Dependence of the transport properties on the long range order of β -phase Co 0.50 Ti0.50 alloy filmsY. P. Lee, K. W. Kim, J. Y. Rhee, Y. V. Kudryavtsev, V. V. Nemoshkalenko, and V. G. Prokhorov Citation: Journal of Applied Physics 89, 3315 (2001); doi: 10.1063/1.1346649 View online: http://dx.doi.org/10.1063/1.1346649 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Local atomic order, electronic structure and electron transport properties of Cu-Zr metallic glasses J. Appl. Phys. 115, 203714 (2014); 10.1063/1.4879903 Structural studies of high-K u metastable CoPt thin films with long-range order J. Appl. Phys. 111, 07A303 (2012); 10.1063/1.3670511 Properties of microstructure on amorphous film of rare earth–transition metal alloy for ultrahigh density recording J. Appl. Phys. 101, 09C522 (2007); 10.1063/1.2714672 On the use of alloying elements for Cu interconnect applications J. Vac. Sci. Technol. B 24, 2485 (2006); 10.1116/1.2357744 Effect of the structural disorder on the magnetic, transport, and optical properties of B2 -phase Ni 0.50 Al 0.50alloy films J. Appl. Phys. 91, 4364 (2002); 10.1063/1.1456964

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Dependence of the transport properties on the long range order of b-phaseCo0.50Ti0.50 alloy films

Y. P. Leea)

Department of Physics, Hanyang University, Seoul, 133-791, Korea

K. W. KimDepartment of Physics, Sunmoon University, Asan, Choongnam, 336-840, Korea

J. Y. RheeDepartment of Physics, Hoseo University, Asan, Choongnam, 336-795, Korea

Y. V. Kudryavtsev, V. V. Nemoshkalenko, and V. G. ProkhorovInstitute of Metal Physics, National Academy of Sciences of Ukraine, 36 Vernadsky Str, 252680,Kiev-142, Ukraine

~Received 14 January 2000; accepted for publication 9 December 2000!

The influence of the structural disorder on the transport properties of Co0.50Ti0.50 alloy films in atemperature range of 4.2–300 K has been investigated without and with a magnetic field of 0.5 T.The absence of translational invariance in a disordered state leads to an increase in the resistivityand causes a change from the positive to negative temperature coefficient of resistance. This fact isexplained by the partial localization of electronic states near the Fermi level. It was established thata partial structural disordering enhances the role of the electron-phonon-vibrating impurityscattering in the transport properties and also makes the spin-diffusive scattering rather noticeable.The appearance of a low-temperature resistivity minimum for the disordered Co0.50Ti0.50 alloy filmarises from the quantum corrections to the electron–electron interactions in the presence of weaklocalization. © 2001 American Institute of Physics.@DOI: 10.1063/1.1346649#

I. INTRODUCTION

In the perfectly ordered equiatomic CoTi alloy the Coand Ti atoms form interpenetrating primitive cubic sublat-tices where each Co atom has eight Ti atoms as the nearestneighbors and vice versa. In contrast to the ordered state ofCoTi alloy, the Co and Ti atoms can randomly occupy thesites of the bcc lattice in the disordered state, which changesthe local environment@the antistructure Co atoms~Co–ASA;the Co atoms at the Ti sites! and/or their clusters can beformed# and leads to changes in the electron–energy struc-ture of the alloy.

The perfectly ordered equiatomic CoTi alloy is known tobe paramagnetic even at very low temperatures.1 The appear-ance of the Co–ASA and/or their clusters, however, causesthe changes in the magnetic state of the alloy. In the per-fectly ordered equiatomic alloy, a Co atom is surrounded byeight nearest-neighbor Ti atoms, occupying the eight cubecorners, and thus has no nearest-neighbor Co atoms. Thelack of nearest-neighbor Co atoms results in suppression ofthe direct exchange interactions between the Co atoms and,consequently, reduction of the localized spin moment of Coatoms. Furthermore, there is a significant charge transferfrom the Ti to Co atom. The charge transfer plays an impor-tant role to determine not only the structural stability of thealloy but the magnetic properties. It is known that there is aclose relationship between the charge transfer and the mag-netic properties of the transition-metal nontransition-metal

alloys.2 The charge transfer from the Ti to Co atoms furthercancels out the magnetic moment of the Co atoms by com-pensating the unpaired electron spin. As a consequence, theperfectly ordered equiatomic alloy comes to be nonmagnetic.

