We have studied  										  the transport and magnetic properties  										  of a metallic granular CoxCu1-x system, with 0.2≤x≤0.25, prepared by  										  using an ion-beam sputtering  										  technique. Giant magnetoresistance  										  (GMR) has been observed for the all  										  studied concentration with the average  										  amplitude around 2%. at 300K. The  										  maximum value has been observed for Co21Cu79 granular alloy after annealing at  										  450°C for 2h. Above this concentration  										  the MR decreases which is attributed  										  to the percolation effect, which  										  causes a concurrent reduction in  										  magnetic coercivity. The variation of  										  magnetic and transport properties with  										  the concentration and annealing time  										  is discussed.We have studied  										  the transport and magnetic properties  										  of a metallic granular CoxCu1-x system, with 0.2≤x≤0.25, prepared by  										  using an ion-beam  sputtering  										  technique. Giant magnetoresistance  										   (GMR) has been observed for the all  										  studied concentration  with the average  										  amplitude around 2%. at 300K. The  							 			  maximum value has been observed for Co21Cu79 granular alloy after annealing at  										  450°C for 2h. Above this  concentration  										  the MR decreases which is attributed  							 			  to the percolation effect, which  										  causes a concurrent  reduction in  										  magnetic coercivity. The variation of  							 			  magnetic and transport properties with  										  the  concentration and annealing time  										  is discussed

Berrada, A.

Dinia, A.

Errahmani, H.

Schmerber, G.

English

We have studied  										  the transport and magnetic properties  										  of a metallic granular CoxCu1-x system, with 0.2≤x≤0.25, prepared by  										  using an ion-beam  sputtering  										  technique. Giant magnetoresistance  										   (GMR) has been observed for the all  										  studied concentration  with the average  										  amplitude around 2%. at 300K. The  							 			  maximum value has been observed for Co21Cu79 granular alloy after annealing at  										  450°C for 2h. Above this  concentration  										  the MR decreases which is attributed  							 			  to the percolation effect, which  										  causes a concurrent  reduction in  										  magnetic coercivity. The variation of  							 			  magnetic and transport properties with  										  the  concentration and annealing time  										  is discussed.We have studied  										  the transport and magnetic properties  										  of a metallic granular CoxCu1-x system, with 0.2≤x≤0.25, prepared by  										  using an ion-beam sputtering  										  technique. Giant magnetoresistance  										  (GMR) has been observed for the all  										  studied concentration with the average  										  amplitude around 2%. at 300K. The  										  maximum value has been observed for Co21Cu79 granular alloy after annealing at  										  450°C for 2h. Above this concentration  										  the MR decreases which is attributed  										  to the percolation effect, which  										  causes a concurrent reduction in  										  magnetic coercivity. The variation of  										  magnetic and transport properties with  										  the concentration and annealing time  										  is discussed

Revues Scientifiques Marocaines

Annealing effect on the structural, magnetic and transport properties in the CoxCu1-x granular alloys with 0.20 ≤ x ≤ 0.25 prepared by the ion-beam sputtering technique

