Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry
The magnetic and magnetoresonance properties of a new single-crystal compound Ni₅(TeO₃)₄Cl₂ are studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5–300 K) made it possible to conclude that Ni₅(TeO₃)₄Cl₂ is a quasi-two-dimensional...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2008
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nasplib_isofts_kiev_ua-123456789-1173922025-02-09T17:56:52Z Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry Gnatchenko, S.L. Kobets, M.I. Khatsko, E.N. Baran, M. Szymczak, R. Lemmens, P. Berger, H. Низкотемпеpатуpный магнетизм The magnetic and magnetoresonance properties of a new single-crystal compound Ni₅(TeO₃)₄Cl₂ are studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5–300 K) made it possible to conclude that Ni₅(TeO₃)₄Cl₂ is a quasi-two-dimensional antiferromagnet with the easy magnetization axis a* directed perpendicular crystallographic plane bc and a magnetic ordering temperature TN≈21 K. The resonance measurements at 4.2 K in wide range of frequencies (25–105 GHz) and magnetic field (up to 200 kOe) permitted us to obtain the frequency–field dependence of AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that the magnetic field directed along the antiferromagnetism axis (H||a*) induces the magnetic phase transition of a spin-flop type which is found to be Hsf≈120 kOe. The magnetic resonance experimental data are described qualitatively in model of the biaxial antiferromagnet. The authors would like to thank Dr. V.I. Fomin and Dr. V.P. Gnezdilov for fruitful discussions. This work has been supported in part by Ukrainian-Russian grant № 8-2008. 2008 Article Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry / S.L. Gnatchenko, M.I. Kobets, E.N. Khatsko, M. Baran, R. Szymczak, P. Lemmens, H. Berger // Физика низких температур. — 2008. — Т. 34, № 8. — С. 798–803. — Бібліогр.: 7 назв. — англ. 0132-6414 PACS: 76.50.+g;75.50.Ee https://nasplib.isofts.kiev.ua/handle/123456789/117392 en Физика низких температур application/pdf Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| topic |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
| spellingShingle |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Gnatchenko, S.L. Kobets, M.I. Khatsko, E.N. Baran, M. Szymczak, R. Lemmens, P. Berger, H. Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry Физика низких температур |
| description |
The magnetic and magnetoresonance properties of a new single-crystal compound Ni₅(TeO₃)₄Cl₂ are
studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature
range (5–300 K) made it possible to conclude that Ni₅(TeO₃)₄Cl₂ is a quasi-two-dimensional antiferromagnet
with the easy magnetization axis a* directed perpendicular crystallographic plane bc and a magnetic
ordering temperature TN≈21 K. The resonance measurements at 4.2 K in wide range of frequencies
(25–105 GHz) and magnetic field (up to 200 kOe) permitted us to obtain the frequency–field dependence of
AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that the magnetic field
directed along the antiferromagnetism axis (H||a*) induces the magnetic phase transition of a spin-flop type
which is found to be Hsf≈120 kOe. The magnetic resonance experimental data are described qualitatively
in model of the biaxial antiferromagnet. |
| format |
Article |
| author |
Gnatchenko, S.L. Kobets, M.I. Khatsko, E.N. Baran, M. Szymczak, R. Lemmens, P. Berger, H. |
| author_facet |
Gnatchenko, S.L. Kobets, M.I. Khatsko, E.N. Baran, M. Szymczak, R. Lemmens, P. Berger, H. |
| author_sort |
Gnatchenko, S.L. |
| title |
Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry |
| title_short |
Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry |
| title_full |
Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry |
| title_fullStr |
Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry |
| title_full_unstemmed |
Magnetic and resonance properties of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂ with frustrated geometry |
| title_sort |
magnetic and resonance properties of the two-dimensional s = 1 compound ni₅(teo₃)₄cl₂ with frustrated geometry |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2008 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/117392 |
| citation_txt |
Magnetic and resonance properties
of the two-dimensional S = 1 compound Ni₅(TeO₃)₄Cl₂
with frustrated geometry / S.L. Gnatchenko, M.I. Kobets, E.N. Khatsko, M. Baran, R. Szymczak, P. Lemmens, H. Berger // Физика низких температур. — 2008. — Т. 34, № 8. — С. 798–803. — Бібліогр.: 7 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
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2025-11-29T03:29:16Z |
| last_indexed |
2025-11-29T03:29:16Z |
