Low temperature resistivities of FexNi80-xP14B6 metallic glasses

Solid State Communications, Vol. 27, pp. 441—444. © Pergamon Press Ltd. 1978. Printed in Great Britain.

0038—1098/78/0722—0441 $02.00/C)

LOW TEMPERATURE RESIST!VITIES OF Fe~Ni~..xP14B6 METALLIC GLASSES E. Babid, 2. Marohnid and J. Ivkov Institute of Physics of the University, Zagreb, P.O. Box 304, Yugoslavia (Received 20 February 1978 &v J.L. Olsen) The resistivities of six Fe~Ni~.~P14B6 alloys have been measured between 1.5 and 50K. It is found that the resistivity variations both below and above the resistivity minima depend on the transition metal composition. The room temperature coefficients of the resistivity indicate the existence of the magnetic contribution to the resistivity.

IN THE LAST FEW YEARS amorphous ferromagnetic alloys have been a subject of considerable interest. This interest has arisen both because of the unusual physical properties connected with the structural disorder and because of potential industrial applications of these materials. One of the most striking features is a Kondo like resistance minimum which seems to appear in all amorphous ferromagnets. However in an amorphous ferromagnet such a resistivity behaviour cannot be caused by a single impurity Kondo effect because the large internal fields inherent to a ferromagnet quench any spin flip scatteringprocesses. As resistivity minima also appear in other (non-magnetic) amorphous alloys it seems rather appealing to associate them with the structural disorder. A recent calculation [11,based on the excitations of a two level system (TLS) (originally proposed [2, 3] for the explanation of the low temperature specific heat of amorphous insulators) is reported to describe adequately the resistance anomalies both in magnetic and nonmagnetic amorphous alloys. From the experimental side however the situation is not so clear, there are still claims [1, 4] and counterclaims [5] as regards the validity of this model for metallic glasses. One of the reasons for such different observations is probably the very different amorphous alloys which were investigated. In our opinion it may be more important to investigate systematically one particular alloy system than to investigate very different systems which are probably better explained by different scattering mechanisms [6]. Here we wish to present the first systematic study of the low temperature resistivities of Fe~Ni~_~P14B6 alloys. Although the magnetic properties of this system have been investigated in some detail [7, 8] there was no systematic study of the changes in the electrical resistivity when varying the transition metal composition. In the same time this system may be particularly interesting in order to distinguish whether the resistance 441

anomaly is caused by the structural or various magnetic mechanisms [6] because a rather sudden loss of ferromagnetism is reported [8] for these alloys with less than 10 at.%Fe. The resistivities of six Fe~Ni~_~Pi4B6 alloys (where x varied in steps of 10) were measured in the temperature interval 1.5—50 K. The samples were in a form of ribbon 1—2mm wide and up to 50~zmthick. In each resistivity run two samples of a few centimeters in length were mounted between the insulated copper plates in a special cryostat. The temperature was measured by the calibrated Ge thermometer and contrailed by the resistance heater and a small pressure of helium exchange gas. The resistivity measurements were performed by using a potentiometric set up with the resolution better than 1 part in 1 o~. The absolute resistivity values (accurate to about 5%) were determined from the geometrical shape factor deduced from the measured mass and density of the sample. The densities were obtained by weighing the samples both in air and bromophorm. The densities (D) and the resistivities at 4.2 K (p4.2) are given (together with some other relevant data) in Table 1. We note that the p~values deduced in that manner are considerably lower than those obtained from directly measured dimensions of the samples which is due to roughness (small indentations) of the surfaces of the samples caused by the preparation technique. Therefore we believe that most of the resistivity values of the metallic glasses quoted in the literature are probably somewhat too high. Here it is interesting to note that the p~values of all our samples appear to be approximately the same within the experimental error. The low temperature resistivities (1.5—50K) of Fe~Ni~~ P14B6 alloys are shown in Fig. 1. Here the quantity plotted is s~p/p~j~ (where ~ is the resistivity at the temperature of the resistivity minimum, Tmj~)which because the low temperature resistivities

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LOW TEMPERATURE OF Fe~Ni8o~Pi4B6

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Table 1. Data relevant to our alloys. D is density, p~is the resistivity at 4.2K, Tmjn is the temperature of the resistivity below and above 5K respectively, T~is the Curie temperature and (dp/dT) RTdenotes the room temperature coefficient ofthe resistivity AllOY Fe 10Ni~P14B6 Fe20Ni~,P14B6 Fe~Ni50P14B6 Fe~Ni~P14B6 Fe~Ni20P14B6 Fe~Ni10P14B6 *

3) D (gcm

(j.~Z-cm) P4.2

(K) Tmjn

(K)

(l/p~j~,J A K iO~ (dplogT) A’x iO~

(n~cmK) (dp/dT)RT

7.86 7.78 7.71 7.57 7.43 7.37

135 138 136 138 140 138

32 17 18.5 27 30.5 25

60 230 444 533 619 628

3.7 2.6 1.7 1.7 1.7 1.5

5.4 12.3 27.8 27.2 22.1 17.4

7.0 2.9 1.8 2.0 1.8 1.8

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Fig. I. The relative changes in the resistivity = p(T) Pmjnl of Fe~Ni~_~P 14B6 alloys vs log r. The numbers denote Fe content (x). Full curve is obtained from equation (1) with ~.IK = 1 K and A adjusted to give the resistivity decrease observed in Fe20Ni~P14B6alloy. In the inset the two logarithmic slopes of the resistivity below and above 5 K (full and open symbols respectively) are plotted vs iron concentration.

