Magnetic properties of carbon-coated, ferromagnetic ...

Magnetic properties of carbon-coated, produced by a carbon-arc method

ferromagnetic

nanoparticles

E. M. Brunsman, R, Sutton, E. Bortz, S. Kirkpatrick, K. Midelfort, J. Williams, P. Smith, M. E. McHenry, S. A. Majetich, J. 0. Artman, M. De Graef, and S. W. Staley Carnegie Mellon University, Pittsburgh, Pennsylvania 15213~3890

The Kratschmer-Huffman carbon-arc method of preparing fullercnes has been used to generate carbon-coated transition metal (TM) and TM-carbide nanocrystallites. The magnetic nanocrystallites were extracted from the. soot with a magnetic gradient field technique. For TM=Co the majority of nanocrystals exist as nominally spherical particles, 0.5-S nm in radius. Hysteretic and temperature-dependent magnetic response, in randomly and magnetically aligned powder samples frozen in epoxy, correspond to fine particle magnetism associated with monodomain TM particles. The magnetization exhibits a unique functional dependence on H/T, and hysteresis below a blocking temperature T, . Below TR, the temperature dependence of the coercivity can be expressed as Hc=Hcu[ 1 - (T/TB)1’2], where HcO is the (1 K coercivity.

1. INTRODUCTION We report here on the synthesis and separation of carbon-coated ferromagnetic transition metal (TM) and TMcarbide nanocrystallites.’ These were produced by the Kratschmer et uZ.” carbon-arc process commonly used to synthesize fullerenes.” Endohedrally doped4 fullerenes have been produced by modifying the carbon electrodes. It was subsequently found that carbon-coated metal or metal carbide nanocrystallites could also be generated with these modified electrodes. The first of these nanoparticles to be produced was LaC, .‘,’ We have recently described the preparation of rare-earth carbide nanocrystallites7 and their isolation. The magnetic particles produc.ed by our carbon-arc process are predominately monodomain. These nanocrystallites are of potential interest for applications for which ferromagnetic iron oxide particles are currently used, i.e., in data storage, for toner in xerography, in ferrofiuids, and as contrast agents in magnetic resonance imaging. The carbon coating provides an effective oxidation barrier. We describe magnetic measurements on carbon-coated TM nanocrystals which have revealed interesting manifestations of fine particle magnetism, including superparamagnetic response. Our data provide the first link between fullerene-related nanocrystals and fine particle magnetism with the aim of producing materials interesting for magnetic applications. II. EXPERIMENTAL Graphite rods (0.25 in. diamj were drilled and packed with a mixture of transition metal oxide (TM=Fe, Co, and Ni) powder and a combination of graphite powder and graphite cement and baked to drive off water vapor and to cure the graphite cement. Our starting materials had an -0.fJ4 TM/C molar ratio, consistent the LaC2 preparation scheme.“,” These rods were set in the upper electrode position of a dc carbon arc, with a disc-shaped graphite anode.’ Rods were consumed under 100 A, 30 V arc conditions, in 125 Torr of He. In general, a metal oxide, II~c~O~ 7 is reduced in the plasma arc according to the reaction Me,Oy+yC*xMe+yCO, 5882

J. Appt. Phys. 75 (IO), 15 May 1994

ill

which is favored at the high plasma temperature. The nature of the carbide or supersaturate.d metal present in the soot depends on the metal/carbon phase diagram. The raw soot is ground to a fine, pm-sized powder and passed through a magnetic field gradient to separate the magnetic from the nonmagnetic species. The shear action of the grinding process has not been seen to damage the nanoparticles. Aferromagnetic particle, with magnetization M, sees a force: PAf=~b~~VjK.

(2)

Ln the first pass through the separator 95% by volume of TM-containing soot was retrieved; however, only a small fraction of this magnetically responding powder is ferromagnetic. Thus it appears that this “magnetic” powder contains ferromagnetic TM nanocrystallites embedded in larger carbon particles. Small amounts of fullerenes, including endohedral species, are also present. The nonmagnetic soot contams amorphous carbon, graphitic nanoparticles, and empty fullerenes, Most of the fullerenes were removed by extraction in carbon disulfide.” We have produced Fe. (carbide), Co, and Ni nanocrystals by this procedure. The structure and morphology of the magnetic powde.r were examined by x-ray diffraction and electron microscopy. X-ray diffraction revealed the fee rather than hcp cobalt phase, typical of fine particles. Energy dispersive x-ray fluorescence spectroscopy (EDS) indicates the transition metal to be uniformly distributed. Scanning electron microscopy (SE.M) reveals submicron-sized nominally spherical particles. Closer inspection was made with a JEOL 3c)OO,400 keV, high-resolution transmission electron microscope (IIRTEM). The 0.5-S nm range of radii observed for the encapsulated Co is about a fifth of the 5-25 nm range previously observed for Gd2C3 nanocrystallites. Our particle sizes are roughly equivalent to those observed by Bethune et al,” but without the spider web-like morphology of carbon nanotubes reported in their paper. Magnetization data for the TM nanocrystallite powder samples have been obtained with a Quantum Design superconducting quantum interference device (SQUID) magnetometer. Powders containing nanocrystallites were stabilized by epoxy. Samples containing randomly aligned particles and

