Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields
Ultrasound and magnetization studies of bond frustrated spinel HgCr₂S₄ are performed as a function of temperature in static magnetic fields. Beside the anharmonic effect, the sound velocity shows pronounced anomaly at the antiferromagnetic (AFM) transition at TN = 23 K with an additional significant...
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Felea, V. Prodan, L. Stefanet, E. Cong, P.T. Zherlitsyn, S. Tsurkan, V. 2018-01-19T18:54:47Z 2018-01-19T18:54:47Z 2017 Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields / V. Felea, L. Prodan, E. Stefanet, P.T. Cong, S. Zherlitsyn, V. Tsurkan // Физика низких температур. — 2017. — Т. 43, № 5. — С. 702-706. — Бібліогр.: 27 назв. — англ. 0132-6414 PACS: 43.35.+d, 62.65.+k, 72.55.+s, 75.50.Ee, 75.60.Jk https://nasplib.isofts.kiev.ua/handle/123456789/129493 Ultrasound and magnetization studies of bond frustrated spinel HgCr₂S₄ are performed as a function of temperature in static magnetic fields. Beside the anharmonic effect, the sound velocity shows pronounced anomaly at the antiferromagnetic (AFM) transition at TN = 23 K with an additional significant increase of the order of 0.5% indicating a strong spin-lattice coupling. External magnetic fields enhance the ferromagnetic (FM) correlations and shift the anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. The constructed H– T phase diagram beside the long-range AFM states reveals the state with induced FM order and regimes with short-range AFM and FM correlations as well. We acknowledge financial support via the institutional project 15.817.02.06F and projects for young researchers 15.819.02.01F. This work was partially supported by DFG via SFB 1143 and via the collaborative research center TRR 80 “From Electronic Correlations to Functionality” (Augsburg, Munich, and Stuttgart). We acknowledge the support of the HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL). en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур К 70-летию со дня рождения С.Л. Гнатченко Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields Article published earlier |
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Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields |
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Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields Felea, V. Prodan, L. Stefanet, E. Cong, P.T. Zherlitsyn, S. Tsurkan, V. К 70-летию со дня рождения С.Л. Гнатченко |
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Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields |
| title_full |
Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields |
| title_fullStr |
Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields |
| title_full_unstemmed |
Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields |
| title_sort |
ultrasound propagation in bond frustrated hgcr₂s₄ spinel in magnetic fields |
| author |
Felea, V. Prodan, L. Stefanet, E. Cong, P.T. Zherlitsyn, S. Tsurkan, V. |
| author_facet |
Felea, V. Prodan, L. Stefanet, E. Cong, P.T. Zherlitsyn, S. Tsurkan, V. |
| topic |
К 70-летию со дня рождения С.Л. Гнатченко |
| topic_facet |
К 70-летию со дня рождения С.Л. Гнатченко |
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2017 |
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English |
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Физика низких температур |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Ultrasound and magnetization studies of bond frustrated spinel HgCr₂S₄ are performed as a function of temperature in static magnetic fields. Beside the anharmonic effect, the sound velocity shows pronounced anomaly at the antiferromagnetic (AFM) transition at TN = 23 K with an additional significant increase of the order of 0.5% indicating a strong spin-lattice coupling. External magnetic fields enhance the ferromagnetic (FM) correlations and shift the anomalies to lower temperatures concomitantly with the reduction of the Néel temperature. The constructed H– T phase diagram beside the long-range AFM states reveals the state with induced FM order and regimes with short-range AFM and FM correlations as well.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/129493 |
| citation_txt |
Ultrasound propagation in bond frustrated HgCr₂S₄ spinel in magnetic fields / V. Felea, L. Prodan, E. Stefanet, P.T. Cong, S. Zherlitsyn, V. Tsurkan // Физика низких температур. — 2017. — Т. 43, № 5. — С. 702-706. — Бібліогр.: 27 назв. — англ. |
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| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 5, pp. 702–706
Ultrasound propagation in bond frustrated HgCr2S4 spinel
in magnetic fields
V. Felea1, L. Prodan1, E. Stefanet1, P.T. Cong2, S. Zherlitsyn2, and V. Tsurkan1,3
1Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028, Chişinǎu, R. Moldova
2Hochfeld-Magnetlabor Dresden (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
3Experimental Physics V, Center for Electronic Correlations and Magnetism, Institute of Physics,
University of Augsburg, D 86159 Augsburg, Germany
E-mail: vtsurkan@yahoo.co.uk
Received October 17, 2016, published online March 24, 2017
Ultrasound and magnetization studies of bond frustrated spinel HgCr2S4 are performed as a function of tem-
perature in static magnetic fields. Beside the anharmonic effect, the sound velocity shows pronounced anomaly
at the antiferromagnetic (AFM) transition at TN = 23 K with an additional significant increase of the order of
0.5% indicating a strong spin-lattice coupling. External magnetic fields enhance the ferromagnetic (FM) correla-
tions and shift the anomalies to lower temperatures concomitantly with the reduction of the Néel temperature.
