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Physica B 369 (2005) 160–167
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Large negative magnetoresistance in capsulated Co–Cupowder prepared by mechanical alloying
W. Rattanasakulthong, C. Sirisathitkul
Magnet Laboratory, Experimental Physics Research Unit, Walailak University, Nakorn Si Thammarat 80160, Thailand
Received 1 April 2005; accepted 11 August 2005
Abstract
Large negative magnetoresistance (MR) in mechanically alloyed, Cobalt–copper (Co–Cu) powder was observed at
room temperature. Cu–Co powder (atomic ratio 30:70) milled for 30, 60 and 120 h was packed in plastic capsules with
the bulk densities of 2.68–3.96 g/cm3. At room temperature, the alloyed Co–Cu capsules showed up to 70% decrease in
electrical resistance when they were subjected to 11 kOe applied magnetic field. It was proposed that the movement of
loosely packed Co-rich cluster under the magnetic field, was the origin of this MR. The movement provided the more
efficient electron pathways resulting in resistance decrease. The variation of the MR ratio was closely related to the
milling time and the packed bulk density. With the same milling time, the optimum packed bulk density led to the
highest MR ratio and the effect was suppressed at higher densities such as in pressed pellets. By virtue of fracturing and
cold welding, the milling changed the cluster size distribution and intercluster distance. The structural, magnetic and
thermal properties were also studied in order to understand the effect of the milling time on the alloyed powder.
r 2005 Elsevier B.V. All rights reserved.
PACS: 72.15.Eb; 75.20.En; 75.50.Tt; 75.70.Pa
Keywords: Magnetoresistance; Mechanical alloy; Capsulated Co–Cu powder; Particles-ferromagnetic; Magnetic clusters
1. Introduction
A variety of alloyed powders are successfullyprepared by mechanical alloying because the highenergy supplied by milling balls leads to crystalline
e front matter r 2005 Elsevier B.V. All rights reserve
ysb.2005.08.010
ng author. Tel.: +667567 3230;
004.
sses: [email protected],
h (W. Rattanasakulthong).
or amorphous solid solutions. The mechanicalalloying can also prepare atomic scale mixtures ofthe normally immiscible materials. The phase,microstructure and other interesting propertiesof mechanical alloys (MA) are attributed to theprocess parameters such as milling time, composi-tion, ball to powder mass ratio and ball size. Oneof the noteworthy applications of mechanicalalloying is preparation of giant magnetoresistive(GMR) materials. After the discovery of GMR in
d.
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multilayers [1], the research has been extended tomechanical alloys produced by milling the fineferromagnetic powder (e.g. Fe, Co) with a non-ferromagnetic metallic powder (e.g. Au, Ag, Cu).After the milling, the mixture becomes nanoscaleferromagnetic granules embedded in a non-mag-netic matrix. The powder is then compressed underhigh pressure and magnetoresistance (MR) mea-surements are performed on bulk samples. GMRarises dominantly from spin-dependent scatteringat the interfaces between ferromagnetic granulesand non-magnetic medium. In 1994, Coey et al. [2]reported that pressed Co30Ag70 MA exhibited3.2–4.2% GMR at room temperature. Eilonet al. [3] found 5% room temperature GMR under120 kOe applied field in Fe25Cu75 with a pressedbulk density of 7.4 g/cm3. For Co–Cu MA,observations of GMR were reported by Aizawaet al. [4] and Champion et al. [5]. Champion et al.[5] observed the maximum GMR of 4% at 4.2K ina pressed pellet with rather high-cobalt volumefraction of 40%. Later on, Socolovsky et al. [6]and Gomez et al. [7] reported GMR in themechanically alloyed Fe–Au and Fe–Ag, respec-tively. Furthermore, ternary alloys also attractedmuch attention and GMR in such structures werereported by Ikeda et al. [8] and Nagamine et al. [9]on Fe–Co–Cu, Cohen et al. [10] on Fe–Co–Ag,Nash et al. [11] on Fe–Cu–Ag and Zhang et al. [12]on Ni–Co–Cu. According to these works, pressedMA powder exhibited 1–10% room temperatureGMR which were smaller than those exhibited bymultilayers [13].The focus of this paper is a very large negative
MR at room temperature in Co–Cu MA powderpacked in commercial drug capsules. Filling thepowder in plastic capsules instead of compressingthe powder under high pressure increases MR. Thedependence of structural, thermal and magneticproperties of mechanically alloyed Co–Cu powderon the milling time were also studied in order tounderstand the effect of alloying process.
