Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0)
Magnetization studies for FeSe₁–xTex (x≃0, 0.5, and 1.0) compounds were carried out in magnetic fields up to 50 kOe and in the temperature range 2–300 K. The superconducting transition was observed at Tc≃8 K and 13.6–14.2 K in FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, respectively. For the most samples, a nonline...
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| Zitieren: | Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) / A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, A.N. Vasiliev // Физика низких температур. — 2010. — Т. 37, № 1. — С. 100–107. — Бібліогр.: 39 назв. — англ. |
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Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Gnatchenko, S.L. Tsurkan, V.V. Deisenhofer, J. Krug von Nidda, H.-A. Loidl, A. Chareev, D.A. Volkova, O.S. Vasiliev, A.N. 2017-05-30T11:03:10Z 2017-05-30T11:03:10Z 2011 Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) / A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, A.N. Vasiliev // Физика низких температур. — 2010. — Т. 37, № 1. — С. 100–107. — Бібліогр.: 39 назв. — англ. 0132-6414 PACS: 74.70.Xa, 74.20.Pq, 74.25.Ha, 75.30.Cr https://nasplib.isofts.kiev.ua/handle/123456789/118439 Magnetization studies for FeSe₁–xTex (x≃0, 0.5, and 1.0) compounds were carried out in magnetic fields up to 50 kOe and in the temperature range 2–300 K. The superconducting transition was observed at Tc≃8 K and 13.6–14.2 K in FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, respectively. For the most samples, a nonlinear behavior of the magnetization curves in the normal state gives evidence of a commonly observed substantial presence of ferromagnetic impurities in the compounds under study. By taking these impurity effects into account, the intrinsic magnetic susceptibility χ of FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN≃70 K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe within the local spin density approximation. The authors dedicate this work to the 100th anniversary of David Shoenberg, who was a pioneer of lowMagnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) Fizika Nizkikh Temperatur, 2011, v. 37, No. 1 107 temperature physics and studies of electronic structure of solids This work has been supported by the Russian-Ukrainian RFBR-NASU project 43-02-10 and 10-02-90409. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Сверхпроводимость Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| title |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) |
| spellingShingle |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Gnatchenko, S.L. Tsurkan, V.V. Deisenhofer, J. Krug von Nidda, H.-A. Loidl, A. Chareev, D.A. Volkova, O.S. Vasiliev, A.N. Сверхпроводимость |
| title_short |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) |
| title_full |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) |
| title_fullStr |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) |
| title_full_unstemmed |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) |
| title_sort |
magnetic and superconducting properties of fese₁–xtex (x≃0, 0.5, and 1.0) |
| author |
Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Gnatchenko, S.L. Tsurkan, V.V. Deisenhofer, J. Krug von Nidda, H.-A. Loidl, A. Chareev, D.A. Volkova, O.S. Vasiliev, A.N. |
| author_facet |
Fedorchenko, A.V. Grechnev, G.E. Desnenko, V.A. Panfilov, A.S. Gnatchenko, S.L. Tsurkan, V.V. Deisenhofer, J. Krug von Nidda, H.-A. Loidl, A. Chareev, D.A. Volkova, O.S. Vasiliev, A.N. |
| topic |
Сверхпроводимость |
| topic_facet |
Сверхпроводимость |
| publishDate |
2011 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
Magnetization studies for FeSe₁–xTex (x≃0, 0.5, and 1.0) compounds were carried out in magnetic fields up to 50 kOe and in the temperature range 2–300 K. The superconducting transition was observed at Tc≃8 K and 13.6–14.2 K in FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, respectively. For the most samples, a nonlinear behavior of the magnetization curves in the normal state gives evidence of a commonly observed substantial presence of ferromagnetic impurities in the compounds under study. By taking these impurity effects into account, the intrinsic magnetic susceptibility χ of FeSe₀.₉₆₃ and FeSe₀.₅Te₀.₅, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN≃70 K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe within the local spin density approximation.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118439 |
| citation_txt |
Magnetic and superconducting properties of FeSe₁–xTex (x≃0, 0.5, and 1.0) / A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, A.N. Vasiliev // Физика низких температур. — 2010. — Т. 37, № 1. — С. 100–107. — Бібліогр.: 39 назв. — англ. |