The constructed H–T phase diagram beside the long-range AFM states reveals the state with induced FM order
and regimes with short-range AFM and FM correlations as well.
PACS: 43.35.+d Ultrasonics, quantum acoustics, and physical effects of sound;
62.65.+k Acoustical properties of solids;
72.55.+s Magnetoacoustic effects;
75.50.Ee Antiferromagnetics;
75.60.Jk Magnetization reversal mechanisms.
Keywords: geometrical frustration, H–T phase diagram, spinel, magnetization.
Ternary chromium oxide and chalcogenide spinels with
the formula ACr2X4 have been intensively studied in last
decades. They manifest unusual phenomena and exotic
ground states including complex spin degrees of freedom,
relaxor multiferroicity and colossal magnetoresistance [1–6].
Strong competition of antiferromagnetic (AFM) and ferro-
magnetic (FM) interactions and geometrical frustration [7]
establish a fascinating phase diagram of ACr2X4 spinels
with complex magnetic ground states [8]. The magnetism in
chromium oxide spinels is governed by a strong nearest
neighbor Cr–Cr AFM exchange within the corner-sharing
tetrahedral network of the magnetic ions, a prototypical ex-
ample of a geometrically frustrated pyrochlore lattice. The
oxide spinels undergo an antiferromagnetic spin order transi-
tion at temperatures TN much lower than the Curie–Weiss
temperature ΘCW characterizing the strength of the dominat-
ing exchange. The magnetic transitions in the chromium
oxide spinels are frequently accompanied by structural dis-
tortions which have been interpreted in terms of a spin-
driven Jahn–Teller (JT) effect [9,10]. In sulfide and selenide
spinels the direct Cr–Cr exchange is less effective and the
indirect Cr–X–Cr exchange becomes more important lea-
ding to a competition of the AFM and FM interactions, or
bond frustration. In the chromium sulfide and selenide
spinels, ZnCr2S4 and ZnCr2Se4, the observed splitting of the
phonon modes on entering into the antiferromagnetic state
[11,12] was attributed to a strong spin-lattice coupling. In
these materials the Cr3+ ions in the octahedral crystal field
reveal a nearly spherical charge distribution with the g factor
close to 2 [13]. Significant spin-phonon coupling was also
found in CdCr2S4 ferromagnet and in HgCr2S4 metamagnet
with the AFM ground state. These two sulfide compounds
are dominated by the strong ferromagnetic exchange.
Altghough no splitting of the phonon modes was detected at
the magnetic phase transitions in CdCr2S4 and HgCr2S4, IR
spectroscopy studies documented significant effects in the
temperature dependences of the plasma frequencies indicat-
ing changes of the nature of the bonds and charge transfer
[14]. Moreover, the specific polar modes in HgCr2S4 reveal
the shifts exactly correlated with the magnetic-field-
© V. Felea, L. Prodan, E. Stefanet, P.T. Cong, S. Zherlitsyn, and V. Tsurkan, 2017
Ultrasound propagation in bond frustrated HgCr2S4 spinel in magnetic fields
dependent magnetization indicating strong magnetodie-
lectric effect [14]. In a view of colossal magnetocapacitance
and relaxor multiferroicity observed in CdCr2S4 and
HgCr2S4 [4,15] this underlines the importance of the spin-
lattice coupling in understanding the nature of the multi-
ferroic behavior of the chromium sulfide spinels. Multifer-
roic materials with concurrent magnetic and ferroelectric
orders are the subject of intensive current theoretical and
experimental investigations. These interesting materials not
only challenge the understanding of the ordering phenomena
in solids but also provide new functionalities in spintronics
since the dielectric and magnetic polarizations can be tuned
by either external magnetic or electric fields [16–19].