M. J. CONDENSED MATTER                    VOLUME 2, NUMBER                                                2000                                                                                         2                              2000 The Moroccan Statistical Physical Society.Annealing effect on the structural, magnetic and transport propertiesin the  CoxCu1-x granular alloys with 0.20 ≤ x  ≤ 0.25 preparedby the ion-beam sputtering techniqueH. Errahmani*, A. Dinia**, G. Schmerber**, A. Berrada**L.P.M. Faculté des Sciences Université Mohammed V,  B.P 1014 Rabat, Morocco.**Institut de Physique et Chimie des Matériaux de Strasbourg, 23 rue du Loess F-67037Strasbourg cedex France We have studied the transport and magnetic properties of a metallic granular Cox Cu1-x system, with 0.2 ≤ x ≤ 0.25,prepared by using a ion-beam sputtering technique. Giant magnetoresistance (GMR) has been observed for the all studiedconcentration with the average amplitude around 2%. at 300K. The maximum value has been observed for Co21Cu79granular alloy after annealing at 450°C for 2h. Above this concentration the MR decreases which is attributed to thepercolation effect, which causes a concurrent reduction in magnetic coercivity. The variation of magnetic and transportproperties with the concentration and annealing time is discussed.I. IntroductionGranular films are magnetic composite materialsthat usually consist of nanoscale magnetic granulesembedded in an immiscible metallic matrix, or inan insulator 1,2. Those systems, which include Cu-Co3-5, Cu-Fe4, Co-Ag6.7, Fe-Ni-Ag8.9, and Fe-Co-Ag10 alloys, exhibit giant magnetoresistance(GMR) effect3.4, and have interesting potentialapplication in microelectronic devices.Several techniques, such as evaporation,sputtering, mechanical alloying, and melt spinning,can be used to produce immiscible granular alloys;subsequent heat treatment of the sample allows theformation of nanoscale magnetic granules. Thestudies of granular materials to date have beenfocused on systems in which the granules areelemental metals1-4, while granular metals in aninsulating medium have been explored for quitesome time, those in a metallic medium are morerecent ventures 11.12.The GMR effect in granular metals, as well as inmetallic multilayers, is well known to be related tothe spin-dependent scattering of the conductionelectrons and to the reorientation of the magneticmoments by an external magnetic field13.14. In azero applied field, the magnetic moments of thegranules are supposed to be oriented randomly inspace, and the resistance, in both the spin-up andspin-down channels, is considerably high. Thealignment of the moments, however, leads to ashort-circuit effect in one of the spin channels,with a consequent decrease of the total resistanceof the system. It is shown that the amplitude of theMR depends strongly on the size of the magneticgranules and on the fabrication procedure.Therefore, appropriate annealing procedure is animportant requirement for obtaining highperformance materials.In this work, we have used a number ofexperimental techniques including X-ray diffraction,alternating-gradient force magnetometer (AGFM)and four-terminal magnetoresistivity geometry tocharacterise the ion beam sputtered Co1-xCuxgranular alloys. This allows the investigation ofthe Co phase separation process, magnetic andtransport properties and their relation with thecomposition and annealing conditions. Thevariation of these properties with the concentrationand annealing time is reported and discussed.II. Experimental detailsCo-Cu thin film samples were prepared using theion-beam sputtering (IBS) technique. The highvacuum system is equipped with a kauffman iongun. The base pressure was 1×10-8 mbar andsputtering was carried out using 2×10-4 mbar argonions. After being ionised in the high voltagechamber, the argon ion beam bombards the targetat the incidence angle of 45°. The beam voltagewas 400 V and the beam current around 1mA. Thegrowth rates were typically around 0.7 nm/min and1.6 nm/min for Co and Cu layers respectively. Thesamples are deposited at room temperature on aglass substrate covered by a 5 nm Fe layer. Aseries of samples with different Co concentrationwere fabricated. The film thickness was fixedaround 1000Å.III. Results and discussionThe X-ray measurements were performed atroom temperature using a Siemens powderdiffractometer with monochromatic Cu or Co Kα1radiation. The geometry of the diffractometerallows only experiments in reflection mode. X-rayspectra of the as sputtered CoxCu1-x samples