Low-temperature heat-capacity studies of R
2
Ni
3
Si
5
„ R Pr, Nd, Sm, Gd, Tb, Dy, Ho…
Chandan Mazumdar*
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
K. Ghosh,
†
R. Nagarajan, and S. Ramakrishnan
Tata Institute of Fundamental Research, Bombay 400 005, India
B. D. Padalia
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
L. C. Gupta
Tata Institute of Fundamental Research, Bombay 400 005, India
Received 14 September 1998
We report here our low temperature (2 -30 K) heat capacity, C
p
measurements of R
2
Ni
3
Si
5
( R
=Pr, Nd, Sm, Gd-Ho). Large peaks in heat capacity data at magnetic transition temperatures ( T
N
) confirm
the bulk nature of magnetic order in these compounds. In Nd
2
Ni
3
Si
5
, Gd
2
Ni
3
Si
5
, and Dy
2
Ni
3
Si
5
, magneti-
zation studies indicated only one magnetic transition, whereas, heat-capacity data show two transitions. T
N
of
the heavier rare-earth member, Tb
2
Ni
3
Si
5
, showing significant deviation from de Gennes scaling is notable.
Magnetic entropy, S , estimated from heat-capacity data suggest that the magnetic ground state is a doublet in
R
2
Ni
3
Si
5
( R =Pr, Nd, Sm, Dy, Ho) and a quartet in Tb
2
Ni
3
Si
5
. In all the cases, S , at T
N
is slightly less
than that expected due to the suggested states, which we attribute to the occurrence of precursor effects of
magnetic order above T
N
. Except for Gd
2
Ni
3
Si
5
, S of the compounds does not reach the saturation limit of
R ln(2J+1) even at 30 K, indicating the presence of crystalline electric field CEF effects. A hump in C
p
is
observed below T
N
in Gd
2
Ni
3
Si
5
which is interpreted in terms of a possible amplitude-modulated magnetic
spin structure. S0163-18299912605-5
I. INTRODUCTION
Physical properties of rare-earth intermetallic materials
have been of interest due to the effect of hybridization of 4 f
orbitals with the 3 d orbitals resulting in anomalous phenom-
ena such as, valence fluctuation, heavy fermion behavior,
Kondo effect, magnetic transition temperature higher than
expected, etc. In this context we had investigated the series
R
2
Ni
3
Si
5
( R =Y , rare earth.
1–7
Our synthesis and magnetic
susceptibility studies of R
2
Ni
3
Si
5
( R =Pr, Nd, Sm, Gd, Tb,
Dy, Ho Refs. 4–7 showed that they crystallize in
U
2
Co
3
Si
5
structure
8
and order antiferromagnetically.
Tb
2
Ni
3
Si
5
revealed a well-separated double magnetic transi-
tion ( T
N
1
=19.5 K and T
N
2
=12.5 K).
6
We had found posi-
tive giant magnetoresistance GMR in R
2
Ni
3
Si
5
( R =Tb,
Sm, Nd Ref. 9 and anomalous magnetoresistance in
R
2
Ni
3
Si
5
( R =Pr, Dy, Ho).
10
The positive GMR in
Tb
2
Ni
3
Si
5
at 4.4 K and at 45 kG ( 85%) is the largest
among polycrystalline materials.
9
From our studies of the
temperature dependence of magnetoresistance, we had sug-
gested the possibility of the existence of short-range ferro-
magnetic correlations above T
N
in these materials.
10
In order
to get further insight into the nature of the magnetic ordering
in these materials, we have studied the low-temperature heat
capacity of the series R
2
Ni
3
Si
5
( R =Pr, Nd, Sm, Gd, Tb,
Dy, Ho, the results of which are reported here.
II. EXPERIMENT
The compounds, R
2
Ni
3
Si
5
( R = rare earth), were pre-
pared by the standard arc melting procedure. Details of
sample preparation and unit-cell parameters are given
elsewhere.
3–6
The heat-capacity measurements were per-
formed at T.I.F.R., using a home built, fully automated adia-
batic calorimeter
11
in the temperature range 1.5–30 K. The
absolute accuracy of our heat-capacity measurements is bet-
ter than 1% in this temperature range. The measurements
reported here have been performed on samples obtained from
the same batch on which the magnetic measurements were
reported earlier.
4–6
III. RESULTS AND DISCUSSION
The heat-capacity results of our measurements on
R
2
Ni
3
Si
5
( R =Pr, Sm, Ho, Gd, Nd, Tb, Dy) are given
in Figs. 1, 2, 3, 4, 5, 6, and 7. In R
2
Ni
3
Si
5
( R =Pr, Sm, Ho,
a single large peak at 8.5, 10.4, and 6.7 K, respectively and
in R
2
Ni
3
Si
5
( R =Nd,Gd,Tb,Dy) two peaks at 9.2 and
9.5K, 13.5 and 15.0 K, 12.4 and 19.2 K, and 3.9 and 9.0 K,
respectively are seen in the heat capacity. The temperature
at which the peaks occur the higher temperature peak where
two peaks are seen agree with the magnetic ordering tem-
perature, T
N
, as seen in magnetic susceptibility data con-
firming the magnetic origin of the peaks in heat capacity. In
the case of Tb
2
Ni
3
Si
5
, two peaks are seen in magnetic
susceptibility
6
as well as in resistivity
12
and they correspond
to the peaks seen in heat-capacity data. In the case of
Dy
2
Ni
3
Si
5
, two clear peaks are seen in heat-capacity, but in
the magnetic susceptibility, only the higher transition at
9.5 K is seen clearly; the lower transition appears as a very
PHYSICAL REVIEW B 1 FEBRUARY 1999-II VOLUME 59, NUMBER 6
PRB 59 0163-1829/99/596/42156/$15.00 4215 ©1999 The American Physical Society