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2025-11-25T22:20:34Z |
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2025-11-25T22:20:34Z |
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| fulltext |
© A.V. Fedorchenko, G.E. Grechnev, V.A. Desnenko, A.S. Panfilov, S.L. Gnatchenko, V.V. Tsurkan, J. Deisenhofer, H.-A. Krug von
Nidda, A. Loidl, D.A. Chareev, O.S. Volkova, and A.N. Vasiliev, 2011
Fizika Nizkikh Temperatur, 2011, v. 37, No. 1, p. 100–107
Magnetic and superconducting properties of FeSe1–xTex
(x 0, 0.5, and 1.0)
A.V. Fedorchenko1, G.E. Grechnev1, V.A. Desnenko1, A.S. Panfilov1, S.L. Gnatchenko1,
V.V. Tsurkan2,3, J. Deisenhofer2, H.-A. Krug von Nidda2, A. Loidl2, D.A. Chareev4,
O.S. Volkova5, and A.N. Vasiliev5
1 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: panfilov@ilt.kharkov.ua
2 Experimental Physics 5, Center for Electronic Correlations and Magnetism, Institute of Physics
University of Augsburg, Augsburg 86159, Germany
3 Institute of Applied Physics, Academy of Sciences of Moldova, MD-2028 Chisinau, Republic of Moldova
4 Institute of Experimental Mineralogy, Russian Academy of Sciences
Chernogolovka, Moscow District 142432, Russia
5 Moscow State University, Physics Department, Moscow 119991, Russia
Received October 12, 2010
Magnetization studies for FeSe1–xTex (x 0, 0.5, and 1.0) compounds were carried out in magnetic fields up
to 50 kOe and in the temperature range 2–300 K. The superconducting transition was observed at Tc 8 K and
13.6–14.2 K in FeSe0.963 and FeSe0.5Te0.5, respectively. For the most samples, a nonlinear behavior of the mag-
netization curves in the normal state gives evidence of a commonly observed substantial presence of ferromag-
netic impurities in the compounds under study. By taking these impurity effects into account, the intrinsic mag-
netic susceptibility χ of FeSe0.963, FeSe0.5Te0.5, and FeTe was estimated to increase gradually with Te content.
For FeTe a drastic drop in χ(T) with decreasing temperature was found at TN 70 K, which is presumably re-
lated to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of
the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe within the local spin density
approximation.
PACS: 74.70.Xa Pnictides and chalcogenides;
74.20.Pq Electronic structure calculations;
74.25.Ha Magnetic properties;
75.30.Cr Saturation moments and magnetic susceptibilities.
Keywords: FeSe, FeTe, high-temperature superconductivity, magnetization, electronic structure.
1. Introduction
Following recent discovery of the iron-pnictide high cT
superconductors (SCs) [1,2], a search for the new SCs ra-
pidly extended to a large variety of iron-based planar com-
pounds [3–8]. Among them, iron chalcogenides FeSe1–xTex
are distinguished by their structural simplicity [9]. They
belong to so-called «11»-type iron-based SCs and consist
of the iron-chalcogenide layers with square planar sheets
of Fe in a tetrahedral Se (or Te) environment, maintaining
the same Fe+2 charge state as the iron pnictides. The SC
with modest transition temperature about 8 KcT was ob-
served for Se deficient FeSe compounds [9–11], whereas
partial replacing of Se with Te has provided 15 KcT
at about 50% Te substitution [12,13]. However, the recent
reports on SC of FeSe under high pressures with
27 KcT [14], 34 K [15], 35 K [16], and 37 K [17,18]
have stimulated considerable interest to physical properties
of FeSe1–xTex.
Magnetic and superconducting properties of FeSe1–xTex (x 0, 0.5, and 1.0)
Fizika Nizkikh Temperatur, 2011, v. 37, No. 1 101
The electron-phonon interaction is estimated to be too
small in the iron-based SCs to provide the conventional
superconductivity, and there is growing anticipation that
superconductivity in the iron-based SCs is driven by spin-
fluctuations due to proximity to magnetic instability in
FeSe and related compounds [7,19,20]. The itinerant spin-
density-wave (SDW) transitions were established in parent
compounds of the Fe based SCs, which are resulted in rela-
tively small ordered magnetic moments, and in essentially
non-Curie-Weiss behavior of magnetic susceptibility with
temperature above SDWT [3–6]. On the other hand, the
undoped FeTe compound is not superconducting but mag-
netically ordered [13,21,22]. Moreover, the magnetic struc-
ture found in the FeTe compound is rather different from
that of parent iron-arsenide SC compounds, despite the
same Fermi surface nesting predicted by DFT calculations
[7,19]. It was suggested that the electrons in FeTe1–xSex
system are localized and close to Mott–Hubbard transition,
with the local magnetic moments interacting via short-
range superexchange [23], and the superconductivity is
promoted by a combination of resonant valence bond and
excitonic insulator physics [8].
