PHYSICAL REVIEW B
94
, 064522 (2016)
Mn-doping induced ferromagnetism and enhanced superconductivity
in Bi
4
−
x
Mn
x
O
4
S
3
(0
.
075
x
0
.
15)
Zhenjie Feng,
1
,
2
,
*
Xunqing Yin,
1
Yiming Cao,
1
Xianglian Peng,
1
Tian Gao,
3
Chuan Yu,
1
Jingzhe Chen,
1
Baojuan Kang,
1
Bo Lu,
4
Juan Guo,
5
Qing Li,
1
Wei-Shiuan Tseng,
6
Zhongquan Ma,
1
Chao Jing,
1
Shixun Cao,
1
,
2
,
†
Jincang Zhang,
1
,
7
and N.-C. Yeh
6
,
‡
1
Department of Physics, Shanghai University, Shanghai 200444, China
2
Shanghai Key Laboratory of High Temperature Superconductors, Shanghai 200444, China
3
Department of Physics, Shanghai University of Electric Power, Shanghai 201300, China
4
Laboratory for Microstructures, Shanghai University, Shanghai 200444, China
5
School of Physical Engineering, Zhengzhou University, Zhengzhou 450001, China
6
Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
7
Materials Genome Institute, Shanghai University, Shanghai 200444, China
(Received 24 January 2016; revised manuscript received 6 August 2016; published 30 August 2016)
We demonstrate that Mn doping in the layered sulfides Bi
4
O
4
S
3
leads to stable Bi
4
−
x
Mn
x
O
4
S
3
compounds that
exhibit both long-range ferromagnetism and enhanced superconductivity for 0
.
075
x
0
.
15, with a possible
record superconducting transition temperature (
T
c
)
∼
15 K among all BiS
2
-based superconductors. We conjecture
that the coexistence of superconductivity and ferromagnetism may be attributed to Mn doping in the spacer Bi
2
O
2
layers away from the superconducting BiS
2
layers, whereas the enhancement of
T
c
may be due to excess electron
transfer to BiS
2
from the Mn
4
+
/
Mn
3
+
substitutions in Bi
2
O
2
. This notion is empirically corroborated by the
increased electron-carrier densities upon Mn doping, and by further studies of the Bi
4
−
x
A
x
O
4
S
3
compounds
(
A
=
Co, Ni;
x
=
0.1, 0.125), where the
T
c
values remain comparable to that of the undoped Bi
4
O
4
S
3
system
(
∼
4
.
5 K) due to lack of 4
+
valences in either Co or Ni ions for excess electron transfer to the BiS
2
layers. These
findings therefore shed new light on feasible pathways to enhance the
T
c
values of BiS
2
-based superconductors,
although complete elucidation of the interplay between superconductivity and ferromagnetism in these anisotropic
layered compounds awaits the development of single crystalline materials for further investigation.
DOI:
10.1103/PhysRevB.94.064522
I. INTRODUCTION
One of the commonalities among the cuprate and iron-based
high-temperature superconductors is their layered structures
[
1
,
2
]. Interestingly, even for conventional superconductors, the
highest superconducting transition temperature (
T
c
) has been
found in layered magnesium diboride MgB
2
[
3
]. Recently,
superconductivity with
T
c
=
4
.
5 K was discovered in a new
superconductor Bi
4
O
4
S
3
[
4
]. This compound has a layered
structure composed of two superconducting BiS
2
layers
and spacer layers of Bi
4
O
4
(SO
4
)
1
−
x
, where
x
indicates the
deficiency of (SO
4
)
2
−
ions at the interlayer sites. Since the
discovery of Bi
4
O
4
S
3
, several other BiS
2
-based superconduc-
tors
Ln
O
1
−
x
F
x
BiS
2
(
Ln
=
La, Ce, Pr, Nd) with the highest
T
c
∼
10
.
6 K have been reported [
5
–
11
]. Both experimental
and theoretical studies to date have indicated that the BiS
2
layers play the role of the superconducting planes in these
sulfide superconductors, similar to the CuO
2
planes in the
cuprate superconductors and the Fe
2
An
2
(
An
=
P, As, Se, Te)
layers in the iron-based superconductors [
1
,
2
,
12
].