Since the Co–ASA and their clusters can be consideredas additional scattering centers for charge carriers, the trans-port properties should be sensitive to the change in the long-range order,h. Because the Co–ASA and their clusters car-ries a magnetic moment, they are considered to be not onlythe scattering centers for the structural disorder but also themagnetic-impurity-scattering centers. Therefore, it is desir-able to investigate the transport properties of the alloys withvarioush. The effect of the structural disorder on the resis-tivity of CoTi alloy has not been investigated yet. The pur-pose of this article is to obtain the equiatomic CoTi alloy inthe disordered state and to study experimentally the influenceof the structural order-disorder transformation on its trans-port properties.

II. EXPERIMENTAL PROCEDURE

Co0.50Ti0.50 alloy films with total thickness of;100 nmwere prepared by flash evaporation onto glass substrates in ahigh vacuum condition better than 531025 Pa. An equi-atomic orderedb phase in these films was obtained by depo-sition onto substrates heated up to 730 K~state 1!. In order toprepare a disordered state of the film, a vapor quenchingdeposition technique was employed, where a chaos of gasphase is condensed onto substrates cooled down to 150 K byliquid nitrogen~state 2!. All the films prepared at 150 K werea!Electronic mail: yplee@email.hyu.ac.kr

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 6 15 MARCH 2001

33150021-8979/2001/89(6)/3315/4/$18.00 © 2001 American Institute of Physics

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cut into two parts and one set of halves was put into avacuum chamber and annealed at 730 K for 45 min in a highvacuum of 131025 Pa ~state 3!. Structural analyses of thefilms were performed by using transmission electron micros-copy and thus it was known that a set of Co0.50Ti0.50 alloyfilms with various degrees of long range order (h1;1.h3

.h2;0) was prepared.3 The resistivity measurements werecarried out by the four-probe technique in a temperaturerange of 4.2–300 K with and without an external magneticfield of 0.5 T directed normally to the film plane. A moredetailed description of the sample preparation is givenelsewhere.3

III. RESULTS AND DISCUSSIONS

Figure 1 presents the temperature dependence of the nor-malized resistivity for the Co0.50Ti0.50 alloy films with vari-ous h. As can be seen, the temperature dependence of thetransport properties of the films varies withh. The ordered~state 1! and moderately disordered~state 3! films exhibittypically a metallic behavior with positive temperature coef-ficient of resistivity~TCR!, while the disordered film has anegative TCR. The negative TCR of the disordered film~state 2! can be easily understood, since the resistivity of thefilm at room temperature is 625mV cm, which is well abovethe Mooij criterion for the negative TCR. The negative TCRfor a disordered~or highly resistive! metallic alloy is not anunusual phenomenon. A threshold residual resistivity ofr0

max51/s0min5150 mV cm, which divides the positive and

negative TCR regimes, has often been treated as the univer-sal boundary.4

In order to carry out the analyses of the experimentaldata more precisely, the experimental values of the normal-ized resistance,R(T)/R300 K, for the ordered Co0.50Ti0.50 al-loy film ~state 1! and the film with an intermediate degree oflong range order~state 3! were fitted by a function ofA

1BT5F1(T)1CT2F2(T)1DT3/2, which describes the mostprobable mechanisms of the electron scattering. HereR300 K

is the resistance at room temperature,A is the normalizedresidual resistance,B, C andD are fitting constants,

F1~T!5E0

QD /T x5dx

~ex21!~12e2x!,

and

F2~T!5E0

QD /Tex~x21!

~ex21!2xdx.