In this paper we present the results of the experimental
study of sound propagation in HgCr2S4 spinel aimed to
provide further insight into magnetoelastic coupling of this
compound. The ultrasound technique is known as a power-
ful tool for study of the magnetoelastic properties of solids
being extremely sensitive to structural and magnetic
changes [20].
The material under the study, HgCr2S4, crystallizes in a
normal cubic ( 3 )Fd m spinel structure with diamagnetic
Hg2+ ions occupying the tetrahedral A sites and magnetic
chromium ions (Cr3+, 3d3 with S = 3/2) occupying the oc-
tahedral B sites. At low temperature this compound devel-
ops a long-range AFM spin order in spite of high positive
Curie–Weiss temperature ΘCW = +142 K that indicates the
dominance of strong FM exchange at high temperatures
[21]. Earlier neutron-diffraction investigations at low tem-
peratures and in zero magnetic field revealed a spiral spin
configuration [22] suggesting that HgCr2S4 is an anti-
ferromagnet below a critical temperature of 60 K. A simi-
lar conclusion was provided by the optical studies [23].
Further detailed analysis of the properties of HgCr2S4 by
magnetization, electron-spin resonance, and specific-heat
documented the appearance of strong ferromagnetic fluc-
tuations below 50 K and the occurrence of a complex long-
range antiferromagnetic order only below TN = 22 K [24].
A highly unconventional behavior was observed, which
resembles properties of a noncollinear antiferromagnet and
of a soft ferromagnet depending on temperature and mag-
netic field. It was shown that even weak external magnetic
fields disturb the antiferromagnetic order and strongly en-
hance the ferromagnetic correlations. Further high-resolu-
tion powder neutron diffraction investigations [25] estab-
lished that the long-range incommensurate magnetic order
with propagation vector (0,0,∼0.18) sets in at TN ∼ 22 K,
in agreement with the results of the studies of the macro-
scopic properties in [24]. On cooling below TN, the propa-
gation vector increases and saturates at the commensurate
value (0,0,0.25). The magnetic structure below TN consists
of ferromagnetic layers in the ab plane stacked in a spiral
arrangement along the c axis. The symmetry analysis per-
formed in [25] revealed a point group symmetry in the
ordered magnetic phase of 422 (D4) which is incompatible
with the macroscopic ferroelectricity, indicating that the
spontaneous dielectric polarization observed experimentally
in [15] cannot be coupled to the magnetic order parameter.
We have performed the ultrasound propagation and
magnetic measurements of single crystalline HgCr2S4 spi-
nel aimed to study magnetoelastic coupling and to deduce
the thermodynamic H–T phase diagram.
The single crystals of HgCr2S4 have been grown by the
chemical transport reaction method using the ternary poly-
crystalline material prepared by solid-state reactions and
chlorine as the transport agent. The growth experiments were
performed at temperatures between 850 and 900 °C. Struc-
tural analysis of the grown crystals was performed by x-ray
powder diffraction on crushed single crystalline samples. The
analysis confirmed the single phase composition and the ab-
sence of foreign phases. The x-ray data were analyzed by
standard Rietveld refinement using FULLPROF program
[26]. For the crystallographic structure, the following param-
eters have been refined: the lattice parameter a0 = 10.246 Å,
the sulfur positional parameter x = 0.266 f.c., the three iso-
tropic temperature factors, Biso = (1.16/1.35/2.31), for Hg,
Cr, and S atoms, respectively. RBrаgg is 3.9 %.
The magnetization measurements were done utilizing a
commercial SQUID magnetometer MPMS-5 (Quantum
Design) in fields up to 5 T. The ultrasound-velocity and
attenuation experiments were performed in the temperature
range 2–300 K in static magnetic fields up to 14 T using a
phase-sensitive setup [27].
