Anomalous magnetoresistance in antiferromagnetic polycrystalline
materials R2Ni3Si5 (R=rare earth)
Chandan Mazumdar, A. K. Nigam, R. Nagarajan, L. C. Gupta, G. Chandra et al.
Citation: J. Appl. Phys. 81, 5781 (1997); doi: 10.1063/1.364666
View online: http://dx.doi.org/10.1063/1.364666
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Published by the American Institute of Physics.
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Anomalous magnetoresistance in antiferromagnetic polycrystalline
materials R2Ni3Si5 (R5rare earth)
Chandan Mazumdar
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076, India
A. K. Nigam, R. Nagarajan, L. C. Gupta, and G. Chandra
Tata Institute of Fundamental Research, Colaba, Bombay 400 005, India
B. D. Padalia
Department of Physics, Indian Institute of Technology, Powai, Bombay 400 076 India
C. Godart
L.C.M.S.T.R., U.P.R. 209, C.N.R.S., F-92195 Meudon, Cedex, France
R. Vijayaraghaven
Tata Institute of Fundamental Research, Colaba, Bombay 400 005, India
Magnetoresistance ~MR! studies on polycrystalline R2Ni3Si5, ~R5Y, rare earth! which order
antiferromagnetically at low temperatures, are reported here. MR of the Nd, Sm, and Tb members
of the series exhibit positive giant magnetoresistance, largest among polycrystalline materials ~85%,
75%, and 58% for Tb2Ni3Si5, Sm2Ni3Si5, and Nd2Ni3Si5, respectively, at 4.4 K in a field of 45 kG!.
These materials have, to the best of our knowledge, the largest positive GMR reported ever for any
bulk polycrystalline compounds. The magnitude of MR does not correlate with the rare earth
magnetic moments. We believe that the structure of these materials, which can be considered as a
naturally occurring multilayer of wavy planes of rare earth atoms separated by Ni–Si network, plays
a role. The isothermal MR of other members of this series (R5Pr,Dy,Ho) exhibits a maximum and
a minimum, below their respective T N ’s. We interpret these in terms of a metamagnetic transition
and short-range ferromagnetic correlations. The short-range ferromagnetic correlations seem to be
dominant in the temperature region just above T N . © 1997 American Institute of Physics.
@S0021-8979~97!74808-4#
We had earlier reported interesting phenomena in several members of the series R2Ni3Si5. Our work had earlier
revealed that Ce and Eu ions in Ce2Ni3Si5 and Eu2Ni3Si5 ,
respectively, exhibit valence fluctuation behavior;1,2
Lu2Ni3Si5 becomes a superconductor below 2 K ~Ref. 3! and
Tb2Ni3Si5 exhibits a double magnetic transition with
T N 1 519.5 K and T N 2 512.5 K.4 Considering these interesting
phenomena, we studied magnetic properties of other members of the series. We found that Pr2Ni3Si5 , Nd2Ni3Si5 ,
Sm2Ni3Si5 , Gd2Ni3Si5 , Dy2Ni3Si5 , and Ho2Ni3Si5 undergo
antiferromagnetic transition at 8.7, 9.5, 11.1, 14.7, 9.5, and
6.5 K, respectively.4–6 x(T) of Dy2Ni3Si5 also shows a signature of a second magnetic transition around 4 K ~lower
inset of Fig. 1!. Unlike other members of this series, x(T) of
Ho2Ni3Si5 exhibits a tail below its T N ~Fig. 1!. We believe
that this may be arising from a change in magnetic structure
at low temperatures. The likelihood of such a situation is also
indicated by the hysteresis ~not exactly that of ferromagnetism! seen in isothermal magnetization at 6 K, which is
just below magnetic ordering.7 A significant magnetic entropy seen from specific heat measurements below 4 K also
supports this view.8 Here we present our longitudinal magnetoresistance ~MR! results on polycrystalline R2Ni3Si5 in
the temperature range 4.4–100 K and in the magnetic field
range 0–45 kG. Some aspects of this work have been reported in our very recent publications.7,9
Since magnetic measurements of all the materials show
the materials to order antiferromagnetically, one expects the
MR of the materials to be positive.10 The MR of R2Ni3Si5
J. Appl. Phys. 81 (8), 15 April 1997
~R5Nd,Sm,Tb! has been found to exhibit such a behavior,
although the magnitude of MR is anomalously large.9 At
4.4 and a field of 45 kG, the magnetoresistance of
Nd2Ni3Si5 , Sm2Ni3Si5 and Tb2Ni3Si5 is 58%, 75%, and 85%,
respectively.9 Figure 2 ~left! shows the MR behavior of
Nd2Ni3Si5 . For a bulk polycrystalline antiferromagnetic material, these values are quite high, and to the best of our
knowledge, the largest positive GMR reported ever for any
bulk polycrystalline compounds. We also find that the magnetoresistance values do not scale with the magnetic moment
of the rare earth.9 We believe that the structure of these materials, which can be considered as a naturally occurring
multilayer of wavy planes of rare earth atoms separated by
Ni–Si network @Fig. 2 ~right!# plays a role.
