Mesot1, H. Berger2, and P.J. Brown3
1Laboratory for Neutron Scattering, ETHZ & PSI, CH-5232 Villigen PSI, Switzerland
E-mail: oksana.zaharko@pci.ch
2Institute de Physique de la Mati�re Complexe, EPFL, CH-1015 Lausanne, Switzerland
3Institute Laue-Langevin, 156X, F-38042 Grenoble Cedex, France
Received January 25, 2005
Neutron scattering studies on powder and single crystals have provided new evidences for uncon-
ventional magnetism in Cu2Te2O5Cl2. The compound is built from tetrahedral clusters of S = 1/2
Cu2+ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN = 18.2 K, leads to a
very complex multi-domain, most likely degenerate, ground state, which is characterized by an in-
commensurate (ICM) wave vector k � [0.15, 0.42, 1/2]. The Cu2+ ions carry a magnetic moment
of 0.67(1) �B/Cu2+ at 1.5 K and form a four helices spin arrangement with two canted pairs
within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the
tetragonal plane (Hc � 0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized
by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing
to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometri-
cal frustration of the Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings hav-
ing similar strengths and strong Dzyaloshinski–Moriya anisotropy. Candidates for the dominant
intra- and inter-tetrahedral interactions are proposed.
PACS: 75.30.–m, 61.12.Ld, 75.10.Jm
Introduction
Reduced dimensionality, geometrical frustration
and low spin values lead to quantum fluctuations of-
ten resulting in interesting new ground states and spin
dynamics [1]. The most famous examples are based on
triangular units (triangular and kagome lattices [2])
in two dimensions (2D) and tetrahedral clusters
(FCC and pyrochlore lattices [3]) in 3D. The nature
of the ground state in such systems is a subject of cur-
rent strong interest, especially for the extreme quan-
tum mechanical case S = 1/2.
The copper tellurate Cu2Te2O5X2 (X = Cl, Br,
space group P-4) [4] belong to a new family of such
compounds. Their structure can be viewed as a stacking
of layers of Cu4O8Cl4 clusters along c. The four Cu2+
ions within such a cluster, Cu1 (x, y, z), Cu2 (1–x,
1–y, z), Cu3 (y, 1–x, –z), and Cu4 (1–y, x, –z), form
an irregular tetrahedron with two longer (Cu1–Cu2,
Cu3–Cu4) and four shorter edges. The tetrahedra
have a 2D square arrangement within the ab-layers.
The Cu2Te2O5Cl2 system attracted much attention
as the first magnetic susceptibility data fitted well a
model of isolated tetrahedra [4]. The observed broad
maximum between 20–30 K and a rapid drop at lower
temperatures indicated a strength of the intra-tetrahe-
dral coupling J � 38.5 K. Raman spectroscopy, how-
ever, indicated a substantial inter-tetrahedral cou-
pling along the c-axis [5,6] and the data analysis have
been performed [7] in terms of a dimerized model with
the two pairs of Cu2+ spins: Cu1–Cu2 and Cu3–Cu4.
Further magnetic susceptibility and specific heat mea-
surements [5] indicated an onset of antiferromagnetic
(AF) order at TN = 18.2 K, thus confirming the impor-
tance of inter-tetrahedral couplings. Recent neutron
diffraction studies [8] revealed the details of the mag-
netic order: it is incommensurate and very complex.
© O. Zaharko, H.M. Ronnow, A. Daoud-Aladine, S. Streule, F. Juranyi, J. Mesot, H. Berger, and P.J. Brown, 2005
In spite of considerable progress in experimental
studies, the relevant intra- and inter-tetrahedral mag-
netic interactions in Cu2Te2O5Cl2 remain a puzzle due
to the rather complex 3D exchange topology (Fig. 1).
