*Corresponding author. Tel.: #90 212 285 32 60; fax: #90212 285 63 86; e-mail: oner@sariyer.cc.itu.edu.tr.

Journal of Magnetism and Magnetic Materials 185 (1998) 305—308

Resistance fluctuations in amorphous Cr74

Fe26

films

Y. O® ner!,*, A. Kilic", M. O® zdemir#, S. Senoussi$! Department of Physics, Istanbul Technical University, 80626, Maslak, Istanbul-TR, Turkey

" Department of Physics, Karaelmas University, Zonguldak-TR, Turkey# Department of Physics, Marmara University, Go( ztepe, Istanbul-TR, Turkey

$ Laboratoire de Physique des Solides, Universite de Paris-Sud, 91405 Orsay Cedex, France

Received 1 October 1997; received in revised form 6 January 1998

Abstract

Magnetoresistance measurements *o/o have been carried out on an amorphous Cr74

Fe26

film in the temperaturerange of 4—120 K in a magnetic field up to 120 kOe. We observed that the isotropic component of the magnetoresitanceat low temperatures first decreases with increasing field due to the decrease in the spin fluctuations (negative mag-netoresistance), goes through a minimum at a certain field value, H

#and then starts to increase (positive magnetoresis-

tance) as the magnetic field is further increased. As the temperature increases, H#

shifts to a higher field and themagnetoresistance on both sides of this field (negative and positive magnetoresistance) decreases. The magnetoresistancebecomes almost zero at temperatures between 10—16 K. However, as the temperature is further increased, the mag-netoresistance starts to increase again. This overall behaviour with the resistivity data have been interpreted successfullyin terms of localization effects. We have also observed huge oscillations in the resistivity at low frequencies (7—70 mHz) inthe temperature range where *o/o becomes weaker. Starting with the temperature at about ¹"15 K, the amplitudes ofthese oscillations reduce with both increasing and decreasing temperatures. However, the frequencies of these oscillationsincrease with decreasing temperature. No detectable oscillation in the resistivity has been observed at temperaturesbelow ¹"8 K and above ¹'¹

#(the Curie temperature, ¹

#"43.8 K). We have no full explanation for this

phenomenon, however, it is tempting to ascribe it to some kind of spin density wave. ( 1998 Elsevier Science B.V. Allrights reserved.

PACS: 74.40.#k; 73.50.Td; 73.50.Jt

Keywords: Amorphous systems — TM alloys; Anderson localization; Magnetoresistance — amorphous alloys

CrFe binary alloys among other ‘transitionmetal—transition metal’ binary systems are mostlystudied partly because of their unusual electrical

and magnetic properties, and partly because of thepractical importance. Although a large number ofthe publications about this alloy system in thepolycrystalline phase appear in the literature, thereare only a few publications in the amorphousphase. Recent studies [1,2] show that amorphousCrFe exhibits highly different properties concerning

0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved.PII S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 0 4 5 - 6

Fig. 1. The temperature dependence of the resistivity nor-malized to that at 273 K for Cr

74Fe

26. Inset shows the temper-

ature derivative of the resistivity at around the Curie point.

electrical resistivity and magnetization comparedto those for the corresponding alloys in the poly-crystalline phase.

The flash-evaporation method was employed toobtain an amorphous alloy in the form of a thinfilm deposited onto a Corning glass substrate. Thefollowing procedures were used to prepare thefilms. First, Cr and Fe as a bulk materials withhigh-purity 99.9% or better were weighed carefullyand put into a sealed quartz tube under 0.3 atmargon. They were melted several times using an RFinduction furnace and quenched into water. Theresulting bulk alloy material was then ground topowder form. We have placed an extra arrange-ment in the VEECO-400 evaporation system toenable us to drop the fine powder on to a substratelocated at 15 cm away from the W-basket. A verysmall amount alloy powder was dropped manuallyon this W-basket heated by an AC current supplyup to &2000°C. The powders were evaporatedmaking a flash before reaching the heater and de-posited on the substrate in 0.3—0.4]2.4 cm]cmdimensions. During the evaporation, it was seenthat the vacuum was about 1]10~6 Torr and thesubstrate temperature reached up to 100°C. In eachrun, eight films of the material were obtained. Afterthe films were taken out of the vacuum chamber,they were covered by an organic compound toprevent oxidation. Ag was evaporated as a contactpad for electrical measurements. The thicknesses ofthe films are about 3200 A_

X-ray diffraction patterns have been used to de-termine whether the films are amorphous or poly-crystalline. The patterns show that these sampleshave a highly disordered and non-crystalline struc-ture. (see Ref. [2] for further detail).

