Low-lying magnetic excitations in Ni
3
Al and their suppression by a magnetic field
Anita Semwal and S. N. Kaul*
School of Physics, University of Hyderabad, Central University P.O., Hyderabad 500 046, Andhra Pradesh, India
Received 3 May 1999; revised manuscript received 21 July 1999
Results of high-resolution magnetization M measurements performed on well-characterized polycrystalline
Ni
3
Al sample over wide ranges of temperature and external magnetic field are presented and discussed in the
light of existing theoretical models. Contrary to the earlier claims that either Stoner single-particle excitations
or nonpropagating spin fluctuations solely determine the temperature dependence of spontaneous magnetiza-
tion M ( T ,0), at low temperatures, we find that propagating transverse spin-density fluctuations spin waves
almost entirely account for the thermal demagnetization of both M ( T ,0) and ‘‘in-field’’ magnetization
M ( T , H), at temperatures T 0.28T
C
( T
C
=Curie point. The spin-wave stiffness possesses a field-independent
value of 69.6(14) meV Å
2
which conforms well with those determined earlier from small-angle and inelastic
neutron-scattering experiments. In the temperature range 0.32T
C
T 0.92T
C
, enhanced nonpropagating spin-
density fluctuations SF give a contribution to M ( T ,0) and M ( T , H) that completely overshadows the one
arising from spin waves. In accordance with the predictions of a modified spin-fluctuation theory, proposed by
the authors recently, the thermally excited SF’s get strongly suppressed by magnetic field H while the zero-
point SF’s are relatively insensitive to H. S0163-18299901142-X
I. INTRODUCTION
Out of the intermetallic compounds that exhibit weak
itinerant-electron ferromagnetism, ordered cubic L 1
2
crystal
structure Ni
3
Al has captured maximum experimental and
theoretical attention during the past three decades, and yet
certain aspects of magnetism in this compound have eluded a
complete understanding so far. One such aspect pertains to
the nature of low-lying magnetic excitations. Magnetic prop-
erties of Ni
3
Al have been extensively studied
1–11
and the
results discussed in the light of either
Stoner-Wohlfarth
1–8,12,13
model or the spin fluctuation
model.
9–23
On the one hand, de Boer and co-workers
1,2,4
claim that the temperature dependence of spontaneous mag-
netization M ( T ,0) in the temperature interval 0.1T
C
T
0.75T
C
( T
C
= Curie point is very well described by the
expression M ( T ,0) = M (0,0) -aT
2
yielded by the Stoner-
Wohlfarth model,
12,13
which holds the Stoner single-particle
spin-flip excitations solely responsible for the thermal de-
magnetization of M ( T ,0). On the other hand, Sasakura, Su-
zuki, and Masuda
10
assert that M ( T ,0) follows the relations
M
2
( T ,0) = M
2
(0,0) -a ' T
2
and M
2
( T ,0) =a ( T
C
4/3
-T
4/3
),
predicted by the spin-fluctuation SF model,
14–21
in the tem-
perature ranges 0.1T
C
T 0.4T
C
and 0.42T
C
T T
C
, re-
spectively. According to the SF model, nonpropagating ther-
mally excited longitudinal and transverse spin-density
fluctuations completely account for the decline of spontane-
ous magnetization with increasing temperature. In conflict
with both the above-mentioned observations concerning the
actual functional form of M ( T ,0), small-angle
neutron-scattering
24
SANS and inelastic neutron-
scattering
25
INS experiments provide direct evidence for
well-defined spin-wave excitations i.e., for propagating ther-
mally excited transverse spin-density fluctuations in Ni
3
Al
at temperatures in the range 0.1T
C
T 0.8T
C
. Though the
existence of spin waves at low temperatures in itinerant-
electron magnetic systems has been recognized
26
for a long
time now, no indication for such excitations in Ni
3
Al has
been found to date from magnetization measurements.
The spin-fluctuation SF theories henceforth referred to
as conventional SF theories proposed hitherto are, to some
extent, limited in scope in that they are unable to clarify the
role of zero-point quantum spin fluctuations and fail to
yield an expression which quantifies the suppression of local
spin-density fluctuations by external magnetic field H
ext
.
Obviously, the theoretical limitations of this kind seriously
hamper the understanding of magnetism in weak itinerant-
electron WI ferromagnets such as Ni
3
Al. Recently, Kaul
and co-workers
27,28
have addressed these deficiencies of the
conventional SF theories from the theoretical point of view
and remedied them by a self-consistent treatment of the SF
model, which makes use of the Ginzburg-Landau formalism.
Kaul and co-workers
27,28
have explicitly calculated the zero-
point ZP and thermally excited TE contributions to spin
fluctuations in WI ferromagnets in the presence and absence
of H
ext
and the results briefly summarized in the next sec-
tion; the details are given in Ref. 28 demonstrate the follow-
ing. ZP spin fluctuations i have a major share in renormal-
izing the Landau coefficients of the Stoner-Wohlfarth theory,
ii are relatively insensitive to H
ext
, and iii make an ap-
preciable contribution to the temperature dependence of
magnetization. By contrast, TE collective electron-hole pair
excitations almost entirely account for the dependences of
magnetization on temperature and field, and get strongly sup-
pressed by H
ext
. In addition, this theoretical approach
27,28
for the first time, yields an analytical expression for the sup-
pression of TE spin fluctuations by magnetic field for tem-
peratures just outside the critical region but below the Curie
point T
C
.
Extensive high-resolution bulk magnetization measure-
ments were undertaken on well-characterized Ni
3
Al sample
with a view to resolve the controversy surrounding the nature
of low-lying magnetic excitations and to test the validity of
PHYSICAL REVIEW B 1 NOVEMBER 1999-II VOLUME 60, NUMBER 18
PRB 60 0163-1829/99/6018/1279911/$15.00 12 799 ©1999 The American Physical Society