The intra-tetrahedral spin interactions are mediated
via the superexchange paths Cu–O–Cu and can be de-
scribed by J1 and J2 exchange constants. It was sug-
gested [9] according to the Goodenough rules, that the
J2 interaction should be weakly antiferromagnetic if not
ferromagnetic, while J1 is antiferromagnetic and rather
weak [9]. From the band structure calculations [10] it is
expected that within the ab-layers the Ja and Jd
inter-tetrahedral couplings are substantial (see Fig. 1)
and mediated by the halogen p orbitals. Between the
adjacent layers the tetrahedra interact through the
super-superexchange paths Cu–O…O–Cu and the
corresponding inter-tetrahedral couplings are as well
important.
The nature of the magnetic ordering transition is
also still unclear. A strong spin-lattice coupling near
TN has been suggested from magnetic susceptibility
and thermal conductivity studies [11]. However, the
infrared spectroscopy study [12], as well as high-reso-
lution neutron diffraction study [8] gave no evidence
of any lattice distortion.
We present neutron diffraction and inelastic scat-
tering results, which supply new information on the
magnetic ground state, spin-orbit coupling and spin
dynamics. We hope this will provide a better starting
point for theoretical modeling [13–17].
Experimental
High-purity powder and single crystals of
Cu2Te2O5Cl2 were prepared by the halogen vapor
transport technique, using TeCl4 and Cl2 as transport
agents. Neutron powder diffraction (NPD) patterns
were collected in the temperature range 1.5–30 K on
the DMC instrument, with a neutron wavelength of
� = 4.2 � and on the high-resolution HRPT instru-
ment (� = 1.889 �) at SINQ, Villigen, Switzerland.
The neutron single crystal diffraction (NSCD) experi-
ments on two crystals of dimensions 2.5�3�5 mm and
2�3.5�6 mm were carried out using the diffractometers
TriCS at SINQ (� = 1.18 �) and D15 (� = 1.17 �) at
the high-flux ILL reactor, France. The NSCD with ap-
plied magnetic field were performed on a larger 1 cm3
crystal on TriCS (� = 1.18 and 2.4 �) for three field
directions H||a, b and c.
The inelastic neutron scattering experiment was
carried out on 15 g powder on neutron time-of-flight
spectrometer FOCUS (� = 2.8 and 4 �) at SINQ and
on a large 1.5 cm3 crystal on the IN8 thermal neutron
three-axis spectrometer at ILL. IN8 was configured
with doubly focusing monochromator and analyzer
with a 5 cm graphite filter in kf and fixed final neu-
tron energy of 14.7 meV.
Results
Diffraction
Below TN = 18 K tiny magnetic peaks appeared
(Fig. 2) in DMC neutron diffraction patterns of
Cu2Te2O5Cl2, as already reported in [8]. Moreover,
the onset of magnetic order at TN can be followed
from the structural peaks in the HRPT data. The lattice
constant a(b) significantly decreases with temperature
above TN and changes very little below TN (Fig. 2, in-
set). This implies that the spin-lattice coupling is sub-
stantial, but no changes of the crystal structure could
be determined from the neutron patterns.
To determine correctly the magnetic ground state it
is very important to elucidate the magnetic symmetry.
The [001] projection of neutron diffraction pattern of
a typical Cu2Te2O5Cl2 single crystal is presented in
Fig. 3. Up to 16 magnetic satellites of the (0,0,0)
reflection have been observed. The reflection set
denoted by black circles arises from four arms of the
star of the incommensurate (ICM) wave vector
k�[0.150, 0.422, 1/2]. The reflections denoted by
dotted circles correspond to the star of another wave
vector k’�[–0.150, 0.422, 1/2]. The k and k’ vectors
are not related by the symmetry elements of the group
P-4, and could belong to growth crystallographic
twins. Since for several studied crystals the k’ reflec-
Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9 1069
Fig. 1. The xy-projection of the Cu2Te2O5Cl2 crystal
structure (z = – 1/2�1/2) with the J1, J2 and possible
intertetrahedral exchange paths [10]. The double-line seg-
ment is normal to the incommensurate component of the
wave vector k’ [8].
tions are absent, we conclude that the k and k’ sets are
independent. Interestingly, the intensity ratio be-
tween the two first magnetic reflections for the k and
k’ sets is different, implying that the magnetic struc-
tures associated with these two stars are different.
