ARTICLE IN PRESS
Double magnetic transition and anomalous magnetoresistance
in Er2Ni3Si5
Chandan Mazumdara,*, R. Nagarajanb, A.K. Nigamb, K. Ghoshb,
S. Ramakrishnanb, L.C. Guptab, B.D. Padaliac
a
Experimental Condensed Matter Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700 064, India
b
Department of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research,
Colaba, Bombay 400 005, India
c
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
Abstract
Synthesis and magnetic properties of a new material, Er2 Ni3 Si5 ; are reported here. This compound exhibits a double
magnetic transition (Tm B4 K and 3 K) as revealed by heat capacity measurement. The magnetic ground state in
Er2 Ni3 Si5 is found to be a doublet. Magnetoresistance of Er2 Ni3 Si5 above Tm exhibits anomalous behaviour, suggesting
occurrence of short range magnetic correlations above TN :
Keywords: Rare-earth compounds; Effects of material synthesis; Magnetically ordered materials; Magnetic measurements
1. Introduction
Chabot and Parthe [1] first reported the synthesis of single-phase compounds RE2 Ni3 Si5
(RE ¼ Ce; Dy and Y). These materials form in
orthorhombic U2 Co3 Si5 -type structure (space
group Ibam). This structure can be considered as
intergrowth of well-known tetragonal REM2 X2
blocks and REMX3 (M = transition metals;
X = Si, Ge, Sn, B, etc.) blocks of unknown
structure [1]. Subsequently, we had succeeded in
synthesizing several members of this rare-earth
series and investigated their properties [2–11]. The
compounds RE2 Ni3 Si5 (RE ¼ Ce–Ho) form in
orthorhombic U2 Co3 Si5 -type of structure, whereas
Lu2 Ni3 Si5 seems to form in a monoclinic structure
[4]. It should be noted that in the RE2 Co3 Si5
series, while the lighter rare-earth analogues form
in orthorhombic U2 Co3 Si5 -type structure, the
heavier rare-earth analogues form in monoclinically distorted Lu2 Co3 Si5 -type structure (space
group C2/c) [12]. A variety of physical phenomena
have been observed in the members of RE2 Ni3 Si5
series: valence fluctuation behaviour in Ce2 Ni3 Si5
[2] and Eu2 Ni3 Si5 [3]; superconductivity in
Lu2 Ni3 Si5 ðTc E2 KÞ [4]; antiferromagnetic ordering at low temperature in most of the members
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217
with moment bearing RE-atoms [5–7]; double
magnetic transition in Nd-, Gd-, Tb- and Dyanalogues [7,8]; positive giant magnetoresistance
(GMR) in Tb-, Sm- and Nd-analogues (the largest
observed in polycrystalline compound) [9,10];
anomalous magnetoresistance (MR) in Pr-, Dyand Ho-analogues [10,11], etc. Here, we report the
synthesis of another member, Er2 Ni3 Si5 ; of the
RE2 Ni3 Si5 series and report the results of
magnetic, heat capacity and magnetoresistance
measurements on this material.
2. Experimental
Er2 Ni3 Si5 was prepared by arc melting together
high-purity (Er > 99.9%, Ni > 99.9% and Si >
99.999%) constituent elements in a water-cooled
copper hearth under flowing argon atmosphere.
The total weight loss during the process of melting
was less than 1%. The ascast button was then
wrapped in a tantalum foil, sealed in a quartz
capsule in vacuum and given heat treatment. A
commercial energy dispersive analysis of X-ray
(EDAX) equipment (KEVEX 8000 Microanalyzer, USA), attached with a scanning electron
microscope (SEM) equipment (Jeol 840, Japan),
was used to check for the homogeneity and
chemical composition. Room temperature powder
X-ray diffraction (XRD) measurements were
carried out in a commercial diffractometer (JDX
8030, Jeol, Japan). Magnetic measurements were
carried out in a commercial SQUID magnetometer
(Quantum Design Inc., USA), in the temperature
range 2–300 K and in the magnetic field range
0–55 kG: The heat capacity measurement was
performed in a home-built, fully automated
adiabatic calorimeter [13] in the temperature range
1.5–15 K: Longitudinal magnetoresistance measurements were carried out in a home-built set-up
in the temperature range 4.4–100 K and in the
magnetic field range 0–45 kG [14].