At the present time, there is considerable controversy
regarding an interplay between electronic structure, mag-
netism and superconductivity in FeSe1–xTex compounds,
and their complex magnetic properties are still not well
characterized and understood. The experimental data on
magnetic susceptibility behavior of FeSe1–xTex systems in
the normal state are still incomplete and contradicting
[12,13,21]. Also, the magnetic behaviors of FeSe1–xTex
systems are presumably related to the presence of magnetic
impurities and secondary phases. Therefore, further studies
of magnetic and superconducting properties and their evo-
lution with doping, pressure, and temperature can help to
elucidate a mechanism of the high- cT superconductivity in
this family of the Fe-based SCs.
In order to elucidate the superconducting mechanism
and its relation with the expected effect of spin fluctua-
tions, it is very important to obtain the intrinsic susceptibi-
lity of the Fe-based SCs. In this contribution we report the
experimental results on magnetic susceptibility studies for
the FeSe1–xTex compounds in the normal state. The main
objective of this study is to reveal and separate magnetic
properties of the parent phase from contributions of secon-
dary phases and impurities. The experimental study is sup-
plemented by ab initio calculations of the electronic struc-
ture and magnetic susceptibility of FeSe and FeTe within
the density functional theory (DFT). Therefore, the aim of
this investigation is to shed more light on the relation be-
tween magnetic properties and the chemical and structural
composition, and also on the interplay between supercon-
ductivity and magnetic instability in the FeSe1–xTex sys-
tem.
2. Experimental details and results
The polycrystalline FeSe 0.963 and FeTe 0.95 samples
were obtained by conventional solid-state synthesis. The
starting chemicals were powder iron (Merck, 99.5%, 10
lm) and crystalline selenium and tellurium cleaned by the
floating zone method. These chemicals were mixed in pro-
portions consistent with the stoichiometry of reaction,
Fe:Se = 1:0.963 and Fe:Te = 1:0.95, sealed in an evacuated
(10 4− bar) silica glass capsule, and annealed at 700 K for
14 days. The reacted mixture was ground in an agate mor-
tar under acetone and then pressed into pellets of 6 mm in
diameter at the load of 1–1.2 tons, followed by annealing
in the evacuated silica glass capsule at 700 K for 20 days.
Both synthesized substances were examined under a mi-
croscope in reflected light and analyzed by x-ray powder
diffraction (XRD, Co Ka radiation, Fe filter) and by elec-
tron microanalysis (CAMECA SX100, 15 kV).
The single crystals with x 0.5 and 1 were grown by
a slow cooling with the self-flux method [24], and two
series of samples have been prepared. The phase content of
the samples was checked by x-ray diffraction. Hereafter,
we will refer to the polycrystalline and single-crystalline
samples as P and S, respectively, followed by the series
number. The dc magnetization studies were carried out in
the magnetic field up to 50 kOe and the temperature range
2–300 K using a superconducting quantum interference de-
vice (SQUID) magnetometer. For single crystals the mag-
netic field was applied along the tetragonal c-axis.
The temperature dependences of the magnetic suscepti-
bility ( )Tχ measured in the low magnetic field (see Fig. 1)
exhibit few clear peculiarities. The low temperature pecu-
liarities are related to the superconducting transitions at
8cT ∼ and 13.5 K for FeSe 0.963 (P) and FeSe 0.5 Te 0.5
(S1), respectively. The detailed data on SC transition for
the FeSe 0.5 Te 0.5 single crystals are shown in Fig. 2. The
value 14.2cT K resulted for the sample of the second
series is close to the maximum cT value observed at ambi-
ent pressures in the FeSe1–xTex family for 0.5x∼ [12,25].
Fig. 1. Temperature dependence of the magnetic susceptibility for
FeSe0.963, FeSe0.5Te0.5, and FeTe (FeTe0.95) measured in the
magnetic field = 200H Oe and zero field cooling (ZFC).