2. Materials and methods
Co–Cu mechanical alloys were synthesized bythe ball milling technique. Starting materials were
cobalt powder of 99.8% purity with an averageparticle size less than 2 mm and copper powder of99% purity with an average particle size less than10 mm. Cobalt and copper powders were mixedtogether with the atomic ratio 30:70 in a steel vialloaded with various-sized steel balls (ball topowder mass ratio 6.6:1). In order to prevent thepowder from sticking to the vial during themilling, ethanol was added to the mixture. Thevial was sealed and then spun (220 rpm) on ahome-made milling machine for 30, 60 and 120 h.After the milling, ethanol was removed by bakingthe mixture in an oven at 200 1C for 18 h. Nofurther heat treatment was performed to modifythe structure. The powder structure was character-ized by X-ray diffraction (XRD) with cobalt Karadiation. Thermal properties were characterizedby differential thermal analysis (DTA) in thetemperature range from 50 to 1300 1C at a heatingrate of 10 1C/min. Magnetic properties weremeasured by a vibrating sample magnetometer(VSM). Microanalysis and size distribution of MApowder were examined in a laser particle sizeanalyzer and scanning electron microscope (SEM)with an energy-dispersive spectroscopy (EDS)probe.To produce bulk samples, we introduced a new
method by filling the Co–Cu MA powder in emptycommercial 500mg drug capsules (18.9mm inlength, 6.65mm in diameter). In order to retainthe capsule shape, only powder of mass from 1.50to 2.22 g (corresponding to the packed bulkdensities from 2.68 to 3.96 g/cm3) was appropriate.Thirteen Co–Cu capsules of different masses andmilling times were prepared and measured. Theroom temperature MR measurements were per-formed on these capsules by the four-point probetechnique. The capsule was mounted on the end ofmeasurement probe where four pins piercedthrough a capsule and made electrical contacts tothe powder inside. The probe was then installedbetween electromagnetic poles as shown in Fig. 1.A programmable current source ‘Keithley 220’forced a constant DC current through a pair ofouter contacts and a voltage drop across two innercontacts was measured with a nanovoltmeter‘Agilent 34420A’. A computer-controlled powersupply continuously swept the magnetic field upto
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11 kOe. The voltage reading was transferred fromthe nanovoltmeter to a computer by a GPIBinterface. The electrical resistance was thereforededuced and plotted as a function of the magneticfield. The MR magnitude is defined as 100ðRðHÞ Rð0ÞÞ=Rð0Þ; where Rð0Þ and RðHÞ areresistances in zero and 11 kOe field, respectively.As control experiments, measurements were per-formed on unmilled cobalt and unmilled copperpowder packed in similar capsules.
Fig. 1. Co–Cu capsule in magnetic field.