Resistivity measurements were made with con-ventional AC lock-in techniques, using a probecurrent at a frequency of 282 Hz. The voltage accu-racy was better than one in 10~6. The data weretaken once in every 1

3second averaging every ten

adjacent values. In addition, the measurementswere made on both films of CrFe and NiMn whichhave almost the same resistance values; that is,measurements were carried out in a multiplexedfashion. The fluctuations observed for all sampleswere found to be uncorrelated, showing that theobserved resistance oscillation in CrFe was indeed

intrinsic to it. The magnetic field was supplied froma superconducting magnet of 160 kOe. A carbon-glass thermometer was used for the temperaturemeasurements.

Fig. 1 shows the resistivity normalized too273 K

as a function of the temperature. As seen inthe figure, the resistivity first increases monotoni-cally with decreasing temperature and then exhibitsan anomaly at about ¹"44 K. As the temperatureis further decreased, it stays almost constant in therange of 8—40 K and rises dramatically at thelowest temperatures. The later behaviour, whichrequires further study will be discussed elsewhere.We have ascribed this anomaly to the Curie (¹

#)

point. That the sample has become more orderedmagnetically at temperatures below ¹"40 K isalso confirmed by the magnetization measurementstaken on the same sample (see Fig. 7 in Ref. [2]).We have determined the Curie temperature moreaccurately from resistivity data. For this reason, thetemperature derivative of the resistivity at around¹

#was calculated (see inset to Fig. 1). These results

were approximately represented by ordinary equa-tions of the form A#Bq~a (q"(¹!¹

#)/¹

#, ¹

#being the Curie temperature). The four parameterswhich appear in the equation at the temperaturesabove and below (¹

#) were determined by the

method of least squares, using a computer. Thecloseness of the values of B and B@ is very impor-tant. The coincidence of these values implies that

306 Y. O$ ner et al. / Journal of Magnetism and Magnetic Materials 185 (1998) 305—308

Fig. 2. Magnetoresistance of the Cr74

Fe26

amorphous film forvarious temperatures is indicated in the figure. Note that theaveraged values of the magnetoresistance data were taken for¹"15 K (dashed line), owing to resistance oscillations.

Fig. 3. The variation of resistance relative to its mean value asa function of time at ¹"8, 15 and 21 K in zero magnetic field.

the observed discontinuity in the temperature de-rivative of the resistivity is independent of (¹!¹

#)

which is indispensible for the concept of a discon-tinuity. The measurement were made in the rangeq&5]10~4—2]10~3. The least-squares fit gives allthe five parameters (A, A@, B"B@, ¹

#, and a"a@) at

both sides of ¹#,

dod¹

"G0.63!0.75q0.18 l) cm/K at ¹(¹

#,

1.09!0.75q0.18 l) cm/K at ¹'¹#.

Here ¹#"43.8 K. The parameter values obtained

for this sample are very close to those for an Fecrystal [3].

Fig. 2 shows the longitudinal magnetoresistivity(the magnetic field parallel to both the film surfaceand the measuring current) of the Cr

74Mn

26amorphous film up to a field of 120 kOe at differenttemperatures such as ¹"4.2, 15, 30, 60, 95 and120 K. For ¹'15 K, the sample displays onlypositive magnetoresistance over the entire range ofmagnetic field. At the lowest temperatures(¹(10 K), it undergoes a transition from negativeto positive magnetoresistance as the magnetic field,H increases. The position of H

#(which corresponds

to the minimum of *o/o) shifts toward higher mag-netic fields as the temperature increases, while thenegative magnetoresistance vanishes. It is also in-teresting to note the temperature, ¹"15 K, atwhich the deep minimum is observed in *o/o fora given magnetic field.

