Comparative investigation of structural and transport ...

J . phys. F: Met. Phys. 18 (IYSX)

2033-2051. Printed in the LK

Comparative investigation of structural and transport properties of L10 NiPt and Copt phases; the role of magnetism C Leroux?. M C Cadevillet. V Pierron-Bohnes'i, G I n d e n t and F Hinz:i .: Groupe d'Etude dc\ MatCriiiux MCtalliques. IPCMS ( U M CNRS n o 380046).

1. rue Blaise Pascirl. 67070 Str;i\hourg Cedes. France j Max-Planck lnstitut fur Eisenforschung GmbH, Max-Planck Strasse 1, D-4000 Dusseldorf. FRG Keceitcd IC)Januar! IOSX. in linal tomi S March 19SS Abstract. The re\tilt\ 01 :I compar;itivc investigation of strueturiil and transport properties of equiatomic NiPt and C'oPt ph;i\es. in their ordered a n d disordered states. iire presented a n d di\cus\etl. The t h o compounds displa) the same hind o l order-disorder transformation L I , , - h , i n rhcir pir;imagnetic \tate. I n thi\ stud!. atrcntion \viis paid to make iiieii\iiremcnts o n \ampies in well detined thermod~n,imiestate\. U hich can he either equilihriuin statcs o r quenched one\. Thi\ in~estig;ition\ h i ) & \ that the atomic disorder gencr;ited hy the thermal effect o r h) con\titution:il deleet\ (o('C-\toichiometr! ) hiis jimilur consequences o n the structure (tetragon;ility ratio) irnd o n the re\idual rc\istivit) in the t h o compounds. A grcat ditt'erence occur\ i n the high-Tmistivity. he lo^ a n d iit the 0 - 1 ) translormation temperature ( T , ) , ) )Whcre;i\ . NiPt displa)s ;I hehakiour typical of this kind of transforinat i o n . Copt \how\ unexpected rc\ults. in particular ii decrease in its resistibity ;it To,,. Such ;I dil'lerence can he e.;pl;iincd by taking into account the \pin-disorder \cattcring contribution to the resisti\ity. which I\ larger in Copt than in NiPt. A process hased o n the existence o l Huctu;iting Ioc,il magnetic moment\ and magnetic short-range order in the p;iramagnetic state of C'oPt i \ propowd to expl;iin the ohserved hehaviour qualitati\ely.

1. Introduction

The electrical resistivity of metallic alloys o r compounds is often used to get relevant information on either the ordered states o r the time dependence of the degree of order. Most current investigations of ordering kinetics o r relaxations are based on resistivity methods. ming either low-temperature ( 3 K ) experiments (see Balanzat et ril 1983 and references therein) o r iri sirii experiments (Dahmani et a1 198%). At low temperatures. the resistivity of an alloy is mainly sensitive t o the atomic distribution o n the lattice sites. i.e. to t h e presence of short- o r long-range order. In solid solution phases. the sign o f the change in residual resistivity associated with an increase in the short-range order (SRO) can be positive, as in NiAl (Afyouni et a1 1988) or negative ;IS in FeV (Pierron-Bohnes et ril 1983). depending on many factors such as the electronic band structure o r the sign and order of magnitude of S K O parameters. In ordered phases an increase in the long-range order ( L K C ) ) parameter always leads t o a decrease in the residual resistivity. This comes from the quantum effect associated 0305-4608/88/092033+ 19 $02.50

0 1988 IOP Publishing Ltd

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with the improvement of the crystal periodicity with ordering. At high temperatures, the problem is not so simple. The effect of atomic ordering can be partially hidden by contributions from other electron diffusion processes. Previous investigations of atomic relaxations in several ordered phases of the Ni-Pt and Co-Pt systems (Ni,Pt, NiPt, Copt,, Co,,,Pt,(,, Co, curve obtained is shown in figure 11. O n e sees that in the ferromagnetic range pmincreases rapidly with T, reaching 2 8 p Q cm at Tch,.This value lies between the corresponding values for Ni (20,uQcm) and Fe (70uQ cm), as does the average moment of Copt ( 1 . 2 p B ) ,in qualitative agreement with the estimate of the theoretical model (equation (3)).

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Figure 11. T dependence of the magnetic contribution ( + ) deduced from equation ( 5 ) , compared with that of the chemical disorder ( 0 )in Copt. The curves are to guide the eye. The inset shows the Tdependence of the magnetic susceptibility at the 0-D transformation of Copt. as measured at increasing and decreasing temperatures.

Striictiiral utid trutisport properties of NiPt arid CoPt

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"CO

Figure 12. Probability for CO atoms of having n CO atoms in their first nearest neighbour shell in the ordered states (hatched areas) and disordered states (as deduced from a binomial law).

