- Email: [email protected]
The resistivity of Ni1-xMnx in the reentrant and spin-glass phases
Journal
of Magnetism
and Magnetic
THE RESISTIVITY Y. ONER,
Materials
54-57
OF Ni, _ x Mn,
D. ELKHATOURI,
159
(1986) 159-160
IN THE REENTRANT
AND
SPIN-GLASS
PHASES
S. SENOUSSI
Lahoratoire de Physique des Solrdes, Uniuersitk Purrs - Sud, 91405 0r.r~~. France
We report measurements of the electrical resistivity of Ni, ~, Mn, alloy as a function of Mn concentration (0.05 < x < 0.33) and temperature (T= 4.2, 77 and 295 K). Contrary to the spontaneous magnetization which seems to vanish at a critical concentration x, = 0.24. no anomaly is seen in the resistivity curves at any x. The residual resistivity per at% Mn increases in the framework of a monotoneously with x from = 0.75 PQcm for x * 0 to = 3.2 PQcm for x = 0.33. This is interpreted two-current like model in which mixing mechanism is produced by spin-glass disorder.
Magnetic properties of disordered Ni, _,Mn, alloys have been extensively investigated in the concentration range 20 to 30 at% Mn [l-4]. It has been found that both the spontaneous magnetization [4] and the magnetoresistance [3] exhibit large change around the so-called multicritical point (x, - 0.24, T, = 80 K) defined by the intersection of the transition lines between ferromagnetism, paramagnetism and spin-glass (SG) order. Direct magnetic measurements are of essential and fundamental interest for the study of such transition lines. Nonetheless, their interpretation is extremely difficult because magnetization is sensitive both to shortrange and long-range magnetic orders as well as to coercivity effects. Transport measurements, on the other hand, are less direct but have the advantage of being only sensitive to relatively short-range order, probably on the scale of the conduction electron mean free path. Moreover, the resistivity probes the zero-field state (H = 0) and is certainly totally insensitive to coercivity and domain rotation effects on any scale. Recent studies [5] suggest that the anomalous behaviour of the magnetization, in the “mixed spin-glass-ferromagnetic” region, is dominated by coercivity and domain rotation effects. In order to get more information on the transition lines between spin-glass and ferromagnetic orders we have prepared and measured the resistivity of about 15 NiMn alloys as a function of Mn content from x = 0.05 to x = 0.33 for different temperatures (4.2, 77 and 295 K). See ref. [3] for experimental details. Fig. 1 shows p vs. x curves at 4.2 and 295 K for Ni , .Mn,. The most striking feature is a large and unusual growth in ~~(4.2 K) over the whole range of Mn concentration. That such a behaviour is unusual is illustrated in the insert on the same figure where we compare the residual resistivity of NiMn with those of NiCr, NiFe and NiCo [6]. For NiFe and NiCo, p0 is very low and approaches saturation around x = 0.25. Marked saturation effects are also exhibited by p0 of NiCr alloy above this concentration. In the case of NiMn, p0 varies almost quadratically with x and shows no sign of any saturation up to the highest concentration (x = 0.33) considered here. The peculiar behaviour of the NiMn system is also 0304-8853/86/$03.50
0 Elsevier
Science
Publishers
illustrated in fig. 2 which displays the reduced resistivity pO/.x and ~(295 K)/x vs. x. As can be seen, pO/x increases with x from = 0.75 PQcm per at% Mn (for the dilute or pure ferromagnetic limit) to about 3.2 yficm per at% (for the SG limit). Obviously, this is a SG disorder term. It is to be stressed that the variation of pO/x is quite gradual over the whole range of Mn concentration. In particular the curve does not show any special feature around the critical concentration (x, - 24%) between spin-glass and ferromagnetic orders. This would mean that the degree of short-range (or local) order changes quite continuously with x over the whole range of Mn concentration considered: our data
295K 100
-75 5 c =t At%Cr,Mn.Fe.Co
Q50
25
At o/m M n
concenfration
Fig. 1. Resistivity of NiMn alloy vs. Mn content for the indicated temperatures. The insert compares p for Cr. Mn. Fe and Co in Ni.