| _version_ |
1850093833105178624 |
| fulltext |
Fizika Nizkikh Temperatur, 2008, v. 34, No. 8, p. 798–803
Magnetic and resonance properties
of the two-dimensional S = 1 compound Ni5(TeO3)4Cl2
with frustrated geometry
S.L. Gnatchenko, M.I. Kobets, and E.N. Khatsko
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine
47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: gnatchenko@ilt.kharkov.ua
M. Baran and R. Szymczak
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02–688 Warsaw, Poland
P. Lemmens
Institute for Condensed Matter Physics, TU Braunschweig, D–38106 Braunschweig, Germany
H. Berger
Institute of Physics of Complex Matter, EPFL, Lausanne, Switzerland
Received February 4, 2008, revised April 8, 2008
The magnetic and magnetoresonance properties of a new single-crystal compound Ni (TeO ) Cl5 3 4 2 are
studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temper-
ature range (5–300 K) made it possible to conclude that Ni (TeO ) Cl5 3 4 2 is a quasi-two-dimensional antifer-
romagnet with the easy magnetization axis a* directed perpendicular crystallographic plane bc and a mag-
netic ordering temperature TN � 21 K. The resonance measurements at 4.2 K in wide range of frequencies
(25–105 GHz) and magnetic field (up to 200 kOe) permitted us to obtain the frequency–field dependence of
AFMR spectrum for a field applied along the easy magnetization axis a*. It is shown that the magnetic field
directed along the antiferromagnetism axis (H||a*) induces the magnetic phase transition of a spin-flop type
which is found to be Hsf �120 kOe. The magnetic resonance experimental data are described qualitatively
in model of the biaxial antiferromagnet.
PACS: 76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance;
75.50.Ee Antiferromagnetics.
Keywords: low-dimensional magnet, frustration, magnetization, magnetic susceptibility, AFMR, magnetic
phase transition.
Recent increasing attention of scientists has been fo-
cused on «nonclassical» magnets in which of consider-
able importance are different kinds of topological
inhomogeneities, spin fluctuations, competition and frus-
tration of magnetic interactions, and so on. These pecu-
liarities result in a number of specific features of mag-
netic properties, a new type of excitation, peculiar
magnetic structures, nonlinear effects.
The above peculiarities are most pronounced for a sys-
tem of reduced dimension. Hence, the investigation of
two- and one-dimensional magnets is of obvious interest
and way involve the development of new models for
low-dimensional many-sublattice magnetic systems, es-
pecially as modern chemical technologies make it possi-
ble to produce new compounds of a given lattice symme-
try and dimension.
The paper concerns study of magnetic and resonance
properties of a new recent synthesized compound tellu-
rium oxychloride Ni (TeO ) Cl5 3 4 2 [1], belong to the
Ni (TeO ) X5 3 4 2 (X = Cl, Br, J).
© S.L. Gnatchenko, M.I. Kobets, E.N. Khatsko, M. Baran, R. Szymczak, P. Lemmens, and H. Berger, 2008
The x-ray diffraction analysis [1] shows that
Ni (TeO ) Cl5 3 4 2 is of monoclinic symmetry with the space
group C2/c. The cell parameters are as follows: a =
= 19.5674 �, b = 5.2457 �, c = 16.3084 �, � � 125°29�,
z = 4.
The compound has a clearly pronounced layered struc-
ture formed by the connected corners of [Ni5O17Cl2]
blocks that consist of five face-coupled octahedrons of
nickel. Nickel layers lie in bc plane and form complicated
three angular structure. In such geometry effects of mag-
netic frustration may play significant role.
The previous studies of temperature dependences of
magnetic susceptibility �( )T and optical far IR spectra
(10–100 cm–1) of Ni (TeO ) Cl5 3 4 2 [1,2] demonstrate that
at rather high temperatures the susceptibility can be de-
scribed by the Curie–Weiss law with a negative constant
� � �50 K. This suggests that exchange interaction is
of antiferromagnetic character. The temperature depend-
ence of susceptibility at T � 23 K exhibits an anomaly
which the authors relate to the transition to an antifer-
romagnetic state. The effective g factor determined from
the dependence �( )T was 2.21. In addition, at tempera-
tures below 23 K the authors of Ref. 1 observed the pro-
nounced dependence of magnetic susceptibility on exper-
imental conditions (ZFC and FC dependences �( )T ). The
investigation of magnetic susceptibility �( )T was carried
out on powdered samples. The optical measurements in
magnetic fields made it possible to recognize absorption
electron bands and to calculate the value of spin-flop tran-
sition field that appeared to be equal 100 kOe. It must
be noted, that magnetic and resonant properties of iso-
structural single crystal Ni (TeO ) Br5 3 4 2 were studied in
Refs. 3, 4.
