Magnetic and acoustic properties of CoCr₂S₄
We report results of magnetic and ultrasound studies of the sulfide spinel CoCr₂S₄, for which the multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been observed at TN = 222 K and in applied magnetic fields evidencing the important role of magn...
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irk-123456789-1753192021-02-01T01:26:44Z Magnetic and acoustic properties of CoCr₂S₄ Felea, V. Cong, P.T. Prodan, L. Gritsenko, Y. Wosnitza, J. Zherlitsyn, S. Tsurkan, V. Специальный выпуск. К 80-летию со дня рождения А.И. Звягина We report results of magnetic and ultrasound studies of the sulfide spinel CoCr₂S₄, for which the multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been observed at TN = 222 K and in applied magnetic fields evidencing the important role of magnetoelastic interactions in this material. In contrast, no anomalies have been detected at TC = 28 K, where a spontaneous electric polarization and isostructural distortions have been reported. We have extracted the H–T phase diagram of CoCr₂S₄ from our experiments for magnetic fields applied along the ⟨111⟩ direction. We discuss our observations in relation to our earlier results obtained for the oxide multiferroic spinel CoCr₂O₄. 2017 Article Magnetic and acoustic properties of CoCr₂S₄ / V. Felea, P.T. Cong, L. Prodan, Y. Gritsenko, J. Wosnitza, S. Zherlitsyn, V. Tsurkan // Физика низких температур. — 2017. — Т. 43, № 11. — С. 1618-1621. — Бібліогр.: 25 назв. — англ. 0132-6414 PACS: 72.55.+s, 62.65.+k, 75.50.Ee http://dspace.nbuv.gov.ua/handle/123456789/175319 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Специальный выпуск. К 80-летию со дня рождения А.И. Звягина Специальный выпуск. К 80-летию со дня рождения А.И. Звягина |
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Специальный выпуск. К 80-летию со дня рождения А.И. Звягина Специальный выпуск. К 80-летию со дня рождения А.И. Звягина Felea, V. Cong, P.T. Prodan, L. Gritsenko, Y. Wosnitza, J. Zherlitsyn, S. Tsurkan, V. Magnetic and acoustic properties of CoCr₂S₄ Физика низких температур |
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We report results of magnetic and ultrasound studies of the sulfide spinel CoCr₂S₄, for which the multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been observed at TN = 222 K and in applied magnetic fields evidencing the important role of magnetoelastic interactions in this material. In contrast, no anomalies have been detected at TC = 28 K, where a spontaneous electric polarization and isostructural distortions have been reported. We have extracted the H–T phase diagram of CoCr₂S₄ from our experiments for magnetic fields applied along the ⟨111⟩ direction. We discuss our observations in relation to our earlier results obtained for the oxide multiferroic spinel CoCr₂O₄. |
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Article |
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Felea, V. Cong, P.T. Prodan, L. Gritsenko, Y. Wosnitza, J. Zherlitsyn, S. Tsurkan, V. |
| author_facet |
Felea, V. Cong, P.T. Prodan, L. Gritsenko, Y. Wosnitza, J. Zherlitsyn, S. Tsurkan, V. |
| author_sort |
Felea, V. |
| title |
Magnetic and acoustic properties of CoCr₂S₄ |
| title_short |
Magnetic and acoustic properties of CoCr₂S₄ |
| title_full |
Magnetic and acoustic properties of CoCr₂S₄ |
| title_fullStr |
Magnetic and acoustic properties of CoCr₂S₄ |
| title_full_unstemmed |
Magnetic and acoustic properties of CoCr₂S₄ |
| title_sort |
magnetic and acoustic properties of cocr₂s₄ |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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2017 |
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Специальный выпуск. К 80-летию со дня рождения А.И. Звягина |