We further tried to clarify if the magnetic structure
is single-k or multi-k. In the case of a single-k mag-
netic structure the k(k’) set contains contributions of
four configuration domains. The configuration do-
mains must all have the same structure but possibly
different populations. Each of them could have two
180 deg domains and two chiral domains. In the case
of multi-k structure, the four arms of the star build
one magnetic structure.
It is possible to distinguish the single-k or multi-k
cases by studying the variation of magnetic intensities
as a function of an applied magnetic field H. We per-
formed such study for fields along a, b, and c crystal
directions. For H along c the intensities of the mag-
netic reflections hkl with l > 0 increase and for l < 0
they decrease, but all reflections persist up to 4 T.
This implies a change of the spin arrangement with re-
spect to the zero-field magnetic structure, but without
domain redistribution and/or meta-magnetic transi-
tion. For H||a a transition occurs at Hc � 0.5 T: inten-
sities of the (±�,±�,±l/2) reflections vanish, while
the magnetic reflections of the type (±�,±�,±l/2)
change their intensities smoothly (Fig. 4). Flipping
the field to –a results in the same behavior. Switching
off the field restores partly the vanished peaks. A si-
milar picture is observed for H||b, but now the
(±�,±�,±l/2) reflections vanish at � 0.5 T. Our re-
sults imply that 0.5 T applied in the tetragonal plane is
enough to depopulate the domains with the propaga-
tion vector nearly normal (90 ± 10 deg) to the field di-
rection. This supports the idea that the magnetic struc-
ture is a single-k and not a multi-k structure.
The model for the k’ magnetic structure has been
recently elaborated in [8]. The only symmetry con-
straint is imposed by the commensurate component of
the wave vector. It implies that the ab-layers of tetra-
hedra alternating along c carry oppositely oriented
spins. The magnetic moments of the four Cu2+ ions in
the unit cell can be independent and a generalized
helix characterizing spin arrangement of each of them
in the crystal is expressed as
S
j
= A cos(k·r
j
+ �) + B sin(k·r
j
+ �).
The spin components are modulated by the wave vec-
tor k, rj is the radius vector to the origin of the j-th
1070 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9
O. Zaharko et al.
N
e
u
tr
o
n
in
te
n
si
ty
, a
rb
. u
n
its
2500
2000
1500
1000
500
20 30 40 50 60 70
2 , deg
4 K
18 K
7.5915
7.5925
6.3180
6.3184
6.3188
0 5 10 15 20 25 30
T, K
TN
a,
Å
b
,Å
Fig. 2. The 18 and 4 K DMC NPD patterns of Cu2Te2O5Cl2 (� = 4.2 �), arrows point to magnetic reflections. Inset:
temperature evolution of the lattice constants from HRPT NPD data (� = 1.889 �).
b
a
Fig. 3. The [001] projection of the reciprocal space of a
Cu2Te2O5Cl2 single crystal. The black circles correspond
to the k magnetic reflections and the dotted circles – to
the k’ set; � = 0.15, � = 0.422.
unit cell. A and B are orthogonal vectors, which de-
fine the magnitude and direction of the axes of the he-
lix, whilst � defines its phase. For the most general
case 27 independent parameters should be considered.
Since the available number of experimental observa-
tions does not allow to refine with confidence all of
them, we imposed physically sound constraints: iden-
tical moment value for all four independent Cu2+ ions
and a circular envelope of the helices. This lowered
the number of independent parameters to 12.
Very good fits were obtained for a model (Fig. 5)
in which the 4 Cu2+ moments in each tetrahedron form
two canted pairs: Cu1–Cu2 and Cu3–Cu4. The two
spins of the pair share a common (A, B) plane, but the
associated helices have different phases �. The differ-
ence between the phases defines the canting angle
between spins of the pair �. The canting angle for the
first pair is �12 = 38(6) deg and for the second pair
�34 = 111(14) deg. The amplitude of the magnetic mo-
ment carried by each Cu2+ ion is 0.67(1) �B at 1.5 K.