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1000 2 2000 T~K ~ = ~p(T) A log T Fig. 2. The corrected resistivities (see text) vs T2 temperature scale. The numbers denote —

—

of all our alloys are approximately equal represents also well absolute resistivity variation. Beforethe discussing thesethe results in detail we briefly summarize main results of the structural model. In this model the origin

Fe content. In the inset the room temperature coefficients of the resistivity dp/dT RT are plotted vs Fe concentration. of the resistance anomaly is sought in the indeterminacy in local atomic configurations of the amorphous system. The calculation [1] based on a simple case of two equivalent atomic positions separated by a small barrier gives for the decreasing part of the resistivity 2 T? + &) (1) Pd = A In (k where A is a constant depending on the number of contributing sites and the strength of Coulomb interaction.

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LOW TEMPERATURES OF Fe~Ni~~P~4B6

k is Boltzmann constant and ~ is the energy difference between the two atomic tunnelling states. It is clear that equation (1) can reproduce the Kondo like resistivity behaviour. isAtobtained the temperatures well below ~/k a 2 saturation while at temperatures above ~/kT the resistivity is expected to decrease logarithmically The slope of this decrease is governed by A. From Fig. I can be seen that our measurements do not extend to low enough temperatures to observe the saturation of the resistivity. It was reported.recently [5] that the resistivity of Fe~Ni~P 14B6 alloy (Metglass 2826) does not saturate down to 70 mK. On the other hand a clear saturation of the resistivity of Co76P24 amorphous alloy, measured down to 0.6 K, is reported. We note however that in the latter case due to rather low = 0.6 K one would expect the real saturation to set in at the temperatures considerably below 0.6 K. In any case our results would put an upper limit for ~/k values of all our samples to about 1.5 K. At somewhat higher temperatures (1.5—15 K) our data are detailed enough to enable us to follow precisely the expected logarithmic resistivity variation. We note however that in some samples a change of the slope (an increase) is observed (Table 1)’ at the temperatures above 4K. (A similar change in the slope of the resistivity vs temperature has been observed in several other metallic glasses [4, 5] -) As a consequence we expect that the value of ~ depends on the temperature interval in which a fit to equation (1) is attempted and that no single values of ~ and A can give reliable fit to our data. To illustrate this we have also shown in Fig. 1 the resistivity variation derived from equation (1) with ~/k = 1 K and the slope observed in FeA selected to give the resistivity decrease 20Ni~P16B6alloy. the structural model such a behaviour could be Within qualitatively explained by the contribution to the resistivity arising from the sites with higher ~ which at higher temperatures come into play. It is not clear to us however why in amorphous system such a process does not lead to smoother resistivity variation. In order to clarify these points an independent estimate of ~ values from the low temperature specific heat measurements would be neccessary. There is another interesting point in our results. The slopes of the resistivity both at lower (T c~5 K) and somewhat higher (5
443

why the change in the slopes is nonlinear with Fe concentration. In particular a big change in the number of contributing sites occuring in a rather narrow concentration interval at.%) seem to us alloys rathervary Unlikely. We note that(~the10densities of these slowly and approximately linearly with Fe (or Ni) concentration. We also note that the biggest change in the slopes occurs near to Fe concentration around which the ferromagnetism seems to disappear [8]. In that sense the variation in the low temperature resistivity slopes can be compared with the manner in whcih the Curie temperatures (also given in Table 1) decrease with decreasing Fe content in these alloys. In what follows we want to discuss briefly the ternperatures of the resistivity minima and the resistivities immediately above the minima. It can be seen that in contrast to the low temperature resistivity slopes the Tmjn values (Table 1) are not simply correlated with the transition metal concentration. This indicates that the positive (i.e. increasing with temperature) contribution the resistivity is not the same in all our alloys but it also depends on the transition metal composition. Thus we would expect the following expression for the contribution to the resistivity which is increasing with ternperature, B~ — —

where the coefficient B is a function of the transition metal composition (which is not neccessarily singlevalued) and n is the temperature exponent. From Fig. I. can be seen the resistivities above Tmjn increase (except for Fe1oNi~P14B6)alloy with decreas2 temperature ing Fe content. However if plotted 2vsinT any significant scale they dointerval not seem vary astoTwhat reported temperature in to contrast earlier [5]. In any case one would not expect a pure T2 dependence of the resistivity at temperatures rather near to Tmjn because the resistivity minimum is obtained where the coefficients and not the magnitudes [9] of the increasing and decreasing contributions to the resistivity are equal.* In order to verify this point in Fig. 2 we have plotted the resistivities (above about 20K) corrected for the negative resistivity contribution (obtained by the extrapolation of the logarithmic resistivity variation between 5 and 12 K to higher temperatures) vs T2 scale. It can be seen that the corrected resistivities seem to follow reasonably well a T2 variation. This T2 variation may be due to electron—electron scattering. However the adopted correction procedure ____________