ZFC
200
150
100
50
0
0
50 100 150 200 250 300
T, K
FeTe (SI)
�
,
1
0
em
u
/m
o
l
–
3
FeSe (P)0.963
FeSe Te (SI)0.5 0.5
FeTe (P)0.95
A.V. Fedorchenko et al.
102 Fizika Nizkikh Temperatur, 2011, v. 37, No. 1
Also, the pronounced anomalies of ( )Tχ are seen in
Fig. 1 at 125 K. Below this temperature ( )Tχ exhibits a
remarkable irreversibility between zero-field cooling
(ZFC) and field cooling (FC, not shown in the figure)
magnetization data. Such behavior may be due to the mag-
netite (Fe3O4) impurities and related to the Verwey transi-
tion, which is observed in magnetite at 120–125VT ∼ K
(see [26] and references therein).
In addition, a threshold cusp in ( )Tχ appears near 70 K
for FeTe (S1) and FeTe 0.95 (P). According to the recent
neutron-scattering measurements for FeTe [21,22] this
peculiarity corresponds to an antiferromagnetic (AFM)
ordering with a rather complex magnetic structure and the
simultaneous structural transition from a tetragonal lattice
(at high temperatures) to a distorted orthorhombic phase.
A relatively large content of ferromagnetic (FM) impu-
rities in the studied samples is readily illustrated by the
magnetization data ( )M H in Fig. 3. Generally, at high
magnetic fields the ( )M H dependencies show a linear
behavior (dashed lines in Fig. 3) with a slope determined
by the host (i.e. intrinsic) magnetic susceptibility of the
sample. By their extrapolation to the zero field we obtained
the saturation moment values of FM impurities for our
samples, which fall in the range from 25 to 300 emu/mol,
being weakly dependent on temperature.
Despite the pronounced FM impurity effects, the ob-
tained magnetization data in Fig. 3 make it possible to es-
timate with sufficient accuracy the host magnetic suscepti-
bilities hostχ for our samples from the slope of linear part
of corresponding ( )M H dependence at high fields. The
resulted values of hostχ at some fixed temperatures are
shown by full circles in Fig. 4. In Fig. 4 we also presented
the detailed host ( )Tχ data, which were obtained according
to the equation,
host( ) ( ) = ( ( ) ) / ,sT T M T M Hχ ≡ χ − (1)
from temperature dependence of the magnetization ( )M T
measured in magnetic field of 30 kOe. Here the saturation
moment value sM of FM impurity is assumed to be con-
stant and equal to its temperature-averaged value for a
given sample.
Fig. 2. Low field magnetic susceptibility in vicinity of the super-
conducting transition for FeSe0.5Te0.5 single crystals of two series
S1 and S2.
0
–2
–4
–6
–8
0 5 10 15 20 25
T, K
S1
S2
FC
ZFC
H = 10 Oe
�
,
em
u
/m
o
l
FeSe Te0.5 0.5
Fig. 3. Magnetization data for some FeSe1–xTex compounds at different temperatures.
M
,
e
m
u
/m
o
l
M
,
e
m
u
/m
o
l
20 K
110 K
200 K
4.2 K
110 K
200 K
5 K
110 K
250 K
5 K
110 K
200 K
270 K
H || c
H || c
FeTe (SI)
M
,
e
m
u
/m
o
l
M
,
e
m
u
/m
o
l
T, K
T, K
T, K
T, K
300
300
200
200
100
100
0
0
0
0
a b
c d
10
10
10
10
20
20
20
20
30
30
30
30
40
40
40
40
50
50
50
50
400
300
200
100
250
200
150
100
50
FeSe (P)0.963 FeSe Te (SI)0.5 0.5
FeTe (P)0.95
Magnetic and superconducting properties of FeSe1–xTex (x 0, 0.5, and 1.0)
Fizika Nizkikh Temperatur, 2011, v. 37, No. 1 103
In Fig. 5 the magnetization data are shown for some se-
lected temperatures for FeSe 0.5 Te 0.5 and FeTe single
crystals of the second series. Compared to the first series
(see Fig. 3,b), the FeSe0.5Te0.5 sample appeared to have
much smaller saturation moment of FM impurities. In ad-
dition, the temperature dependence of its host magnetic
susceptibility (inset in Fig. 5,a) is distinctly different from
that of the first series sample (Fig. 4) both in character and
in magnitude of the susceptibility values. As is evident
from a linear ( )M H dependence for FeTe in Fig. 5,b,
there are no any detectable FM impurities in this sample.