3. Results and discussions
The XRD pattern of Er2 Ni3 Si5 sample (Fig. 1—
bottom) prepared using the standard heat treat-
Fig. 1. Room temperature powder X-ray diffraction pattern of
Er2 Ni3 Si5 : Bottom: Sample prepared with the standard heat
treatment 1100 C for 1 day and 1000 C for subsequent 7 days
[1]. This pattern cannot be indexed on the basis of orthorhombic U2 Co3 Si5 -type structure (space group Ibam) Top: Sample
prepared with the heat treatment 1100 C for 8 days. The points
represent the experimental data points and solid line represents
the fit using Rietveld refinement method, for the orthorhombic
U2 Co3 Si5 -type structure (space group Ibam). The difference
between the calculated and the experimental XRD patterns is
also shown. The vertical bars denote the positions of the peak in
the XRD pattern permitted by the space group Ibam. Note the
good fit of the XRD pattern to orthorhombic U2 Co3 Si5 -type
structure (space group Ibam) after annealing at 1100 C for 8
days.
ment process (1100 C for 1 day and 1000 C for
subsequent 7 days [1]), employed for the other
members of the series, showed that the sample is
not of U2 Co3 Si5 -type structure. We could succeed
in obtaining an essentially single-phase sample
with the expected U2 Co3 Si5 -type structure
(Fig. 1—top) with heat treatment of 1100 C for 8
days. EDAX analysis confirmed the homogeneous
nature of the sample, with a composition close to
the ideal stoichiometric composition. All the
measurements were performed on this sample.
Powder XRD pattern of the sample could be
satisfactorily fit to orthorhombic U2 Co3 Si5 -type
structure (space group Ibam) through the Rietveld
procedure (Fig. 1—top), except for a few peaks of
small intensity (less than 5% of most intense
peak). The lattice parameters, derived from the
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218
( b¼
analysis was found to be: a ¼ 9:544 A;
(
(
11:054 A and c ¼ 5:607 A: These values fit well
in the systematics of lattice parameters of other
members of this rare-earth series [1,2,5–7].
Magnetic susceptibility, ðMðTÞÞ=H; and isothermal magnetization, MðHÞ; measurements on our
sample of Er2 Ni3 Si5 are shown in Figs. 2 and 3,
respectively. The magnetic susceptibility measurements show that in the high-temperature region
(5–300 K), ðMðTÞÞ=H follows the Curie–Weiss
law, ðMðTÞÞ=H ¼ C=ðT Yp Þ þ w0 ; where C is
the Curie constant, Yp is the paramagnetic Curie
temperature and w0 is the temperature-independent contribution. In the fitting, the value of w0
obtained from the susceptibility data of
Gd2 Ni3 Si5 ; where Gd-compounds is not expected
to have any crystalline electric field (CEF) effect,
has been used. The fit yields the values, Yp B1 K
and effective paramagnetic moment, meff ¼
9:0mB =Er-ion, which is slightly less than that
of free trivalent Er3þ ion ð¼ 9:6mB Þ: We note
that in this series of materials, Ni ion does not
seem to carry any significant magnetic moment.
We believe that CEF effect may be responsible for
observed reduction in the magnetic moment.
Below 4 K; ðMðTÞÞ=H shows a tendency to
saturate (Fig. 2) indicating the possibility of the
existence of magnetic order in this compound
below 4 K: The positive value of Yp and the
tendency of ðMðTÞÞ=H to saturate suggests
the magnetic order to be ferromagnetic. Since the
lower limit of our observation is 2 K; the conclu-
Fig. 2. Magnetic susceptibility (measured in 4 kG field) and its
inverse in Er2 Ni3 Si5 : The solid line is a fit to the Curie–Weiss
law. The inset shows the expanded region near the magnetic
transition temperature ðB4 KÞ:
Fig. 3. Low-temperature isothermal magnetization (as a function of magnetic field) for Er2 Ni3 Si5 measured at different
temperatures above the magnetic transition temperature. Data
were recorded both while increasing and decreasing the field. It
should be noted that the magnetization shows a tendency to
saturate at H > 20 kG field at 6 K (TN B4 K; see Fig. 4). The
solid lines are guides to the eye.
sion on the nature of magnetic order is not
definite. The other magnetic members of this
rare-earth ternary series (RE ¼ Pr; Nd, Sm and
Gd–Ho) order antiferromagnetically [5–7]. We
may note that the expected magnetic ordering
temperature in Er2 Ni3 Si5 ; as calculated using the
deGennes scaling w.r.t. TN of Gd2 Ni3 Si5 ; is about
2:4 K: Heat capacity measurements (see below)
show that in fact this material has complex
magnetic ordering with two ordering temperatures
(TN B4:2 and 2:9 K).
The isothermal magnetization, MðHÞ; was
measured at low temperatures (Fig. 3). Because
of our instrument’s limitation, MðHÞ measurements could be performed only above 5 K; i.e.,
above the magnetic ordering temperature ðB4 KÞ:
While the data taken at 40 and 25 K depend
linearly on field, as it should be in the paramagnetic region, those taken at 12 and 6 K; are nonlinear with respect to field and show a tendency to
saturate, although these temperatures are above
the magnetic ordering temperature. This shows
that strong magnetic correlations are present right
from 12 K and below. MR measurements confirm
this (see below).