In the paramagnetic state pi,,does not remain constant as the model predicts, but decreases with T . reaching about 15pR cm in the disordered state where it remains nearly constant. This behaviour seems to be inversely correlated to the effect of the thermal disorder on the resistivity, which is also shown in figure 11. Now. to understand the behaviour of p,,,(T ) in the paramagnetic state, o n e has to go beyond the oversimplified description of magnetism in terms of a simple molecular field approximation. It is now well established from theoretical approaches ( U e d a and Moriya 1975. Prange and Korenmann 1975, Moriya 1979, 1983, Korenmann and Prange 19x4. Clauberg et (11 1986) and from experiments (Shirane et ul 1984, Brown er a1 1984. Kisker et ul 1984, Capellmann 1985). that. at high temperature, in the paramagnetic state of strong itinerant ferromagnets, as in C o p t ( i ) fluctuating local magnetic moments exist and ( i i ) these moments are not totally disordered but present some magnetic short-range order ( h i s i w ) . To interpret the resistivity data, we suggest that the Msno increases simultaneously with the increase in the chemical disorder and that. correspondingly. the contribution of the spin-disorder scattering to the resistivity decreases according to the following simple scheme. In binary systems the L K O is characterised by a strong heterocoordination whereas disorder favours homocoordination. This is illustrated in figure 12 for the chemical environment of CO atoms in their nearest neighbour ( N N ) shell in an equiatomic alloy: while in the LI,, structure there are only clusters of CO atoms with four CO atoms in their N N shell. in the disordered state the probability of finding clusters with ti CO atoms ( I < t i < 12) in their N N shell is given by a binomial distribution and is a maximum for 11 = 5. Because of the cooperative character of magnetism, the magnetic interactions and hence the MSKO must be enhanced in such Co-rich clusters ( t i > 4 ) with respect t o interactions in Co-poor clusters. This assumption is strengthened by the increase in the magnetic susceptibility at the 0--Dtransition (Ax/x= 17%) (figure ll), which is large compared with the corresponding small increase in NiPt (Ax/x= 6%). Thus. if the MSKO is enhanced b y the increase in chemical disorder. then the contribution of the spin-disorder scattering t o the resistivity will decrease. Therefore, the phenomena of MSKO and chemical disorder contribute with opposite sign to the resistivity variation when T approaches To,,. as illustrated by the two curves of

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figure 11. The two contributions may even cancel each other out. The data observed on Copt, i.e. the slow decrease of pm(T)between TcMand To, followed by an abrupt decrease at To,, indicate that the effect of MSRO is the more important the higher is the atomic disorder. On the whole it contributes to a decrease of the magnetic contribution by a factor of two. However, a part of this decrease could be due to effects associated with the change in ,Iphand nc,,at To,, which we have assumed to be equivalent in NiPt and Copt. In the disordered paramagnetic state the T dependence of the chemical disorder (SRO) is negligible, so that the correlated MSRO effect no longer varies with T , in agreement with the nearly constant value of pmabove TO,. Our analysis shows that the small decrease in the total resistivity of Copt at TO, is due to the fact that. in this particular case, the M S R O effect prevails over all other effects at TOD.This is consistent with the fact that in Copt the 0--Dtransformation occurs relatively close to the Curie temperature ( T O J T c M =1.5). so that the MSRO is still important at TOD.However, different situations can arise at other compositions. For example, in Copt,, previously investigated by Dahmani (1985), it seems that magnetic and chemical disorder effects compensate exactly at the 0-D transformation, where only a change of slope of p( T ) is observed. According to our simple scheme, the effect of chemical disorder on magnetic correlations is expected to be more important in Copt, than in Copt since CO atoms in ordered Copt, have no cobalt in their N N shell. On the other hand, the mean magnetic moment in Copt, is only 0 . 6 7 , ~ and ~ moreover the 0-D transformation occurs at a temperature well above the Curie = 3.43). At such high values of TIT,,,, magnetic moments and temperature ( ToD/TcM short-range magnetic correlations become much weaker (Brown er a1 1984) and thus the effect on the resistivity expected through our simple description should be smaller. The behaviour of the resistivity of Copt, at the 0--Dtransformation is therefore compatible with our interpretation. To conclude, this investigation of the structural and transport properties of NiPt and Copt compounds in the vicinity of their order-disorder transformation and their comparative analysis in a qualitative scheme show that the two compounds present great similarities when considering the effect of constitutional and thermal disorder on their structural and transport properties. In Copt the presence of an important magnetic contribution associated with spin-disorder scattering causes its resistivity to be higher than that of NiPt at the same temperature. Moreover it is necessary to take into account the existence of an important MSRO in the paramagnetic state of Copt to explain the unusual decrease in its resistivity at TOD.One can say that the present work supports the argument of experimentalists and theoreticians who militate for the presence of short-range magnetic correlations in the paramagnetic state of itinerant ferromagnets. It is obvious that measurements of paramagnetic neutron scattering at the order-disorder transformation of such Co-Pt or other similar compounds would be desirable to check our interpretation of their transport properties. However, such experiments, which necessitate high-flux neutron sources and single-crystalline specimens, are not trivial to perform and are very time consuming. Acknowledgments

The authors of IPCMS are greatly indebted to Professor E Daniel for stimulating and valuable discussions.

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