B.V.
Here P t is not provided by the well-known inelastrc spin-flip scattering mecanism (since T = 0)but could he represented as follows: A given conduction electron. with spin up to say. will experience (during its life time) regions of the sample where the local spins are “ mixed” up and down according to the degree of spin-glass disorder. Then in the dilute (or pure ferromagnetism) such a mixing effect is negligible and 1
PO ,-
A+%
I
0
I
10 At % Mn
Fig. 2. Reduced
resistivity
and 295 K. The insert K)/p(4.2
20 concentration
per at9 Mn vh. Mn content Y for 4.2 shows the .Y dependence of ~(77
suggest that SG effects persist down to .Y= 10% at least. On the other hand, the increase in pa/x which persists up to the highest concentration considered here (X = 0.33) indicates that the degree of .rhort-runge-ferromag-
stu.v.r quite .significunt
1
-
(0.7 * 0.1) p8cm%.
p&x-
= .i (
p , + p I ) = (2.06 * 0.7) pficm%.
This is not very different from our experimental \,alue (3.2 p8cm). The agreement must be considered as quite reasonable in view of the large uncertainties in p t and PI, From the present 11.3 pQcrn% Mn.
data we get p , = 0.72 and p L b
up to thut cwKentrutron.
The absence of any anomaly in the resistivity curves at .Y, ih not inconsistent with the idea that the degree of long range ferromagnetic order (to which p,, is not sensitive) changes suddenly at this point. Concerning the ~(77 K)/.u curve (not shown here) we also find no sign of any anomaly near the multicritical point (x~ = 0.24. T, = 80 K). Our above analysis is consistent with the insert of fig, 2 which represents the x-dependence of the resistivity ratio ~(77 K)/p(4.2 K). Clearly this term drops very rapidly (but continuously) as a function of x from the lowest s shown to about X, = 0.24 and then level off. Th1.s prohah(v mean.~ the existence of u lurge den.rity of mugnon.s, ussociutrd with long runge ferromugnetic order. which freeze out rapidly and becomes negligeable for .X.2 I,. The variation of pa/x can also be analysed in terms of a two-current-like model in which: PO/-U = [ P r P , + P t
PTPl
pr ‘Pl
where p T = (0.75 k 0.2) and p I = (7.5 k pf22.5) kltcm are taken from the literature [7.8]. In the SC limit the mixing mecanism is fully efficient so that it is no longer possible to distinguish between the two currents. Using the above values for p f and p 1 we get:
Mn !
K).
nrtic order
~
(P t +PJ]/[PrPI
+4p,
I]
It is found that the degree of short-range order varies smoothly and regularly over the whole range of Mn content (0 to 33 at%) in particular near the critical point ( .r< = 0.24). This point is associated with a rapid change in the degree of long-range ferromagnetic order to which p,, is not sensitive but is well reflected in the (~(77 K) - po)/p,, vs. .Y relationship which is probably related to the density of magnons at 77 K in the alloy. [II ‘T. Satoh, R.B. Goldfarb
and C.E. Patton.
IEEE Tram
cm
Magn. MAG-13 (1977) 1454. 121S. Senouasl. Phys. Rev. Lett. 51 (1983) 221X: .I. de Phys. 45 (1984) 315.