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0.20 0.15 0.10 a05 0.00

l

F

1WK

5

5.05

z

-0.10

0.00 5.25 5.50

-0.19 -0.20

0.25

-0.75

-I -2

-1

0 Hfr

1

2

-1.00 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.2

cm)

H(T)

FIG. 1. Magnrtiz4on data plottrd as a function of HiT at temperatures of 5, 10, IS, 2tJ 5, 50, 100. and m F; for randomly oriented oanocrystalline C&Y particles immobilized by epoxy.

those aligned in the field provided by a pair of FeNdB permanent magnets were prepared for observation. I%4(H,T) then was determined in solenoidal fields between lt.5 T at temperatures ranging from 4 to 300 K.

; + 9 . .

0.25

2 E s!

o-00 5.25

t : i :

Ill. RESULTS AND DISCUSSION \IFe have synthesized nanocrystalline Gd2C3 (Ref. 7) and No2CS paramagnets and nanocrystalline Fe, Co,’ and Ni ferromagnets. The ferromagnets display textbook superparamagnetic response.w*’ The hysteretic and temperaturedependent magnetization, in randomly and magnetically aligned powder samples froncn in epoxy, are characteristic of fine particle magnetism. As an example, we illustrate the behavior of monodomain C.!o particles. Magnetization data for the randomly aligned nanocrystallinc Co/C particles are plotted versus II/T in Fig. I.. Data were taken at eight temperatures covering the range between 5 and 200 K. In Fig. 2ia’) an expzmded view of the 5 K curve is shown. In Fig. 3’bj the corresponding plot for the magnetically aligned specimen is given. Ilysteresis is present in both . The coercivity Iii. present at the low temperatures was plotted versus T and fit to the expression I-p,=I3,,,rl-!T/“TR)1:‘].

(3)

See Fig. 3. The intrinsic II,,, was determined to be -450 Oe and the blocking temperature TR, to be -160 K. Note that the coercivity data for both aligned and randomly oriented specimens lie on the same curve. Direct observation of particles by TEM gives a sense of the distribution of particle sizes. These particle sizes can then be compared with those inferred independently from other types of magnetic measurements. The scaled magnetization data (e.g., Fig. 1,) can be fit to a Langevin function L using the relation M F- =Qni=coth(o)-;; I)

>

(4

where M, is the 0 K saturation magnetization and n =$-I/ kT. The effective moment ,X is given by the product &fs( V}, where ~%f,~ is the saturation magnetization and (I/> is the avJ. Appl. Phys., Vol. 75, No. IO, 15 May 1994

RG. 2. Low field magnetization curves at 5 K for Co/C particles (a) rimdomly oriented and (hi magnetically aligned.

erage particle volume. Assigning the bulk value to M, we can infer a particle volume from the Langevin fit. Also coercivity data exists for elemental and other common magnets as a function of size.13 Comparison of our magnetically determined coercivities Zl, with these data offers another way of estimating mean particle radius. Finally, the blocking temperature TB also offers information as to the magnetic particle size. TR is the temperature at which metastable hysteretic response is lost for a particular experimental time frame.

0.05 ,

I

dT P/K)

FIG. 3. H&T) vs &? for aligned and unaligned Co/C particles. The hlocking temperature, TR, is 460 K. Brunsman et al.

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TABLE I. Anisotropy, coercivity, and moment data for certain nanocrystallites.

co Fe Ni Y-~-W&

Crystalline anisotropy energy W ( lo” ergs/cm3)

Blocking temp. TB tK)

Coercivity FI,.,, (Oej

Cluster moment (10” &?j

-27” (4 Kj ~!.7~ (77 Kj -6’ i 100 K) -0.45d CRT)

I60 x0 1so 80

450 350 260 370

11 10 25 6.7

“References 15, If>. “Reference 17. ‘Reference 18. “Reference 19.

Below T, , hysteretic response is observed since thermal acis insufficient to allow the alignment of particle moments with the applied field in the time scale of the measurement. For spherical particles the rotational energy barrier to alignment is given by the magne.tocrystallinc anisotropy energy per unit volume K multiplied by the particle volume V. For hysteresis loops taken over -1 h, the blocking temperature should roughly satisfy the relationship: tivation

T =W) R 3Ok, . The factor of 30 represents ln(w,Jw), where OJ is the inverse of the experimental time (--IO-” Hzj and tie an attempt frequency (--1 GHz). If K can be estimated (i.e., from bulk values) and the value of T, determined then the mean particle volume can be estimated. The error in TB was 230 K, yielding a particle radius of (4.11?0.33jXl@ 7 cm for cobalt. Radii determined this way were larger than estimated by other methods, suggesting an anisotropy greater than bulk . We have characterized fine particle magnetism in Fe[carbide], Co, and Ni nanocrystals produced by the Kritschmer arc process. For comparison we also have made similar measurements on nanocrystalline y-Fe,O, nanocrystals’” of a similar size, produced by a matrix-mediated synthesis route and provided by Xerox Corporation. In Table I we summarize the bulk K values for each of these materials as well as values of TR and E-i,,, determined from fits to H, [Eq. [3)]. We also include the cluster moment p as determined Eq. (4j. In Table II we summarize the mean-particle radius for our nanocrystailine. samples as inferred from the Langevin function fits, r, data analysis and comparison with previous coercivity measurements in tine particle magnets. Wherever

