Anomalous low temperature magnetoresistance in polycrystalline CeFe2
S. Radha, S. B. Roy, and A. K. Nigam
Citation: J. Appl. Phys. 87, 6803 (2000); doi: 10.1063/1.372847
View online: http://dx.doi.org/10.1063/1.372847
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JOURNAL OF APPLIED PHYSICS
VOLUME 87, NUMBER 9
1 MAY 2000
Anomalous low temperature magnetoresistance in polycrystalline CeFe2
S. Radhaa)
Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-40076, India
S. B. Roy
Low Temperature Physics Section, Centre for Advanced Technology, Indore-452012, India
A. K. Nigamb)
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400005, India
Results of magnetoresistance measurements on a polycrystalline sample of CeFe2 are presented. The
sample has been characterized as a ferromagnet below 230 K. A fairly large positive
magnetoresistance is observed below 50 K that is indicative of the presence of an antiferromagnetic
interaction over the mean free path length. At higher temperatures the magnetoresistance is
characteristic of a ferromagnet. The study suggests the existence of stable antiferromagnetic
correlations in high fields at low temperatures in this compound. © 2000 American Institute of
Physics. 关S0021-8979共00兲94008-8兴
has also been suggested from nonlinear susceptibility9 and
magnetoresistance 共MR兲 measurements10–12 on Ce共Fe, T兲2
compounds.
Recently, experiments on neutron diffraction, nonlinear
ac susceptibility, and magnetic x-ray circular dichroism13–15
on single crystals of the CeFe2 compound have been reported. However, there have been few transport measurements on these alloys. The brittleness of the sample renders
difficulties in measurement of electrical resistivity. In this
article, we present the first results obtained for the MR measurements of polycrystalline CeFe2. Among the macroscopic
measurements, MR has the advantage of probing the microscopic spin correlations occurring over the mean free path
length. The occurrence of FM to AF transition in the
Ce共Fe, T兲2 compounds could be clearly identified by MR
measurements.10–12 The present article investigates the magnetic state of the parent compound, CeFe2, through transport
property measurements.
I. INTRODUCTION
There has been considerable interest in the study of rare
earth-transition metal intermetallic compounds because of
several interesting physical phenomena such as valence fluctuation, Kondo effect, heavy fermion behavior, etc., exhibited by them. One such system is the binary Laves phase
compound, RM2 共R: rare earth and M: transition metal兲.
Among these compounds, CeFe2 is particularly interesting
because it exhibits anomalously low ferromagnetic ordering
temperature 共⬃230 K兲, low Fe moment, and smaller lattice
constants compared to other isostructural compounds.1 This
difference has been attributed to 3d – 4 f hybridization and
has been verified by x-ray absorption studies.2 The relativistic band structure calculations have shown that Ce has a
magnetic moment, which is antiferromagnetically coupled to
the Fe moment.3 The magnetic susceptibility study on CeFe2
showed a ferromagnetic 共FM兲 state below around 230 K.1
However, the neutron scattering measurements on a powder
sample suggested an antiferromagnetic 共AF兲 coupling between Ce and Fe moments below 60 K, which confirmed the
theoretical prediction.4 Further, a polarized neutron study of
a single crystal5 and a recent Compton scattering study6 confirmed the existence of a 4 f spin moment antiparallel to the
Fe moment. The measurement of ac susceptibility at low
fields shows a structure around 80 K, which was totally
washed out in fields above 250 Oe.7 This structure was identified as a precursor to an incipient magnetic instability and
showed up on alloying with Al, Ru, etc. The substitution of
Fe by even small concentrations of Al, Ru, or Co destabilizes
the ferromagnetism in CeFe2 leading to a second magnetic
transition at a lower temperature.8 The neutron scattering
measurements on Ce共Fe, T兲2 共T⫽Co, Ru, and Al兲 indicate
that the low temperature phase is an antiferromagnetic one,
passing through a region of coexistent ferro- and antiferromagnetic moments giving rise to a spin canted phase.4 This
II. EXPERIMENT
The samples were prepared by argon arc melting of constituent elements having 99.99% purity. They were suction
chill cast to obtain rods of rectangular cross section. The
annealing treatment is as reported in an earlier article.4 The
longitudinal magnetoresistance measurements were carried
out in the temperature range 4.5–300 K in magnetic fields up
to 45 kOe generated by a homebuilt superconducting magnet. The resistance was measured by a standard dc four probe
technique. The electrical contacts to the sample were made
with Indium solder using ultrasonic soldering. The temperature of the sample was controlled and monitored by a lake
shore carbon glass sensor 共in magnetic field兲 up to 60 K and
by a silicon diode sensor above 60 K, employing a lake shore
DRC-82C temperature controller. The stability in resistance
measurements was better than 50 ppm.
