Results on electrical resistivity, magnetoresistance, magnetic Results on
electrical resistivity, magnetoresistance, magnetic susceptibility, heat
capacity and Gd Mossbauer measurements on a Gd-based intermetallic compound,
Gd$_{2}$PdSi$_{3}$ are reported. A finding of interest is that the resistivity
unexpectedly shows a well-defined minimum at about 45 K, well above the long
range magnetic ordering temperature (21 K), a feature which gets suppressed by
the application of a magnetic field. This observation in a Gd alloy presents an
interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long range
magnetic order as a mechanism for the electrical resistance minimum.Comment: 4 pages, 4 figure

E. V Sampathkumaran

Elliot R. J.

G Wortmann

Gratz E.

M Strecker

Mallik R.

Maple M. B.

R Mallik

English

arXiv

Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat capacity and 155Gd M&#246;ssbauer measurements on a Gd-based intermetallic compound, Gd2PdSi3, are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents an interesting scenario. On the basis of our results, we propose electron localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as a mechanism for the electrical resistance minimum

Mallik, R.

Sampathkumaran, E. V.

Strecker, M.

Wortmann, G.

Indian Academy of Sciences

EUROPHYSICS LETTERS 1 February 1998
Europhys. Lett., 41 (3), pp. 315-320 (1998)
Observation of a minimum in the temperature-dependent
electrical resistance above the magnetic-ordering
temperature in Gd2PdSi3
R. Mallik
1
, E. V. Sampathkumaran
1(∗), M. Strecker2 and G. Wortmann2
1 Tata Institute of Fundamental Research
Homi Bhabha Road, Colaba, Mumbai-400 005, India
2 Fachbereich 6-Physik, Universität Paderborn - D-33095, Germany
(received 30 June 1997; accepted in final form 4 December 1997)
PACS. 72.15−v – Electronic conduction in metals and alloys.
PACS. 75.20Hr – Local moment in compounds and alloys; Kondo effect, valence fluctuations,
heavy fermions.
PACS. 75.30Et – Exchange and superexchange interactions.
Abstract. – Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat
capacity and 155Gd Mössbauer measurements on a Gd-based intermetallic compound, Gd2PdSi3,
are reported. A finding of interest is that the resistivity unexpectedly shows a well-defined
minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a
feature which gets suppressed by the application of a magnetic field. This observation in a
Gd alloy presents an interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as
a mechanism for the electrical resistance minimum.
Ever since a minimum in the electrical resistance (R) at a characteristic temperature in noble
metals containing 3d magnetic impurities was reported several decades ago, the consequences
of this phenomenon, known as the Kondo effect, in metallic solids remains an active area
of research. In concentrated Kondo alloys of Ce, Sm, Yb or U [1], if the strength of the
intersite indirect exchange interaction (given by TRKKY) is comparable to that of the Kondo
effect (given by the Kondo temperature, TK), one also observes magnetic ordering well below
the temperature at which resistivity shows a minimum. However, such a feature in R is not
expected for Gd alloys, since the Gd 4f orbital is so well localized that it cannot exhibit the
Kondo effect. In this article, we report the observation of a pronounced minimum in the
plot of R vs. temperature (T ) well above the magnetic-ordering temperature in a Gd-based
intermetallic compound, Gd2PdSi3 (which is presumably antiferromagnetic [2], TN = 21 K),
resembling the behavior in magnetic Kondo lattices. This finding implies that a minimum in
(∗) E-mail: SAMPATH@tifrvax.tifr.res.in
c© EDP Sciences
316 EUROPHYSICS LETTERS
R(T ) can also occur from a completely different mechanism as a magnetic precursor effect,
which, we believe, is an exchange-interaction–induced electron localization.
The polycrystalline sample, Gd2PdSi3, was prepared by arc melting followed by homog-
enization at 750 ◦C in an evacuated sealed quartz tube. We have also investigated alloys
with Gd substituted by Y , i.e. (Gd1−xYx)2PdSi3 (x = 0.2, 0.5 and 0.8). The X-ray powder
diffraction patterns (Cu Kα) confirm that these alloys are single phase forming the hexagonal
AlB2-type structure [2]. The homogeneity of the samples was checked by scanning electron
microscopy. The electrical-resistance measurements (4.2–300 K) were performed by a con-
ventional four-probe method. Additional experiments carried out to support the line of our
arguments and conclusions are: i) The magnetoresistance measurements in the longitudinal
geometry in a magnetic field (H) of 50 kOe as a function of T and as a function of H at selected
temperatures for the x = 0.0 alloy; ii) 155Gd(5/2 → 3/2, 86.5 keV transition) Mössbauer
measurements in the transmission geometry with a 2 mCi 155Eu(SmPd3) source for x = 0.0
alloy below 25 K at selected temperatures; iii) Heat-capacity (C) measurements as a function