QD is the Debye temperature. The second term in the fittingfunction describes the ‘‘pure’’ electron-phonon~e-ph! scat-tering in the disordered~or dirty! systems~the modifiedBloch–Gruneisen function!,5 the third term indicates theelectron-phonon-vibrating impurity scattering~VIS!,6 andthe last one represents the spin-diffusive scattering withmagnons~SD! in dilute ferromagnetic alloys.7 There are fiveparameters in this fitting procedure:A, B, C, D, and QD ,where the Debye temperature,QD , determines the upperlimit of the integral in functionsF1 and F2 . The resultantfitting parameters are summarized in Table I, and the ob-tained curves are presented in Fig. 2. In Fig. 2, (Dr(T)/r0)vs T curves are plotted, whereDr(T)[r(T)2r0 and r0

5Ar300 K is the residual resistivity obtained by the fitting.It is seen that the resistance of the ordered Co0.50Ti0.50

alloy film ~state 1! above 60 K is dominated by the e-phscattering with aQD of 297 K, since the VIS term is rela-tively small and the SD term is negligible.. The partial dis-order ~transition from state 1 to 3! leads to a noticeable in-crease in the residual resistivity of the alloy.A’s are 0.656and 0.769 for the state 1 and 3, respectively. According toRef. 8, in the films with a large residual resistance, the VISscattering can exceed the contribution of the e-ph scatteringto the resistance at relatively high temperatures or even nearroom temperature. By comparing the ratio of coefficientBandC for these states in Table I, it is apparent that a partialdisorder can cause an increase in contribution of such a scat-tering to the total resistivity of the Co0.50Ti0.50 alloy films~see Fig. 2!.

In contrast to the ordered state of the alloy film, the SDterm plays an important role in the total resistivity of theCo0.50Ti0.50 alloy films with an intermediate degree of longrange order~state 3!. The coefficient of SD term for state 1 isalmost zero, while that of state 3 is 5.5831026 K21.5. Thisresult allows us to presume the existence of a spin-glass statein such a film in a temperature range shown in Fig. 2. Ac-cording to our magnetization study using a vibrating samplemagnetometer, it was observed that a small magnetic mo-ment appeared in the disordered Co0.50Ti0.50 alloy films ~state2! below 100 K.3 We attributed such a change in the mag-

FIG. 1. Variation of the normalized resistivity with temperature for theordered~1!, intermediately disordered~3!, and disordered~2! Co0.50Ti0.50

alloy films.

TABLE I. Fitting parameters to theR(T)/R300 K data for two states of the Co0.50Ti0.50 alloy films.

Fit range B C D QD

Alloy state ~K! A (10213 K25) (1026 K22) (1026 K21.5) ~K!

1 30–300 0.656 7.68 5.75 ;0 2973 40–300 0.769 2.15 2.69 5.58 346

3316 J. Appl. Phys., Vol. 89, No. 6, 15 March 2001 Lee et al.

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netic state of the disordered alloy to the appearance of theCo–ASA and/or their nearest-neighbor-clustering Co atomscoupled ferromagnetically below 100 K. Although the num-ber of these clusters in the Co0.50Ti0.50 alloy films with anintermediate degree of long-range order~state 3! is notenough to be coupled ferromagnetically, a substantial contri-bution of the SD scattering, which obeys theT3/2 law, isobserved in addition to the two kinds of the electron-phononscattering~e-ph and VIS!.

The structural disorder in the Co0.50Ti0.50 alloy filmsleads to the change in sign of the temperature coefficient ofresistance from positive to negative as seen in Fig. 1. Theresistivity also increases significantly from 218mV cm forthe ordered film~state 1! to 625 mV cm for the disorderedfilm ~state 2! at room temperature. The temperature depen-dence of the resistivity for the Co0.50Ti0.50 alloy film with anintermediate degree of long-range order~state 3! comes inbetween the ordered and disordered ones. Taking into ac-count the results of our previous study of the optical proper-ties of these alloys,3 the observed temperature dependence ofthe resistivity for the disordered Co0.50Ti0.50 alloy films isexplained by the partial localization of the electronic statesnear the Fermi level.