The aim of the work under consideration was to
study comprehensively the static magnetic properties
Ni (TeO ) Cl5 3 4 2 single crystal in wide temperature range,
as well as magnetoresonance properties investigation of
this compound in a low-frequency diapason (25–100 GHz) at
helium temperatures to determine the values of typical
magnetic interaction.
Experimental technique
The magnetization measurements were performed in a
temperature range 5–300 K at a constant magnetic field
up to 50 kOe with the use of a SQUID magnetometer
MPMS 5 Quantum Design. The resonance experiments
were carried out with a pulsed tuned-frequency radio-
spectrometer in a magnetic field up to 200 kOe at helium
temperature.
The sample of Ni (TeO ) Cl5 3 4 2 was a single crystal
plate. The b and c axes lie in plate plane. The sample
mass and the volume were m = 13.927 mg and V �
= 2.6·10–3 cm3, respectively.
Magnetic measurements
The experimental data on temperature dependence of
magnetization in temperature range from 5 to 300 K are
shown in Figs. 1–3. The measurements were carried out
in magnetic fields of 100 and 1000 Oe.
The dependences M(T) obtained for Ni (TeO ) Cl5 3 4 2
permitted us to determine the easy magnetization axis a*
which is perpendicular to the bc plane layers. The high-
temperature region of the temperature dependence of
magnetic susceptibility for the easy magnetic axis from
80 to 300 K can be described by the Curie–Weiss law
�( ) / ( )T C T� �� (see Fig. 2,b) with a negative constant
� � �49 K which is typical of antiferromagnetic exchange
interaction. On further decreasing temperature below
80 K, � continues its monotonic rise reaching its peak at
Tmax � 30 K and then it reduces (Figs. 1 and 2,a). The per-
pendicular susceptibility also have some peculiarity at
temperature 30 K (see Fig. 3). At T � 21K there is a kink
in the parallel susceptibility curve (Fig. 1) while the per-
pendicular one exhibits a local minimum (Fig. 3). We as-
sociate in accordance with [1] anomaly at 21 K with phase
transition in antiferromagnetic state. Anomalies at 30 K
remain unclear. Most probably they related to low
dimensionality of this system, but the ultimate answer
calls for further investigation (for example specific heat).
As mentioned above, the experiments in Ref. 1 re-
vealed that below the magnetic ordering temperature
the temperature behavior of magnetic susceptibility was
dependent on cooling method. The ZFC and FC
dependences measured in fields of 100 Oe, 1 kOe and
10 kOe were substantially different. As for our similar
studies (Fig. 1), we did not observe any dependence on
cooling method. As is evident from the figure, the ZFC
Magnetic and resonance properties of the two-dimensional S = 1 compound
Fizika Nizkikh Temperatur, 2008, v. 34, No. 8 799
0 20 40 60 80 100 120 140
2 0.
2 5.
3 0.
3 5.
4 0.
2.0
2 5.
3.0
3 5.
4.0
T K,
TN
Tmax
M
,
1
0
G
–
2
1
0
4
�
H = 100 Oe; H || a*
ZFC
FC
Fig. 1. The temperature dependence of magnetization and
magnetic susceptibility of the single crystal Ni (TeO ) Cl5 3 4 2.
The measurements were made in H = 100 Oe directed along
the easy magnetization axis a* after the sample cooling down
to 5 K without magnetic field (the ZFC dependence) and in the
field of 100 Oe (the FC dependence).
and FC dependences of parallel susceptibility of the
Ni (TeO ) Cl5 3 4 2 single crystal measured in a field of 100 Oe
show a good coincidence. It must be noted, that analogous
measurements for isostructural Ni (TeO ) Br5 3 4 2 [4] don’t
display any dependence of results from cooling regime. It
may be that the effect observed in Ref. 1 is related to the
powdered sample structure.