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http://dspace.nbuv.gov.ua/handle/123456789/175319 |
| citation_txt |
Magnetic and acoustic properties of CoCr₂S₄ / V. Felea, P.T. Cong, L. Prodan, Y. Gritsenko, J. Wosnitza, S. Zherlitsyn, V. Tsurkan // Физика низких температур. — 2017. — Т. 43, № 11. — С. 1618-1621. — Бібліогр.: 25 назв. — англ. |
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Физика низких температур |
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11, pp. 1618–1621
Magnetic and acoustic properties of CoCr2S4
V. Felea1, P.T. Cong2, L. Prodan1, Y. Gritsenko2,3, J. Wosnitza2,3,
S. Zherlitsyn2, and V. Tsurkan1,4
1Institute for Applied Physics, Academy of Science of Moldova, Chisinau MD-2028, Republic of Moldova
2Hochfeld-Magnetlabor Dresden (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, Dresden D-01314, Germany
E-mail: s.zherlitsyn@hzdr.de
3Institut für Festkörperphysik, Technische Universität Dresden, Dresden D-01062, Germany
4Experimental Physics 5, Center for Electronic Correlations and Magnetism, Institute of Physics,
Universität Augsburg, Augsburg D-86159, Germany
Received May 25, 2017, published online September 25, 2017
We report results of magnetic and ultrasound studies of the sulfide spinel CoCr2S4, for which the
multiferroicity has recently been suggested. Clear anomalies in the magnetic and acoustic properties have been
observed at TN = 222 K and in applied magnetic fields evidencing the important role of magnetoelastic interac-
tions in this material. In contrast, no anomalies have been detected at TC = 28 K, where a spontaneous electric
polarization and isostructural distortions have been reported. We have extracted the H–T phase diagram of
CoCr2S4 from our experiments for magnetic fields applied along the <111> direction. We discuss our observa-
tions in relation to our earlier results obtained for the oxide multiferroic spinel CoCr2O4.
PACS: 72.55.+s Magnetoacoustic effects;
62.65.+k Acoustical properties of solids;
75.50.Ee Antiferromagnetics.
Keywords: sulfide spinel CoCr2S4, magnetic and acoustic properties, electric polarization.
Frustrated magnetic systems are of great interest exhibiting
exotic magnetic states and unusual cooperative phenomena. In
three dimensions, a typical example of such frustrated systems
is a pyrochlore-lattice material with the magnetic ions located
on the vertices of corner-sharing tetrahedra and having com-
peting magnetic interactions. Spinels with Cr3+ ions, ACr2X4
(A = Mn, Cd, Co, Zn and X = O, S, Se) are ideal candidates
for studying various magnetic frustration effects. For these
spinels a strong spin-phonon coupling frequently leads to
clearly visible spin-lattice effects [1–10]. The oxide spinels
are geometrically frustrated with a direct antiferromagnetic
(AFM) exchange of the order of a few hundred Kelvin,
whereas the sulfide and selenide spinels are bond frustrated
due to a competition between the direct AFM exchange and
the 90° ferromagnetic (FM) exchange interactions, still having
an AFM ground state [11].
Remarkable example of the oxide spinels is CoCr2O4.
This material exhibits a number of unique magnetic states
including commensurate and incommensurate spin configu-
rations, a high-field phase with a disordered transverse com-
ponent of the magnetization above 43 T, and a huge metasta-
ble region in the H–T phase diagram [3]. Furthermore, a
spontaneous dielectric polarization appears in the incom-
mensurate-spiral state below 27 K [12]. This multifer-
roicity in CoCr2O4 was assigned to the inverse Dzyalo-
shinskii–Moriya interaction based on the fact that the ob-
served modulations of the spins and the lattice have the
same wave vector [13].
