The change in the temperature coefficient of resistivity with Mn addition in
Fe and Corich amorphous alloys
S. N. Piramanayagam, Shiva Prasad, S. N. Shringi, A. K. Nigam, Girish Chandra et al.
Citation: J. Appl. Phys. 73, 5595 (1993); doi: 10.1063/1.353662
View online: http://dx.doi.org/10.1063/1.353662
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Published by the American Institute of Physics.
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The change in the temperature coefficient
in Fe- and Co-rich amorphous alloys
of resistivity
with Mn addition
S. N. Piramanayagam, Shiva Prasad, and S. N. Shringi
Department of Physics, LL T. Powai, Bombay-400016, India
A. K. Nigam and Girish Chandra
LTP, TIFF
Coiaba, Bombay-400 005, India
N. Venkataramani
ACRE, l2.K
Pou~i, Bombay-400 076, India
R. Krishnan
CXR.S..
9395 Meudon, France
Electrical resistivity studies have been carried out on a-Coso_.~_yFe.~n~,,Sis
alloys for
lO<x<7c and y=O,4,8. It has been found that the decrease of temperature coefficients of
resistivity (TCR) with Mir substitution is significantly more in Fe-rich alloys than in Co-rich
alloys. A 2-2 term is not found in Co-rich alloys even up to the addition of 8 at. % of Mn.
Although Fe-rich alloys show a T2 dependence for 8 at. % of Mn, a second minimum is not
observed. In the absence of Mn, the substitution of Fe for Co does not decrease the TCR while
in the presence of Mn, the substitution of Fe does decrease the TCR.
I. INTRODUCTION
II. EXPERIMENTAL
The temperature dependence of electrical resistivity in
amorphous ferromagnetic materials containing early 3d
transition met& like Cr and V’” has been a subject of
study for a very long time. Even so, a clear understanding
of the subject has not yet evolved. The study poses difficulty because in some of these alloys, two minima are ob*served in the temperature dependence of resistivity instead
of the usual single minimum. There have been various attempts to understand the negative and positive tempeature coefficients of resistivity (TCR) in these alloys, which
manifest themselves in the form of two minima.3T4 However, it has been difficult to even understand the systematits of them. A recent study of Mn containing FeNi-based
amorphous alloys has shown that the higher temperature
minimum in the resistivity may not be due to an entirely
different mechanism. It can be explained as being caused
by a change of sign in one of the two terms in the usually
positive TCR?
In order to further understand the role played by Mn
in amorphous alloys, it is desirable to study the effect of
Mn inclusion in other systems also. In this paper we report
resistivity studies on u-CosO-X-yFeMn$l,Sis
alloys with
Y= 10, 15, 25,40, 55, and 70 andy=O, 4, and 8. The study
was carried out with a specific aim to understand the effect
of Mn on the functional form and magnitude of the positive TCR, so that an eventual correlation could be made to
the problem of observation of two minima in certain types
of alloys. The present system is interesting for more reasons. It is known that the changes caused in magnetic
properties by Mn are quite different in the Co-rich alloys as
compared to the Fe-rich onesst6 Hence, the present study
could answer the natural query whether the transport
properties are also different in the two extremes.
5595
J. Appl. Phys. 73 (101, 15 May 1993
DETAILS
The alloys were prepared in the form of ribbons by
melt spinning technique. The other details are mentioned
elsewhere.” As the error in determining the geometrical
factor is large, the data for all the samples have been presented as m(T)=[R(T)-R(T,i,)]/R(T,i,>,
where
R(T)
is the resistance at temperature T and R( TIrlifl) is
the minimum resistance and r,( T) is the normalized resistivity.
Ill. RESULTS AND DISCUSSION
The temperature dependence of resistivity of all the
samples show a single minimum in the resistivity at a temperature (Tmi,) in the order of 20 K except for x=40,
y=8 sample which showed a single but very shallow minimum at 90 K. Two minima, a feature common in Febased alloys cont.aining Cr or V, is not observed for any of
our samples. Figure 1 shows the variation of r,,(T) with
temperature for samples containing largest amount of Co
(x= lo), and largest amount of Fe (x=70), for different
Mn concentrations.
A. Functional
form of positive
TCR
The positive temperature coefficient of resistivity in
amorphous alloys, based on various theories,7 is described
by the following equation:
Yn( T) =a+bT”,
(1)
with n= 1, 1.5, or 2. We, therefore, tried to fit our resistivity data on all the samples in the temperature range 150300 K to Rq. (1) using the three different values of “E.”
The fitting indicated that most of the samples with y<4
show the best fit for n = 1.5. For y= 8 samples, on the other
hand, the best tit is obtained for n= 1 when x920, for
B= 1.5 when x=25, and for n =2 when x > 25. This feature
is more obvious from Fig. 2 where s=&,~ ( T)/d( T3’2) has
0021-8979/93/105595-03$06.00
0 1993 American Institute of Physics
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5595
a-Ca,,-,_,Fe,MnyB12SiB
0
*
a
.
*
,.I-0
9
008
x
v
0 .6
‘3
i _,
x=lO.y-0
r=1o,j-4
x-1o.y-8
x=?O.y-0
*
*
*
*
$:
n
Y- 0.4
‘C
0
ii] 2
ai
il.
..o 0.0
*
*
n
*
n
----r----x-i;or~~entration
3, Plot of b in r,J T) --a+bT””
30
y=o
* y=4
f3 y=8
n
Q’j
FIG.
alloys
*
l-l
n
-1
8(]
60
of
Fe(QtJg j
as a function of Fe concentration
(-+)a
300
I
Tem$%&”
(K)
FIG. 1. Plot of normalized resistivity as a function of temperature for
a-Corn-,... 1Fe,Mn,J3,LSiaalloys.
been plotted as a function of T312. It is seen here that for
lower values of x, s decreases with T3’2 indicating a value
of n < 1.5~~For higher values of x, on the other hand, s is
observed to increase with T”j2 implying n > 1.5.