Of particular importance for magnetic ordered systems
is the existence of magnetic spin-reorientation phase tran-
sitions. For such transitions to be detected, the field
dependences of magnetization were measured at different
temperatures in an external magnetic field directed along
the easy magnetization axis. The measurement data are il-
lustrated in Fig. 4. As is seen, the field dependences M(H)
exhibit a slight nonlinearity. But at fields up to 50 kOe the
dependences M(H) do not demonstrate any dramatic
changes in magnetization or kinks, suggesting that there
are no magnetic phase transitions in Ni (TeO ) Cl5 3 4 2
within the magnetic field range studied. For comparison,
Fig. 4 shows the dependence M(H) measured at T = 10 K
in a magnetic field perpendicular to the easy magnetic
axis. This dependence is linear in a filed range of 0 to
50 kOe.
When treating the experimental data obtained on the
powdered Ni (TeO ) Cl5 3 4 2 samples, the authors of Ref. 1
suggested that there was a weak ferromagnetic moment in
the antiferromagnetic studies. For such a weak ferromag-
netic moment to be detected in our single crystal
Ni (TeO ) Cl5 3 4 2 the dependences M(H) were measured in
low magnetic field at T � 5 K with an external field ap-
plied along and perpendicular to the easy magnetization
axis. The dependence M(H) at a magnetic field directed
800 Fizika Nizkikh Temperatur, 2008, v. 34, No. 8
S.L. Gnatchenko, M.I. Kobets, E.N. Khatsko, M. Baran, R. Szymczak, P. Lemmens, and H. Berger
0 50 100 150 200 250 300
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
T, K
ZFC
–50 0 50 100 150 200 250 300
0
2.0
4.0
6.0
8.0
1.0
T, K
H = 1000 Oe; H || a*
a
H = 1000 Oe; H || a*
b
M
,
G
1
0
1
/
–
3
�
� �= C/(T – )
C = 0.0378 K; = – 49 K�
Fig. 2. (a) The temperature dependence of magnetization for
Ni (TeO ) Cl5 3 4 2. The measurements were made in the magnetic
field H �1000 Oe directed along the easy magnetization axis
a* after the sample cooling down to 5 K without magnetic
field (the ZFC dependence). (b) The temperature dependence
of inverse magnetic susceptibility calculated from the data
shown in Fig. 2,a.
0 20 40 60 80 100
0.20
0.25
0.30
0.35
0.40
2.0
2.5
3.0
3.5
T, K
ZFC
4.0
M
,
G
TN
Tmax
1
0
4
�H || a*
H a*�
H = 1000 Oe
Fig. 3. The temperature dependence of magnetization and
magnetic susceptibility for the Ni (TeO ) Cl5 3 4 2 single crystal
measured at H �1000 Oe parallel and perpendicular to the easy
magnetization axis a*. Prior to the measurements the sample
was cooled down to 5 K without magnetic field (the ZFC de-
pendence).
0 10 20 30 40 50
5
10
15
20
H, kOe
T = 20 K
T = 15 K
T = 10 K
T = 5 K
H || a*
H a*�
M
,
G
Fig. 4. The field dependences of magnetization for
Ni (TeO ) Cl5 3 4 2 measured at temperatures of 5, 10, 15 and 20 K
in the magnetic field applied along the easy magnetization axis
a*. For comparison, shown also is the dependence M(H) mea-
sured at T �10 K in the field perpendicular to the easy axis.
along the easy magnetization axis is shown in Fig. 5,a.
When measuring, the magnetic field was varied from
–100 to 100 Oe and then from 100 to –100 Oe. As is evi-
dent from the figure, there is no magnetic hysteresis in the
M(H) curve. All the experimental points fit in well
straight line that passes through the point M � 0 at H � 0.
The dependence M H( ) measured at a magnetic field nor-
mal to the easy magnetization axis is illustrated in
Fig. 5,b. In this case the field was varied from 0 to
1000 Oe, and the experimental points also fit in well the
straight line that passes through the origin of coordinates.
Thus, the magnetically ordered single crystal of
Ni (TeO ) Cl5 3 4 2 did not display any weak ferromagnetic
moment either along the easy magnetization axis a* or in
the plane normal to it.