A closely related material is the sulfide counterpart
CoCr2S4, which has a cubic structure ( 3 )Fd m with Cr3+
(3d3, S = 3/2) ions situated at octahedral positions and
Co2+ (3d7, S = 3/2) occupying tetrahedral sites. CoCr2S4
undergoes a ferrimagnetic transition at TN = 222 K in a
state with the Co2+ and Cr3+ sublattices aligned antiparallel
to each other since the Co–Cr interactions are stronger than
the Co–Co or Cr–Cr ones [14]. The magnetic properties of
CoCr2S4 have been studied in Refs. 14–16. Hydrostatic
pressure results in a linear increase of the ordering tempera-
ture by 0.5 K/kbar due to the increase of the dominant nearest-
neighbor Co–Cr superexchange interaction [17], hinting at
the important role of the spin-strain coupling in this mate-
rial. Recently, in polycrystalline CoCr2S4 polar order be-
low 28 K has been reported [18] suggesting multiferroicity
of this material with a spontaneous electric polarization 60
times larger than that in the oxide spinel CoCr2O4. Moreo-
ver, a strong magnetoelastic coupling at TN and isostructural
© V. Felea, P.T. Cong, L. Prodan, Y. Gritsenko, J. Wosnitza, S. Zherlitsyn, and V. Tsurkan, 2017
Magnetic and acoustic properties of CoCr2S4
distortions at 28 K, enhanced by external magnetic fields,
have been suggested based on synchrotron powder-
diffraction studies [18]. The occurrence of polar order in-
volves the expansion of the Co tetrahedra and contraction
of the Cr octahedra in the spinel structure. The appearance
of a spiral spin order below 28 K has been suggested as
well [18].
In this work we report results of magnetization and ul-
trasound experiments on CoCr2S4 single crystals. We
compare our observations with those of the closely related
multiferroic spinel CoCr2O4 [3]. We further investigated
the role of the magnetoelastic couplings on the physics of
CoCr2S4. The ultrasound technique is a sensitive tool for
detecting magnetoelastic couplings and structural phase tran-
sitions [19]. Spin-lattice effects originate mainly from the
exchange-striction mechanism caused by the renormalization
of inter-atomic magnetic interactions due to spin-phonon cou-
pling. In this case the sound velocity and sound attenuation
changes are related to the magnetization and non-uniform
magnetic susceptibilities with the renormalization being pro-
portional to the spin-phonon coupling constants [2,6,20,21].
High-quality CoCr2S4 single crystals were grown by
chemical transport reactions. The phase purity of the sample
was checked by x-ray analysis. The shape of the single crys-
tal allowed us to propagate acoustic waves along the <111>
direction. Two opposite (111)-crystal surfaces were pol-
ished for the ultrasound experiments. The sample thick-
ness along the <111> direction was 1.28 mm. The elastic
properties were studied by measurements of the velocity
and attenuation of longitudinal waves with the wave vec-
tor k and polarization u parallel to the <111> axis, which
for a cubic crystal correspond to the elastic constant
11 12 44= ( 2 4 ) / 3Lc c c c+ + . A phase-sensitive detection
technique based on a pulse-echo method [21] was used. The
sound velocity ( )k,uv is related to the elastic modulus via
2= [ ( )]ijc ρ k,uv , where ρ is the mass density of the crystal
and 0 0/ [ ( , –) /]T H∆ =v v v v v , with the sound velocity 0v
at the initial value of the external parameters T and H. Note,
that cL is a pure acoustic mode which involves all three elas-
tic constants of the cubic crystal. Wide-band polyvinylidene
fluoride (PVDF) films were used to generate and detect
longitudinal acoustic waves. The magnetic field was applied
along the <111> direction. The measurements in static mag-
netic fields up to 6 T were performed for temperatures be-
tween 1.5 and 300 K. Temperatures down to 1.5 K were
reached by use of 4He cryostats placed inside of a 20 T super-
conducting magnet or a 65 T pulsed magnet with the total
pulse duration of ∼ 150 ms [22,23]. RuO2 and PT100 ther-
mometers were thermally coupled to the sample. The magnet-
ization was measured by use of a SQUID magnetometer
(Quantum Design MPMS-5) in static fields up to 5 T.