Another rather interesting feature of the influence of thestructural disorder on the transport properties of theCo0.50Ti0.50 alloy films is the appearance of the resistivityminimum atTmin515– 16 K for the completely and partiallydisordered alloys~states 2 and 3, respectively!. The details ofthese dependencies at low temperatures~4.2–30 K! with zeroand 0.5 T of magnetic field are shown in Fig. 3. Since the

R(T) of the ordered Co0.50Ti0.50 alloy film ~state 1! does notshow any peculiarity at least above 4 K, except for a slightslope change around 30 K, it is evident that the observedresistivity minimum originates from the structural disorder-ing.

Obviously, the resistivity extremum appears as a resultof the interplay among several mechanisms of scattering.The usual e-ph scattering, local spin density fluctuations andelectron-magnon scattering make a positive TCR, i.e., theylead to an increase in resistivity with temperature, while thehopping or tunneling conductivity, the Kondo effect, and thequantum corrections to conductivity due to the electron–electron interactions in the presence of weak localization in-duce a negative TCR.

Taking into account the influence of the magnetic fieldon the resistivity minimum, its nature was examined by fit-ting the experimentalR(T) curves. Altshuler and Aronov9

have shown that the quantum corrections to conductivity dueto the electron–electron interactions in the presence of weaklocalization can be expressed asDr[r(T)2r0'2bT0.5,where b is a constant. Although the temperature range forfitting is rather narrow, it is seen in Fig. 3 that there is areasonable agreement between experimental and fittingcurves inTmax,T,Tmin , whereTmin (Tmax) is the tempera-ture at which the resistivity minimum~maximum! occurs.

FIG. 2. Experimental temperature dependence of the normalized resistivity~circles!, Dr/r0 vs T, for the Co0.50Ti0.50 alloy films in the~a! ordered and~b! intermediate states together with the resultant fitting curves~solid lines!obtained by taking into account various mechanisms of electron scattering:the e-ph~dashed!, VIS ~dotted!, and SD~dotted! mechanisms of scattering.

FIG. 3. Resistance of the~a! ordered~state 1!, ~b! intermediately disordered~state 3! and~c! disordered~state 2! Co0.50Ti0.50 alloy films as a function oftemperature measured at zero~circles! and 0.5 T~triangles! magnetic field.Dashed lines in~b! and~c! are the fits of the functionR5R02b8T0.5 whereR0 andb8 are constants. Solid line in~c! is a guide to the eyes only.

3317J. Appl. Phys., Vol. 89, No. 6, 15 March 2001 Lee et al.

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Therefore, the resistivity minimum can be attributed to thequantum corrections for the electron–electron interactions inthe presence of a weak localization. There is one more thingto be mentioned; the variable-range hopping conductivity,which is one of the possible candidates for the explanation ofthe resistivity growth with decreasing temperature and obeysT21/4 law, gives a much worse agreement between experi-mental and fitting curves.

IV. SUMMARY

It was established that the e-ph scattering and VIS arethe dominant contributions to the resistivity of the orderedand partially disordered Co0.50Ti0.50 alloy films, while for theordered state of alloy the main contribution originates fromthe e-ph scattering. The partial structural disordering en-hances the role of the electron-phonon-vibrating impurityscattering and makes the spin-diffusive scattering rather no-ticeable. The negative temperature coefficient of the resistiv-ity for the disordered Co0.50Ti0.50 alloy films is explained bythe partial localization of the electronic states near the Fermilevel. The appearance of the low-temperature resistivityminimum for the disordered Co0.50Ti0.50 alloy films can beascribed to the quantum corrections for the electron–electroninteractions in the presence of a weak localization.

ACKNOWLEDGMENTS

This work was supported by the Korea Science and En-gineering Foundation through the Atomic-scale Surface Sci-ence Research Center and Project Nos. 97-0702-01-01-3 and995-0200-004-2, and also by the Korea Research Foundationthrough the BSRI Program~1998-015-D00087! and GrantNo. KRF-99-D00048.

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3Y. P. Lee, K. W. Kim, J. Y. Rhee, Y. V. Kudryavtsev, and V. V. Nem-oshkalenko, Phys. Rev. B60, 8067~1999!.

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3318 J. Appl. Phys., Vol. 89, No. 6, 15 March 2001 Lee et al.

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