Magnetic properties of iron-polyethylene nanocomposites prepared by high energy ballmillingAnit Kumar Giri

Citation: Journal of Applied Physics 81, 1348 (1997); doi: 10.1063/1.363870 View online: http://dx.doi.org/10.1063/1.363870 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/81/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic properties of iron nanoparticle J. Appl. Phys. 107, 103913 (2010); 10.1063/1.3428415 Synthesis and magnetic properties of monodisperse Fe 3 O 4 nanoparticles J. Appl. Phys. 95, 7121 (2004); 10.1063/1.1682783 One-step processing of spinel ferrites via the high-energy ball milling of binary oxides J. Appl. Phys. 94, 496 (2003); 10.1063/1.1577225 Preparation and magnetic properties of Ba 2 Co 2 Fe 28 O 46 nanocrystals J. Appl. Phys. 88, 519 (2000); 10.1063/1.373689 Structural and magnetic properties of ball milled copper ferrite J. Appl. Phys. 84, 1101 (1998); 10.1063/1.368109

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Magnetic properties of iron-polyethylene nanocomposites preparedby high energy ball milling

Anit Kumar Giria)Departamento de Fisica, Universidad de los Andes, Cra. 1E # 18A-10, Bogota, Colombia

~Received 5 August 1996; accepted for publication 28 October 1996!

Metal-polymer nanocomposites in the system iron-polyethylene have been prepared by high energyball milling. Minimum average grain size obtained is of the order of 9 nm. Enhanced coercivity of230 Oe at room temperature and 510 Oe at 5 K have been obtained for the sample containing 0.1volume fraction of iron in polyethylene after milling for 200 h. This high value of the coercivitycould be ascribed to the presence of a fraction of single-domain particles in the sample as evidencedfrom the thermal dependence of the magnetization and ac susceptibility measurements. ©1997American Institute of Physics.@S0021-8979~97!05003-2#

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I. INTRODUCTION

Interest in the magnetic properties of nanocrystallmaterials having sizes of the order of a few nanometersmains high1 because of its probable applications in informtion storage, ferrofluids, permanent magnets, paint pigmeetc. Magnetic nanoparticles show superparamagnetismlow a certain size. Ultrafine iron particles having sizes inrange of 2–10 nm embedded in an insulating matrixlike S2or Al2O3 or in a conducting matrixlike Cu exhibit coercivitvalues orders of magnitude higher than that of bulk iron.2 Inrecent times, high-energy ball-milling technique has beused to prepare a variety of materials having nanosidimensions3–7 embedded in some ceramic or immiscibmetal matrices. Little attention has been given to the preration of metal-polymer nanocomposites by ball millinOnly recently Ishida and Tamaru8 have prepared particles oCu and Ni of size around 20mm in a polymer matrixlikepolytetrafluroethylene by high energy ball milling. The higcoercivity of iron in the nanocrystalline form has made itprobable potential candidate for practical use in recordmaterials,2 and this use will be facilitated if iron nanopaticles could be synthesized in polymer matrices and in bamounts. In this regard, we have taken up the preparationanocrystalline iron in polyethylene matrix by ball millingwhich is essentially a bulk preparational technique. Herereport the preparation and some magnetic studies of ipolyethylene nanocomposites.

II. PREPARATION OF SAMPLES AND EXPERIMENT

The starting materials consist of commercially availa2325 mesh Fe powders~typical size about 40mm! and poly-ethylene~PE! powders~molecular weight about 106!. Foursamples of the series Fex-~PE!12x ~x denotes volume fraction! with x50.05, 0.1, 0.2, 0.3 have been prepared by takthe appropriate amounts of Fe and PE into a hardened sless steel vial with four stainless steel balls so that the rof the weight of balls to that of powder is 14:1. The vialsealed in argon atmosphere to prevent oxidation of iron.material is milled for 200 h in a high-energy planetary b

a!Electronic mail: agiri@uniandes.edu.co

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mill. To avoid excessive heating, milling is interrupted ea30 min for 15 min to cool. The vial is opened periodicallyargon atmosphere to take out a small portion of sampleanalysis. The samples are characterized by x-ray diffrac~XRD! with CuKa radiation. Magnetic measurements acarried out in a susceptometer and vibrating-sample magtometer~2–300 K!.

III. RESULTS AND DISCUSSION

Figure 1 shows the XRD pattern of the sample wx50.1 at different milling times. The presence of the refletions due to Fe and PE is observed~only the Fe peaks havebeen marked in Fig. 1, all other reflections arise due to P!.No reflection corresponding to the oxides of iron is noticin the XRD patterns. For the unmilled sample, peaks ofand PE are observed. As the milling time increases, thetensities of the peaks due to Fe decreases and these pbecome broader, indicating the lowering of grain size ofThe peaks of PE also become less intense and broadermilling times and finally after 150 h of milling the less intense peaks could not be observed. The results of osamples are similar. We have calculated the average gsize of Fe for~110! reflection at different milling times fromthe line broadening using the Scherrer equation given by

d50.9l/b cosu, ~1!

whered is the grain size,l is the wavelength of the radiatioused,b is the full width at half-maximum after making thcorrection due to instrumental broadening, andu is the scat-tering angle. Figure 2 shows the milling time dependencethe average grain size of the sample withx50.1. The size ofFe in the unmilled sample could not be calculated by Schrer equation because of its extremely small peak width. Ding the first 50 h of milling, the grain size reduces rapidand the size continues to decrease as the milling timecreases reaching a final value~after 150 h of milling! afterwhich it remains constant. This finding is similar to one oserved by Ambroseet al.3 for Fe–Al2O3. During milling, theFe particles are fractured and cold welded together whleads to the grain size reduction. After some time of millinthe particles become work hardened and increase their abto withstand deformation. This leads the particles to attai

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final grain size. The final average grain size after 200 hmilling for all the samples have been plotted in the insetFig. 2.