The inset of Fig. 1 shows the temperature dependence
of the magnetization obtained below 400 K in a magnetic
field of 50 G under field-cooled (FC) and zero-field-cooled
(ZFC) conditions. A finite spontaneous magnetization ap-
pears below TN = 222 K with a different response for the
FC and ZFC conditions. Such magnetization anomalies are
typical for the ferrimagnetic ordering reported in CoCr2S4
at about the same temperature earlier [14,15]. Similar magnet-
ization results have been previously obtained for a
polycristalline sample [18] as well as for a single crystal [16].
No magnetization anomalies have been observed down to 2 K
neither in our investigation nor in previous works [15,16],
although clear anomalies have been detected in CoCr2O4 at
the transition into the incommensurate spiral-spin state at
TS ≈ 27 K with a spontaneous dielectric polarization [3].
The main panel of Fig. 1 shows the field-induced magneti-
zation obtained at 2 K. The magnetization saturates above
≈ 1 T at the level of 2.7 Bµ per formula unit. This is in a
good agreement with the calculated saturation magnetiza-
tion of 2.4–2.7 Bµ per formula unit with 3.3–3.6 Bµ of
Co2+ opposing about 6 Bµ on the Cr3+ ions [15].
Figure 2 shows the temperature dependence of the sound
velocity for the acoustic mode cL in CoCr2S4. Due to usual
anharmonic contributions, there is a characteristic increase
in the sound velocity by lowering the temperature. The
sound velocity exhibits a slope change (kink) at TN with an
increased stiffness appearing in the ordered state. These
features are clear manifestations of the magnetoelastic in-
teractions in CoCr2S4. Moderate magnetic fields shift TN to
higher temperatures broadening the corresponding sound-
velocity anomaly (inset of Fig. 2). No acoustic anomaly
has been detected at 28 K where a change of magnetic
structure, polar order, and an isostructural distortion have been
suggested [18]. This is puzzling, since the acoustic properties
are very sensitive to structural and magnetic transitions as
shown in various spinels [2–6]. The specific heat results
obtained in polycrystalline CoCr2S4 do not show any anomaly
at 28 K as well [24]. Note, that a pronounced anomaly in the
Fig. 1. Magnetization of CoCr2S4 vs magnetic field measured at
2 K for the magnetic field applied along the <111> direction.
The inset shows the temperature dependence of the magnetiza-
tion measured in a magnetic field of 5 mT applied along the
<111> direction. Results obtained for the FC and ZFC condi-
tions are shown. The ferrimagnetic ordering at TN = 222 K is
marked by an arrow.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1619
V. Felea, P.T. Cong, L. Prodan, Y. Gritsenko, J. Wosnitza, S. Zherlitsyn, and V. Tsurkan
sound velocity has been observed in CoCr2O4 at the transition
into the incommensurate spiral-spin state at TS ≈ 27 K, where
a spontaneous dielectric polarization appears [3]. Moreover, a
strong softening of cL of about 4% has been observed as a
precursor of the multiferroic state in CoCr2O4. One may
speculate whether another acoustic mode might be stronger
coupled to the spin subsystem of CoCr2S4 than cL.
The field dependence of the acoustic properties at 4 K
is shown in Fig. 3. The sound velocity exhibits a sharp,
jump-like increase at ∼ 0.5 T followed by a shallow mini-
mum at about 1.2 T. A negligibly small hysteresis has been
observed near 0.5 T. The sound attenuation increases in the
same field range, passes through a sharp maximum at 0.75
T, and levels off above 1.5 T. The attenuation maximum
coincides with the decrease in /∆v v after the jump-like
increase. The oscillations in the acoustic properties above 2
T are apparently the experimental artefacts. At higher
fields, a softening of the acoustic mode occurs above ∼ 6 T
such that cL has approximately the same stiffness at 0 and
62 T (inset of Fig. 3). The observed softening at high mag-
netic fields is most probably related to the Zeeman splitting
of the two low-lying spin-doublets [14] and related level
crossovers. These observations evidence that the high field
acoustic properties of CoCr2S4 are quite different from
those of CoCr2O4 [3].
Figure 4 shows the sound velocity versus magnetic field
measured at selected temperatures. The jump in the sound
velocity survives in the ordered state at least up to 200 K.
