Exchange-enhanced Pauli spin paramagnetism in nanocrystalline Ni
3
Al
S. N. Kaul* and Anita Semwal
School of Physics, University of Hyderabad, Central University P.O., Hyderabad-500 046, Andhra Pradesh, India
H.-E. Schaefer
Institut fu ¨r Theoretishe und Angewandte Physik, Universita ¨t Stuttgart, D-70550 Stuttgart, Germany
Received 30 March 2000; revised manuscript received 11 July 2000
The results of a detailed comparative bulk magnetization study of nanocrystalline average crystallite size
25 nm) and polycrystalline samples of Ni
3
Al reported here clearly demonstrate that unlike crystalline
Ni
3
Al, which is a well-known weak itinerant-electron ferromagnet, no long-range ferromagnetic order exists in
the nanocrystalline counterpart at any temperature 5 K. Instead, nanocrystalline Ni
3
Al exhibits exchange-
enhanced Pauli spin paramagnetism.
In recent years, magnetism in nanostructured materials
has emerged as one of the most active areas of research
because magnetic properties undergo fundamental changes
as the crystallite size reduces to nanometer nm range. For
instance, the saturation magnetization and Curie temperature
of the strong itinerant-electron ferromagnet Ni decrease
1,2
by
about 15% and 4%, respectively, compared to their polycrys-
talline values, when Ni is in the nanocrystalline form. Sub-
stantial decrease in M
s
reflects a significant reduction in the
total density of states at the Fermi level, N ( E
F
). According
to the Stoner criterion for ferromagnetism, long-range ferro-
magnetic order in itinerant-electron spin systems can be sus-
tained only when the product of Stoner parameter intratomic
exchange constant I and N ( E
F
) is larger than unity. Now
that this product barely exceeds unity in the case of weak
itinerant-electron ferromagnets, even a slight change par-
ticularly reduction in N ( E
F
) is expected to have profound
effect on the magnetic properties of such systems. Though
weak itinerant-electron ferromagnets such as Ni
3
Al have
been extensively studied
3
in the polycrystalline form in the
past, no data are presently available on their nanocrystalline
counterparts. The above consideration prompted us to under-
take a detailed comparative magnetization study of Ni
3
Al in
polycrystalline and nanocrystalline forms.
The intermetallic compound Ni
3
Al was prepared in the
polycrystalline form by radio frequency induction melting
stoichiometric proportions by weight of 99.98% pure Ni and
99.998% pure Al under an inert atmosphere of 99.999% pure
argon gas. From a portion of the cylindrical ingot,
3
ultrafine
grains nanocrystalline of Ni
3
Al were produced by the inert
gas helium condensation technique
4
and the nanocrystal-
lites were in situ compacted under pressures 1 GPa to form
discs of diameter 3 mm and thickness 0.5 mm. The discs thus
prepared were analyzed for the main constituents as well as
for the 3 d transition metal impurities Cr, Mn, Fe, Co, by
x-ray flourescence, inductively coupled plasma and optical
emission spectroscopy and their concentration in atomic per-
cent was found to be Ni=75.47(1), Al=24.34(1), Cr
=0.11(1), Mn=0.003(1), Fe=0.04(1), and Co=0.041.
X-ray diffraction patterns, taken at room temperature on
nanocrystalline n and polycrystalline p disks of Ni
3
Al
using Cu K
radiation and shown in Fig. 1, could be com-
pletely indexed on the basis of L 1
2
cubic structure with the
lattice parameter values a
n
=3.570(2) Å and a
p
=3.564(2)
Å, respectively. It is evident from these diffraction patterns
that i the polycrystalline sample exhibits the fundamental
fcc-type as well as the superstructure Cu
3
Au-type Bragg re-
flection peaks whereas the nanocrystalline counterpart shows
only the fundamental 110, 200, and 220 Bragg reflec-
tions, and ii all the three fundamental diffraction lines are
FIG. 1. X-ray diffraction patterns of polycrystalline ( p -) and
nanocrystalline ( n -) samples of Ni
3
Al at room temperature.
PHYSICAL REVIEW B 1 DECEMBER 2000-I VOLUME 62, NUMBER 21
PRB 62 0163-1829/2000/6221/138924/$15.00 13 892 ©2000 The American Physical Society