Magnetic and transport properties of SmCoAsO1-xFx

Article abstract of DOI:10.1016/j.jpcs.2010.10.040

A series of SmCoAsO_1-xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.

Full text of DOI:10.1016/j.jpcs.2010.10.040

Journal of Physics and Chemistry of Solids 72 (2011) 453–456
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Magnetic and transport properties of SmCoAsO1ꢀx x
F
n
Y.L. Chen a, Y.J. Cui a, Y. Yang a, C.H. Cheng b, Y. Zhao a,b,
a Key Laboratory of Advanced Technology of Materials (Ministry of Education of China), Superconductivity R&D Center (SRDC),
Mail Stop 165#, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
b School of Materials Science and Engineering, University of New South Wales, Sydney, 2052 NSW, Australia
a r t i c l e i n f o
a b s t r a c t
Available online 15 October 2010
A series of SmCoAsO1ꢀxFx (with x¼0, 0.05, 0.1, and 0.2) samples have been prepared by solid state
reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type
structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has
been doped into the lattice. The magnetic and transport properties of the samples have been investigated.
Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism,
ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Ne´el
temperatures were almost independent of the F content and metamagnetic transitions were observed
Keywords:
A. Magnetic materials
D. Magnetic properties
D. Transport properties
´
below antiferromagnetic Neel temperatures. With increasing F content, high temperature (below 142 K)
ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction
in the region of 2–300 K and Fermi liquid behavior at low temperatures were shown in all samples.
Transport properties at applied magnetic fields showed anomalies at low temperatures.
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
RECoAsO and demonstrated that multiple magnetism exists in
the SmCoAsO and GdCoAsO systems [14,15]. Several independent
Since the discovery of superconductivity with Tc¼26 K in F
doped LaFeAsO [1], the quaternary compounds RETMPNO (RE¼rare
earth ions, TM¼transition metals, and PN¼P, As), which possess the
work about magnetic properties of RECoAsO (RE¼La–Gd) has been
also reported [16–18]. However, some different magnetic
properties are presented in them.
In the present work, we investigated the F doping effect on the
structure, magnetic, and resistivity properties in SmCoAsO1ꢀxFx
systems. This will provide an evidence that multiple magnetic
transitions in SmCoAsO system are mainly attributed to the Co
sublattice.
tetragonal ZrCuSiAs-type structure composed of
a
stack of
alternating REO- and TMPN-layers along the c-axis [2,3], have
attracted much interest in experiments and theoretical studies.
Except for the superconductivity in doped systems, the parent
compounds RETMPNO exhibit anomaly magnetic properties. The
magnetic susceptibility and electrical resistivity of REFeAsO shows
an anomaly near 130–150 K [1,4–6], and neutron-scattering
experiments demonstrate that LaFeAsO at 137 K, develops long-
range spin-density-wave type antiferromagnetic (AFM) order [7].
2. Experimental
´
CeZnPO shows AFM order with Neel temperature of 7.4 K and a spin
Polycrystalline samples with the nominal composition of
SmCoAsO1ꢀxFx with x¼0, 0.05, 0.1, 0.2 were prepared by a solid
state reaction of SmAs, CoAs (99.9%), Co (99.99%), Co2O3 (99.9%),
and SmF3 (99.99%). SmAs was obtained by reacting Sm pieces and
As powders at 500 1C for 15 h and then at 900 1C for 12 h. CoAs was
obtained by reacting Co and As powders at 500 1C for 15 h and then
at 800 1C for 24 h. Powders of the raw materials were well mixed
and then pressed into pellets in an inert atmosphere (inside the
glove box). The pellets were wrapped with Ta foil and sealed in an
evacuate quartz tube. They were then annealed at 1150 1C for 48 h
in Ar atmosphere.
´
flop transition well below the Neel temperature [8]. LaMnPO is
proved to be an AFM semiconductor [9]. CeFePO is a heavy Fermion
metal with ferromagnetic (FM) correlations [10]. In addition, the
properties of ‘‘CoAs-based’’ compounds RECoAsO were widely
investigated. LaCoPO and LaCoAsO are itinerant ferromagnets
with Curie temperatures (TC) of 43 and 66 K, respectively
[11,12]. CeCoPO shows a FM ordering below TC¼75 K [13]. Last
year we had synthesized several ‘‘CoAs-based’’ compounds
n Corresponding author at: Superconductivity R&D Center (SRDC), Mail Stop
165#, Southwest Jiaotong University, Chengdu, Sichuan 610031, China.
Tel.: +86 28 87600786; fax: +86 28 87600581.
The crystal structure was studied by powder X-ray diffraction
(XRD) using an X’Pert MRD diffractometer with Cu K
Lattice constants were determined from LeBail refinements. DC
a radiation.
E-mail address: yzhao@swjtu.edu.cn (Y. Zhao).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.10.040