Our experimental data on temperature dependence of
magnetization M(T) were used to estimate the value of ex-
change interaction. The exchange field H e may be esti-
mated in the framework of the molecular field theory [5]
from the phase transition temperature TN . The exchange
field H e can be calculated by the expression:
H
k T
g S S
e
B N
B
�
3
1
( )
, (1)
where k B is the Bolzmann constant, TN = 21 K is the Neel
temperature, S �1 is the spin,
B is the Bohr magneton,
g � 2.2 is the g factor. The calculation gives estimate
value H e � 213 kOe. This value is close to the estimation
obtained in Ref. 2 by the same method.
The above experimental data indicate that the com-
pound Ni (TeO ) Cl5 3 4 2 is a close realization of quasi-
two-dimensional antiferromagnet.
Magnetic resonance measurements
In the paramagnetic state the ground term of a Ni2+ ion
is F (S = 1), and the lowest lying orbital level is a singlet.
Therefore, for low-symmetry crystals there is a triplet
structure with a large initial splitting (up to ten or even
more inverse centimeters) at a zero magnetic field what is
due to the single-ion anisotropy. Experimental study of
magnetic resonance in such systems requires high fre-
quencies and strong magnetic field both in paramagnetic
and magnetically ordered states.
The resonance measurements of the magnetically or-
dered single crystal of Ni (TeO ) Cl5 3 4 2 were carried out in
a frequency region of 25 to 105 GHz at helium tempera-
tures in high magnetic fields. Magnetic fields up to
200 kOe produced by the pulsed technique were much
higher than the magnetic anisotropy field. In our experi-
ments the polarization H h� was used and the external
magnetic field H was applied along the easy magnetiza-
tion axis. These experimental conditions permitted us to
observe two antiferromagnetic resonance (AFMR) lines
corresponding to collinear and spin-flop magnetic phases
and to study the frequency–field dependences of the spec-
trum. The behavior of the frequency–field dependence of
the AFMR spectrum in an external magnetic field di-
rected along the easy magnetization axis is shown in
Fig. 6. As is evident from the figure, the AFMR lines
moves forwards each other with decreasing frequency,
and at � � 30.8 GHz and H � �( )120 2 kOe they are coin-
cident. The value of magnetic field corresponds to the
experimental value of spin-flop transition field. The fre-
quency–field dependence can be described by the theoret-
ical relations for a biaxial antiferromagnet in collinear
and spin-flop phases with an external magnetic field di-
rected along antiferromagnet vector L.
Magnetic and resonance properties of the two-dimensional S = 1 compound
Fizika Nizkikh Temperatur, 2008, v. 34, No. 8 801
–100 –50 0 50 100
–2.0
–1.0
0
1.0
2.0
H, Oe
H, Oe
0 200 400 600 800 1000
0.05
0.10
0.15
0.20
0.25
0.30
a
b
M
,
1
0
G
–
2
T = 5 K; H||a*
T = 5 K; H a*�
M
,
G
Fig. 5. (a) The field dependence of magnetization for the sin-
gle crystal Ni (TeO ) Cl5 3 4 2 measured at T � 5 K in the magnetic
field applied along the easy magnetization axis a*. Points (�)
and (�) correspond measuring with varying magnetic field
from –100 to 100 Oe and from 100 to –100 Oe, respectively.
(b) The field dependence of magnetization for Ni (TeO ) Cl5 3 4 2
measured at T � 5 K in the magnetic field normal to the easy
magnetization axis.
For fields lower than the field of spin-flop transition
H sf the observed AFMR can be given by the expression
[6,7]
( / )
{ [ ( ) ( ) ] },/
� 2
2
1 2
4 2
1 2 2 1
2 1 21
2
2
�
� � �H C C H H C C C C
(2)
where � � g
B/�, C H e1 2� H a1 , C H He a2 22� .
