Transport, magnetic, and 119Sn Mössbauer studies on magnetically
ordered valence fluctuating compound SmRuSn3
Chandan Mazumdar, Z. Hossain, R. Nagarajan, C. Godart, S. K. Dhar et al.
Citation: J. Appl. Phys. 79, 6349 (1996); doi: 10.1063/1.361996
View online: http://dx.doi.org/10.1063/1.361996
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Transport, magnetic, and 119Sn Mössbauer studies on magnetically ordered
valence fluctuating compound SmRuSn3
Chandan Mazumdara)
Department of Physics, Indian Institute of Technology, Bombay 400 076, India
Z. Hossain and R. Nagarajan
Tata Institute of Fundamental Research, Bombay 400 005, India
C. Godart
L. C. M. S. T. R., U.P.R. 209-C.N.R.S., 92195 Meudon Cedex, France
S. K. Dhar and L. C. Gupta
Tata Institute of Fundamental Research, Bombay 400 005, India
B. D. Padalia
Department of Physics, Indian Institute of Technology, Bombay 400 076, India
R. Vijayaraghavan
Tata Institute of Fundamental Research, Bombay 400 005, India
SmRuSn3 is a unique compound among the known Sm-based valence fluctuation ~VF! systems. Its
crystallographic structure has two inequivalent Sm sites and Sm ions at only one of them are in VF
state while the Sm ions in the other site orders magnetically. Our 119Mössbauer studies show a
quadrupolar splitting at the Sn site, consistent with the noncubic symmetry of the Sn site. A
broadening of the Mössbauer spectrum is seen due to magnetic ordering of the material. The
transferred hyperfine field at Sn site at 4.2 K is small. © 1996 American Institute of Physics.
@S0021-8979~96!25308-X#
SmRuSn3 was reported to be the first ternary Sm-based
valence fluctuating ~VF! compound.1 This compound forms
in the cubic ~space group Pm3n! crystal structure. Sm ions
occupy two distinct crystallographically inequivalent sites,
i.e., 2a(0,0,0) and 6d( 41 , 14 ,0).2 From the lattice constant,
transport and magnetic properties measurements, Fukuhara
et al.1 concluded that all Sm ions are in VF state. They also
showed that SmRuSn3 undergoes a magnetic transition at
6 K. Simultaneous occurrence of VF and magnetic ordering
is noteworthy as this is the only Sm based material exhibiting this behavior. Only two Sm-based compounds ~both binary!, SmS ~Ref. 3! and SmB6 ,4 are known to exhibit VF
phenomena and they do not order magnetically. In view of
this unusual behavior, we reinvestigated5 the physical properties of SmRuSn3 . Our magnetic, specific heat and L III edge
results of SmRuSn3 established the mixed valence nature of
Sm in this system. Here we briefly highlight our earlier work
to emphasize the uniqueness of the system and present the
results of our investigations of 119Sn Mössbauer spectroscopy in this system.
Details of sample preparation of SmRuSn3 are given
elsewhere.5 The lattice parameter, a, of our sample of
SmRuSn3 is 9.666 Å5 implying that the valence state of Sm
ions does not differ significantly from 31. We may point out
that Fukuhara et al. reported a value of a~59.73 Å!1 which
deviates from the lanthanide contraction expected for Sm31.
Electron microprobe analysis showed that our sample is
largely homogeneous having a composition Sm0.98RuSn3.10
~normalized to Ru!. Small inclusions ~'50 mm2!, of Sn
a!
Present address: Solid State Physics Group, Tata Institute of Fundamental
Research, Bombay 400 005, India.
J. Appl. Phys. 79 (8), 15 April 1996
metal and some inclusions of composition SmRu were observed occasionally.
Our magnetic susceptibility data show a cusp around 6 K
~inset Fig. 1!, which indicate antiferromagnetic ordering of
the material. Since only Sm31 ions carry a magnetic moment, the magnetic order must be due to Sm31 ions. The
magnetic susceptibility ~Fig. 1! at room temperature is larger
than what one would expect for a Sm31 material. Our theoretical calculations6 showed that the observed susceptibility
cannot be accounted by mixing of excited state and/or crystal
field contributions. We could account for the observed room
temperature susceptibility ~also taking into account the temperature independent van Vleck susceptibility! if about 14%
of Sm ions are in divalent state ~Fig. 1!. We note here that
FIG. 1. Temperature dependence of dc magnetic susceptibility ~s! of
SmRuSn3 ~corrected w.r.t. dc magnetic susceptibility data of LaRuSn3!.
Short dashed lines are the calculated susceptibility for free Sm31 and
Sm21 ions. Long dashed line represents the susceptibility,
(12x) x ~Sm31!1x x ~Sm21!, with x having a value 0.14. Inset shows the
expanded region near the magnetic ordering temperature.
0021-8979/96/79(8)/6349/3/$10.00
© 1996 American Institute of Physics
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6349
FIG. 2. Specific heat of SmRuSn3 ~s! and LaRuSn3 ~d!. Solid line is the
calculated magnetic entropy for SmRuSn3 ~obtained by subtracting specific
heat data of LaRuSn3 from that of SmRuSn3!. R is the molar gas constant.
though Sm21 ions in the ground state do not carry a magnetic
moment, it has a relatively large susceptibility due to mixing
of low lying excited state. Thus, susceptibility data indicate
mixed valence behavior of Sm ions in the material.