0
500
1000
1500
2000
2500
0 10 20 30 40 52-T
Cou
nts
CoO(200)
Co(100)
CoO(111)
30 hr
60 hr
120 hr
Fig. 2. XRD results of 30, 60 and
3. Results and discussion
From XRD analysis of starting materials, thecobalt powder consists of FCC and HCP phasesand the copper is FCC. After milling for 30, 60and 120 h, the powder is characterized as HCP Co,FCC Cu and FCC CoO as indicated in Fig. 2. Theformation of cobalt oxide at the surfaces isexpected since the milling takes place in ethanol.Because HCP Co peaks remain sharp and clearafter the milling, it is evident that Co forms asegregated phase from Cu. For FCC Co, the Braggpeaks are not clearly identified because theyoverlap with those of Cu and HCP Co and aFCC Co–Cu solid solution may be formed.According to reports on Co–Cu MA by Modderet.al [14] and Gente et al. [15], FCC Co partiallydissolves into Cu lattice during the milling process.The XRD patterns do not display large varia-
tions with milling times. Results by Modder et al.[14] similarly showed that the overall structuresdid not change after 10 h milling. However, theXRD indicates a reduction of copper latticeparameters for both Cu (1 1 1) and Cu (2 2 0) forlonger milling times, shifting Cu peaks to higherangles. This shift implies the segregation of Cofrom Cu matrix in those planes. On the other
0 60 70 80 90 100heta
CoO(220)
Cu(200)
Co(101)
Co(002)
Cu(111)
Cu(220)
120-h-milled Co–Cu powder.
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hand, a slight shift to a lower angle of Cu (2 0 0)peak can be explained in terms of the interdiffu-sion of Co into Cu (2 0 0) plane. Copper peakbroadening is also observed in the 60- and 120-h-milled powder as a result of the decrease incrystallite size and increase of strain effect. Theoverall results agree well with those previouslyreported in Co–Cu MA [14,15], with the exceptionof high oxide contents in our samples. By millingin ethanol, longer milling times promote thegrowth of the CoO phase at the expense of Copeaks.From the SEM micrograph exemplified in
Fig. 3, the powder tends to agglomerate asspherical microscale clusters. The compositionsof clusters, analyzed by an EDS probe, are copper,cobalt and oxygen. For example, the clusters atpoint A are mainly composed of Cu. The imagealso shows smaller Co-rich clusters (about 1 mm insize as seen at point B) dispersed around andembedded in Cu-rich clusters. Focusing imagesbecomes difficult in oxide-rich powders such as a120-h-milled powder because of the charge accu-mulation. The cluster size distribution is accuratelyportrayed by a laser particle size analysis.From the particle size analysis in Fig. 4, all
unimodal curves agree with SEM images byshowing peaks around 20 mm. Both, cluster sizeand size distribution (quantitatively expressed interms of span, the width of distribution which isindependent of median size) are modified by the
Fig. 3. SEM micrograph of a 30-h-milled Co–Cu powder.
milling time. The milling affects the cluster sizeand distribution, by virtue of fracturing and coldwelding. The fracturing reduces the size of clustersby breaking them into smaller clusters, but thecold welding tends to embed small clusters intolarger ones. The milling process can then bedivided into 3 stages: fracturing, intermediateand cold welding. A 30 h milling remains in thefracturing stage where the fracturing is a dominanteffect. The cluster size distribution is therefore thebroadest with a span of 2.578 and the peak is atthe largest diameter. A 60 h milling belongs to theintermediate stage in which the fracturing stilldominates, reducing the span to 1.612 but it is alsoslightly influenced by the cold welding process.The cold welding stage as seen in a 120 h milledsample results in agglomeration of fine clustersinto larger clusters, increasing the span to 1.958. Itis noted that the number of submicron clustersdecreases with milling times indicating that themilling indeed embeds smaller clusters into largerones.From DTA analysis in Fig. 5, all samples show a
melting transition at the melting point of Cu about1100 1C and two exothermic transition peaks. Theminor peak occurs around 500 1C indicating atransition from FCC to HCP Co phase and themajor peak around 700 1C corresponds to a latticerecovery process. The 120-h-milled powder with arather flat DTA curve, has the highest thermalstability since substantial energy is already storedduring the milling process. In the 30-h-milledpowder, a DTA curve indicates the substantial
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0.0 0.1 1.0 10.0 100.0 1000.0
Particle Diameter (µm)
Inte
nsity
(ar
b. u
nit)
30 hour
60 hour
120 hour
Fig. 4. Particle size analysis of 30, 60 and 120-h-milled Co–Cu
powder.
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0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0
T (°C )
∆T (
arb
.un
its)
exo
th.
----
----
----
--->
30 hour
60 hour
120 hour
Fig. 5. DTA results of 30, 60 and 120-h-milled Co–Cu powder.