The temperature dependences of the relative change of
the sound velocity, Δv/v, (frame a) and sound attenuation
(frame b) in HgCr2S4 are presented in Fig. 1 for different
magnetic fields applied along the <111> axis. On decreas-
ing temperature below 100 K in zero magnetic field, the
sound velocity shows a continuous increase indicating a
growing stiffness due to anharmonic effects as usually ob-
served in solids. At 24 K the sound velocity exhibits a
step-like upturn on approaching the magnetically ordered
state followed by a more smooth growth on further de-
creasing temperature. The observed significant value of the
additional upturn in Δv/v of about 0.3 % on entering the
AFM state indicates a strong magnetoelastic coupling. The
transition into the long-range AFM state is accompanied
by a sharp anomaly in the sound attenuation α (Fig. 1(b))
as well. Under the application of a magnetic field of 0.3 T
the temperature dependence of the sound velocity becomes
nonmonotonic. The anomaly in Δv/v at TN is transformed
into a deep minimum that strongly shifts to lower tempera-
tures reflecting the respective suppression of the transition
temperature. The sharp anomaly in the sound attenuation is
also strongly displaced to lower temperatures under the
application of magnetic fields in correlation with ultra-
sound velocity data. Beside the sharp anomalies at TN both
quantities, Δv/v and α, develop an additional broad anoma-
ly which is shifted to higher temperatures with increasing
magnetic fields. In the fields above 1 T the temperature
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 5 703
V. Felea, L. Prodan, E. Stefanet, P.T. Cong, S. Zherlitsyn, and V. Tsurkan
dependence of the sound velocity retains its monotonic
character showing only continuous increase on decreasing
temperatures.
In Fig. 2 the temperature dependences of the magne-
tization and derivative of the magnetization measured in
different magnetic fields are shown. In the lowest field
(0.01 T) the magnetization exhibits nonmonotonic temper-
ature dependence with a strong increase of the magnetiza-
tion below 70 K followed by a broad maximum at 30 K
and a significant decrease at lower temperatures on enter-
ing the long-range AFM ordered state. The transition tem-
perature TN = 23 K corresponds to the maximum of the
derivative of the magnetization, showing a sharp anomaly
at this temperature. With increasing fields this anomaly is
shifted to lower temperatures in correlation with the ultra-
sound data. In magnetic fields above 1 T the temperature
dependence of the magnetization shows only monotonous
increase on decreasing temperatures indicating induced
ferromagnetic state. Beside the sharp anomaly at TN anoth-
er broad anomaly develops in the magnetization at high
temperatures. It corresponds to a minimum in the deriva-
tive of the magnetization at Tmin which shifts to higher
temperatures with increasing magnetic fields. We assume
that at Tmin a balance of the competing ferromagnetic and
antiferromagnetic interactions takes place. The increase of
Tmin with temperature on increasing magnetic field might
be explained by an increase of the FM correlations. We
must additionally notice that the observed considerable
increase of the magnetization in HgCr2S4 under the appli-
cation of moderate magnetic field is unique among the
chromium sulfide spinels.
To get further insight into the evolution of the AFM
state with magnetic fields we studied the field dependences
of the ultrasound and magnetization. The relative change
of the sound velocity, Δv/v, at several temperatures meas-
ured in static magnetic fields is presented in Fig. 3. The
data are shown for increasing and decreasing fields. At 2 K
the acoustic mode shows an initial softening with increas-
ing magnetic field. At a field of 0.6 T, Δv/v reaches a min-
imum followed by an increase with further increasing field.
This anomaly (minimum in Δv/v) manifests a pronounced
hysteresis for field sweeps up and down indicating proba-
Fig. 1. Temperature dependences of the relative change of the
sound velocity, Δv/v, (a) and sound attenuation (b) for HgCr2S4
single crystal measured in different magnetic fields applied along
the á111ñ axis. The vertical arrows mark the magnetic phase tran-
sition at TN in zero field and the anomaly at Tmax. The ultrasound
frequency was set to 62 MHz. Here, k is the wave vector and u is
the polarization of the longitudinal sound wave.
Fig. 2. Temperature dependences of the magnetization (a) and
of the derivative of the magnetization (b) for HgCr2S4 single
crystal measured in different magnetic fields applied along the
á111ñ axis. The vertical arrows mark the magnetic phase transi-
tion at TN in zero field and the anomaly at Tmin. Dashed line is
guide to eye.
