On magnetic ordering in NdCu2 Sn2 and PrCu2 Sn2
Chandan Mazumdar a,∗ , R. Nagarajan b , S.K. Dhar b , L.C. Gupta b ,
C. Godart c,d , B.D. Padalia e
a
Experimental Condensed Matter Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700 064, India
b Tata Institute of Fundamental Research, Bombay 400 005, India
c CNRS, UPR 209, Thiais Cedex, France
d CNRS, LURE-Universite de Paris Sud, 91405 Orsay, France
e Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
Abstract
We report here our results showing the occurrence of magnetic order in the intermetallic compounds NdCu2 Sn2 and PrCu2 Sn2 . For
comparison, non-magnetic analogue, LaCu2 Sn2 , was also prepared. X-ray diffraction (XRD) result confirms that all the three compounds
form in the tetragonal CaBe2 Ge2 -type structure. A cusp is observed at ∼3 K in the magnetic susceptibility of NdCu2 Sn2 and PrCu2 Sn2 ,
which arises due to the antiferromagnetic ordering of the magnetic moments in these two compounds. The bulk nature of magnetic order in
both the compounds is confirmed from the observation of peaks in the heat capacity at the magnetic transition temperature. X-ray absorption
measurements at the LIII edge show that Pr ions are in the trivalent state in PrCu2 Sn2 .
Keywords: Rare earth compounds; Magnetically ordered materials; Magnetic measurements
1. Introduction
The ternary intermetallic compounds RM2 X2 (R = rare
earth, M = transition metal, X = Si, Ge, P, Sn, As, Sb, etc.)
are interesting compounds because many members of this
family exhibit anomalous physical properties such as, heavy
fermion behaviour and superconductivity in CeCu2 Si2 [1],
non-magnetic heavy fermion behaviour in CeRu2 Si2 [2],
cooperative valence fluctuation/transition in EuPd2 Si2 [3],
coexistence of superconductivity, magnetism and heavy
fermion phenomenon in URu2 Si2 [4], high magnetic ordering temperature in CeRh2 Si2 [5], complex magnetic
structure in RM2 Si2 (R = Tb, Ho; M = Rh, Ru, Ir) [6],
re-entrant ferromagnetism and large magnetoresistance in
SmMn2 Ge2 [7,8], etc. Majority of these compounds form
in the body-centred-tetragonal ThCr2 Si2 -type structure
(space group I4/mmm), while some adopt the primitive
tetragonal CaBe2 Ge2 -type structure (space group P4/nmm).
There are also a few compounds, such as RPt2 Ge2 [9],
RPt2 Sn2 (R = La, Ce) and EuCu2 Sn2 [10–12] which form
in monoclinic (space group P21 ) variants of CaBe2 Ge2 -type
or ThCr2 Si2 -type structure. While physical properties of
RM2 X2 with X = Si and Ge have been investigated extensively, only a few studies have been reported on the
properties of the series with X = Sn, such as, CeCu2 Sn2
(antiferromagnetic order, TN = 1.6 K [13]), SmCu2 Sn2
[14], LaCu2 Sn2 [15], CeM2 Sn2 (M = Ni, Cu, Rh, Pd, Ir,
Pt) [16], etc. We had prepared RCu2 Sn2 (R = La–Eu) and
investigated their properties [10]. We found that NdCu2 Sn2
orders antiferromagnetically around 3 K. Recently, Baran
et al. [17] have reported from neutron diffraction results
that PrCu2 Sn2 orders below TN ∼3.8 K but NdCu2 Sn2 does
not order magnetically down to 1.6 K. The latter result is in
contradiction with our result. Slebarski et al. [18] have also
reported magnetic ordering in PrCu2 Sn2 at TN ∼3.6 K. Here
we present our studies of structural, magnetic and transport properties of PrCu2 Sn2 and NdCu2 Sn2 and show that
bulk magnetic order exists in both these compounds. We
have also prepared and studied LaCu2 Sn2 as a nonmagnetic
comparison material.