arereported in figure 1, and are characteristics ofsingle-phase metastable alloys. The structure ofthis single-phase is fcc, and the diffraction peaksobserved in these spectra for the wholecomposition are almost identical, with except the2                                                 H. Errahmani, A. Dinia, G. Schmerber, A. Berrada                                                    progressive shift toward high angles observed forthe Cu diffraction peaks with increasing the Cocontent in the film. Two diffraction peaks areclearly distinguished at 2θ = 43.2° and 2θ = 50.2°,which correspond to the (111) Cu and (200) Cuplanes respectively. The lattice parameters of theCu matrix calculated using the 2θ angle of thediffraction peaks is intermediate between those ofbulk Co and bulk Cu11.36 39 42 45 48 51 54 57 60As-sputteredCoxCu1-x(d)(c)(b)(a)(a)  x = 0.20(b)  x = 0.21(c)  x = 0.22(d)  x = 0.25   Intensity (a.u)2Θ (°)Figure 1 : High angles X-ray diffraction spectrum takenat 300 K with Cu Kα1 radiation for  the as-depositedCoxCu1-x granular samples. In the as sputtered state, the lattice parameters ofCu matrix decreases with increasing Coconcentration due to smaller Co atoms dissolved inCu matrix beyond the equilibrate limit leading tothe lattice shrinkage. This effect is furtherconfirmed in the figure 2, which shows thevariation of the spacing of (111) planes, d111, as afunction the Co concentration. Such a systematicdecrease in the d111 with increasing the Coconcentration is consistent with a metastable solidsolution.0,20 0,21 0,22 0,23 0,24 0,252,0582,0602,0622,0642,0662,0682,0702,072As-sputteredCoxCu1-xd(111) (Å)Concentration (at. Co )Figure 2 : d111 spacing of the (111) Cu planes as afunction of the Co concentration for  the as-depositedCoxCu1-x samples.The magnetic properties of the CoxCu1-x sampleshave been studied using an alternating gradientforce magnetometer (AGFM) with the magneticfield applied in the plane of the films. Figure 3shows typical hysteresis loops obtained at roomtemperature on the as-sputtered samples forvarious Co concentrations. Very small hysteresisand small remanent magnetisation are observed inthese systems for the whole concentration. Thesaturation of the magnetiszation has not beenachieved even in magnetic field as large as 15 kOe.-15000 -10000 -5000 0 5000 10000 15000-120-90-60-300306090120 M(emu/gCo)(d)(c)(b)(a)CoxCu1-x(a) x = 0.20(b) x = 0.21(c) x = 0.22(d) x = 0.25Magnetic field (Oe)Figure. 3 : Hysteresis loops at 300K for the as-deposited CoxCu1-x granular samples.This is not clearly distinguishable due the slowslope approach to the saturation and will beconfirmed later using MR measurements. Notwithstanding, we will use in the next part the termsaturation magnetization, which means themagnetization at 15 kOe, in order to make theunderstanding easier for the reader. Figure 4 showsthe variation of the saturation magnetization, MSper gram of Co, with the Co concentration for theas-sputtered and annealed CoxCu1-x samples at450°C during 30 min. As already shown in figure3, in the as-sputtered state, MS increasessignificantly with increasing Co content to reachthe value of 122 e.m.u. /g of Co for 22 % at Co.This value is smaller than the bulk Co value 164e.m.u./g11. This means that an important amount ofCo particles has not precipitate in the as-sputteredstate. Therefore, after annealing the as-depositedsamples at 450°C during 30 min, a strong increasein the magnetization saturation has been observed,to reach an average value of 142 e.m.u./g for thewhole concentration.0,20 0,21 0,22 0,23 0,24 0,25020406080100120140160Annealing at 450°C for 30 minMs (emu/gCo)Concentration (at.Co)CoxCu1-xAs-sputteredFigure 4: Saturation magnetization Ms,per gram of Co,versus of the Co concentration for the as-depositedCoxCu1-x samples (full circles) and annealed CoxCu1-xsamples at 450°C for 30 min (full squares).This value is still below the bulk value even after2 ANNEALING EFFECT ON THE STRUCTURE, MAGNETIC, AND TRANSPORT PROPERTIES…annealing at 450°C for 2h, and indicates that theCo phase precipitation occurred during annealingprocess is not perfect. As a consequence, themagnetic phase separation is incomplete, and someCo atoms are still dissolved in the Cu matrix.In order to have more information on the phaseseparation, magnetoresistance measurements asfunction of the magnetic field have beenperformed at room temperature for the as-deposited and annealed sample. Figure 5 shows theMR results for the as-sputtered CoxCu1-x granularalloys. All the MR curves are similar presenting