The temperature dependence of its magnetic susceptibility
given in Fig. 6,a exhibits almost the same behavior in vici-
nity of the phase transition as ( )Tχ of the polycrystalline
FeTe0.95 sample and FeTe single crystal of the first series
(see Fig. 4) but has more pronounced magnitude of the
effect. A small hysteresis in ( )Tχ dependence is observed
after heating the sample to about 200 K and subsequent
cooling below the transition temperature (Fig. 6,b). A simi-
lar ( )Tχ behavior for FeTe was also reported in Ref. 22.
The experimentally obtained basic superconducting and
magnetic characteristics of the studied samples are summa-
rized in Table 1.
Fig. 4. Temperature dependence of the host magnetic susceptibi-
lity for some FeSe1–xTex compounds. Full circles correspond to
values derived from the high field magnetization data in Fig. 3.
0 50 100 150 200 250 300
T, K
FeTe (SI)
�
,
1
0
em
u
/m
o
l
–
3
FeSe (P)0.963
FeSe Te (SI)0.5 0.5
FeTe (P)0.95
5
4
3
2
1
Fig. 5. Magnetization data for FeSe0.5Te0.5 (a) and FeTe (b) sin-
gle crystals of the second series (S2) at different temperatures. In
inset are the values of the host magnetic susceptibility versus
temperature for FeSe0.5Te0.5, derived from its high field magneti-
zation data.
10
10
20
20
30
30
40
40
50
50
0
0
100
200
300
10
20
30
40
50
0 100 200 300
0,4
0,8
b FeTe (S2)
H, kOe
H, kOe
2 K
70 K
20 K
120 K
250 K
a
T, K
M
,
em
u
/m
o
l
M
,
em
u
/m
o
l
FeSe Te (S2)0.5 0.5
�
,
1
0
em
u
/m
o
l
–
3
Fig. 6. Temperature dependence of the magnetic susceptibility for
FeTe (S2) single crystal (a) and its hysteresis behavior (b) in
vicinity of the phase transition at 70T K.
0 100 200 300
4
6
8
10
50 60 70 80 90
6
8
10
T, K
T, K
FeTe (S2)
H = 10 kOe
H = 100 Oe
�
,
1
0
em
u
/m
o
l
–
3
�
,
1
0
em
u
/m
o
l
–
3
a
b
A.V. Fedorchenko et al.
104 Fizika Nizkikh Temperatur, 2011, v. 37, No. 1
Table 1. Superconducting transition temperature Tc (in K),
FM impurity saturation magnetic moment Ms (emu/mol) and host
(intrinsic) magnetic susceptibility χ ( 310− emu/mol) at room
and zero temperatures for FeTe1–xSex compounds.
Compound Tc Ms
χ
290 K 0 K
FeSe0.963 (P) 7∼ 214 0.5 0.1± 0.75 0.1±
FeSe0.5Te0.5 (S1) 13.5 280 1.3 0.2± 1.45 0.2±
FeSe0.5Te0.5 (S2) 14.2 9 0.85 0.1± 0.4 0.1±
FeTe0.95 (P) − 24 2.7 0.2± 2.65 0.2±
FeTe (S1) − 103 2.9 0.2± 3.6 0.2±
FeTe (S2) − 0∼ 5.7 0.2± 5.45 0.2±
3. Computational details and results
To gain a further insight into magnetic properties of the
FeSe1–xTex system in the normal state, the ab initio calcu-
lations of the electronic structure and exchange enhanced
magnetic susceptibility are performed for FeSe and FeTe
parent compounds within DFT and the local spin density
approximation.
At ambient conditions the FeSe1–xTex compounds pos-
sess the tetragonal PbO-type crystal structure (space group
4/P nmm ), which exhibits strong two-dimensional fea-
tures. The crystal lattice is composed by alternating triple-
layer slabs, which are stacked along the c-axis. Each iron
layer is sandwiched between two nearest-neighbor chalco-
gen layers, which form edge-shared tetrahedrons around
the iron sites. The positions of Se (or Te) sheets are fixed
by the internal parameter Z , which represents the height
of chalcogen atoms above the iron square plane. This pa-
rameter also determines the chalcogen-Fe bond angles.
Crystal structure parameters of FeSe1–xTex compounds
were established in a number of works by means of x-ray
and neutron diffraction studies [11,13,14,21,22].