of temperature (2-70 K); iv) Magnetic-susceptibility (χ) measurements (2–300 K) in a field of
100 Oe and in 2 kOe.
The temperature dependence of the normalized electrical resistivity below 200 K is plotted
in fig. 1 for all compositions investigated. The values of ρ at 300 K and 4.2 K for Gd2PdSi3
are of the order of 400 and 280 mΩ cm, respectively. Due to the presence of microcracks in
the sample, the error of these values might be smaller by about 40%. For this reason, the
data shown in fig. 1 are normalized to the values at 300 K. It is obvious from fig. 1 that the
ρ(T ) of the Gd2PdSi3 compound gradually decreases as the temperature is lowered down to
60 K, followed by an upturn below about 45 K, thus giving rise to a resistivity minimum at
about (Tmin =)45 K; the value before the onset of magnetic ordering (say, at 22 K) is about 5%
higher compared to the minimum resistivity. There is a kink in ρ(T ) vs. T at about 20 K, close
to the temperature where the Gd sublattice exhibits long-range magnetic ordering presumably
of antiferromagnetic-type, as one may conclude from the well-defined peak in the magnetic
susceptibility and from the shoulder in the heat-capacity data (vide infra). Further lowering of
temperature does not result in a drop in ρ(T ) as expected due to the loss of the spin-disorder
contribution below the Néel temperature (TN). We presently attribute this to the opening of
a pseudogap at some portions of the Fermi surface due to the onset of antiferromagnetism
in this compound [3]. The magnetic-ordering temperatures for the compositions x = 0.2, 0.5
and 0.8 are lowered (see the heat-capacity data described below) as expected on the basis
of indirect exchange mechanism; however, the resistivity minimum persists well above TN
for all compositions. Even in the diluted limit of our investigation, x = 0.8, the resistance
increases by about 2% as the temperature is lowered from Tmin towards 4.2 K. The fact that
the minimum is also observable in the sample with the highest Y content (x = 0.8) may exclude
the explanation based on the opening of an energy gap (above 21 K).
Now turning to the influence of the application of a magnetic field, the negative dρ/dT
behavior in zero field vanishes in the presence of a field of 50 kOe (shown in fig. 2 (a) for
x = 0.0 only for the sake of clarity). The exact dependence of TN on H could not be inferred
from the present data and it is not relevant to the main conclusion of this article. The
magnetoresistance, defined as ∆ρ/ρ = {ρ(H) − ρ(0)}/ρ(0), is obtained from this data as a
function of T (shown in fig. 2 (b) for all the compositions). For x = 0.0, it is found that ∆ρ/ρ
is as large as about −10% at 40 K in the presence of H = 70 kOe (fig. 2 (c)). For comparison,
the magnitude of the magnetoresistance values for the isostructural Ce-based Kondo lattice
Ce2PdSi3 (ref. [4]) as well as for the alloys, (Ce1−xYx)2PdSi3, is less than 1% down to 4.2 K.
This establishes that the observed large magnetoresistance effect is Gd-related, as noted by us
for some other Gd alloys [5].
r. mallik et al.: observation of a minimum etc. 317
Fig. 1 Fig. 2
Fig. 1. – The electrical resistance (R) of the alloys, (Gd1−xYx)2PdSi3 (x = 0.0, 0.2, 0.5 and 0.8),
normalized to 300 K values.
Fig. 2. – (a) The electrical resistivity (ρ) as a function of temperature for Gd2PdSi3 in the presence
and in the absence of a magnetic field of 50 kOe. (b) The magnetoresistance (∆ρ/ρ) obtained from
the data shown in (a) for x = 0.0 is plotted; in addition, the data obtained in a similar way for other
compositions are also shown. (c) For x = 0.0, the ∆ρ/ρ as a function of magnetic field at selected
temperatures; the lines through the data points serve as guides to the eye. The error in ∆ρ/ρ is
estimated to be less than 0.5%.
It is not out of place to add that, as the temperature is lowered below TN, the ρ(T ) curve at
50 kOe does not show an upturn. If the upturn in ρ below TN is attributed to magnetic
Brillouin-zone boundary gaps, this finding implies that these gaps are essentially getting
washed out by the application of a magnetic field; there is a feeble rise below 7 K in the
data in the presence of a field, due to possible non-vanishing gap at low temperatures. The
magnitude of ∆ρ/ρ thus increases below 20 K for the x = 0.0 alloy (fig. 2 (b)), attaining a peak
value of about −32% at 7 K. In order to highlight this feature further, the ∆ρ/ρ is plotted as
a function of H in fig. 2 (c) for various temperatures. The overall shape of the plot of ∆ρ/ρ
vs. H gets significantly modified as the temperature is lowered across TN due to suppression
of the proposed magnetic Brillouin-zone boundary gap. The observation of a large negative
value of ∆ρ/ρ is of importance considering the current interest in the phenomenon of giant
magnetoresistance.
318 EUROPHYSICS LETTERS
Fig. 3 Fig. 4
Fig. 3. – 155Gd Mössbauer spectra of Gd2PdSi3 at selected temperatures. The continuous lines
represent standard least-square fit of the data. At the bottom, the effective magnetic-hyperfine-field