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Y.L. Chen et al. / Journal of Physics and Chemistry of Solids 72 (2011) 453–456
magnetization and resistivity were measured with a physical
property measurement system (PPMS, Quantum Design).
3. Results and discussion
The XRD patterns of SmCoAsO1ꢀxFx samples are shown in Fig. 1.
The main peaks can be well indexed based on the tetragonal ZrCuSiAs-
type structure with the space group P4/nmm. The undoped sample
SmCoAsO is detected to be single phase with the refined lattice
constant of a¼0.3957(8) nm and c¼0.8252(0) nm. The lattice
constant a is slightly larger and c is smaller than the values of
a¼0.3936(9) nm and c¼0.8503(7) nm for SmFeAsO sample [19].
Small amount of SmOF emerges when x¼0.05 and enhances with
increasing doping level, which is similar to F doped REFeAsO system.
The variation of a-axis and c-axis lattice parameters with F doping
level is shown in the inset of Fig. 1. A clear shrinkage of the lattice
constants a and c was observed with increasing F content, indicating
that F has been doped into the lattice.
Fig. 2 shows the temperature (T) dependence of dc magne-
tization (M) for SmCoAsO under 100 Oe magnetic field (H) with
zero field cooling (ZFC) and field cooling (FC) mode. A strong
irreversibility is observed and multiple magnetic transitions occur.
During ZFC process, AFM transition firstly takes place at TN1¼39 K.
With increasing temperature, another peak appears at Tp¼64.5 K,
and then M sharply drops, FM transition takes place at TC¼71 K,
which is defined as the minimum of dM/dT vs. T curve. With further
increasing temperature, M decreases to a minimum value at 125 K,
and then increases to a maximum value at TN2¼142 K, but during
FC process, spin reorientation has been observed, the M increases as
the temperature decreases below 142 K. Above results indicated
that ferrimagnetic transition occurs at TN2, furthermore under
10 Oe fields, SmCoAsO shows the negative M between 86 and 138 K
(ZFC process), which are the lower and upper compensation
temperatures of N-type ferrimagnetism, respectively, [14]. The
isothermal M vs. H for 10 KrTr40 K are shown in the inset of
Fig. 2 and metamagnetic transition from AFM to FM is observed for
ToTN1. The M–H curves for T4TN1 are similar to SmCoAsO1ꢀxFx
sample with x¼0.05 as shown in Fig. 4.
Fig. 2. M–T curves for SmCoAsO sample under H¼100 Oe fields with ZFC and FC
mode. The inset shows M–H curves between 10–40 K. metamagnetic transition is
seen for all temperatures below 39 K.
Fig. 3. M–T curves under H¼100 Oe fields for SmCoAsO1ꢀxFx samples with ZFC
(filled symbols) and FC (open symbols) mode, dashed lines 1 and 2 denoted where
AFM transition occur and peak appear, respectively, for x¼0.05 sample. The inset
shows M–T curves under H¼1 T fields.
Fig. 3 shows the temperature dependence of dc magnetization for
SmCoAsO1ꢀxFx samples under 100 Oe magnetic fields with ZFC and
FC mode. TN1 are independent of the F content as demonstrated by the
dashed line one, the same as under 1 T fields (see the inset of Fig. 3),
indicating AFM transition at TN1 is attributed to the interaction
between CoAs layers but not SmO layers. The peak feature around
Tp are observed for all of SmCoAsO1ꢀxFx samples and gradually
suppressed by applied field (see the inset of Fig. 3), indicating that it is
ascribed to weak AFM ordering with FM components and a possible
noncollinear spin structure existing in the ferromagnetic phase
[20,21]. It is believed that adjacent spins align with a finite angle
between them, and such a spin configuration results in spin canting,
and thus, metamagnetic transition. With increasing the F content, Tp
decreased as demonstrated by the dashed line two, which probably
corresponds to the increase of ferromagnetic phase. Another
characteristic feature is that the magnetization between Tp and TN2
increases with increasing F content. Specially for x¼0.2 sample, the
magnetization exhibits an abrupt increase close to TN2, which is strong
Fig. 1. Powder XRD patterns of SmCoAsO1ꢀxFx samples, and no impurity phases are
detected in undoped SmCoAsO sample. The inset shows the lattice constants (a and
c) as functions of F content x.