The previous ab initio calculations of the electronic
structure of the «11»-type iron-based chalcogenides were
predominantly related to studies of the AFM and SDW
ordering [19,20,23,31–34]. In this paper the electronic
structure calculations are carried out for FeSe and FeTe
compounds with the aim to study a paramagnetic response
in an external magnetic field, and to elucidate a nature of
paramagnetism and magnetic instability in the parent
phases of «11» systems. The ab initio calculations are car-
ried out by employing a full-potential all-electron relativis-
tic linear muffin-tin orbital method (FP-LMTO, code RSPt
[35,36]). No shape approximations were imposed on the
charge density or potential, what is especially important for
the anisotropic layered crystal structures. The exchange-
correlation potential was treated within the local spin den-
sity approximation (LSDA, [37]) of the density functional
theory. The calculations were based on the experimental
lattice parameters from Refs. 10, 11, 13, 14, 21, 22.
The calculated basic features of electronic structure of
FeSe and FeTe are in a qualitative agreement with results
of earlier calculations [19]. In particular, the detailed den-
sity of states (DOS) ( )N E of FeSe is presented in Fig. 7.
In the vicinity of the Fermi level FE the d-states of Fe
provide the dominant contribution to DOS in the range
–2 eV and 2 eV around = 0FE . The p states of chalcogen
atoms are predominantly extended in the interstitial region,
and their partial contributions to DOS in vicinity of FE
are substantially smaller for both FeSe and FeTe. As seen
in Fig. 7, in FeSe the Fermi level lies at the steep slope of
( )N E , in the beginning of a pseudogap of about 0.7 eV.
In fact, there is a van Hove singularity in ( )N E at about
0.05 eV below FE (see Fig. 6). The calculated ( )FN E for
FeSe can be related to the measured electronic specific
heat coefficient, γ =9.17 mJ/mol K2 [9], by means of the
Sommerfeld coefficient formula, 2 2=2 ( )(1 )/3B Fk N Eγ π +λ .
This provides the estimation for the enhancement factor in
FeSe: λ =3.8. Also, the evaluated for FeSe and FeTe vol-
ume derivatives d ln ( ) / d lnFN E V are found to be posi-
tive and equal to 1.25 and 1.42, respectively, what suggests
the reduction of ( )FN E with pressure.
The FP-LMTO-LSDA calculations of the field-induced
spin and orbital (Van Vleck) magnetic moments were car-
ried out for FeSe and FeTe self-consistently within the
procedure described in Ref. 36 by means of the Zeeman
operator,
ˆˆ= (2 ) ,Z Bμ +H s lH (2)
which was incorporated in the original FP-LMTO Hamil-
tonian. Here H is the external magnetic field, ŝ and l̂ the
Fig. 7. Total density of states of the paramagnetic FeSe around
FE (solid line) and the partial contribution of the iron d-states
(dashed line). The Fermi level position (at 0 eV) is marked by a
vertical line.
–2 –1 0 1 2
Energy, eV
0
2
4
6
8
N
,
st
at
es
/e
V
f.
u
.
FeSe
Magnetic and superconducting properties of FeSe1–xTex (x 0, 0.5, and 1.0)
Fizika Nizkikh Temperatur, 2011, v. 37, No. 1 105
spin and orbital angular momentum operators, respec-
tively. The field induced spin and orbital magnetic mo-
ments were calculated in the external field of 10 T and
provided estimation of the related contributions to the
magnetic susceptibility, spinχ and orbχ .
For the tetragonal crystal structure of FeSe, the para-
magnetic contributions spinχ and orbχ were derived from
the magnetic moments obtained in an external field, ap-
plied both parallel and perpendicular to the c axis. The
evaluated magnetic anisotropy, which is determined by the
orbital contribution, appeared to be negligible, in compari-
son with the dominant spinχ contribution. The orbital Van
Vleck contribution itself is substantially smaller than the
strongly enhanced spin susceptibility, and comes from the
d-states of Fe.
In the course of calculations, we found that magnetic
response to the external field is very sensitive to the height
Z of chalcogen species from the Fe plane. The corres-
ponding calculated dependences of magnetic susceptibility
for FeSe and FeTe are given in Figs. 8 and 9, respectively.