Heat capacity measurements, CðTÞ; at low
temperature, were performed in Er2 Ni3 Si5 to
establish the bulk magnetic ordering, and it show
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219
two peaks, at TN1 ¼ 4:2 K and TN2 ¼ 2:9 K; with
large heat capacity values, 1 and 1:5 R=Er mol (R
is the universal gas constant), respectively, indicating the existence of double magnetic transition in a
short interval of temperature (Fig. 4). In this
context, it may be mentioned that many members
of this series, RE2 Ni3 Si5 exhibit double magnetic
transition (RE ¼ Nd; Gd, Tb and Dy) [7,8]. The
peak at the higher temperature, is more l-like,
whereas the peak at lower transition temperature,
is quite sharp. The shape of the heat capacity
peaks of Er2 Ni3 Si5 resembles well to those
observed in Tb2 Ni3 Si5 [8], suggesting similar
magnetic structures in both the materials. In the
case of Tb2 Ni3 Si5 ; neutron diffraction measurements have shown that first the material undergoes
(a transition form) paramagnetic to an incommensurate magnetic structure, which becomes commensurate below TN2 [16].
The magnetic entropy in Er2 Ni3 Si5 was calculated using Lu2 Ni3 Si5 [4] as a reference material.
For this purpose, the heat capacity data of both
the materials were extrapolated to 0 K (in the case
of Lu2 Ni3 Si5 ; the measured heat capacity data
were considered down to 3 K only, as Lu2 Ni3 Si5
exhibits a superconducting transition below 2 K
[4], and then extrapolated to 0 K). The estimated
magnetic entropy at TN2 is 0:43 R=Er-ion and at
Fig. 4. Low-temperature specific heat data ðC=RÞ of Er2 Ni3 Si5
ð3Þ: The data indicate two magnetic transitions (at B4:3 and
B2:9 K). The dashed line is a guide to the eye. The solid line
denotes the magnetic entropy as a function of temperature. The
magnetic entropy at the higher magnetic ordering temperature
is 0:7 RBR ln 2; which indicates a doublet magnetic ground
state in this compound.
TN1 is 0:70 R=Er-ion. The latter value is very
nearly equal to R ln 2 which indicates that the
magnetic ground state of the material is a doublet.
We note that the ground state is a doublet in other
members viz., Pr-, Nd, Sm-, Dy- and Ho- of the
RE2 Ni3 Si5 ; series too [8,15].
Fig. 5 shows the MR ð¼ Dr=r; where Dr ¼
r r0 ; r0 is the resistivity in the absence of
external magnetic field), of Er2 Ni3 Si5 as a function
of magnetic field. Since the lower limit of our MR
measurements is 4:4 K; the investigations were
limited to the paramagnetic region only. The
observed MR behaviour is anomalous and similar
to those observed in RE2 Ni3 Si5 (RE ¼ Pr; Dy and
Ho) [10,11]. Just above the magnetic ordering
temperature, Dr=r initially decreases with field,
exhibits a minimum at a field Hmin ðB15 kGÞ; and
Fig. 5. Field dependence of magnetoresistance for Er2 Ni3 Si5 at
a few selected temperatures. The inset shows the anomalous
Dr=r vs. T behaviour at 43 kG field. The lines drawn through
the data points are guides to the eye.
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220
then starts increasing again. At higher temperatures, Hmin does not change significantly, but MR
at Hmin reduces. At T > 10 K; the minimum
disappears and the MR exhibits monotonic
behaviour with H: For T > 25 K; MR starts
decreasing again. The MR at high field ð43 kGÞ;
when plotted as a function of temperature (Fig.
5—inset), exhibits a maximum around 25 K and
then decreases again with the increase in temperature. This is also an anomalous behaviour and is
not expected in a material in the paramagnetic
state. We attribute this behaviour to the existence
of short-range ferromagnetic correlations among
Er-magnetic spins even above the magnetic ordering temperature, as found in the cases of Pr-, Dyand Ho-analogues [10,11]. As pointed out above,
there is a tendency of MðHÞ curve towards
saturation at 6 K ð> Tm Þ; which supports the
suggestion of the presence of short-range ferromagnetic correlations in this material. Neutron
scattering study, both elastic and inelastic, will be
rewarding to understand the details of magnetic
structure in this compound.
4. Conclusion
In conclusion, we have synthesized and studied
the physical properties of a new compound,
Er2 Ni3 Si5 : Unlike most of the members of this
rare-earth series, formation of this compound in
single phase with U2 Co3 Si5 structure requires
different heat treatment. Heat capacity measurements of the material reveal the existence of a
double magnetic transition in this compound and
also show that the magnetic ground state is a
doublet. Magnetization with respect to field is
non-linear even above the magnetic ordering
temperature. MR as a function of temperature
exhibits a broad hump (at B25 K) above Tm in
this compound. The MR behaviour is very
anomalous and suggests the presence of shortrange magnetic correlations well above the
magnetic ordering temperature.
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