*

That this is also true in our alloys can be seen from the fact that the relative depths of the resistivity minima (pi.s Pmin)/Pmin vary more smoothly with Fe content than Tmjn values. —

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LOW TEMPERATURES OF Fe~Ni~~Pj4B6

is up to some extent arbitrary. We also note that in Fe~Ni~B20 amorphous alloys both Tmjn and P depend to some extent on the preparation conditions [10]. Therefore we believe a more detailed analysis of the resistivities above Tmm would at this point be premature. Still we note that the increasing low ternperature resistivity parts in Fe~Niao_~Pi4B6 alloys seem to behave like their decreasing counterparts (except for Fe1oNi~Pj4B6),i.e. they both increase with decreasing Fe content (or Curie temperature). This seems to mdicate that both of these terms are connected to some extent to the magnetic state of the alloy, In order to verify this we have also measured the room temperature coefficients of the resistivity (dp/dT)RT which are shown in the inset to Fig. 2. It can be seen that the behaviour of (dp/dT)RT is in some sense opposite to that of P~at low temperatures. The alloys with higher Curie temperatures (less Ni) have higher (dp/dT)RT values. A sudden drop in (dp/dT)RT occurs for the Fe concentration at which Curie temperature falls below room temperature. This seems to us as a rather clear evidence of the magnetic contribution to the resistivity around room temperature. On the other hand recently was reported no magnetic contribution to the resistivity of Metglass 2826A alloy at higher temperatures [11]. We note however that this alloy is not a very good candidate for the investigation of the magnetic contribution to the resistivity due to giant negative contribution which still does not seem to be well under-

Vol. 27, No. 4

stood [11]. It is interesting to note that (dp/dT)RT values of the alloys which are ferromagnetic at room temperatures slowly decrease with increasing Fe content. Also their values are comparable but somewhat lower than (dp/dT)RT of pure (crystalline) Fe and Ni (50.3 and 38.4 n~2-cm/Krespectively) what is probably associated with their amorphous structure. In conclusion our measurements of the low ternperature resistivities and room temperature coefficients of the resistivity of amorphous ferromagnetic alloys Fe~Ni~_~Pj4B6 show a rather strong dependence of both of these properties on transition metal cornposition. The calculation [1] based on the structural model (at least in its present form) does not seem to be able to account for such dependencies. This may indicate that the observed behaviour is partially of the magnetic origin. However the existing magnetoresistance measurements [11, 12] (although not detailed) seem to rule out any bigger magnetic effects. We believe that for a better understanding of ferromagnetic metallic glasses further systematic studies of resistivities and magnetoresistivities in a wide temperature intervals (particularly at the lowest temperatures) are required. Some of these measurements are already in progress. Acknowledgements We wish to thank Dr. F.E. Luborsky for giving us the samples. We also had useful ~is~u:sic~)ns with Drs. l.A. Campbell, J.R. Cooper and —

1.

REFERENCES COCHRANE R.W., HARRIS R., STROM-OLSON J.O. & ZUCKERMANN M.J., Phys. Rev. Lett. 35, 676 (1975).

2.

ANDERSON P.W., HALPERIN B.I. & VARMA C.M.,Phil. Mag. 25, 1(1972).

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PHILLIPS W.A.,J. Low Temp. Phys. 7,351(1972). BAIBICH M., COCHRANE R.W., MUIR W.B. & STROM-OLSON J.D., Amorphous Magnetism II (Edited by LEVY R.A. & HASEGAVA R.), p. 297. Plenum Press, New York (1977). RAPP 0., BHAGAT S.M. & JOHANNESSON Ch., Solid State Commun~21, 83 (1977).

5. 6.

TSUEI C.C., Amorphous Magnetism II (Edited by LEVY R.A. & HASEGAWA R.), p. 181. Plenum Press, New York (1977).

7. 8.

BECKER J.J., LUBORSKY F.E. & WALLER J.L., IEEE Trans. Magn. 13,988 (1977). CHIEN C.L., MUSSER D.P., LUBORSKY F.E., BECKER J.J. & WALTER J.L., Solid State Commun. 24, 231 (1977). BABIC E., KRSNIK R. & HAMZI~~ A., Solid State Commun. (to be published) (1978).

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BABIC E. & MAROHNIC 2. (unpublished results) (1977). COCHRANE R.W. & STROM-OLSON J.O.,J. Phys. F. Metal Phys. 7, 1799 (1977).

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MAROHNIC 2., BABIC E. & PAVUNA D., Phys. Lett. 63A, 348 (1977).