FIG. 1. Magnetic susceptibility and its inverse of Ho2Ni3Si5 . Insets shows
the expanded region near the magnetic ordering temperature of Ho2Ni3Si5
and Dy2Ni3Si5 .
0021-8979/97/81(8)/5781/3/$10.00
© 1997 American Institute of Physics
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5781
FIG. 2. ~Left and middle!: Field dependence of magnetoresistance at various temperature for Nd2Ni3Si5 and Dy2Ni3Si5 , respectively. Solid lines in the middle
figure are the fit to Eq. ~1!. Field dependence of magnetization of these two materials are also shown in the same range of magnetic field ~bottom figures!.
Insets show the temperature dependence of Dr/r at 43 kG ~data taken from the corresponding main figures!. Dotted lines are the guide to the eye. ~Right
figure!: Crystal structure of R2Ni3Si5 ~size: 2a32b32c!; ~top! projection in the a-c plane and ~bottom! projection in the a-b plane. Horizontal axis
represents the a axis in both the projections. Circles with the largest diameter represent rare earths, the smallest diameter represent silicon, and the others are
nickel.
In contrast, MR of other magnetic members of this series
R2Ni3Si5 ~R5Pr,Dy,Ho! exhibit anomalous behavior in some
other way. Figure 2 ~middle! shows the MR behavior of
Dy2Ni3Si5 . Below their respective T N ’s, the isothermal MR
exhibits a maximum and a minimum. The peak in MR correspond to metamagnetic transition. The peak disappears
above T N , although the minimum still persists. At temperatures well above T N , MR exhibits a monotonic behavior
w.r.t. magnetic field.
The observed minimum in the field dependence of MR
may be arising from two competing contributions: A term
linear in magnetic field and another, a negative term due to a
ferromagnetic component. For example, a ferromagnetic
component has been seen in the structurally related compound U2~Ru0.65Rh0.35!3Si5 , where neutron diffraction study
have shown that the magnetic moments are arranged ferromagnetically in a plane, whereas these planes are coupled
antiferromagnetically.11
We find a significant linear component in the MR of
these materials which is an important observation. In the
nonmagnetic analogue, Y2Ni3Si5 has an unusually large linear component,7,9 and we expect it to be present in all the
compounds. We believe that this linear term, arises from the
RNi2Si2 blocks ~R2Ni3Si5 structure can be considered to be
consisting of RNi2Si2 and RNiSi3 type blocks12!. A positive
linear MR has earlier been observed in PrCu2Si2 .13 We believe that this linear term could be a general feature of
RM 2 X 2 type of compounds.