values obtained from the data are shown as a function of temperature.
Fig. 4. – (a) The magnetic susceptibility (H = 2 kOe) as a function of temperature, (b) the
heat capacity and (c) the 4f contribution (Cm) to C as a function of temperature for the alloys,
(Gd1−xYx)2PdSi3. The lines through the data points in (a) serve as guides to the eyes.
In order to be sure that the upturn in R below Tmin is not due to the opening-up of
magnetic Brillouin-zone boundary gaps, it is desirable to confirm the TN value by a microscopic
technique as well. We therefore show the results of 155Gd Mössbauer (only for x = 0.0), χ and
C measurements in fig. 3, 4 (a) and 4 (b), respectively. The Mössbauer spectra below 20 K are
found to undergo Zeeman splitting, which is well-resolved below 15 K. The magnetic hyperfine
field at the Gd nucleus obtained by a standard least-square fitting procedure of the Mössbauer
spectra are also shown in fig. 3 for x = 0.0. The temperature at which the magnetic hyperfine
field extrapolates to zero (near 20 K) marks the magnetic-ordering temperature and this serves
as a conclusive microscopic proof for paramagnetism above 20 K (which is also confirmed by
our Mössbauer thermal scan experiment) in this compound. The spectral analysis of the data
for 25 K gave evidence for a quadrupolar splitting only (the magnitude of which is 0.88 mm/s).
We have also probed the magnetic behavior by other bulk measurements. With respect
to χ (H = 2 kOe), the low-temperature data show a distinct cusp in χ at about 20 K in
this alloy, indicative of antiferromagnetic-like ordering. The feature due to magnetic ordering
shifts to lower temperatures with dilution of the Gd sublattice (fig. 4 (a)). The plot of the χ−1
vs. temperature (not shown in a figure) is found to be linear down to the critical temperature,
r. mallik et al.: observation of a minimum etc. 319
thus suggesting that the peak in χ(T ) marks the onset of long-range magnetic order. Consistent
with this, in the heat capacity there is a peak attributable to the onset of magnetic ordering
at different temperatures (fig. 4 (b)) for each of the three compositions, x = 0.0, 0.2 and 0.5
with magnetic-ordering temperatures of 20, 15 and 8 K, respectively. For x = 0.8, there is no
clear-cut anomaly due to the onset of long-range magnetic ordering above 2 K. We obtained
the 4f contribution (Cm) to C, shown in fig. 4 (c), using the procedure suggested in ref. [6]
employing the C values of Y2PdSi3 as a reference for the lattice contribution. A point to be
noted [7] is that there is a broad shoulder (marked by a vertical arrow in fig. 4 (c)) at about
22 K (which is a measure of TN from heat capacity [6]) for x = 0.0, beyond which Cm falls
gradually with temperature, extending to about 20 K above TN, similar to that in other Gd
alloys [8]. The magnetic entropy (Sm) at TN obtained from our data is only about 75% of
the saturation value of R ln 8 (R = gas constant) for all the magnetically ordered alloys; the
full value is reached far above TN only, for instance, only around 45 K in Gd2PdSi3. Clearly,
the heat-capacity tail persists for about 20 K due to magnetic precursor effects [8] even in the
case of the dilute alloy, x = 0.8, mimicking heavy-fermion behavior, though this alloy does not
undergo long-range magnetic order above 4 K.
Thus, all the results, including those from the Mössbauer spectroscopy, conclusively estab-
lish the onset of long-range magnetic ordering in the vicinity of 21 K for x = 0.0. Hence
the upturn in R below 45 K is not a consequence of a long-range antiferromagnetic-ordering
(gap) effect. It is important to note that the temperature till which the heat-capacity tail
above TN persists and the one below which a negative ∆ρ/ρ is noticeable beyond experimental
error shift down with increasing x (compare the data for x = 0.0 and 0.8 in fig. 2 (b)) [9].
These observations suggest that these anomalies arise from a common physical origin and thus
the minimum in R is a manifestation of a Gd 4f magnetic precursor effect. We would like
to add that there is no difference between the field-cooled and zero-field-cooled susceptibility
(H = 100 Oe) above TN in these alloys and hence spin-glass effects need not be considered in
the temperature region of interest.
It should be mentioned that the temperature-dependent X-ray diffraction studies do not
show any structural change down to 12 K; no anomaly is apparent in the lattice constants across
TN as well [10]. This rules out structural anomalies in the vicinity of TN as a cause of this
R minimum. It is not clear whether anomalous critical scattering effects or spin fluctuations
involving Pd 4d [11] and/or Gd 5d conduction electrons are responsible for the minimum in
R(T ). The spin fluctuations cannot be of the conventional Kondo type, as the 4f level of
Gd in general is believed to be situated well below the Fermi level. It may be added that
reports confirming Pd 4d ion Kondo effect exist only in the single-ion limit [12]; if the effect