-80
-60
-40
-20
0
20
40
60
80
-12 -8 4 12
Magnetic Field (kOe)
Mag
net
izat
ion
(em
u/g
)
30 hr
60 hr
120 hr
0-4 8
Fig. 6. VSM results of 30, 60 and 120-h-milled Co–Cu powder.
W. Rattanasakulthong, C. Sirisathitkul / Physica B 369 (2005) 160–167164
heat absorption between 125 1C and 650 1C inorder to increase interdiffusion reaction of the Coand Cu. It implies that the energy obtained from arather short milling time is not enough to inducethe interdiffusion reaction. In addition to exother-mic peaks at 500 and 700 1C, the 30-h-milledpowder exhibits another exothermic peak under100 1C. The heat release of this transition may be aresult of the segregation between Co and Cu.Fig. 6 shows magnetization curves obtained by
VSM from the powder of different milling times. Itis understood that the milling time affects themagnetic properties by changing the Co clustersize and intercluster distance. The saturation isachieved around 9 kOe. The longer milling timeleads to faster rate of saturation and lowersaturation magnetization. The saturation magne-tization depends on magnetic concentration, this isclearly explained by the replacement of cobalt byoxides indicated in XRD pattern. Modder et al.[14] also suggested two other factors reducing thesaturation induction, a formation of solid solu-tion, and a reduction in number of Co–Co nearestneighbors. The faster rate of saturation is also aresult of lower Co contents. Another significantobservation is a softness of the 60-h-milled
powder. Its low coercivity and remanence coin-cides with its narrowest cluster size distribution.The similar cluster sizes apparently optimize theCo intercluster distance. The optimum distanceencourages the coupling between Co clusters andreduces the coercivity as a result.The encapsulated powder conducts electrical
current through interconnecting clusters. Conduc-tion electrons travel through the less resistive pathsavoiding oxides and highly porous areas. The high
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resistivity, of the order of 104Om can then beattributed to high-oxide contents, lattice disorderand porosity. In Fig. 7, the resistivity deducedfrom an I– V curve is not constant but dependenton the test current. The resistance increases athigher current as a result of Joule heating. Anotherobservation regarding its temperature sensitivity isthe large thermal drift.In Fig. 8, the electrical resistance of 13 capsules
are shown as functions of different milling timesand packed bulk densities. Different densities areobtained from differently milled powders becauseof different cluster size distributions and oxidecontents. The variation of electrical resistance withthe packed bulk density can be explained in termsof the porosity of the packing. As predicted, thedensely packed capsules generally have a betterelectrical conduction. In addition to the density,the milling time also affects the resistance. At thesame density, the 60-h-milled capsules are moreresistive than the 30-h-milled. Despite having thelargest density, the 120-h-milled capsules have
17.50
18.00
18.50
19.00
19.50
20.00
20.50
0 10 15 20 25 30 35 40 45
Current (x 10-9 A)
Zer
o-fie
ld r
esis
tanc
e (M
Ω)
5
Fig. 7. Dependence of zero-field resistance ðRð0ÞÞ of Co–Cu
capsule on current.
0.0010.0020.0030.0040.0050.0060.0070.0080.0090.00
100.00
Density (g/cm3)
Zer
o-F
ield
Res
ista
nce
(MΩ
)
30hr60 hr120 hr
2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1
Fig. 8. Zero-field resistance of 30, 60 and 120-h-milled Co–Cu
capsules as a function of density.