A quantitative estimate of the Sm ions taking part in
magnetic order was obtained from magnetic entropy estimated from specific heat measurements on SmRuSn3 .5 Specific heat measurements confirmed the magnetic ordering
around 6 K ~Fig. 2!. The presence of strong crystal field
effects is evident from the specific heat data of SmRuSn3 . If
all the Sm ions would have contributed to the magnetic entropy, at the magnetic transition temperature, one would expect a minimum magnetic entropy of R ln 2 ('0.693R) as
observed for a material having a crystal field doublet ground
state. The magnetic entropy at the ordering temperature is
much lower ~;0.5R at 6 K! and can be accounted for properly, if one assumes that only the Sm ions at the 6d crystal
site ~having a crystal field doublet ground state! contribute
toward magnetism, resulting a total minimum magnetic entropy of ~3/4!R ln 2 at the magnetic transition temperature,
which matches very well with the experimental observation.
The Sm ions at 2a site is nonmagnetic or weakly magnetic
and hence do not contribute toward magnetic entropy. The
specific heat results in conjunction with the magnetic susceptibility results can be consistently interpreted if Sm ions at
6d sites are in stable trivalent state and those at 2a sites are
not in stable divalent state but are in VF state with an average valency of ;2.6 at room temperature, making the material a unique one in the field of VF.
Our x-ray absorption ~L III edge! spectroscopic measurements on SmRuSn37 exhibited a temperature dependent bimodal structure which confirm that some of the Sm ions in
the material are in VF state with the average valence varying
from 2.88 at 10 K to 2.91 at 300 K. This total average valence is consistent with the above distribution of the valence
of Sm at the two sites of the material.
Mössbauer spectroscopy, with a probing time of '1028 s
~which is slower than the usually encountered fluctuation
time '10213 s! is a complementary technique with respect to
L III edge absorption spectroscopy ~probing time '10216 s! in
the investigation of valence fluctuation phenomenon. This
fact has been successfully utilized in the case of Eu-based
VF systems using 151Eu Mössbauer spectroscopy.8 Although,
fluctuation effects primarily take place at the rare earth site,
6350
FIG. 3. 119Sn Mössbauer spectroscopic result on SmRuSn3 at different temperatures. The solid lines are fit to experimental data.
119
Sn Mössbauer spectroscopy has been fruitfully used in the
investigation of certain VF systems.9–11 One can also obtain
information on magnetic ordering of Sm ions through transferred hyperfine interactions at the Sn site. Mössbauer spectra of SmRuSn3 were taken against a 119Sn source in CaSnO3
matrix in the temperature range 4.2–300 K. A conventional
constant acceleration-type spectrometer in conjunction with
a home built multiscaler analyzer was used in the studies.
Measurements were made with absorbers of different thickness ~12 mg/cm2 and 30 mg/cm2! to estimate thickness
broadening effects in the line width of the resonance.
Figure 3 shows the 119Sn Mössbauer spectra on
SmRuSn3 at different temperatures. The spectrum at all temperatures is a doublet but the intensities of the two components are not equal. Since there is only one crystallographic
site, 24k, for Sn in this compound, a doublet structure would
primarily arise from quadrupole interaction from the presence of electric field gradient at the Sn site. Furthermore, for
the same reason, the asymmetry of intensity in the doublet
cannot originate from different Sn environments. One may
consider the possibility of attributing the asymmetry to fluctuation effects. However, a careful analysis of the spectra
reveals that the position of the more intense peak of this
doublet is very close to that of pure Sn-metal. It should be
noted here, as mentioned earlier, the electron microprobe
measurements on SmRuSn3 reveals a few small inclusions of
Sn metal in our sample of SmRuSn3 . Considering this, we
explain the spectrum in terms of a superposition of a doublet,
arising out of the non-cubic site symmetry of Sn site, and a
single line of small intensity due to Sn metal.
We have analyzed our spectra in terms of a singlet due to
Sn metal @~fixed isomer shift ~IS! ~2.5 mm/s w.r.t. CaSnO3!
and fixed width ~1.2 mm/s!# and a quadrupole doublet due to
SmRuSn3 . The fit yields the IS as 2.15 mm/s ~w.r.t. CaSnO3!,
e 2 Qq/2 ~where e is the electronic charge, Q is the nuclear
quadrupole moment, and eq is the electric field gradient
along the principle axis! as 1.75 mm/s and width of each line
as 1.5 mm/s for the Sn-Mössbauer spectra of SmRuSn3 . The
J. Appl. Phys., Vol. 79, No. 8, 15 April 1996
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Mazumdar et al.
result is essentially temperature independent above the magnetic ordering temperature. At 4.2 K, a small broadening of
119
Sn Mössbauer line indicates the magnetic ordering taking
place in this compound. However, the transferred hyperfine
field, measured at 4.2 K, at the Sn site is very small. Apart
from the antiferromagnetic nature of the ordering, the measurement temperature ~4.2 K! being very close to magnetic
ordering temperature ~6 K! may also be one of the reasons
responsible for the small transferred hyperfine field. Sm-ions
at 2a sites, being in non ordered state, do not contribute to
the transferred field. Investigations, such as elastic neutron
scattering, will be useful to understand the detailed magnetic
structure.
To conclude, specific heat and magnetic susceptibility
measurements on SmRuSn3 suggest that Sm ions in 2a crystallographic site are valence fluctuating, while Sm ions in 6d
crystallographic site order antiferromagnetically. The valence
fluctuating behavior of Sm ions in is confirmed by L III-edge
spectroscopic measurements. 119Sn Mössbauer spectroscopy
results on SmRuSn3 exhibit a temperature independent isomer shift and electric field gradient ~arising due to noncubic
site-symmetry!. The asymmetry that we observe is not likely
to be due to VF. The transferred hyperfine field at Sn site is
small at 4.2 K.
J. Appl. Phys., Vol. 79, No. 8, 15 April 1996
We would like to thank S. K. Paghdar in Mössbauer
spectroscopy measurements.
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Mazumdar et al.
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6351