atriangular variation of the resistance against themagnetic field and no saturation for appliedmagnetic field as large as 17 kOe, which aretypical characteristics of granular alloys.Figure 5 : Magnetoresistance curves at 300 K forCoxCu1-x alloys: (a) (Co20Cu80); (b) (Co21Cu79); (c)(Co22Cu78); and (d) (Co25Cu75). The magnetic field isapplied in the film plane parallel to the current direction.The MR values are small in the as-deposited stateand increase with the Co concentration to reach amaximum of 2.25% for 21 % at Co, and decreasesabove this concentration to 1.5% for 25 % at Co(figure 6). This drop is mainly due to theincreasing coalescence of the magnetic particles,as the Co concentration increases, and it is morelikely for the magnetic particles to connect toadjacent ones to form clusters networks.Therefore, the interaction among neighbouring Coparticles becomes more important. In addition, thereduction of effective surface area of the formedlarge clusters is also a source of the reduced GMReffect, since the magnetic surface scattering plays adominant role. The observation of a relatively lowMR for the as-sputtered samples is attributed to theformation of small Co particles during thedeposition. As the Co concentration increases, themagnetic clusters become larger in size, andconsequently the MR is reduced due to theappearance of coupling between neighbouring Coclusters, and the system becomes closer to themagnetic percolation.Figure 6 presents the MR ratios measured at roomtemperature for the CoxCu1-x alloys as a function ofthe Co concentration. The evolution of the MR ofthe as-sputtered alloys has been already discussedin the last paragraph. The same evolution has beenobtained for the annealed alloys at 450°C for 30min. Indeed, in the magnetically dilute region, theMR increases with the concentration, due to theincrease of the magnetic scattering centres, toreach 3% for x  = 0.21. Above this concentration,the MR decreases to reach 1.7% for x = 0.25 forthe same reasons as developed above.0,20 0,21 0,22 0,23 0,24 0,250,00,51,01,52,02,53,0Annealing at 450°C for 30 minMR (%)Cocentration (at. Co)T = 300 KCoxCu1-x As-sputteredFigure 6 : Variation of the magnetoresistance as functionof the Co concentration for as-deposited CoxCu1-xsamples (full circles) and annealed CoxCu1-x samples at450°C for 30 min (full squares).All the reported results show that the optimummagnetotransport characteristics are obtained forx = 0.21. For this reason we focus our discussionin the next part on the Co21Cu79 alloy.In figure 7 we report the magnetoresistancecurves measured at room temperature for the as-sputtered and annealed Co21Cu79 alloy at 450°Cwith different annealing time. Already for0,00,51,01,52,0T = 300 K(a)As-sputtered Co20Cu80MR (%)0,00,30,60,91,21,51,82,12,4T = 300 K(b) As-sputteredCo21Cu79MR (%)0,00,30,60,91,21,51,82,1MR(%)T = 300 K(c) As-sputteredCo22Cu78-20000 -15000 -10000 -5000 0 5000 10000 15000 200000,00,20,40,60,81,01,21,41,6T = 300 K(d) As-sputtered Co25Cu75MR(%)Field(Oe)2                                                 H. Errahmani, A. Dinia, G. Schmerber, A. Berrada                                                    annealing during 30 min, the MR increases withrespect to the value obtained for the as-sputteredalloy. As the annealing time increases, the MRincreases. This dependence of the MR on theannealing time is related to the change in themicrostructure of the sample, mainly the size ofthe Co particles and their distribution in the Cumatrix. The maximum of the magnetoresistancevariation reaches 16% after annealing for 2h15. Atthis temperature, we can probably assume that thesample has a higher magnetic particles surface-to-volume ratio that acts as the conduction electronspin-dependent scattering centres16,17,18. The MRremains unsaturated even at 17 kOe probably dueto the slow approach to the saturation of the smallparticles19,20.