comparable resistance to those of the 60-h-milledcapsules. The increased resistance with millingtime is attributable to the growth of oxide phasesby the milling.At room temperature, the Co–Cu capsules
exhibit up to 70% negative MR with saturationin applied magnetic field of 11 kOe. Repeatedsweeps of magnetic field (up and down) results inhysteresis observed in MR curves in Fig. 9. Sincethe magnitude of the effect was unprecedented, afew experiments were performed in order to ruleout any possible artifacts. To confirm that thissizeable MR only occurred in heterogeneoussystems, capsules of unmilled cobalt and unmilledcopper powder were put under tests. According toresults shown in Fig. 9, no MR is observed in Cucapsules but Co capsules show about 5% MR.Measurements with different field sweep rates wereperformed to rule out the induction effect. Fast(37Oe/s) and slow (15Oe/s) sweeps give rise toonly 1% difference proving that the effect is notmagnetic induction. Another evidence to rule outthe induction is an isotropic nature of the effect.The angle between the current and magnetic fieldwas varied and the MR was insensitive to theangle.The origin of this anomalous MR remains
unclear. We propose that it is attributable to themovement of loosely packed Co-rich clusters inresponse to an applied magnetic field. The move-ment may modify intercluster contact areasproviding more efficient electron pathways andreduces the resistance as a result. This model canexplain the exhibition of MR in capsules withhigh-Co contents where GMR should not exist. Inorder to study the effect of milling time anddensity, three repeated MR sweeps were per-formed on 13 capsules with different densitiesand milling times. The average MR magnitude as afunction of density is shown in Fig 10. The 30 and60-h-milled capsules show 20–70% MR dependingon the packed bulk density. At the same density,the 60-h-milled powder has a higher MR thanthose of 30 h milled powder. The maximum MR(over 70%) is found in a 60-h-milled Co–Cucapsule with a density of 3.00 g/cm3. It implies that60 h milling gives rise to the optimum conditionfor MR with the narrowest cluster size distribution
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-120
-100
-80
-60
-40
-20
0
20
0 8 10 12
Magnetic Field (kOe)
MR
(%
)Unmilled Cu
Unmilled Co
Milled Co-Cu
-4
-3
-2
-1
0
1
2
0 4 8 10 12Magnetic Field (kOe)
MR
(%
)
2 4 6
2 6
Fig. 9. Room temperature MR curves of milled Co–Cu capsule compared to unmilled Co , unmilled Cu capsules and Co–Cu pellets
(inset).
0
10
20
30
40
50
60
70
80
90
Density (g/cm3)
MR
(%
)
30h60h120h
2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2
Fig. 10. MR magnitude of 30, 60 and 120-h-milled Co–Cu
capsules as a function of density.
W. Rattanasakulthong, C. Sirisathitkul / Physica B 369 (2005) 160–167166
and the smallest coercivity. The smallest MR(under 30%) is observed in the 120-h-milledcapsules since the lowest magnetic contents andthe highest density impede a reduction in resis-tance. Each milling time shows different optimumdensities where the MR is at the maximum. Ateach milling time, MR magnitude reaches theminimum at the highest density where Co-richclusters are not easily moved under appliedmagnetic field. Finally, we compare MR between
Co–Cu capsules and pressed Co–Cu powder(packed bulk density of 5.44 g/cm3) obtained fromthe same milling. The pressed pellets show smallerroom temperature MR of 2%, which is in the samerange previously reported in mechanical alloys[2–12]. This modest effect is then described asGMR due to spin dependent scattering and thelarge negative MR due to magnetic cluster move-ment that is suppressed.
4. Conclusion
The capsulated, alloyed Co–Cu powder exhibitvery large negative MR (up to 70%) at roomtemperature. We propose that the movement ofloosely packed Co-rich clusters under appliedmagnetic field is the origin of this MR. Themovement tends to provide the more efficientelectron pathways and, as a result, decrease theelectrical resistance. The milling time affectsstructural, magnetic, thermal and MR propertiesby virtue of changing cluster size and distribution.The MR is also sensitive to the packed bulkdensity. With the same milling time, the optimum
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packed bulk density gives rise to the highest MRratio and the effect decreases in highly densecapsules and pressed pellets.
Acknowledgments
This work was supported by Thailand NationalMetal and Materials Technology Center (MTECGrant no. MT-B-46-MET-48-122-G), ThailandToray Science Foundation and Walailak Univer-sity. We are always grateful to Dr. John Gregg ofClarendon Laboratory Oxford for his contributionin setting up our magnet laboratory.
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