131S. Senoussi and Y. Oner, .I. Magn. Magn. Mat. 40 (19X4) 12: J. Appl. Phys. 55 (1984) 1472. and J.S. Kouvel. J. Appl. Phys. 55 (19X4) I41 W. Abdul-Raazaq 1623. 151 S. Senoussi. Phys. Rev. B 31 (19X5) 6086: .I. de Phys. 46 (1985) 1435. Lb1 P. Muth and V. Chrlstoph. J. Phys. F 11 (19X1) 2 II 9. 171 J.W.F. Dorleijn and A.R. Miedena, J. Phys. F 5 4X7 (1975) IX1 J.W.F. Dorleijn. Philips Reu. Rep. 31 (197h) 2X7
of Magnetism
and Magnetic
THE RESISTIVITY Y. ONER,
Materials
54-57
OF Ni, _ x Mn,
D. ELKHATOURI,
159
(1986) 159-160
IN THE REENTRANT
AND
SPIN-GLASS
PHASES
S. SENOUSSI
Lahoratoire de Physique des Solrdes, Uniuersitk Purrs - Sud, 91405 0r.r~~. France
We report measurements of the electrical resistivity of Ni, ~, Mn, alloy as a function of Mn concentration (0.05 < x < 0.33) and temperature (T= 4.2, 77 and 295 K). Contrary to the spontaneous magnetization which seems to vanish at a critical concentration x, = 0.24. no anomaly is seen in the resistivity curves at any x. The residual resistivity per at% Mn increases in the framework of a monotoneously with x from = 0.75 PQcm for x * 0 to = 3.2 PQcm for x = 0.33. This is interpreted two-current like model in which mixing mechanism is produced by spin-glass disorder.
Magnetic properties of disordered Ni, _,Mn, alloys have been extensively investigated in the concentration range 20 to 30 at% Mn [l-4]. It has been found that both the spontaneous magnetization [4] and the magnetoresistance [3] exhibit large change around the so-called multicritical point (x, - 0.24, T, = 80 K) defined by the intersection of the transition lines between ferromagnetism, paramagnetism and spin-glass (SG) order. Direct magnetic measurements are of essential and fundamental interest for the study of such transition lines. Nonetheless, their interpretation is extremely difficult because magnetization is sensitive both to shortrange and long-range magnetic orders as well as to coercivity effects. Transport measurements, on the other hand, are less direct but have the advantage of being only sensitive to relatively short-range order, probably on the scale of the conduction electron mean free path. Moreover, the resistivity probes the zero-field state (H = 0) and is certainly totally insensitive to coercivity and domain rotation effects on any scale. Recent studies [5] suggest that the anomalous behaviour of the magnetization, in the “mixed spin-glass-ferromagnetic” region, is dominated by coercivity and domain rotation effects. In order to get more information on the transition lines between spin-glass and ferromagnetic orders we have prepared and measured the resistivity of about 15 NiMn alloys as a function of Mn content from x = 0.05 to x = 0.33 for different temperatures (4.2, 77 and 295 K). See ref. [3] for experimental details. Fig. 1 shows p vs. x curves at 4.2 and 295 K for Ni , .Mn,. The most striking feature is a large and unusual growth in ~~(4.2 K) over the whole range of Mn concentration. That such a behaviour is unusual is illustrated in the insert on the same figure where we compare the residual resistivity of NiMn with those of NiCr, NiFe and NiCo [6]. For NiFe and NiCo, p0 is very low and approaches saturation around x = 0.25. Marked saturation effects are also exhibited by p0 of NiCr alloy above this concentration. In the case of NiMn, p0 varies almost quadratically with x and shows no sign of any saturation up to the highest concentration (x = 0.33) considered here. The peculiar behaviour of the NiMn system is also 0304-8853/86/$03.50
0 Elsevier
Science
Publishers
illustrated in fig. 2 which displays the reduced resistivity pO/.x and ~(295 K)/x vs. x. As can be seen, pO/x increases with x from = 0.75 PQcm per at% Mn (for the dilute or pure ferromagnetic limit) to about 3.2 yficm per at% (for the SG limit). Obviously, this is a SG disorder term. It is to be stressed that the variation of pO/x is quite gradual over the whole range of Mn concentration. In particular the curve does not show any special feature around the critical concentration (x, - 24%) between spin-glass and ferromagnetic orders. This would mean that the degree of short-range (or local) order changes quite continuously with x over the whole range of Mn concentration considered: our data
295K 100
-75 5 c =t At%Cr,Mn.Fe.Co
Q50
25
At o/m M n
concenfration
Fig. 1. Resistivity of NiMn alloy vs. Mn content for the indicated temperatures. The insert compares p for Cr. Mn. Fe and Co in Ni.