a兲
Electronic mail: [email protected]
Electronic mail: [email protected]
b兲
0021-8979/2000/87(9)/6803/3/$17.00
6803
© 2000 American Institute of Physics
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6804
J. Appl. Phys., Vol. 87, No. 9, 1 May 2000
Radha, Roy, and Nigam
FIG. 2. Temperature 共T兲 dependence of magnetorsistance 共⌬␳/␳兲 in fields of
5, 20, and 40 kOe. 共Solid line connecting the points is a guide to the eye.兲
state as the temperature is decreased.16 Also, in some other
antiferromagnetic rare earth based intermetallic compounds
R2Ni3Si5 (R⫽Sm, Tb, Nd), a similar temperature dependence of ⌬␳/␳ has been observed in the AF state.17 The field
dependence of 共⌬␳/␳兲 at low temperatures 共below 40 K兲 was
fit to the expression
⌬ ␳ / ␳ ⫽ ␣ H⫹ ␤ H 2 ,
FIG. 1. 共a兲 Variation of magnetoresistance 共⌬␳/␳兲 as a function of magnetic
field 共H兲 in the temperature range 4.4–50 K. 共b兲 Magnetoresistance 共⌬␳/␳兲
vs magnetic field 共H兲 in the temperature range 50–300 K.
III. RESULTS AND DISCUSSION
Figures 1共a兲 and 1共b兲 show the magnetic field 共H兲 dependence of magnetoresistance 关 ⌬ ␳ / ␳ ⫽( ␳ (H)⫺ ␳ (0))/ ␳ (0) 兴 ,
where ␳ (H) is the resistivity in field H, at various temperatures in the range 4.4–300 K. It is observed that 共⌬␳/␳兲 is
positive for fields greater than 5 kOe and increases with applied field. The maximum value of 共⌬␳/␳兲 is 22% observed at
4.4 K. It decreases with increasing temperature. Beyond 100
K, 共⌬␳/␳兲 is less than 1% even in the maximum applied field.
A distinct change of curvature in the ⌬␳/␳ vs H curve is
observed at 100 K. At higher temperatures, a small negative
magnetoresistance is observed. The magnitude of negative
⌬␳/␳ increases with increasing temperature until 250 K as
can seen in Fig. 1共b兲 at higher fields. At 300 K 共well above
the ferromagnetic transition temperature兲, the MR is small
and leads to a fluctuation in its sign which could be due to
the resolution limit of our measurements.
Figure 2 shows the temperature dependence of ⌬␳/␳ at
fields of 5, 20, and 40 kOe. In a field of 5 kOe, ⌬␳/␳ is found
to be negative and small 共less than 1%兲 below 50 K. At
higher fields of 20 and 40 kOe, ⌬␳/␳ is positive and decreases fast with increasing temperature, up to around 50 K.
A similar temperature dependence of ⌬␳/␳ has also been observed in a well annealed sample of UCu2Ge2 which is reported to undergo an AF transition from a ferromagnetic
共1兲
where ␣ and ␤ are constants, dependent on temperature. The
value of ␣ is found to be two orders of magnitude higher
than ␤, thereby implying a strong linear dependence of ⌬␳/␳
on field. A similar dependence has been seen in UCu2Ge2 .16
A small local maximum in the ⌬␳/␳ vs T curve is observed
around 70 K and it is difficult to comment about it. Thus, the
low temperature behavior of ⌬␳/␳ reported here suggests the
presence of AF interactions over the length of the mean free
path, which leads to a significant positive magnetoresistance.
The inelastic neutron scattering measurements on a CeFe2
single crystal have revealed the presence of AF fluctuations
extending over several hundred angstroms at low
temperatures.13 The observation of positive ⌬␳/␳ at 20 kOe
and above further suggests that the AF correlations are more
stable in applied magnetic fields. The observation of small
negative ⌬␳/␳ at low fields 共at 5 kOe in Fig. 2兲 could be due
to magnetic frustration in the system leading to a reentrant
spin glass-like behavior, as reported in the frequency dependence of the ac susceptibility of a CeFe2 single crystal.14
The observation of small magnetoresistance above 70 K
is consistent with the ferromagnetic nature of the CeFe2 compound. The ⌬␳/␳ is expected to be small if the electron magnon scattering is weak due to small spin wave excitations in
the system.
To summarize, the present study of magnetoresistance in
CeFe2 suggests the existence of antiferromagnetic interactions at low temperatures. The antiferromagnetic correlations
appear more stable in high fields and it would be of interest
to carry out these measurements at still higher fields than
applied in the present study.
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J. Appl. Phys., Vol. 87, No. 9, 1 May 2000
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