The above results indicate an increase in the value of n
from 1.5 to 2 when 8% Mn is added to Fe-rich alloys.
Similar increase in the value of rz from 1.5 to 2 has been
reported with the addition of other early 3d transition metals like Cr and V’*” in Fe-based alloys. However in the case
of Cr and V additions, this increase takes place at a much
smaller concentration than 8 at. % observed in our case.
This is further
supported by the fact that in
u-Fe~,_,Ni,Mn,B12Sis
alloys with ~~5.5 also, n= 1.5 was
found to give best fit.”
In the Co-rich alloys, on the other hand, no such increase in ?r is observed with Mn addition up to 8%. The
u--Coso-7Fe,Mr~sBi2Sia
alloys
*
*
c
“+.-
-
**
-wu
u
uctr-=**
”0
0
*
0
*
d
x=25
x=40
i-Y55
*=73
4
work of Obi et al. ,s who have found the power of T to be
less < 1.5 in CoMnB alloys, also supports the above conclusion.
B. Dependence
5596
J, Appl. Phys., Vol. 73, No. 10, 15 May 1993
TCR on Mn concentration
Figure 3 shows the coefficient “b” for n = 1.5 for all the
samples. It is seen from this figure that the introduction of
Mn in the system causes a reduction in the coefficient “b”
both in Co- and Fe-rich end. However, as is also apparent
from Fig. 1, the reduction is much smaller in the Co-rich
alloys in comparison to Fe-rich ones. In x= 10 alloy, e.g.,
the reduction in b is only around 25% as compared to 80%
drop for x=70 on addition of 8% Mn. The decrease in the
value of b as y changes from 0 to 4 for our x = 40 sample is
by a similar amount as was observed when x changed from
0 to 5.5 in a-Fe.~_,Ni~~~Mn,BlzSi, alloys.4 However, the
decrease of b with Mn addition is much less compared to
that observed with Cr addition in similar amorphous alloys.9
C. Dependence
of positive
TCR on Fe concentration
As is seen from Fig. 3, the addition of Fe causes TCR
to decrease both for y-=4 and 8. Similar behavior is observed in a-Felo-x-&,Mn,BISis
alloys also, where for
x= 1, the value of b was found to decrease approximately
by 44% when Fe is increased from - 30 to -60 at. 9’6.”
What is interesting from Fig. 1 is that in the absence of
Mn, the values of “b” shows a small increase with the
addition
of Fe. This result is in contrast with
a-Fe40_.~_~i~n,B,LSiR
and other Fe-Ni based alloys,
where even in the absence of Mn, the addition of Fe in
place of Ni causes a decrease in the TCR.‘“,’ i Our results,
however, are somewhat similar to Kettler et al. ‘* in
a-Fe,Coao~~,BzO alloys.
D. Absence
FIG, 2. dr,,( 2-j I/‘& T”“) vs T3R for u-Co,,_,- J?~,MII$,~S~~alloys for
p=8 samples (lines have been drawn only to khow the average slope
clearly. )
of positive
of double minimum
Recent work
ing alloys, where
temperature are
scribed by a T2
has shown that in the Cr and V containtwo minima in resistivity as a function of
observed, r,(T)
is invariably best determ when fitted to Eq. (I). It has even
Piramanayagam
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et al.
5596
been conjectured that in these alloys rn( T) is actually described by an equation of A + BT312 + CT2 type. For such
concentrations of Cr or V, for which B decreases to a
negative value, a second resistivity minimum at a higher
temperature could appear.
In our samples, as discussed earlier, the presence of a
dominant T” term is obtained only for relatively high Mn
concentrations and also at the Fe-rich end. Moreover, fitting rn( T) to an equation containing two temperature dependent terms did not lead to any systematic results which
could indicate a change of sign in one of them. It is, therefore, natural to expect that resistivity of our alloys would
not show two minimum. In general, the Mn causes changes
which are much smaller than Cr and V additions, we expect that in most of the Mn containing alloys of any type,
either no second minimum in resistivity would be observed
or it would be observed only at higher Mn concentrations
in comparison to Cr or V. Such results have indeed been
obtained experimentally. t3-”
It is worth pointing out here that it has not been possible so far to associate a definite scattering mechanism to
the negative term responsible for second minimum. It is
possible, however, that the observed reduction in B to a
negative value with the addition of early 3d-transition metals results from the high temperature tail of maximum
obtained from the recently described electron magnon coupling of impure nature.16
IV. CONCLUSIONS
( 1) In Co-rich alloys, a T2 behavior is not observed
even up to the addition of 8 at. % of Mn while it is observed in Fe-rich alloys only for 8 at. % of Mn. This is
similar to what is observed when Cr or V is added in
Fe-based alloys. However, in Mn containing alloys, this
dependence shows up at significantly larger impurity concentrations.
5597
J. Appl. Phys., Vol. 73, No. IO, 15 May 1993
(2) In both Fe- and Co-rich alloys, Mn decreases the
value of b but the decrease is significantly larger in the case
of Fe-rich alloys.
(3) In the absence of Mn, the addition of Co does not
increase the positive TCR in contrast with what is observed for Ni addition.
(4) In the presence of Mn, the addition of Co leads to
an increase in the positive TCR similar to Ni substitution.
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Piramanayagam
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ef a/.
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