2
2. Experimental
The materials were synthesised by arc melting constituent elements (purity: La, Pr, Nd > 99.9%, Cu > 99.9%,
Sn > 99.999%) under flowing argon atmosphere. The samples were annealed at 800 ◦ C for 10 days. Room temperature powder X-ray diffraction (XRD) data were taken on
a commercial diffractometer (JEOL, Japan). Resistivity
measurements were performed in the temperature range
4.2–300 K by conventional four-probe method in an automated home-built set-up and magnetic susceptibility
measurements were performed using a SQUID magnetometer in the temperature range 2–300 K. Heat capacity
measurements were carried out between 1.7 and 25 K in a
home-built automated calorimeter using semi-adiabatic heat
pulse technique. X-ray absorption studies at the Pr LIII -edge
were carried out at the French synchrotron facility, Laboratoire pour l’Utilisation du Rayonnement Electromagnetique
(LURE) at Orsay, France.
3. Results and discussions
The XRD patterns of LaCu2 Sn2 , PrCu2 Sn2 and NdCu2 Sn2
could be indexed on the basis of the expected primitive
tetragonal CaBe2 Ge2 -type structure. The XRD patterns
of the as cast sample show a few extra lines due to the
presence of parasite impurity phases (maximum intensity
< 5%) were observed which disappear on annealing. The
good quality of our samples can be inferred from their XRD
patterns, shown with Rietveld analysis in Fig. 1. The lattice
parameters (a and c) of the annealed LaCu2 Sn2 , PrCu2 Sn2 ,
and NdCu2 Sn2 are: 4.486 and 10.562; 4.439 and 10.421 and
4.428 and 10.338 Å, respectively. It should be noted that
unannealed PrCu2 Sn2 and NdCu2 Sn2 have slightly smaller
c parameter (10.347 and 10.287 Å, respectively). The lattice parameters agree reasonably well with those reported
in literature [19].
Electrical resistivity, ρ(T), of PrCu2 Sn2 and NdCu2 Sn2
measured in the temperature range 4.2–300 K, exhibits
normal metallic behaviour (Fig. 2). A minor anomaly is
observed around 10 K in ρ(T) of PrCu2 Sn2 which is presumably due to minor impurity phase present in the material
(see below). Apparently, annealing the samples decreases
the parasite phases, but does not remove them completely.
Magnetic susceptibility, M(T)/H, data of the as cast materials are shown in Figs. 3 and 4. The inset shows the susceptibility of the annealed samples in the low temperature
region, both under zero field cooled (ZFC) and field cooled
(FC) conditions. At higher temperature, the susceptibility of
the annealed and as cast samples is practically the same.
A sharp cusp in the susceptibility at 3.5 K, shown both by
ascast as well as annealed samples of NdCu2 Sn2 (Fig. 3),
indicates that NdCu2 Sn2 orders antiferromagnetically. The
susceptibility above the Néel temperature (5–300 K) follows Curie–Weiss (CW) behaviour with effective paramag-
Fig. 1. Room temperature powder XRD pattern of annealed LaCu2 Sn2 ,
PrCu2 Sn2 and NdCu2 Sn2 . The symbols represent the experimental data;
solid line is a fit using the Full Profile Fitting procedure, for the space
group, P4/nmm. Difference between the calculated and the experimental
XRD patterns are also shown in the figure. The positions of the peak in
the XRD pattern permitted by the space group P4/nmm are denoted by
vertical bars.
netic moment, µeff = 3.7µB and paramagnetic Curie temperature, Θp = −7.7 K (obtained from the fit to the expression, M(T)/H = C/(T − Θp )). This value of µeff is very
nearly equal to that of free trivalent Nd-ion (3.62µB ). The
negative sign of Θp is consistent with the antiferromagnetic
ordering.
In PrCu2 Sn2 , a cusp in the susceptibility is seen around 3
K (Fig. 4) which suggests occurrence of antiferromagnetic
order in this material. The susceptibility above 15 K follows
CW behaviour with µeff = 3.6µB and Θp = −2.4 K. This
value of µeff is the same as that of free Pr3+ ion (3.58µB ).
A minor anomaly in the susceptibility occurs around 12
K in PrCu2 Sn2 (Fig. 4) close to which, as mentioned above,
the ρ(T) (Fig. 2) also exhibits an anomaly. In NdCu2 Sn2 , a
similar anomaly occurs around 17 K (Fig. 3) but no corresponding anomaly is seen in the ρ(T) (Fig. 2). However, our
heat capacity results (see below) suggest that in both the ma-
3
Fig. 2. Resistivity of PrCu2 Sn2 (circ) and NdCu2 Sn2 (bullet) as a function
of temperature. The inset shows the expanded region at low temperature.