-18 -15 -12 -9 -6 -3 0 3 6 9 12 15 18024681012141618T  = 300 KMR (%)Field (kOe) Co21Cu79(c)(b)(a)(a) As-sputtered(b) Annealing at 450°C     for 30 min(c) Annealing at 450°C      for 2hFigure 7: Magnetoresistance curves at 300 K forCo21Cu79 granular film : as-deposited (a) and afterannealing at 450°C for 30 min (b) and 2 hours (c).It is now widely accepted that magnetoresistancein the granular systems is a resistance due to spin-dependent scattering from non-alignedferromagnetic particles. Therefore, a correlationbetween MR and magnetization is obvious. Xiao etal.4 suggested that the magnetoresistance in thegranular systems can be expressed as( )MsMFRR −=∆ ,where M is global magnetization and MS thesaturation magnetization. An important factor forthe GMR in the granular systems is the average<cosθij >, where θij is the angle between the axesof the particles magnetic moment. If these particlesare assumed not magnetically correlated, we find21that <cosθij > = <cosθi >2 = (M/Ms)2, where θi isthe angle between the moment of the i-th particleand the magnetic field direction. Thus, the functionF(M/Ms) is proportional to (M/Ms)2, andconsequently ( )MsMARR −=∆ 2, where Adetermines the amplitude of the GMR. Thisparabolic behaviour has been previously reported4.In order to see if such function also applies in ourcase, we have reported in figure 8 the MR curvesas function of the normalised magnetisation,M/MS, for three Co21Cu79 alloys: as-sputtered, andthe annealed at 450°C during 30 min and 2 hours,respectively. The parabolic behaviour predicted bythe independent-moment model is not obtained inthe present situation. Indeed, the deviation fromsuch behaviour and particularly in the low M/Msregion is, however, not unusual, and oftenmentioned as a proof of the existence of magneticinteractions among the magnetic particles. This isin agreement with the magnetization curvesshowing a non-negligible remanent magnetization.Moreover, the remanent magnetisation is stronglyenhanced after annealing at 450°C during 2 hours,which explains the fact that the deviation from theparabolic behaviour is more pronounced in thiscase as shown in figure 8c.  Figure 8: Magnetoresistance curves as function ofnormalised magnetization, M/MS, at 300 K for theCo21Cu79 granular sample : as-deposited (a) and afterannealing at 450°C for 30 min (b) and 2 hours (c).IV. ConclusionWe have investigated the magnetic properties,  andthe  GMR  of  CoxCu1-x  granular  alloys  with 0.20≤ x ≤ 0.25 in the as-sputtered and annealed states.The variation of GMR with Co composition hasbeen shown There is a maximum of GMR atx = 0.21, near the percolation threshold. Thedecrease of GMR at high concentration isattributed to the increasing coalescence of themagnetic particles.0,00,30,60,91,21,51,82,12,4MR (%)(a)As-sputteredCo21Cu790,00,51,01,52,02,53,0Annealuing at 450°C, 30 minCo21Cu79MR (%)(b)-1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,00246810121416(c)Annealing at 450°C for 2 hCo21Cu79MR (%) ( M/Ms )2 ANNEALING EFFECT ON THE STRUCTURE, MAGNETIC, AND TRANSPORT PROPERTIES…[1] B. Abeles, Appl. Solid state sci.6.1(1976).[2] C. L. Chien, J. Appl. Phys 69,5267 (1990).[3] A. E. Berkowitz, J. R. Mitchel, M. J. Carey,A. P. Young, S. Zhang, F. E. Spada, F. T.Parker, A Huutten, and G. Thomas, Phys. Rev.Lett. 68, 3745 (1992).[4] J. Q. Xiao, J. S. Jiang and C. L. Chien, Phys.Rev. Lett.68, 3749 (1992)[5] R. H. Yu, X. X. Zhang, J. Tejada, M. Knobel,P. Tiberto,and G. P. Allia, J. Phys. Condens.Matter 7, 4081 (1995).[6] J. A. Barnard, A. Waknis, M. Tan, E. Haftek,M. R. Parker and M. L. Watson, J. Magn.Magn. Mater. 114, L230 (1992).[7] M. J. Carey, A. P. Young, A. Starr, D. Rao,and A. E. Berkowitz, Appl. Phys. Lett. 61,2935(1992).[8] A. Tsoukatos, A. H. Wan, G. C.Hadjipanayis, and K. M. Unruth, J. Appl. Phus.73, 5509 (1993).[9] M. L. watson, J. A. Barnard, S. Housasain,and M. R, J. Appl. Phys. 73, 5506 (1993).[10] O. Rendon, J pierre, B. Rodmacq, B. Mevel,and B. Dieny, J. Magn. magn. Mater, 149,398(1995).[11] J. R. Childress and C. L. Chien, Phys. Rev.B 43, 8089 (1991)..[12] J. R. Childress and C. L. Chien, J. Appl.Phys. 70, 5885 (1991).[13] J. d'.albuquerque e Castro, M. S. Ferrina andR. B. Muniz, Phys. Rev. B 49,16062(1994).[14] J. Mathon, Murielle villeret, R. B. Muniz, J.d'Albuquerque e Castro, andD. M. Edwards,Phys. Rev. Lett. 74, 3696 (1995).[15] H. Errahmani, A. Berrada, G. Schmerber,and A. Dinia, vacuum journal (1999)[16] R. E. Camely and J. Barnas,Phys. Rev. Lett.63, 664 (1989).[17] P. M. Levy, S. Zhang and al. Fert, Phys.Rev. Lett. 65, 1643 (1990).[18] A. Barthelmey and A. Fert, Phys. Rev. B 43,13124 (1991). [19] J. Q. Xiao, J.S. Jiang, and C. L. Chien,Phys. Rev. Lett. 68, 3749 (1992); Phys. Rev.B 46, 9266  (1992).[20] Jiang-Quing Wang and Gang Xiao, Phys.Rev. B 49, 3982 (1994).[21] Y. Goldstein and J.L. Gittleman, SolidState Commun. 9, 1197 (1971).

https://revues.imist.ma/index.php/MJCM/article/download/51/50

Annealing effect on the structural, magnetic and transport properties in the CoxCu1-x granular alloys with 0.20 ≤ x ≤ 0.25 prepared by the ion-beam sputtering technique

Abstract

Similar works

Full text

Available Versions

Revues Scientifiques Marocaines

Revues Scientifiques Marocaines