is caused by Pd 4d, it is clearly an effect induced by Gd, since no R minimum has been
observed in Y2PdSi3 [4]. Above all, the sign of the Curie-Weiss temperature obtained from
the high-temperature χ data is not negative [2] as it should be negative for conventional
Kondo systems. Therefore, these spin fluctuations, if they exist, are triggered by Gd 4f
short-range correlations prior to long-range magnetic ordering. We propose that the origin
of the R(T ) minimum lies in a localization or trapping of the electron cloud polarized by the
s-f (or d-f) exchange interaction as one lowers the temperature towards long-range magnetic-
transition temperature; this could be triggered by Gd 4f short-range magnetic order [8], just as
crystallographic disorder results in electron localization [13], thereby reducing the mobility of
charge carriers. The application of a magnetic field suppresses the spin fluctuations or alters the
localization length (as proposed for La manganites [14] above the Curie temperature) resulting
in an enhancement of the mobility of the electron cloud; this explains the suppression of the R
minimum in the paramagnetic state in a magnetic field. The concepts of exchange-interaction–
mediated localization, i.e. some kind of magnetic-polaronic effects, have been proposed for
320 EUROPHYSICS LETTERS
semiconducting rare-earth compounds before [15]. Through this article we point out that the
essential concept discussed in refs. [14] and [15] may to some extent be applicable even in
some metallic alloys, particularly to those behaving as poor metals, i.e. in which the residual
resistivity ratio is low, as appears to be the case in Gd2PdSi3.
Summarising, the intermetallic compound, Gd2PdSi3, is shown to exhibit an electrical
resistivity minimum as a function of temperature above TN. We propose that a novel magnetic
precursor effect, possibly electron localization the rootcause of which lies in magnetism even in
metallic systems, may result in an upturn of the electrical resistance before long-range order
sets in. Finally, we have also made several other interesting observations in this compound.
These are: i) Positive Curie-Weiss temperature indicative of canted antiferromagnetism; ii) The
absence of downturn in electrical resistance below TN; iii) A sudden jump in the |Beff | at the Gd
nucleus at 15 K, related to lower ordering temperature for one of the two Gd sites [2], [4] and
iv) Negative magnetoresistance with the magnitude increasing with decreasing temperature
(large near TN), an observation of significance to the field of giant magnetoresistance.
REFERENCES
[1] Maple M. B., DeLong L. E. and Sales B. C., in Handbook on the Physics and Chemistry of
Rare Earths, edited by K. A. Gschneidner, jr. and L. Eyring (North-Holland, Amsterdam)
1978, p. 797 and references therein.
[2] Kotsanidis P. A., Yakinthos J. K. and Gamari-seale E., J. Magn. & Magn. Mater., 87
(1990) 199.
[3] Elliot R. J. and Wedgewood F. A., Proc. Phys. Soc., 81 (1963) 846; Das I., Sampathku-
maran E. V. and Vijayaraghavan R., Phys. Rev. B, 44 (1991) 159 and references therein.
[4] Mallik R. and Sampathkumaran E. V., J. Magn. & Magn. Mater., 164 (1996) L13.
[5] Das I. and Sampathkumaran E. V., Phys. Rev. B, 49 (1994) 3972; Sampathkumaran E. V.
and Das I., Phys. Rev. B, 51 (1995) 8631; Physica B, 223-224 (1996) 149; R. Mallik et al.,
Phys. Rev. B, 55 (1997) R8650.
[6] Blanco J. A., Leuthuilier P. and Schmitt D., Phys. Rev. B, 43 (1991) 13137.
[7] The peak values in fig. 4 (c) (close to 10 mJ/Gd mol) are far less than that expected [6] (27 J/Gd
mol) for an equal-moment antiferromagnetic structure and this proves that the magnetic structure
could be of modulated or of any other complicated type. We retain the notation TN to represent
the onset of magnetic ordering.
[8] Sampathkumaran E. V. and Das I., Phys. Rev. B, 51 (1995) 8178.
[9] The value of Tmin is found to be specimen-dependent (possibly due to slight non-uniform dis-
tribution of Gd in the dilute limit) for the composition x = 0.8, and it falls in the range
15–45 K. Therefore, the magnetoresistance data (fig. 2 (b)) is more instructive to infer the onset
temperature of magnetic-precursor effect for this alloy.
[10] Mallik R., Sampathkumaran E. V., Strecker M., Wortmann G., Paulose P. L. and
Ueda Y., submitted to Phys. Rev. B.
[11] See, for instance, Gratz E., Resel R., Burkov A. T., Bauer E., Markosyan A. S. and
Galatanu A., J. Phys. Condens. Matter, 7 (1995) 6687.
[12] Gross K. D., Riegel D. and Zella R., Phys. Rev. Lett., 65 (1990) 3044.
[13] Lee P. A. and Ramakrishnan T. V., Rev. Mod. Phys., 57 (1985) 287.
[14] Varma C. M., Phys. Rev. B, 54 (1997) 7328; Viret M., Ranno L. and Coey J. M. D., Phys.
Rev. B, 55 (1997) 8067.
[15] Kasuya T., J. Appl. Phys., 77 (1995) 3200; Shapira Y., Foner S., Oliveira N. F., jr. and
Reed T. B., Phys. Rev. B,10 (1974) 4765; Nyhus P., Yoon S., Kauffman M., Cooper S. L.,
Fisk Z. and Sarrow J., Phys. Rev. B, 56 (1997) 2717.