Y.L. Chen et al. / Journal of Physics and Chemistry of Solids 72 (2011) 453–456
455
Fig. 4. M–H curves for SmCoAsO1ꢀxFx samples above TN1
.
evidence for FM transition, indicating ferrimagnetic state gradually
changes to ferromagnetic state.
Fig. 5.
r
/
r
300 K vs. T for SmCoAsO1ꢀxFx samples under zero field. The solid lines fit
to
r(T)¼r
0+AT2. The inset shows F content x dependence of the residual resistivity
r0/r300 K.
The M–H curves below Tp show similarities with the undoped
SmCoAsO sample, as shown in the inset of Fig. 2. However, obvious
difference takes place in a region of TpoToTN2. Fig. 4 shows the
field dependence of magnetization above TN1 for SmCoAsO1ꢀxFx
samples. For example, at 110 K, the magnetization gradually
increases with increasing field for x¼0.05 and 0.1 samples,
which change to FM saturation behavior for x¼0.2 sample. The
magnetic properties illustrated from M–H curves is consistent with
the M–T– curves given above.
There are two Co sublattices in NdCoAsO, as demonstrated by
neutron diffraction [18]. In SmCoAsO, Co magnetic moments are
ordered ferromagnetically in each Co sublattice at high temperature,
and involve canting between them, leading to a ferrimagnetic state.
Below TN1, the reorientation of Co moments occurs and system
transforms into AFM state. As shown in the inset of Fig. 1, F doping in
SmCoAsO results in the shrinkage of the lattice, consequently,
enhances the interaction between two Co sublattices, which
stabilizes the FM state and makes the TC increase.
Fig. 5 shows the temperature dependence of electrical resistivity
Fig. 6.
r vs. T for SmCoAsO1ꢀxFx samples under selected magnetic fields blow 45 K.
(
r
) for SmCoAsO1ꢀxFx samples.
showing metallic behavior over the whole temperature range. No
r decreases with temperature,
4. Conclusions
any magnetic transitions can be observed. At low temperatures,
r
exhibits a T2 dependence below ꢁ70 K, characteristic of a Fermi
r(T)¼r
0+AT2, where r0
Effects of F doping on magnetic and transport properties in
SmCoAsO1ꢀxFx are investigated. Multiple magnetic transitions are
observed in SmCoAsO. F doping enhances the interaction between
CoAs layers, stabilizes the FM state, and makes the TC increase. For
all of SmCoAsO1ꢀxFx samples, TN1 is a fixed value, below which
metamagnetic transition takes place. The resistivity minimums at
low temperature are observed under field for all F doped samples
and shift to low temperature with increasing field. More investiga-
tions on this system are necessary to understand competition or
correlation between magnetism and the Kondo effect.
liquid. We fitted the data to the formula
and
respectively, and obtained r0¼0.01807 m
10ꢀ5 m cm/K2 for the undoped sample. As shown in the inset of
Fig. 5, the residual resistivity r0 r300 K increases with increasing F
A
are residual resistivity and
a
numerical coefficient,
O
cm and A¼1.40196 ꢂ
O
/
content, which resulted from the impurities resistivity induced by F
doping.
The temperature dependence of electrical resistivity under
various fields for SmCoAsO1ꢀxFx samples is shown in Fig. 6. The
resistivity minimums at about 30 K (Tmin) are observed under 1 T
fields for all F doped samples and shift to low temperature with
increasing field, which may be due to AFM transition [17,18]. Tmin is
slightly smaller than TN1 from M–T curves under H¼1 T fields.
However, no resistivity minimum is observed under zero field.
There is another possibility that the resistivity minimum can be
described as Kondo effect. On the other hand, as shown in Fig. 5,
Acknowledgements
The authors are grateful for the financial support of the National
Natural Science Foundation of China under Grant no. 50588201,
50872116, 11004162, PCSIRT of the Ministry of Education of China
(IRT0751), the National High-Tech Program of China (Program 863
r
/
r
300 K vs. T plot shows feeble S-shaped behavior and a broad hump
above 75 K, which can be ascribed to Kondo coherence effects.

456
Y.L. Chen et al. / Journal of Physics and Chemistry of Solids 72 (2011) 453–456
under Grant no. 2007AA03Z203), Research Fund for the Doctoral
Program of Higher Education of China (SRFDP200806130023),
the Fundamental Research Funds for the Central Universities
(SWJTU09ZT24) and Australian Research Council under Grant nos.
DP0559872 and DP0881739. Yajing Cui is grateful for the financial
support of scientific research starting project for young teachers of
Southwest Jiaotong University (2009Q009) and the Fundamental
Research Funds for the Central Universities (SWJTU09CX054).
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