It is interesting that the vector sum of the spins of one
pair is the same for all tetrahedra in the crystal
(m12 = 1.27(6) �B, m34 = 0.76(14) �B), whilst the lo-
cal magnetic moment of the tetrahedra is not zero and
changes from one unit cell to another. For an isotropic
exchange the spin state of the tetrahedra would be
zero, so our model might suggest that the two intra-tet-
rahedral couplings J1 and J2 are different and that the
J2 interaction is stronger.
Such a particular magnetic ground state might be a
consequence of one dominant interaction or result from
contributions of several inter-tetrahedral couplings of
similar strengths. We tried to attribute the observed re-
ciprocal wave vector k’ = (–0.15,0.42,1/2) to some
specific exchange inter-tetrahedral path in the struc-
ture and found a simple correlation not to k’, but to
the (–0.15,–0.58,0) vector. As this vector is relateby a
lattice translation to the (–0.15,0.42,0), the two
Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2
Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9 1071
(–0.15, –0.42, –1/2) 0.5 T
0 T
a
( –0.15, –1/2)–0.42,
0.5 T
0 T
b
0 5 10 15 20 25 30 35
0 5 10 15 20 25
Step, �
Step, �
900
800
700
600
500
400
300
200
3000
2500
2000
1500
1000
500
N
e
u
tr
o
n
co
u
n
ts
,a
rb
. u
n
its
N
e
u
tr
o
n
co
u
n
ts
, a
rb
. u
n
its
Fig. 4. The TriCS �-scans of the (–0.15,–0.42,–1/2) (a)
and (0.42,–0.15,1/2) (b) magnetic reflections (� = 2.4 �)
at H = 0 T and H = 0.5 T along a.
Fig. 5. The xy-projection of the Cu2Te2O5Cl2 magnetic
structure with the spin tetrahedra at the z = 0 layer.
0
5
10
20
25
30
35
40
1 2 3 4 5 6 7 8
Energy, meV
In
te
n
si
ty
1.5 K
14 K
18 K
30 K
Fig. 6. Inelastic powder neutron scattering of Cu2Te2O5Cl2
integrated between 0.8 �–1 and 3.3 �–1 for four tempera-
tures between 2 and 30 K. With incident energy Ei = 10.4
meV inelastic focusing was adjusted to provide optimal res-
olution at 5 meV of 0.45 meV (full-width-half-maximum).
vectors mean a different choice of the unit cell of the
same magnetic structure. The ICM component is or-
thogonal to a specific set of planes containing the Cu2+
ions. One of these planes, presented by a double-line
segment in Fig. 1, passes through the Cu2–Cu4 ions of
the adjacent tetrahedra. This corresponds to the Ja
coupling, which according to Ref. 10 is substantial
and is mediated by the halogen orbitals. Based on
these considerations we suggest that Ja is the domi-
nant inter-tetrahedral coupling.
Inelastic scattering
Several inelastic neutron scattering studies have
been performed to investigate the excitation spectrum
of the Cu2Te2O5Cl2 system. Spectra from powder re-
vealed in the ordered phase below TN a spherically
averaged density of states extending to a maximum at
6 meV, above which no significant scattering was de-
tected up to 15 meV (Fig. 6). Raising the temperature
above TN, spectral weight shifted downwards to a
broad quasi-elastic peak.
For single crystal neutron spectroscopy a single
crystal was aligned with (3,1,0) and (0,0,1) in the
horizontal scattering plane, such that (0.42,0.15,1/2)
and the equivalent magnetic Bragg peaks were accessi-
ble given the wide (� 5 deg) vertical resolution. Scans
performed along the Q = (h,h/3,3/2), (0.45,0.15,l)
and (0,0,l) directions revealed two well defined exci-
tation modes. One mode is completely dispersionless
at 6.0 meV and shows no variation in intensity as a
function of Q. The other mode displays strong disper-
sion along both accessible directions from a maximum
energy close to the flat mode down to a minimum en-
ergy gap of 2.1 meV at the same positions in Q as the
ICM magnetic Bragg peaks.