The anomaly in the ρ(T) of PrCu2 Sn2 near 12 K is due to the magnetic
ordering of very small amount of impurity phase in this material.
terials, these anomalies at higher temperature are very likely
due to a magnetic impurity phase.
Heat capacity, C(T), studies were performed to ascertain
the bulk nature of magnetic ordering in the two compounds.
In NdCu2 Sn2 , a large peak around 3 K, with a peak height
of ∼6.5 J/mol K, was observed both in the ascast as well
as annealed samples (Fig. 5) which confirms bulk magnetic
ordering in the material. We note that there is no anomaly
in the heat capacity around 17 K which suggest that the
anomaly observed in the susceptibility data around the same
temperature (Fig. 3) is due to a small magnetic impurity
phase. A minor upturn observed below 1.8 K in the as cast
sample (not shown) is not observed in the annealed sam-
Fig. 3. DC magnetic susceptibility (measured in a field of 1 kG) and its
inverse of as cast NdCu2 Sn2 as a function of temperature. The solid line
is a fit to CW law. The upper inset is the expanded view of magnetic
susceptibility of annealed sample in low temperature region, both under
ZFC and FC conditions. The cusp in susceptibility at ∼3.5 K indicates
the possible magnetic ordering in this material. The lower inset shows
the isothermal magnetisation behaviour of the material at 2 K.
Fig. 4. DC magnetic susceptibility (measured in a field of 1 kG) and its
inverse of as cast PrCu2 Sn2 as a function of temperature. The solid line
is a fit to CW law. The anomaly in the susceptibility of PrCu2 Sn2 near
10 K is due to magnetic ordering of very small amount of impurity in this
material. The upper inset is the expanded view of magnetic susceptibility
of annealed sample at low temperature region, both under ZFC and FC
conditions showing the magnetic ordering at ∼3 K. The lower inset
shows the isothermal magnetisation behaviour of the material measured
at 2 K.
ple, suggesting that it is also very likely due to an impurity phase, so small that it is not discernable in the XRD
result.
The heat capacity of annealed sample of PrCu2 Sn2 (Fig. 6)
shows a peak of about 5.5 J/mol K, around 3 K, where a cusp
in susceptibility is observed, strengthening the possibility of
a magnetic order below this temperature. However, the net
jump in the heat capacity at the transition temperature is only
∼1 J/mol K. We find that in the ascast sample, the magnetic
order is seen only as a sudden decrease of C(T) around 3 K.
Around 11 K, where an anomaly in M(T)/H and ρ(T) were
seen in the as cast samples, only a minor structure of small
magnitude is observed in the heat capacity data of ascast
sample of PrCu2 Sn2 . However, this feature is not seen in
the heat capacity of the annealed sample (Fig. 6). These
results confirm that the anomaly in the susceptibility of
PrCu2 Sn2 around 11 K, is due to the presence of a magnetic
impurity.
Recently, it has been reported from neutron diffraction
measurements [17] that there is no magnetic order down to
1.6 K in NdCu2 Sn2 and that PrCu2 Sn2 orders magnetically
below TN = 3.8 K. Antiferromagnetic ordering in PrCu2 Sn2
has also been reported by Slebarski et al. [18] from magnetic
measurements. The result of absence of magnetic order
down to 1.6 K in NdCu2 Sn2 is in conflict with our finding
of magnetic order around 3 K in this material. We, therefore, prepared one more independent batch of NdCu2 Sn2 as
well as PrCu2 Sn2 , annealed them at 800 K for 1 week as reported in Ref. [17] and measured the heat capacity of these
samples. The heat capacity peak was reproduced in this
batch of samples also, confirming the occurrence of bulk
magnetic order in NdCu2 Sn2 . The magnetisation data were
also matching with the data taken on samples made earlier.
4
Fig. 5. Temperature dependence of heat capacity (C) of annealed NdCu2 Sn2 and LaCu2 Sn2 . The peak in heat capacity of NdCu2 Sn2 around 3 K indicate
occurrence of magnetic ordering. Continuous line is the magnetic component of heat capacity of NdCu2 Sn2 .