It should be noted here that the itinerant nature of the hy-
bridized 3d-states of Fe is an essential condition for the
described above field-induced calculations of paramagnetic
susceptibility. There is a strong experimental support for
this itinerant picture for FeSe, which is expected to be in a
non-magnetic spin-degenerate state. For FeTe, however, a
validity of the field-induced calculations of χ is question-
able due to the expected more localized nature of the 3d-
states. For this reason, the calculations for FeTe are per-
formed only for volumes smaller than the experimental
volume, and results of these calculations have to be thor-
oughly verified by other methods, and compared with ex-
perimental data.
The enhanced Pauli spin contribution to the magnetic
susceptibility was also calculated within the Stoner model:
2 1
ston = ( )[1 ( )] ,P B F FS N E IN E −χ χ ≡ μ − (3)
where 2 ( )P B FN Eχ = μ , S is the Stoner enhancement fac-
tor, and Bμ the Bohr magneton. The multi-band Stoner
integral I , representing the exchange-correlation interac-
tions for conduction electrons and appropriate for com-
pounds, can be expressed in terms of the calculated pa-
rameters of the electronic structure [38]:
2= 1/ ( ) ( ) ( ).F ql F qll ql F
qll
I N E N E J N E′ ′
′
∑ (4)
Here ( )FN E and ( )ql FN E are the total density of elec-
tronic states and site, q, and angular momentum, l , pro-
jected DOS at the Fermi level. The parameters of the ex-
change interaction qllJ ′ are defined in terms of the intra-
atomic exchange integrals:
2 2= ( ( )) ( ) ( ) ,qll ql qlJ g r r r dr′ ′ρ φ φ∫ (5)
and therefore depend upon the corresponding partial wave
functions ( )l rφ . Here ( ( ))g rρ is a function of the electron
density [37], l and l′ are the corresponding angular-
momentum quantum numbers.
In the framework of an itinerant model of magnetism
the mean field treatment within the Stoner model can be
valid at least to establish trends. This model predicts the
FeTe system to be unstable in a non-magnetic state. For
FeSe the calculated value of the enhanced Pauli suscepti-
bility ( 3
ston 0.4·10−χ ∼ emu/mol) is close to the field-
Fig. 8. Calculated paramagnetic susceptibility of FeSe as a func-
tion of the internal lattice parameter Z . The unit cell volume and
/c a ratio are fixed to their experimental ambient pressure values
(78.4 Å3 and 1.464 [14]). The dashed line is a guide for the eye.
The dashed-dotted line corresponds to the experimental Z .
0.24 0.25 0.26 0.27
Z
0
0.5
1.0
1.5
FeSe
�
P
,
1
0
em
u
/m
o
l
–
3
Fig. 9. Calculated paramagnetic susceptibility of FeTe as a func-
tion of the internal lattice parameter Z for LSDA optimized
(87 Å3) unit cell volume. The /c a ratio is fixed to the experi-
mental ambient pressure value (1.647 [14]). The dashed line is
a guide for the eye.
0,250 0,252 0,254
Z
0
2
4
6
FeTe
�
P
,
1
0
em
u
/m
o
l
–
3
A.V. Fedorchenko et al.
106 Fizika Nizkikh Temperatur, 2011, v. 37, No. 1
induced evaluated spinχ for the same range of lattice pa-
rameters. The calculated susceptibility enhancement factor
S appears to be about 10, and this means nearness to a
quantum critical point in the pure FeSe compound and a
possibility of competition between FM and AFM spin fluc-
tuations.
4. Discussion
The experimental superconducting and magnetic char-
acteristics obtained for the studied FeSe1–xTex compounds
agree reasonably with those reported in Refs. 13, 22, 27–30.
In particular, the intrinsic magnetic susceptibility derived
in our work for the normal state of FeSe0.963 is close to
that cited recently in Ref. 27 for polycrystalline Fe1.11Se.
The inherent feature of the FeSe–FeTe system resulted from
our study is a high sensitivity of the magnetic properties to
quality and composition of the samples. This can be readi-
ly demonstrated here by the data for FeSe0.5Te0.5 and FeTe
compounds in Table 1. However, despite an appreciable
uncertainty, the obtained experimental data suggest a grad-
ual increase of the magnetic susceptibility in FeSe1–xTex
system with increasing of tellurium content.