While more detailed modeling of the excitation
spectrum is under way, several important conclusions
can be read directly off the figure. If the system would
be a collection of very weakly coupled tetramers, one
would expect a series of essentially dispersionless
modes. The strong dispersion observed in our measure-
ments imply strong inter-tetrahedral coupling both
within the ab-plane and along the c-axis. Secondly, a
classical ordered magnetic structure with continuous
symmetry of the order parameter should have gapless
spin waves emerging from the magnetic Bragg peaks.
The rather large energy gap must involve strong aniso-
tropy terms in the Hamiltonian, whose origin remains
to be determined.
Summary
The presented results of neutron diffraction and in-
elastic neutron scattering evidence new details of the
magnetic ordering in the Cu2Te2O5Cl2 compound.
The idea of a single-k magnetic structure is strongly
supported by the observed magnetic domains redistri-
bution in an applied magnetic field. The presence of
two different k and k’ structures suggests that a num-
ber of ground states with equal or close energies might
exist. The discovered relation between the incommen-
surate component of the wave vector and the inter-tet-
rahedral coupling Ja invites for a theoretical revision
of the Cu2Te2O5X2 system. The peculiar features of
the spin excitation spectrum deserve further study.
The work was performed at SINQ, Paul Scherrer In-
stitute, Villigen, Switzerland, at ILL reactor, Grenoble,
France. We thank Prof. F. Mila, Dr. M. Prester, and
Prof. A. Furrer for fruitful discussions and Swiss
NCCR research pool MANEP of the Swiss NSF for
financial support.
1. Quantum magnetism, Lect. Notes Phys. 645,
Springer-Verlag, Berlin, Heidelberg (2004).
2. F. Mila, Eur. J. Phys. 21, 499 (2000).
3. A.P. Ramirez, Geometrical Frustration, in: Handbook
of Magnetic Materials 13, Elsevier Science (2001).
4. M. Johnsson, K.W. Tornroos, F. Mila, and P. Millet,
Chem. Mater. 12, 2853 (2000).
5. P. Lemmens, K.-Y. Choi, E.E. Kaul, C. Geibel, K.
Becker, W. Brenig, R. Valenti, C. Gross, M. Johnsson,
P. Millet, and F. Mila, Phys. Rev. Lett. 87, 227201
(2001).
6. C. Gross, P. Lemmens, M. Vojta, R. Valenti, K.-Y.
Choi, H. Kageyama, Z. Hiroi, N.M. Mushnikov, T.
Goto, M. Johnsson, and P. Millet, Phys. Rev. B67,
174405 (2003).
7. J. Jensen, P. Lemmens, and C. Gross, Europhys. Lett.
64, 689(2003).
8. O. Zaharko, A. Daoud-Aladine, S. Streule, J. Mesot,
P.-J. Brown, and H. Berger, Phys. Rev. Lett. 93,
217206 (2004).
9. M.H. Whangbo, H.J. Koo, and D. Dai, J. Solid State
Chem. 176, 417 (2003).
10. R. Valenti, T. Saha-Dasgupta, C. Gros, and H. Rosner,
Phys. Rev. B67, 245110 (2003).
11. M. Prester, A. Smontara, I. Zivkovi’c, A. Bilusi’c, D.
Drobac, H. Berger, and F. Bussy, Phys. Rev. B69,
180401R (2004).
12. A. Perucchi, L. Deiorgi, H. Berger, and P. Millet,
Eur. Phys. J. B38, 65(2004).
13. W. Brenig and K.W. Becker, Phys. Rev. B64, 214413
(2001).
14. W. Brenig, Phys. Rev. B67, 64402 (2003).
15. K. Totsuka and H.-J. Mikeska, Phys. Rev. B66, 54435
(2002).
16. V.N. Kotov, M.E. Zhitomirsky, M. Elhajal, and F.
Mila, Phys. Rev. B70, 214401 (2004).
17. V.N. Kotov, M.E. Zhitomirsky, M. Elhajal, and F.
Mila, J. Phys.: Condens. Matter 16, S905 (2004).
1072 Fizika Nizkikh Temperatur, 2005, v. 31, Nos. 8/9
O. Zaharko et al.
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