Observation of a minimum in the temperature-dependent electrical resistance above the magnetic-ordering temperature in Gd<SUB>2</SUB>PdSi<SUB>3</SUB>

Crossref

Observation of a minimum in the temperature-dependent electrical resistance above the magnetic-ordering temperature in Gd
                    2

Publications of the IAS Fellows

EUROPHYSICS LETTERS 1 February 1998
Europhys. Lett., 41 (3), pp. 315-320 (1998)
Observation of a minimum in the temperature-dependent
electrical resistance above the magnetic-ordering
temperature in Gd2PdSi3
R. Mallik1, E. V. Sampathkumaran1(), M. Strecker2 and G. Wortmann2
1 Tata Institute of Fundamental Research
Homi Bhabha Road, Colaba, Mumbai-400 005, India
2 Fachbereich 6-Physik, Universita¨t Paderborn - D-33095, Germany
(received 30 June 1997; accepted in nal form 4 December 1997)
PACS. 72.15−v { Electronic conduction in metals and alloys.
PACS. 75.20Hr { Local moment in compounds and alloys; Kondo eect, valence fluctuations,
heavy fermions.
PACS. 75.30Et { Exchange and superexchange interactions.
Abstract. { Results on electrical resistivity, magnetoresistance, magnetic susceptibility, heat
capacity and 155Gd Mo¨ssbauer measurements on a Gd-based intermetallic compound, Gd2PdSi3,
are reported. A nding of interest is that the resistivity unexpectedly shows a well-dened
minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a
feature which gets suppressed by the application of a magnetic eld. This observation in a
Gd alloy presents an interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as
a mechanism for the electrical resistance minimum.
Ever since a minimum in the electrical resistance (R) at a characteristic temperature in noble
metals containing 3d magnetic impurities was reported several decades ago, the consequences
of this phenomenon, known as the Kondo eect, in metallic solids remains an active area
of research. In concentrated Kondo alloys of Ce, Sm, Yb or U [1], if the strength of the
intersite indirect exchange interaction (given by TRKKY) is comparable to that of the Kondo
eect (given by the Kondo temperature, TK), one also observes magnetic ordering well below
the temperature at which resistivity shows a minimum. However, such a feature in R is not
expected for Gd alloys, since the Gd 4f orbital is so well localized that it cannot exhibit the
Kondo eect. In this article, we report the observation of a pronounced minimum in the
plot of R vs. temperature (T ) well above the magnetic-ordering temperature in a Gd-based
intermetallic compound, Gd2PdSi3 (which is presumably antiferromagnetic [2], TN = 21 K),
resembling the behavior in magnetic Kondo lattices. This nding implies that a minimum in
() E-mail: SAMPATH@tifrvax.tifr.res.in
c© EDP Sciences
316 EUROPHYSICS LETTERS
R(T ) can also occur from a completely dierent mechanism as a magnetic precursor eect,
which, we believe, is an exchange-interaction{induced electron localization.
The polycrystalline sample, Gd2PdSi3, was prepared by arc melting followed by homog-
enization at 750 C in an evacuated sealed quartz tube. We have also investigated alloys
with Gd substituted by Y , i.e. (Gd1−xYx)2PdSi3 (x = 0:2, 0.5 and 0.8). The X-ray powder
diraction patterns (Cu K) conrm that these alloys are single phase forming the hexagonal
AlB2-type structure [2]. The homogeneity of the samples was checked by scanning electron
microscopy. The electrical-resistance measurements (4.2{300 K) were performed by a con-
ventional four-probe method. Additional experiments carried out to support the line of our
arguments and conclusions are: i) The magnetoresistance measurements in the longitudinal
geometry in a magnetic eld (H) of 50 kOe as a function of T and as a function of H at selected
temperatures for the x = 0:0 alloy; ii) 155Gd(5=2 ! 3=2, 86.5 keV transition) Mo¨ssbauer
measurements in the transmission geometry with a 2 mCi 155Eu(SmPd3) source for x = 0:0
alloy below 25 K at selected temperatures; iii) Heat-capacity (C) measurements as a function
of temperature (2-70 K); iv) Magnetic-susceptibility () measurements (2{300 K) in a eld of
100 Oe and in 2 kOe.
The temperature dependence of the normalized electrical resistivity below 200 K is plotted
in g. 1 for all compositions investigated. The values of  at 300 K and 4.2 K for Gd2PdSi3
are of the order of 400 and 280 mΩ cm, respectively. Due to the presence of microcracks in
the sample, the error of these values might be smaller by about 40%. For this reason, the
data shown in g. 1 are normalized to the values at 300 K. It is obvious from g. 1 that the
(T ) of the Gd2PdSi3 compound gradually decreases as the temperature is lowered down to
60 K, followed by an upturn below about 45 K, thus giving rise to a resistivity minimum at
about (Tmin =)45 K; the value before the onset of magnetic ordering (say, at 22 K) is about 5%
higher compared to the minimum resistivity. There is a kink in (T ) vs. T at about 20 K, close