A major challenge facing this new class of layered
superconductors is to optimize
T
c
by exploring different
spacer layers. Additionally, the effects of doping by either
*
fengzhenjie@shu.edu.cn
†
sxcao@shu.edu.cn
‡
ncyeh@caltech.edu; Correspondence should be addressed to
N.-C.Y. and requests for materials should be addressed to either Z.-J.F.
or N.-C.Y.
nonmagnetic or magnetic elements are important issues for
investigation. To date, suppression of superconductivity has
been observed in the case of Cu and Ag substitutions for Bi
in the Bi
4
O
4
S
3
superconductor [
13
,
14
], whereas coexistence
of superconductivity and ferromagnetism has been reported in
the CeO
1
−
x
F
x
BiS
2
[
15
,
16
] and Sr
0
.
5
Ce
0
.
5
FBiS
2
[
17
]systems
at low temperatures. However, none of these doping effects
are fully understood.
Aiming at addressing the aforementioned issues, we report
in this work our studies of 3
d
transition-metal substitutions for
Bi in Bi
4
O
4
S
3
by synthesizing Bi
4
−
x
A
x
O
4
S
3
(
A
=
Mn, Co, Ni;
0
.
075
x
0
.
15) compounds with conventional solid-state
reaction. We first focus on the investigation of Bi
4
−
x
Mn
x
O
4
S
3
because these results are most interesting and reveal a possible
record
T
c
∼
15 K, and then perform comparative studies on
Bi
4
−
x
A
x
O
4
S
3
(
A
=
Co, Ni) in the Discussion section to
elucidate the underlying physics. Based on our empirical find-
ings, we suggest that the coexistence of superconductivity and
long-range ferromagnetism in all Bi
4
−
x
A
x
O
4
S
3
(
A
=
Mn, Co,
Ni) compounds may be attributed to the selective doping of 3
d
transition-metal elements in the spacer Bi
2
O
2
layers, whereas
the enhancement of
T
c
found only in Mn-doped samples
may be due to substantial electron transfer from Mn
4
+
/
Mn
3
+
substitutions in Bi
2
O
2
to the superconducting BiS
2
layers.
II. EXPERIMENT
Bulk polycrystalline Bi
4
−
x
Mn
x
O
4
S
3
(
x
=
0
.
075, 0.1,
0.125, 0.15) and Bi
4
−
x
A
x
O
4
S
3
(
A
=
Co, Ni;
x
=
0
.
1, 0.125)
2469-9950/2016/94(6)/064522(9)
064522-1
©2016 American Physical Society
ZHENJIE FENG
et al.
PHYSICAL REVIEW B
94
, 064522 (2016)
samples were synthesized by conventional solid-state reaction
method. For the Bi
4
−
x
Mn
x
O
4
S
3
samples, high purity Bi
(99.99%), Bi
2
O
3
(99.99%), S (99.999%), MnO
2
(99.99%)
were first weighed in stoichiometric ratio and then grounded
thoroughly in a glove box under high purity argon atmosphere.
Next, the mixture was pressed into a pellet shape and sealed
in an evacuated quartz tube (10
−
4
Torr). The pellet was then
heated up to 510
◦
C and kept for 10 hours. After cooling the
pellet to room temperature, the product was well mixed again
by regrinding, pressed into a pellet shape, and then annealed
at 510
◦
C for another 10 hours. The samples thus obtained
appeared black in color and were hard. It is important to note
that the sample may not be heated above 550
◦
C. Otherwise
S-O gas would be produced and could result in explosion of
the quartz tube [
4
]. Similar procedures were applied to the
synthesis of the Bi
4
−
x
A
x
O
4
S
3
(
A
=
Co, Ni;
x
=
0
.
1, 0.125)
samples.