TABIE II, Nanocrystallite particle radii (in nmj determined by various methods. Langevin fit (Y-co Fe carhide Ni y-Fe,O,

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2.5 2.3 4.7 3.3

Blocking temp. Coercivity (,lO) TEM analysis 3.9 5.2 6.0 12

1.5-2 1-2 .. .

J. Appl. Phys., Vol. 75, No. 10, 15 May 1994

0.5-S ... ... 3.25-5.25

possible these are compared with TEM observations of the particle size distribution. In most cases these are seen to be in excellent agreement. The discrepancy between radii determined by TB analysis for y-FezO, suggests a smaller anisotropy constant but not by several orders of magnitude as has been previously suggeste.d.“” IV. CONCLUSIONS The Kr&schme.r-Huffman carbon-arc method has been used to generate carbon-coated TM and TM-carbide nanocrystallites. These were collected by magnetic gradient separation. Particles have a spherical morphology with an -0.5-S nm radius (for Co) from TEM. These monodomain particles exhibit superparamagne.tic response with hysteresis only at temperatures T< TR . Analysis of the TB data, as well as the Langevin fits, allows calculation of mean-particle radii for superparamagnetic particles. The radii so calculated are in agreement with TEM observations and with prior measurements of fine particle magnetism in TM particles. ACKNOWLEDGMENTS M.E.M. and S.A.M. thank the NSF for support through NY1 awards No. DMR-92584.50 and No. DMR-9258308. We are indebted for support from the CMU DSSC REU program. This material is also based (in part) upon work supported by the NSF under Grant No. ECD-8907068. The assistance of the CMU SURG program has been invaluable. Finally, we would also like to thank R. F. Ziolo of Xerox for providing y-FezO, nanocrystals. ‘M. E. McHemy, S. A. Majetich, J. 0. Artman, M. De Graef, and S. W. St&y, Phys. Rev. B (in press). a W. Kratschmer, L. D. Lamb, K. FostiropouIos, and D. R. iiuffman, Nature 347, 354 (1.990). ‘H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R. E. Smaliey, Nature 318, 162 (19%). ‘Y. Chai, T. Guo, C. Jin, R. E. Haufler, I.., P, E Chibante, J. Fure, L. Wang, J. M. Alford, and R. E. Smalley, J. Phys. Chem. 95, 7564 (1991). ‘Rodney S. Ruoff, Donah! C. Dxcnts, Bryan Ghan. Ripudaman Malhotra, and Shekhar Subramoney, Science 259, 346 (1993). ‘M. Tomita, Y. Saito, and T. Hayashi, Jpn. J. Appl. Phys. 32, L2$0 (1993j. 7S. A. Majetich. J. G. Artman, M. E. McHenry, N. T. Nuhfer, and S. W. Staley, Phys. Rev. B (in press). ‘R. S. Ruoff, R. Malhotra. D, L. Huestis, D, S, Tse, and I>. C. Inrents, Nature 362, 141 (1993). “D. S. Bcthune, C. II. Klang, M. S. de Vrics, G. Gorman, R. Savoy, J. Vazquez, and R. Beyers, Nature 363, 605 (1993). ‘“B. D. Cullity, ~n&rodz&ion to Mugnetic Mzteriuls (Addison-We-sley, Reading, MA, 1972). ” C. P. Bean and J, I), Livingston, J. Appl. Phys. 30, 12OS (1959). “I. S. Jacobs and C. P. Bean, in ~f~zg~rrtisrn,edited by G. T. Rado and H. Suhl (Academic, New York, 1963), Vol. 3. r3F E. Luborsky, J. AppI. Phys. 32, 171.5 (19hlj. ” d. F. Ziola, E. P. Giannelis, B.A. Weinstein, M. P. O’Horo, B. N. Ganguly, V. Mchrotra, M. W. Russell, and D. R. Huffman, Science 257,219 i1992). “W . D I Doyle and P. J. Flanders, International Conference on Magnetism, Nottingham. 1964. p. 751, puhlished in the Proc. of The Physical Society. “W A Sucksmith and J. E. Thompson, Proc. R. Sot. London Sect. A 254, 36; il954j. “R. M. Bozorth, J. Appl. Phys. 8, 575 (1.937). ‘sG. Aubcrt, J. Appl. Phys. 39, 504 (1968). 19&fug?zetic&ides, edited by D. J. Craik (Wiley, London, 1975). ‘)J. K. Vassiliou, V. Mehotrd, M. W. Russell, E. P. Giannelis, R. D. McMichael, R. D. ShulI, and R. E Ziolo, J. Appl. Phys. 73, 5109 (1993).

Brunsman et al,