to the temperature where the Gd sublattice exhibits long-range magnetic ordering presumably
of antiferromagnetic-type, as one may conclude from the well-dened peak in the magnetic
susceptibility and from the shoulder in the heat-capacity data (vide infra). Further lowering of
temperature does not result in a drop in (T ) as expected due to the loss of the spin-disorder
contribution below the Neel temperature (TN). We presently attribute this to the opening of
a pseudogap at some portions of the Fermi surface due to the onset of antiferromagnetism
in this compound [3]. The magnetic-ordering temperatures for the compositions x = 0:2, 0.5
and 0.8 are lowered (see the heat-capacity data described below) as expected on the basis
of indirect exchange mechanism; however, the resistivity minimum persists well above TN
for all compositions. Even in the diluted limit of our investigation, x = 0:8, the resistance
increases by about 2% as the temperature is lowered from Tmin towards 4.2 K. The fact that
the minimum is also observable in the sample with the highest Y content (x = 0:8) may exclude
the explanation based on the opening of an energy gap (above 21 K).
Now turning to the influence of the application of a magnetic eld, the negative d=dT
behavior in zero eld vanishes in the presence of a eld of 50 kOe (shown in g. 2 (a) for
x = 0:0 only for the sake of clarity). The exact dependence of TN on H could not be inferred
from the present data and it is not relevant to the main conclusion of this article. The
magnetoresistance, dened as = = f(H) − (0)g=(0), is obtained from this data as a
function of T (shown in g. 2 (b) for all the compositions). For x = 0:0, it is found that =
is as large as about −10% at 40 K in the presence of H = 70 kOe (g. 2 (c)). For comparison,
the magnitude of the magnetoresistance values for the isostructural Ce-based Kondo lattice
Ce2PdSi3 (ref. [4]) as well as for the alloys, (Ce1−xYx)2PdSi3, is less than 1% down to 4.2 K.
This establishes that the observed large magnetoresistance eect is Gd-related, as noted by us
for some other Gd alloys [5].
r. mallik et al.: observation of a minimum etc. 317
Fig. 1 Fig. 2
Fig. 1. { The electrical resistance (R) of the alloys, (Gd1−xYx)2PdSi3 (x = 0:0, 0.2, 0.5 and 0.8),
normalized to 300 K values.
Fig. 2. { (a) The electrical resistivity () as a function of temperature for Gd2PdSi3 in the presence
and in the absence of a magnetic eld of 50 kOe. (b) The magnetoresistance (=) obtained from
the data shown in (a) for x = 0:0 is plotted; in addition, the data obtained in a similar way for other
compositions are also shown. (c) For x = 0:0, the = as a function of magnetic eld at selected
temperatures; the lines through the data points serve as guides to the eye. The error in = is
estimated to be less than 0.5%.
It is not out of place to add that, as the temperature is lowered below TN, the (T ) curve at
50 kOe does not show an upturn. If the upturn in  below TN is attributed to magnetic
Brillouin-zone boundary gaps, this nding implies that these gaps are essentially getting
washed out by the application of a magnetic eld; there is a feeble rise below 7 K in the
data in the presence of a eld, due to possible non-vanishing gap at low temperatures. The
magnitude of = thus increases below 20 K for the x = 0:0 alloy (g. 2 (b)), attaining a peak
value of about −32% at 7 K. In order to highlight this feature further, the = is plotted as
a function of H in g. 2 (c) for various temperatures. The overall shape of the plot of =
vs. H gets signicantly modied as the temperature is lowered across TN due to suppression
of the proposed magnetic Brillouin-zone boundary gap. The observation of a large negative
value of = is of importance considering the current interest in the phenomenon of giant
magnetoresistance.
318 EUROPHYSICS LETTERS
Fig. 3 Fig. 4
Fig. 3. { 155Gd Mo¨ssbauer spectra of Gd2PdSi3 at selected temperatures. The continuous lines
represent standard least-square t of the data. At the bottom, the eective magnetic-hyperne-eld
values obtained from the data are shown as a function of temperature.
Fig. 4. { (a) The magnetic susceptibility (H = 2 kOe) as a function of temperature, (b) the
heat capacity and (c) the 4f contribution (Cm) to C as a function of temperature for the alloys,
(Gd1−xYx)2PdSi3. The lines through the data points in (a) serve as guides to the eyes.
In order to be sure that the upturn in R below Tmin is not due to the opening-up of
magnetic Brillouin-zone boundary gaps, it is desirable to conrm the TN value by a microscopic
technique as well. We therefore show the results of 155Gd Mo¨ssbauer (only for x = 0:0),  and
C measurements in g. 3, 4 (a) and 4 (b), respectively. The Mo¨ssbauer spectra below 20 K are
found to undergo Zeeman splitting, which is well-resolved below 15 K. The magnetic hyperne
eld at the Gd nucleus obtained by a standard least-square tting procedure of the Mo¨ssbauer
spectra are also shown in g. 3 for x = 0:0. The temperature at which the magnetic hyperne