B.V.
Here P t is not provided by the well-known inelastrc spin-flip scattering mecanism (since T = 0)but could he represented as follows: A given conduction electron. with spin up to say. will experience (during its life time) regions of the sample where the local spins are “ mixed” up and down according to the degree of spin-glass disorder. Then in the dilute (or pure ferromagnetism) such a mixing effect is negligible and 1
PO ,-
A+%
I
0
I
10 At % Mn
Fig. 2. Reduced
resistivity
and 295 K. The insert K)/p(4.2
20 concentration
per at9 Mn vh. Mn content Y for 4.2 shows the .Y dependence of ~(77
suggest that SG effects persist down to .Y= 10% at least. On the other hand, the increase in pa/x which persists up to the highest concentration considered here (X = 0.33) indicates that the degree of .rhort-runge-ferromag-
stu.v.r quite .significunt
1
-
(0.7 * 0.1) p8cm%.
p&x-
= .i (
p , + p I ) = (2.06 * 0.7) pficm%.
This is not very different from our experimental \,alue (3.2 p8cm). The agreement must be considered as quite reasonable in view of the large uncertainties in p t and PI, From the present 11.3 pQcrn% Mn.
data we get p , = 0.72 and p L b
up to thut cwKentrutron.
The absence of any anomaly in the resistivity curves at .Y, ih not inconsistent with the idea that the degree of long range ferromagnetic order (to which p,, is not sensitive) changes suddenly at this point. Concerning the ~(77 K)/.u curve (not shown here) we also find no sign of any anomaly near the multicritical point (x~ = 0.24. T, = 80 K). Our above analysis is consistent with the insert of fig, 2 which represents the x-dependence of the resistivity ratio ~(77 K)/p(4.2 K). Clearly this term drops very rapidly (but continuously) as a function of x from the lowest s shown to about X, = 0.24 and then level off. Th1.s prohah(v mean.~ the existence of u lurge den.rity of mugnon.s, ussociutrd with long runge ferromugnetic order. which freeze out rapidly and becomes negligeable for .X.2 I,. The variation of pa/x can also be analysed in terms of a two-current-like model in which: PO/-U = [ P r P , + P t
PTPl
pr ‘Pl
where p T = (0.75 k 0.2) and p I = (7.5 k pf22.5) kltcm are taken from the literature [7.8]. In the SC limit the mixing mecanism is fully efficient so that it is no longer possible to distinguish between the two currents. Using the above values for p f and p 1 we get:
Mn !
K).
nrtic order
~
(P t +PJ]/[PrPI
+4p,
I]
It is found that the degree of short-range order varies smoothly and regularly over the whole range of Mn content (0 to 33 at%) in particular near the critical point ( .r< = 0.24). This point is associated with a rapid change in the degree of long-range ferromagnetic order to which p,, is not sensitive but is well reflected in the (~(77 K) - po)/p,, vs. .Y relationship which is probably related to the density of magnons at 77 K in the alloy. [II ‘T. Satoh, R.B. Goldfarb
and C.E. Patton.
IEEE Tram
cm
Magn. MAG-13 (1977) 1454. 121S. Senouasl. Phys. Rev. Lett. 51 (1983) 221X: .I. de Phys. 45 (1984) 315.
131S. Senoussi and Y. Oner, .I. Magn. Magn. Mat. 40 (19X4) 12: J. Appl. Phys. 55 (1984) 1472. and J.S. Kouvel. J. Appl. Phys. 55 (19X4) I41 W. Abdul-Raazaq 1623. 151 S. Senoussi. Phys. Rev. B 31 (19X5) 6086: .I. de Phys. 46 (1985) 1435. Lb1 P. Muth and V. Chrlstoph. J. Phys. F 11 (19X1) 2 II 9. 171 J.W.F. Dorleijn and A.R. Miedena, J. Phys. F 5 4X7 (1975) IX1 J.W.F. Dorleijn. Philips Reu. Rep. 31 (197h) 2X7