The temperature dependence of MR of these compounds
5782
~in a constant high field!, exhibit a pronounced minimum at
the magnetic ordering temperature of the corresponding
compound. Above T N , the MR exhibit a broad maximum,
which may be due to the presence of short range ferromagnetic correlations in these materials. Precursor effects seen in
the magnetic entropy in our specific heat measurements on
these compounds, also support this conjecture.8
As we have suggested above, for T.T N , short range
ferromagnetic correlations exist in these materials. This effect is more prominent for those having R5Pr,Dy,Ho. To
understand the MR in this regime, we adopt the following
approach assuming that short range ferromagnetic correlations could be treated in the same manner as in superparamagnetic systems. Berkowitz et al.14 and Xiao et al.15
showed that in certain heterogeneous alloys, in thin film
form, where ferromagnetic particles are embedded in a metallic matrix and forming nanocrystalline materials, can produce large negative MR which is proportional to square of
magnetization of the material. Helmolt et al. found this to
hold good for the system, Au70Co20B10 ,16 where they assume
that the small ferromagnetic clusters have a superparamagnetic behavior ~i.e., where the magnetic energy in an applied
field is comparable to thermal energy17,18! and showed that
MR can be expressed as proportional to the square of Langevin function.16
We attempt to apply the same approach in our case of
R2Ni3Si5 ~R5Pr, Dy, Ho! by assuming that the short range
ferromagnetic interactions present in our case also could be
treated as in a superparamagnetic system. From the M vs H
J. Appl. Phys., Vol. 81, No. 8, 15 April 1997
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Mazumdar et al.
F
r ~ T ! 5 r 0 1 a T 2 1 b T 11
FIG. 3. Expanded region of resistivity of Tb2Ni3Si5 near the transition temperatures. Below T N 2 , the solid line represents a fit with Eq. ~2!. In the
temperature range, T N 1 2 T N 2 , the solid line represent a quadratic behavior.
Inset shows the resistivity in the temperature range 4.2–300 K.
curve, using the Langevin function used in the case of superparamagnetic system, we deduced the value of cluster moment. This moment value was used in fitting MR vs H data.
As mentioned earlier, the field dependence of MR also has a
linear term. However, this relation, i.e., square of Langevin
function with a linear field dependence term does not give a
satisfactory fit to the observed MR. In order to get an improved fit we replaced the quadratic Langevin function by a
function with power n, as this is the only parameter which
allows a change. Thus for the MR fit, we used the relation,
S
Dr
m H k BT
2
5a coth
r~ 0 !
k BT m H
D
n
1bH,
~1!
where m is the moment of each ferromagnetic cluster, H is
the magnetic field, k B is the Boltzmann constant and a, b,
and n are numerical constants. In our case, a is negative and
b is positive.
The above expression gives a better fit to the data ~Fig. 2
middle!. Using the value of m obtained from the magnetization curve, we find that just above the transition temperature,
the value of n is nearly 4/3. As the temperature is increased,
the value of n gets reduced and tends toward 1. At present,
we do not know the reason for value of n being different
from 2 and its temperature dependence.
We may also like to comment on the temperature dependence of the resistivity of these antiferromagnetic materials.
The resistivity, r, of these compounds exhibit normal metallic behavior. As a typical example, r(T) of Tb2Ni3Si5 is
given in the inset of Fig. 3. r(T) of Tb2Ni3Si5 exhibits
change of slope at ;19.5 K and at ;13 K ~Fig. 3! which
correspond to T N 1 and T N 2 , respectively, as observed in the
x(T) measurements. Below T N 2 , we find that the resistivity
can be fit to the expression19,20
J. Appl. Phys., Vol. 81, No. 8, 15 April 1997
G S D
2D
2T
exp
.
D
T
~2!
Here the second term originates from the spin fluctuations
coming from Fermi-liquid consideration, and the third term
is due to electron spin wave scattering in an antiferromagnetic material having an energy gap D. The fit results:
r0;3.35 mV cm, a;631023 mV cm K22, b;843
mV cm K21 and D;119 K. The resistivity rises much faster
in the temperature range 12–13 K than predicted by the
model, indicating the presence of some additional scattering
mechanism in this region. In the temperature range 13–19.5
K, a linear fit yields r (T)50.53T mV cm K21, whereas a
better fit can be obtained as r(T)54 mV cm10.17T 2
mV cm K22 ~Fig. 3!.
C.M. acknowledges the financial support from C.S.I.R.,
New Delhi w.e.f. 1 Jan. 1996. The authors also express their
gratitude for the help in experiments by Dr. S. Radha and
Mr. S. K. Paghdar.
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Mazumdar et al.
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