Note that the total change of the sound velocity in the magnet-
ic field of about 10–3 is much less than the velocity change
from room temperature down to 1.5 K in zero magnetic field,
∼ 3·10–2. Interestingly, the data obtained at 20 and 92 K are
quite similar in spite of the suggested polar and spiral spin
order below 28 K in this material [18].
In Fig. 5, we plot the H–T phase diagram with the posi-
tions of the anomalies extracted from our ultrasound data.
The black squares are from the jumps in the sound velocity
(Fig. 4) and the blue triangles reflect the temperature depend-
ent kinks in /∆v v in applied magnetic fields (arrows in
Fig. 2). The ordering temperature, TN = (222±2) K at H = 0,
is extracted from the magnetization (inset of Fig. 1) and the
sound velocity (Fig. 2) as well. The magnetic field shifts TN
to higher temperatures with ∼ 4 K/T (for 0Hµ < 1 T).
In conclusion, we have studied the magnetic and acoustic
properties in the bond-frustrated antiferromagnet CoCr2S4
Fig. 2. (Color online) Temperature dependence of the sound-
velocity changes, ∆v/v, in CoCr2S4 for the longitudinal cL mode
measured in zero magnetic field. The inset shows the data near TN
(marked by arrows) in magnetic fields applied along <111>. The
data for different magnetic fields are shifted along the y axis for
clarity. The ultrasound frequency was 60 MHz.
Fig. 3. (Color online) Field dependence of the sound velocity (a)
and sound attenuation (b) of the mode cL in CoCr2S4 at 4 K. Ar-
rows indicate the field-sweep directions. The magnetic field was
applied along <111>, the ultrasound frequency was 53.6 MHz.
The inset shows the sound-velocity change for the same acoustic
mode measured in pulsed magnetic fields up to 62 T at the same
temperature and frequency as in the main figure.
Fig. 4. (Color online) Field dependence of the sound velocity of
the mode cL in CoCr2S4 measured at selected temperatures. The
data for various temperatures are shifted along the y axis for clari-
ty. The ultrasound frequency was 60 MHz. The magnetic field
was applied along the <111> direction.
1620 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11
Magnetic and acoustic properties of CoCr2S4
by means of magnetization and ultrasound experiments.
The magnetoelastic coupling plays an important role in this
sulfide spinel leading to the renormalization of the acoustic
properties at the magnetic phase transition. The magnetization
and acoustic anomalies are clearly seen at TN with an addi-
tional elastic stiffness appearing in the ordered state. Mag-
netic fields applied along <111> increase the ordering
temperature. A jump-like anomaly has been observed in the
sound velocity at ∼ 0.5 T indicating a field-induced phase
transition. A smooth increase in the magnetization at about
the same fields might be related to the change of the domain
structure. However, the sound-velocity anomaly cannot be
explained by change of the domain structure alone. The
nature of this transition should be subject of further inves-
tigations. The shallow minimum in the sound velocity
above 0.7 T is probably due to the strain interactions with
the spin waves [25]. The obtained results are significantly
different than those for the closely related multiferroic
CoCr2O4 [3]. All ultrasound anomalies revealed in the
sulfide spinel CoCr2S4 are much weaker than in the oxide
counterpart CoCr2O4 [3]. No indication for an isostructural
phase transition and change of the magnetic structure, as
suggested in Ref. 18, has been observed in the magnetic
and elastic properties of CoCr2S4.
We acknowledge the support of HLD at HZDR, member
of the European Magnetic Field Laboratory (EMFL). The
research has been supported by the DFG via TRR 80 (Augs-
burg-Munich) and SFB 1143 (Dresden) and the project for
young researches 16.819.02.03F (ASM Moldova).
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Fig. 5. (Color online) H–T phase diagram of CoCr2S4 extracted
from ultrasound experiments. The magnetic field is applied along
the <111> direction. PM states for the paramagnetic phase.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 11 1621
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