704 Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 5
Ultrasound propagation in bond frustrated HgCr2S4 spinel in magnetic fields
bly irreversible transformation of the helical structure by
magnetic fields. With increasing temperature within the
magnetically ordered phase this anomaly shows a strong
shift to lower fields and fully disappears for temperatures
above 23 K. The ultrasound attenuation exhibits here a
maximum that correlates with the respective anomaly in
the sound velocity and experiences the similar evolution
with temperature.
In Fig. 4 the field dependences of the magnetization and
its derivative for different temperatures are presented. At 2 K,
the magnetization curve M(H) is strongly nonlinear showing
a maximal slope between 0.3 and 0.5 T. With increasing
temperature to 20 K, the initial slope of the M = f(H) curve
increases, but at temperatures above 25 K it decreases again
(Fig. 4(b)). However, even at 40 K, M(H) curve is still non-
linear as expected for induced ferromagnetic state.
The anomalies observed in the temperature and field de-
pendences of the acoustic properties and magnetizations
marks the phase boundaries which are plotted in the H–T
phase diagram (Fig. 5). At temperatures below 23 K a long-
range ordered antiferromagnetic state is established in zero
field. The AFM state is continuously suppressed under the
application of a magnetic field as reflected by a shift to
lower temperatures of the anomalies in the acoustic pro-
perties and magnetization. In the fields above 1 T the AFM
state is fully suppressed and an induced ferromagnetic (IFM)
state is formed. Above 40 K, the broad anomalies that de-
velop in the sound velocity, attenuation, and magnetization
Fig. 3. Relative change of the sound velocity, Δv/v, (a) and attenua-
tion α (b) vs magnetic field at different temperatures in HgCr2S4.
The ultrasound frequency was set to 48.4 MHz. The experimental
geometry is the same as in Fig. 1.
Fig. 4. Magnetization (a) and derivative of the magnetization (b)
vs field for HgCr2S4 single crystals. Magnetic field is applied
along the á111ñ axis.
Fig. 5. (Color online) H–T phase diagram of HgCr2S4 spinel. The
vertical arrow marks the transition into the long-range ordered
AFM state. IFM denotes induced FM state; FMC is the state with
dominating short-range ferromagnetic correlations; AFMC is the
regime with dominating short-range antiferromagnetic correla-
tions. The solid lines are guide to eye.
Low Temperature Physics/Fizika Nizkikh Temperatur, 2017, v. 43, No. 5 705
V. Felea, L. Prodan, E. Stefanet, P.T. Cong, S. Zherlitsyn, and V. Tsurkan
mark the high-temperature H–T phase boundary that sepa-
rates the induced FM state and the disordered magnetic state
with short-range FM correlations. This state is clearly differ-
ent from the true paramagnetic state which is established
only at much higher temperatures of ~ 250 K, as can be con-
cluded from the deviation of the magnetic susceptibility
from the Curie–Weiss law [24]. In between 23 and 40 K a
narrow region (colored by grey) marks the regime with the
antiferromagnetic short range correlations.
In conclusion, our detailed ultrasound propagation and
magnetization studies of HgCr2S4 single crystals have re-
vealed significant anomalies in the ultrasound velocity and
attenuation at the magnetic transition into the spin-spiral
AFM state indicating strong magnetoelastic coupling in
this material. The observed evolution of the ultrasound
velocity with temperature and magnetic field resembles the
respective variation of the magnetization. Magnetic field
strongly enhances the ferromagnetic correlations and sup-
presses the antiferromagnetic state. The nonmonotonic
behavior of the magnetization and sound velocity disap-
pears in magnetic fields above 1 T corresponding to the
induced ferromagnetic state. The revealed strong magneto-
elastic coupling in bond frustrated HgCr2S4 spinel must be
taken into consideration in understanding the origin of the
colossal magnetocapacitive effect and relaxor multiferroic
behavior observed in this compound.
Acknowledgments
We acknowledge financial support via the institutional
project 15.817.02.06F and projects for young researchers
15.819.02.01F. This work was partially supported by DFG
via SFB 1143 and via the collaborative research center
TRR 80 “From Electronic Correlations to Functionality”
(Augsburg, Munich, and Stuttgart). We acknowledge the
support of the HLD at HZDR, a member of the European
Magnetic Field Laboratory (EMFL).
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