eld extrapolates to zero (near 20 K) marks the magnetic-ordering temperature and this serves
as a conclusive microscopic proof for paramagnetism above 20 K (which is also conrmed by
our Mo¨ssbauer thermal scan experiment) in this compound. The spectral analysis of the data
for 25 K gave evidence for a quadrupolar splitting only (the magnitude of which is 0.88 mm/s).
We have also probed the magnetic behavior by other bulk measurements. With respect
to  (H = 2 kOe), the low-temperature data show a distinct cusp in  at about 20 K in
this alloy, indicative of antiferromagnetic-like ordering. The feature due to magnetic ordering
shifts to lower temperatures with dilution of the Gd sublattice (g. 4 (a)). The plot of the −1
vs. temperature (not shown in a gure) is found to be linear down to the critical temperature,
r. mallik et al.: observation of a minimum etc. 319
thus suggesting that the peak in (T ) marks the onset of long-range magnetic order. Consistent
with this, in the heat capacity there is a peak attributable to the onset of magnetic ordering
at dierent temperatures (g. 4 (b)) for each of the three compositions, x = 0:0, 0.2 and 0.5
with magnetic-ordering temperatures of 20, 15 and 8 K, respectively. For x = 0:8, there is no
clear-cut anomaly due to the onset of long-range magnetic ordering above 2 K. We obtained
the 4f contribution (Cm) to C, shown in g. 4 (c), using the procedure suggested in ref. [6]
employing the C values of Y2PdSi3 as a reference for the lattice contribution. A point to be
noted [7] is that there is a broad shoulder (marked by a vertical arrow in g. 4 (c)) at about
22 K (which is a measure of TN from heat capacity [6]) for x = 0:0, beyond which Cm falls
gradually with temperature, extending to about 20 K above TN, similar to that in other Gd
alloys [8]. The magnetic entropy (Sm) at TN obtained from our data is only about 75% of
the saturation value of R ln 8 (R = gas constant) for all the magnetically ordered alloys; the
full value is reached far above TN only, for instance, only around 45 K in Gd2PdSi3. Clearly,
the heat-capacity tail persists for about 20 K due to magnetic precursor eects [8] even in the
case of the dilute alloy, x = 0:8, mimicking heavy-fermion behavior, though this alloy does not
undergo long-range magnetic order above 4 K.
Thus, all the results, including those from the Mo¨ssbauer spectroscopy, conclusively estab-
lish the onset of long-range magnetic ordering in the vicinity of 21 K for x = 0:0. Hence
the upturn in R below 45 K is not a consequence of a long-range antiferromagnetic-ordering
(gap) eect. It is important to note that the temperature till which the heat-capacity tail
above TN persists and the one below which a negative = is noticeable beyond experimental
error shift down with increasing x (compare the data for x = 0:0 and 0.8 in g. 2 (b)) [9].
These observations suggest that these anomalies arise from a common physical origin and thus
the minimum in R is a manifestation of a Gd 4f magnetic precursor eect. We would like
to add that there is no dierence between the eld-cooled and zero-eld-cooled susceptibility
(H = 100 Oe) above TN in these alloys and hence spin-glass eects need not be considered in
the temperature region of interest.
It should be mentioned that the temperature-dependent X-ray diraction studies do not
show any structural change down to 12 K; no anomaly is apparent in the lattice constants across
TN as well [10]. This rules out structural anomalies in the vicinity of TN as a cause of this
R minimum. It is not clear whether anomalous critical scattering eects or spin fluctuations
involving Pd 4d [11] and/or Gd 5d conduction electrons are responsible for the minimum in
R(T ). The spin fluctuations cannot be of the conventional Kondo type, as the 4f level of
Gd in general is believed to be situated well below the Fermi level. It may be added that
reports conrming Pd 4d ion Kondo eect exist only in the single-ion limit [12]; if the eect
is caused by Pd 4d, it is clearly an eect induced by Gd, since no R minimum has been
observed in Y2PdSi3 [4]. Above all, the sign of the Curie-Weiss temperature obtained from
the high-temperature  data is not negative [2] as it should be negative for conventional
Kondo systems. Therefore, these spin fluctuations, if they exist, are triggered by Gd 4f
short-range correlations prior to long-range magnetic ordering. We propose that the origin
of the R(T ) minimum lies in a localization or trapping of the electron cloud polarized by the
s-f (or d-f) exchange interaction as one lowers the temperature towards long-range magnetic-
transition temperature; this could be triggered by Gd 4f short-range magnetic order [8], just as
crystallographic disorder results in electron localization [13], thereby reducing the mobility of
charge carriers. The application of a magnetic eld suppresses the spin fluctuations or alters the