The crystal structures of all samples were characterized by
x-ray powder diffraction (XRD, 18 kW D/MAX 2550) using
the Cu-
K
α
radiation. The lattice constants were calculated
from the 2
θ
values and the Miller indices by using the
JADE
6.5 software. After XRD studies, these polycrystalline samples
were cut into rectangular shape and polished for electrical re-
sistivity measurements. The electrical resistivity was measured
with a standard four-terminal method covering temperature
range from 3–300 K in a physical property measurement
system (PPMS-9, Quantum Design, Inc.). Typical current den-
sities used for the resistive measurements were
∼
100 A
/
m
2
.
No apparent dependence on the current density was found up
to
∼
2000 A
/
m
2
, whereas resistive signals became difficult
to resolve for current densities significantly smaller than
100 A
/
m
2
. The magnetization and specific heat measurements
were conducted using the same PPMS with vibrating sample
magnetometer (VSM) and specific heat options. The zero-
field-cooling (ZFC) and field-cooling (FC) of the magnetic
susceptibility measurements of the samples were performed
in the warming process. Additionally, the carrier densities
of bulk polycrystalline samples were determined from their
normal-state Hall coefficients at 300 K by means of the van der
Pauw method and with the use of the Hall effect measurement
system CVM200 made by the East Changing Company.
III. RESULTS AND ANALYSIS
A. Structural characterization
The XRD patterns of different Mn-doped samples and the
corresponding crystalline structure are shown in Figs.
1(a)
–
1(b)
. These data suggest that all samples acquired the ex-
pected tetragonal phase (space group
I
4
/mmm
) with minor
rhombohedra Bi
2
S
3
and Bi impurities, the latter being common
occurrences in the Bi
4
O
4
S
3
system [
4
,
13
,
14
,
18
] and exhibiting
no superconductivity above 3 K [
19
–
21
]. The nominal Mn-
doping (
x
) dependence of the lattice constants
a
and
c
are
illustrated in the inset of Fig.
1(a)
. The general trend of
decreasing lattice parameters with increasing Mn doping is
reasonable because the ionic radii of Mn are much smaller than
that of Bi
3
+
[
22
]. This trend is also indicative of successful
incorporation of Mn ions into the Bi
4
O
4
S
3
unit cells. Similarly,
XRD studies of Bi
4
−
x
A
x
O
4
S
3
(
A
=
Co, Ni) also indicate that
the lattice constants of Bi
1
−
x
A
x
O
4
S
3
were all reduced relative
to those of Bi
4
O
4
S
3
, as shown in Fig.
1(c)
. Moreover, the lattice
constants for different dopants followed the descending order
of Co, Ni, and Mn, as explicitly shown in the inset of Fig.
1(c)
.
B.
M
-vs-
T
and
ρ
-vs-
T
studies of Bi
4
−
x
Mn
x
O
4
S
3
Temperature-dependent magnetization (
M
)of
Bi
4
−
x
Mn
x
O
4
S
3
with
x
=
0
.
075, 0.10, 0.125, and 0.15
was studied under both zero-field-cool (ZFC) and field-cool
(FC) conditions from 3–300 K and with an external field
H
=
100Oe, as illustrated in Fig.
2
. For each magnetization
curve, three characteristic temperatures are noteworthy:
The N
́
eel temperature (
T
N
) near
∼
125 K for
x
=
0
.
125
and 0.15, below which
M
decreased due to the onset of
FIG. 1. Structural properties of Bi
4
−
x
A
x
O
4
S
3
(
A
=
Mn, Co, Ni): (a) X-ray diffraction (XRD) patterns of Bi
4
−
x
Mn
x
O
4
S
3
(0
.
075
x
0
.
15).
The inset shows the doping-dependent variations of the in-plane and
c
-axis lattice parameters
a
and
c
. (b) Schematics of the layered structure of
Bi
4
−
x
Mn
x
O
4
S
3
. (c) X-ray diffraction (XRD) spectral studies of Bi
4
−
x
Co
x
O
4
S
3
and Bi
4
−
x
Ni
x
O
4
S
3
for
x
=
0
.
125. The XRD spectra indicated
that the lattice constants after 3
d
transition-metal doping were all reduced relative to those of Bi
4
O
4
S
3
, and the values for different dopants
followed the descending order of Co, Ni, and Mn, as shown in the inset of (c).
064522-2