Here we focus our attention on the temperaturerange where the very weak magnetoresistance andthe resistance oscillations are observed. Fig. 3 pres-ents the observed resistance oscillations with timeat the three different temperatures (¹"8, 15 and21 K). The resistance oscillations observed at¹"8 K exhibit at least two modes on the measur-ing time-scale shown in this figure. The mode ap-pearing as ripples at ¹"8 K corresponds to themode observed as a single mode (&70 mHz) at¹"15 K. Close inspection of Fig. 2 also showsanother mode at a much lower frequency(&7 mHz) corresponding to one appearing withlarger amplitude at ¹"8 K. However, as the tem-perature is further decreased, we could not recordany oscillation within our experimental accuracy,owing to much higher and/or much lower frequen-cies and the considerable decrease in the ampli-tudes for higher modes. In brief, we have observedat least two modes of resistance oscillations in ourexperimental observation time at the frequency of282 Hz of the measuring current. Starting with the

Y. O$ ner et al. / Journal of Magnetism and Magnetic Materials 185 (1998) 305—308 307

temperatures at which the magnetoresistance be-comes almost zero up to the fields of about 60 kOe,the amplitudes of the modes observed reduce withboth increasing and decreasing temperature. It isalso found that the resistance oscillations persist atleast up to our available fields of 160 kOe with only10% attenuation in the oscillation amplitudes.

As seen in Fig. 2, the resistivity increases firstsuddenly with the increasing field which is calledthe ferromagnetic anisotropy in the resistivity (dueto spin—orbit effects [4]) following the negativemagnetoresistance at higher fields due to alignmentof spins along the applied magnetic field. This is thewell-known behaviour for most ferromagneticalloys. However, the appearance of positive mag-netoresistance for ferromagnetic materials is veryunusual. Very recently [2], we have interpretedsuccessfully this positive magnetoresistance to-gether with the temperature behaviour of the resis-tivity using the Anderson localization in thepresence of strong spin—orbit coupling. We havealso shown that the establishment of long-rangeferromagnetic order gradually suppresses the local-ization effects.

It appears that the occurrence of huge resistanceoscillations at about the temperatures where thetransition from the ferromagnetic to the paramag-netic state takes place, and the magnetoresistancebecomes weaker for a given magnetic field, is notaccidental. It is likely that, at about the Curietemperature, the film divides into a number ofsimilar clusters in which there are only a few spinscoupled weakly to each other. This coupling isso weak that spin-wave modes may be excited

thermally within such a small cluster. Presumably,these excited spin waves modulate the conductionelectrons. However, it should be noted that theelectron—local spin wave interaction occurs onlywhen the mean free path of the conduction elec-trons (which is about a few 10 A_ for these alloys) iscomparable to the cluster size. The coupling be-tween spins within a small cluster, including onlya very limited number of spins, must be weakenough to yield thermally excited spin-wave modes.Otherwise, such localized spin excitations becomeenergetically unfavourable. We, therefore, concludethat the electrons modulated with such a spin-wavemode may account for the resistance oscillationsobserved in the Cr

74Fe

26amorphous film.

Acknowledgements

This research was supported partly by NATO(Project No: RCG 940539) and partly by TUB-ITAK (Project No: TBAG-1365). The support toA.K. by BAYG-TUBITAK grant is gratefully ac-knowledged.

References

[1] S.K. Xia, E. Baggio-Saitovitch, C. Larica, Phys. Rev. B 49(1994) 927.

[2] Y. O® ner, A. Kilic7 , M. O® zdemir, H. C7 elik, S. Senoussi, J.Phys.: Condens. Matter 8 (1996) 11121.

[3] Ya.A. Kraftmakher, T.Yu. Pinegina, Sov. Phys. Solid State16 (1974) 78.

[4] J.B. Bieri, A. Fert, G. Creuzet, J.C. Ousset, J. Appl. Phys. 55(1984) 1948.

308 Y. O$ ner et al. / Journal of Magnetism and Magnetic Materials 185 (1998) 305—308