The peak in the heat capacity of PrCu2 Sn2 due to magnetic order, is not as prominent as typically seen at a
transition to long-range magnetic order. For example, it is
relatively muted in comparison with NdCu2 Sn2 . We believe
that the prominence of the peak is masked by the presence
of appreciable heat capacity in the paramagnetic region
just above TN . The large heat capacity in the paramagnetic
region could either be due to Schottky contribution arising
from the thermal depopulation of the closely lying excited
crystal electric field level(s) of Pr3+ ions or it may even
arise due to complex short range magnetic order above 3
K. In order to obtain the 4f derived magnetic contribution
to the heat capacity, the heat capacity of non-magnetic
analogue, LaCu2 Sn2 was measured. The 4f-electronic contribution to the heat capacity of PrCu2 Sn2 and NdCu2 Sn2
obtained after subtracting the heat capacity of LaCu2 Sn2
is shown in Figs. 5 and 6. The large 4f-electronic heat capacity in the paramagnetic region 4–25 K is clearly seen in
both the cases and even exhibits a broad maximum centred
around 10 K in PrCu2 Sn2 and around 20 K in the case
of NdCu2 Sn2 . This is typical of presence of appreciable
Schottky contribution to the heat capacity, which arises
due to crystal field splitting of the ionic energy levels. It is
also clear from the magnetic contribution curve (subtracted
curve) of Fig. 6 that the magnetic peak in PrCu2 Sn2 would
have been as prominent as that of NdCu2 Sn2 if the Schottky
anomaly peak had occurred at a higher temperature.
From neutron diffraction measurements, Baran et al. have
reported that in the magnetically ordered state, the moment
on Pr in PrCu2 Sn2 to be 1.17µB [17] which is smaller
than the free Pr3+ ion value of 3.2µB . However, the µeff
in the paramagnetic state is same as that of the free Pr3+
ion. Further, as pointed out by Baran et al. [17], TN (∼3
K) of PrCu2 Sn2 is much higher than that expected from
de Gennes scaling (∼0.56 K) which may also be either
due to f-electron hybridisation with conduction electrons
or due to crystal field splitting of f-electron levels. Pr ions
can have two different valence states, Pr3+ and Pr4+ . To
confirm the trivalent nature of Pr ions in this compound,
we performed X-ray absorption edge measurements of
PrCu2 Sn2 at the Pr–LIII -edge at room temperature. The
result is shown in Fig. 7. A single absorption peak around
5965 eV indicates Pr ions to be in the stable 3+ valence
state and rules out any possibility of mixed valence behaviour in this material. Slebarski et al. [18], have also
concluded from X-ray photoelectron spectroscopic (XPS)
measurements of PrCu2 Sn2 that Pr valence in this material
is close to 3+. Thus, the large TN of PrCu2 Sn2 is not due to
hybridisation effects, but a crystalline electric field (CEF)
effect. Inelastic neutron scattering experiments would provide proper information on the CEF levels in these two
compounds.
Fig. 6. Temperature dependence of heat capacity (C) of annealed PrCu2 Sn2
and LaCu2 Sn2 . A small peak in heat capacity of PrCu2 Sn2 around 2.5 K
indicate occurrence of magnetic transition. Continuous line is the magnetic
component of heat capacity of PrCu2 Sn2 .
5
References
Fig. 7. X-ray absorption (µ) at Pr LIII -edge in PrCu2 Sn2 measured at
300 K.
4. Conclusion
We have studied the structural, transport, and magnetic
properties of two compounds, PrCu2 Sn2 and NdCu2 Sn2 .
Heat capacity measurements on the non-magnetic LaCu2 Sn2
have also been performed. From magnetic susceptibility
studies, antiferromagnetic order is inferred in both the materials around 3 K. Recent neutron scattering investigations
report the absence of magnetic order down to 1.6 K in
NdCu2 Sn2 , which is contrary to our findings. Our heat capacity measurements on independently prepared batches of
NdCu2 Sn2 , with as cast as well as annealed samples have
confirmed bulk magnetic ordering of the material around 3
K. Our heat capacity results also confirm bulk magnetic order in PrCu2 Sn2 . Both the materials show large Schottky
contribution in the heat capacity at low temperatures, which
to some extent masks the anomaly in the heat capacity at
the magnetic transition temperature in PrCu2 Sn2 .
Acknowledgements
We thank S.K. Paghdar for his valuable and timely help
in some of the sample preparation and characterisation.
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