On comparing the experimental χ from Table 1 with
the calculated ones from Fig. 8 we should note that the
experimental internal lattice parameter Z in FeSe is about
0.26 [10,11,14], whereas the optimized DFT calculated
values of Z are 0.234 [19] and 0.26 [33]. Though the cal-
culated paramagnetic susceptibility is very sensitive to
the height Z of Se atoms from the Fe plane, we can esti-
mate the corresponding contributions to χ as spinχ =
= 0.55·10–3 emu/mol and orbχ =0.11 3·10− emu/mol for the
experimental lattice parameters of FeSe (V =78.4 Å3, /c a
=1.464, Z = 0.26 [14]). Therefore the calculated field-
induced magnetic moments are in a qualitative agreement
with the obtained experimental susceptibility of FeSe in
the paramagnetic region (Table 1). Actually, the FeSe
compound is found to be on the verge of magnetic instabil-
ity. The proximity to a quantum critical point is clearly
seen in Fig. 8, and this nearness can result in strong spin
fluctuations.
For FeTe the Stoner criterion is fulfilled for experimen-
tal values of cell volume and parameter Z . Actually, our
self-consistent field-induced LSDA calculations for FeTe
converged to the paramagnetic state only for reduced lat-
tice parameters. This is especially relevant to the parameter
Z , which had to be also reduced for about 10%. Therefore
we should consider the calculated paramagnetic suscepti-
bility of FeTe in Fig. 9 as a rough estimation which can be
valid at least to establish a trend for the effect of Z pa-
rameter. To further address the question to what extent a
qualitative agreement between the calculated χ and ex-
perimental data for FeTe in Table 1 might be fortuitous,
the detailed study of pressure effect on χ is highly desir-
able.
A detailed investigation of ( )Tχ and χ(x) in FeSe1–xTex
compounds merits a separate examination beyond the
scope of this study. In order to elucidate in a systematic
way the effects of isovalent partial substitution of Te for Se
in the system, one has to examine an extended concentra-
tion range. Also, a further development in technology of
samples preparing is desirable. On the theoretical side, a
more rigorous calculations technique for FeTe and the al-
loys is needed, presumably by employing the so-called
disordered local moments (DLM) approach [34,39], which
seems relevant for the localized states of Fe.
5. Conclusions
Magnetic susceptibility of FeSe1–xTex ( 0x , 0.5, and
1.0) compounds was investigated in the temperature range
2–300 K. The superconducting transitions are detected at
8 K and 13.6–14.2 K in FeSe0.963 and FeSe0.5Te0.5 sam-
ples, respectively. For the most samples, a nonlinear be-
havior of the magnetization curves in the normal state
gives evidence of substantial presence of ferromagnetic
impurities. By taking these impurity effects into account,
the intrinsic magnetic susceptibility χ in the series of iron
chalcogenides FeSe0.963, FeSe0.5Te0.5 and FeTe was esti-
mated to increase gradually with Te content in about ten
times.
Ab initio calculations of the electronic structure and
paramagnetic contributions to susceptibility of the FeSe
compound have revealed that this system is in close prox-
imity to the quantum critical point, and this nearness can
result in strong spin fluctuations. It is shown, that the
paramagnetic susceptibility calculated in external magnetic
field appears to be close to the obtained experimental
value. The Van Vleck contribution to χ in FeSe, which
amounts up to 20% of total susceptibility, comes mainly
from d-electrons of Fe, and should not be neglected in
comparisons with the experimental data. In general, the
numerical results point out that itinerant magnetism theory
is relevant to describe magnetic properties of FeSe system.
For FeTe a drastic drop in ( )Tχ with decreasing tem-
perature was found at 70NT K, which is presumably
related to antiferromagnetic ordering. The LSDA calcu-
lated paramagnetic susceptibility (Fig. 9), being of the
same order with the experimental data, reveals a drastic
sensitivity to the structural parameter Z . Therefore, the
detailed study of pressure effect on χ would be very use-
ful to further address the question about a nature of para-
magnetic state in FeTe. Also rigorous calculations of χ are
required for FeTe, which would take into account disor-
dered local magnetic moments above NT . In particular, the
recently ab initio employed DLM approach [34,39] seems
very promising to shed light on behavior of ( , )T Pχ .
The authors dedicate this work to the 100th anniversary
of David Shoenberg, who was a pioneer of low-
Magnetic and superconducting properties of FeSe1–xTex (x 0, 0.5, and 1.0)
Fizika Nizkikh Temperatur, 2011, v. 37, No. 1 107
temperature physics and studies of electronic structure of
solids.
This work has been supported by the Russian-Ukrainian
RFBR-NASU project 43-02-10 and 10-02-90409.
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