localization length (as proposed for La manganites [14] above the Curie temperature) resulting
in an enhancement of the mobility of the electron cloud; this explains the suppression of the R
minimum in the paramagnetic state in a magnetic eld. The concepts of exchange-interaction{
mediated localization, i.e. some kind of magnetic-polaronic eects, have been proposed for
320 EUROPHYSICS LETTERS
semiconducting rare-earth compounds before [15]. Through this article we point out that the
essential concept discussed in refs. [14] and [15] may to some extent be applicable even in
some metallic alloys, particularly to those behaving as poor metals, i.e. in which the residual
resistivity ratio is low, as appears to be the case in Gd2PdSi3.
Summarising, the intermetallic compound, Gd2PdSi3, is shown to exhibit an electrical
resistivity minimum as a function of temperature above TN. We propose that a novel magnetic
precursor eect, possibly electron localization the rootcause of which lies in magnetism even in
metallic systems, may result in an upturn of the electrical resistance before long-range order
sets in. Finally, we have also made several other interesting observations in this compound.
These are: i) Positive Curie-Weiss temperature indicative of canted antiferromagnetism; ii) The
absence of downturn in electrical resistance below TN; iii) A sudden jump in the jBe j at the Gd
nucleus at 15 K, related to lower ordering temperature for one of the two Gd sites [2], [4] and
iv) Negative magnetoresistance with the magnitude increasing with decreasing temperature
(large near TN), an observation of signicance to the eld of giant magnetoresistance.
REFERENCES
[1] Maple M. B., DeLong L. E. and Sales B. C., in Handbook on the Physics and Chemistry of
Rare Earths, edited by K. A. Gschneidner, jr. and L. Eyring (North-Holland, Amsterdam)
1978, p. 797 and references therein.
[2] Kotsanidis P. A., Yakinthos J. K. and Gamari-seale E., J. Magn. & Magn. Mater., 87
(1990) 199.
[3] Elliot R. J. and Wedgewood F. A., Proc. Phys. Soc., 81 (1963) 846; Das I., Sampathku-
maran E. V. and Vijayaraghavan R., Phys. Rev. B, 44 (1991) 159 and references therein.
[4] Mallik R. and Sampathkumaran E. V., J. Magn. & Magn. Mater., 164 (1996) L13.
[5] Das I. and Sampathkumaran E. V., Phys. Rev. B, 49 (1994) 3972; Sampathkumaran E. V.
and Das I., Phys. Rev. B, 51 (1995) 8631; Physica B, 223-224 (1996) 149; R. Mallik et al.,
Phys. Rev. B, 55 (1997) R8650.
[6] Blanco J. A., Leuthuilier P. and Schmitt D., Phys. Rev. B, 43 (1991) 13137.
[7] The peak values in g. 4 (c) (close to 10 mJ/Gd mol) are far less than that expected [6] (27 J/Gd
mol) for an equal-moment antiferromagnetic structure and this proves that the magnetic structure
could be of modulated or of any other complicated type. We retain the notation TN to represent
the onset of magnetic ordering.
[8] Sampathkumaran E. V. and Das I., Phys. Rev. B, 51 (1995) 8178.
[9] The value of Tmin is found to be specimen-dependent (possibly due to slight non-uniform dis-
tribution of Gd in the dilute limit) for the composition x = 0:8, and it falls in the range
15{45 K. Therefore, the magnetoresistance data (g. 2 (b)) is more instructive to infer the onset
temperature of magnetic-precursor eect for this alloy.
[10] Mallik R., Sampathkumaran E. V., Strecker M., Wortmann G., Paulose P. L. and
Ueda Y., submitted to Phys. Rev. B.
[11] See, for instance, Gratz E., Resel R., Burkov A. T., Bauer E., Markosyan A. S. and
Galatanu A., J. Phys. Condens. Matter, 7 (1995) 6687.
[12] Gross K. D., Riegel D. and Zella R., Phys. Rev. Lett., 65 (1990) 3044.
[13] Lee P. A. and Ramakrishnan T. V., Rev. Mod. Phys., 57 (1985) 287.
[14] Varma C. M., Phys. Rev. B, 54 (1997) 7328; Viret M., Ranno L. and Coey J. M. D., Phys.
Rev. B, 55 (1997) 8067.
[15] Kasuya T., J. Appl. Phys., 77 (1995) 3200; Shapira Y., Foner S., Oliveira N. F., jr. and
Reed T. B., Phys. Rev. B,10 (1974) 4765; Nyhus P., Yoon S., Kauffman M., Cooper S. L.,
Fisk Z. and Sarrow J., Phys. Rev. B, 56 (1997) 2717.


Results on electrical resistivity, magnetoresistance, magnetic
susceptibility, heat capacity and $^{155}\rm Gd$ Mössbauer measurements on
a Gd-based intermetallic compound, $\rm Gd_{2} PdSi _{3}$, are reported. A
finding of interest is that the resistivity unexpectedly shows a
well-defined minimum at about 45 K, well above the long-range magnetic-ordering temperature (21 K), a
feature which gets suppressed by the application of a magnetic field. This observation in a Gd alloy presents
an interesting scenario. On the basis of our results, we propose electron
localization induced by s-f (or d-f) exchange interaction prior to long-range magnetic order as a
mechanism for the electrical resistance minimum

R. Mallik

E. V. Sampathkumaran

M. Strecker

G. Wortmann

EDP Sciences OAI-PMH repository (1.2.0)

Observation of a minimum in the temperature-dependent electrical
resistance above the magnetic-ordering  temperature in

http://repository.ias.ac.in/45637/1/12-PUB.pdf