Figure 3 shows typical hysteresis loops ofx50.1 sampleunmilled and milled for 200 h measured at room temperatwith a maximum applied field of 8 kOe. Milling for 200 hresulted in a value of the coercivity of 230 Oe comparedthe value of 36 Oe of the starting material. For all tsamples, the coercivity values increase with the milling tias can be seen in Fig. 4, where the coercivities of allsamples have been plotted against the milling time. Thecrease in coercivity is due to the reduction of grain size ahence signifies the presence of substantial fraction of suparamagnetic particles in the samples studied. To clarifypoint, we choose the sample showing the highest coercivalue at room temperature, i.e., forx50.1 milled for 200 h,for further magnetic measurements at low temperatures.ure 5 shows the temperature dependence of the coercivitthis sample. At 5 K, the value of this parameter was 510Increase of the coercivity with the decrease of the tempture suggests the presence of the superparamagnetic parin the samples. The temperature dependence of coercivinoninteracting single domain ferromagnetic particles is givby9

FIG. 1. XRD patterns of the samplex50.1 for different milling times.~Fereflections are marked; all other reflections correspond to polyethylene!

FIG. 2. Plot of the average grain size (d) against the milling time of thesample withx50.1: ~Inset! Plot of the average grain size (d) vs the Fevolume fraction (x) for the samples milled for 200 h.

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Hc5Hc0@12~T/TB!1/2# ~2a!

for fixed volume of the particle and

Hc5Hc0@12~Vc /V!1/2# ~2b!

at fixed temperature.HereHc andHc0 are the coercivities of the ferromag

netic particles at temperatureT and 0 K, respectively,TB isthe superparamagnetic blocking temperature proportionaKV ~K is effective anisotropy constant andV is the volume!,andVc is the blocking volume.Hc0 is given byHc052K/Ms

whereMs is the saturation magnetization. For Fe,Hc0 hasbeen estimated, for spherical particles, to be around 6001

and at room temperature the value should be a fractionthis. The coercivity values obtained in this sample therefseem to be consistent with the theoretical prediction. Tnanoparticles of Fe in these samples are strongly bondethe PE matrix, contrary to the assumption in the modelferred to where the magnetic particles are assumed to beof stress. The critical size of Fe for single domain is arou15 nm.10 For the samples having grain sizes below this,can be seen from Eq. 2~a!, the coercivity increases with thincrease of grain size forT,TB . For this reason, the coercivity for the sample withx50.1 is higher than that withx50.05 at room temperature. The decrease of coercivity w

FIG. 3. Typical hysterisis loop for the 200 h milled sample withx50.1.Continuous line is for the unmilled sample and dotted line is for the 20milled sample.

FIG. 4. Plot of coercivity vs milling time for all the samples.

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the increase of Fe volume fraction beyond the limitx50.1~Fig. 4! is associated with the percolation effect whereparticles are connected to each other forming magneticsure domains. We should mention here that the temperadependence of the coercivity for thex50.1, 200 h milledsample does not follow theT1/2 law.9 This is due to theoccurrence of interparticle dipolar interactions as evidenby the dependence of coercivity on Fe concentrationcould also be due to the distribution of particle size, escially a ‘‘tail’’ of large particles.

In Fig. 6, we present the temperature dependence ofmagnetization, both field cooling~FC! ~constant applied field8 kOe! and zero field cooling~ZFC! data are shown, for thesample withx50.1. These data also evidence the presenca fraction of superparamagnetic particles having a blocktemperature close to 5 K. A very clear peak at 17 K for t

FIG. 5. Plot of coercivity against temperature for the sample withx50.1after 200 h of milling.

FIG. 6. Plot of magnetization against temperature for the samplex50.1 after 200 h of milling; ZFC~solid circle! and FC~open circle!.

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ZFC curve evidences the existence of a blocking temperaand hence the presence of a fraction of superparamagparticles in the measurement of the thermal dependence osusceptibility~Fig. 7! in this sample. Considering the typicameasuring time for the magnetization and ac susceptibithe two values of the blocking temperatures obtained throtwo measurements are said to be in quantitative agreem

IV. CONCLUSIONS

In conclusion, we have successfully synthesizednanoparticles in polyethylene matrix using a bulk prepational method of ball milling and obtained high coercivity foa sample having 0.1 volume fraction due to the presencsingle-domain particles. Studies of thermal dependencemagnetic properties of these nanocomposites for all ocompositions are in progress and detailed results will be plished elsewhere.

ACKNOWLEDGMENT

Partial financial support from the Committee of Invesgations, Faculty of Science, Universidad de los AndBogota, Colombia, is acknowledged.

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FIG. 7. Plot of ac susceptibility against the temperature for the samplex50.1 after 200 h of milling.

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