For the field H H H Hsf e a� � �( ) /2 1201
1 2 kOe one
can observe a spin-reorientation magnetic phase transi-
tion followed by the formation of the spin-flop phase. Our
experimental value of the sp³n-flop transition is some-
what higher that calculated in Ref. 2, H sf = 100 kOe, be-
cause the authors of the Ref. 2 experienced difficulties
with precise extrapolation of the transition field by the
initial portion of the frequency–field dependence. For a
biaxial antiferromagnetic the spin-flop transition field in
the general case is equal to the low-frequency gap of
magnon excitation ( / )
�1
2
12� H He a . Hence, knowing
the value of exchange interaction effective field, one can
estimate the magnetic anisotropy field H a1 that equals
34 kOe. Using Eq. (2), we estimate the values of magnon
high-frequency gap and magnetic anisotropy field H a2
and obtain 434 GHz and 47 kOe, respectively. At fields
higher than spin-flop transition the frequency–field de-
pendence is of the form [6,7]
( / )
� 2 2 2� �H H sf . (3)
Equations (2) and (3) give the best fit of the experimental
dependences �(H) for the following parameters values:
H e � 213 k O e , H a1 34� k O e , H a2 47� k O e , H sf �
�120 kOe, �1 371� GÍz, �2 434� GHz, g � 2.21. It is ev-
ident that description is rather qualitative.
Now we can try to compare our experimental data in
0.1–3.5 cm–1 frequency range with optical far IR spectra
(10–100 cm–1) taken from paper [2]. Figure 7 represents
these data for lowest resonant mode below the spin-flop
field H sf . Line labeled as 1 is fitting curve from Ref. 2
based on assumption a collinear spin structure. It is seen
that low-frequency region does not described and spin-
flop field is too small comparing with the experiment. The
line 2 corresponds to our expression (2) with mention-
ed above parameters. In this case we obtain energy
gap 371 GHz (12.37 cm–1) 9% less experimental value
405 GHz (13.5 cm–1). The curve 3 also corresponds to
Eq. (2) when constants are fitted so that we obtain experi-
mental value of energy gap. In this case we must suppose
the spin-flop field H sf = 130 kOe (8% more than experi-
ment). As is seen in all cases there is rather qualitative
agreement with experiment. Evidently the model of two-
sublattice biaxial antiferromagnet is too simple for this
compound to say nothing about rough estimated para-
meters.
Thus, the complex study of magnetic and resonance
properties of the Ni (TeO ) Cl5 3 4 2 single crystal made it
possible to obtain the following results:
1. The investigation of the magnetization of the
Ni (TeO ) Cl5 3 4 2 single crystal in a wide temperature
range demonstrates that the crystal is a quasi-two-dimen-
sional antiferromagnetic with the easy magnetization axis
a* directed perpendicular crystallographic plane bc and
a magnetic ordering temperature TN � 21 K. No a weak
ferromagnetic moment has been observed in the
Ni (TeO ) Cl5 3 4 2 single crystal.
2. The frequency–field dependence of the AFMR
spectrum in Ni (TeO ) Cl5 3 4 2 in external magnetic field di-
rected along the antiferromagnetism axis of the crystal
802 Fizika Nizkikh Temperatur, 2008, v. 34, No. 8
S.L. Gnatchenko, M.I. Kobets, E.N. Khatsko, M. Baran, R. Szymczak, P. Lemmens, and H. Berger
0 40 80 120 160 200 240
80
160
240
320
400
H, kOe
T = 4.2 K, H||a*
�
,
G
H
z
Fig. 6. The frequency–field dependence of AFMR spectrum
for the external magnetic field applied along the easy magneti-
zation axis a* of Ni (TeO ) Cl5 3 4 2 , T � 4.2 K.
0 20 40 60 80 100 120 140
2
4
6
8
10
12
14
H, kOe
1
2
3
T = 4.2 K, H||a*
�
,
cm
–
1
Fig. 7. Summary of our date and date of paper [2] for lowest
resonant mode below the spin-flop field Hsf . The length of the
vertical and horizontal bars indicates estimated error. The
line 1 corresponds fitting curve from Ref. 2, the line 2 and 3
are obtained from our expression (2) with different parameters
sets (explanation see in text).
(H||a*) was studied. It was shown that in H||a* there
occurs a spin-flop magnetic phase transition at H sf �
�120 kOe.
3. The magnetic resonance experimental data can be
described qualitatively in model of the biaxial antiferro-
magnet.
Acknowledgment
The authors would like to thank Dr. V.I. Fomin and
Dr. V.P. Gnezdilov for fruitful discussions.
This work has been supported in part by Ukrainian-
Russian grant ¹8-2008.
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Magnetic and resonance properties of the two-dimensional S = 1 compound
Fizika Nizkikh Temperatur, 2008, v. 34, No. 8 803
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