NANO LETTERS
Synthesis, Self-Assembly, and Magnetic Properties of FexCoyPt100-x-y Nanoparticles
2002 Vol. 2, No. 3 211-214
Min Chen and David E. Nikles* Department of Chemistry and Center for Materials for Information Technology, The UniVersity of Alabama, Tuscaloosa, Alabama 35487-0209 Received October 22, 2001; Revised Manuscript Received December 21, 2001
ABSTRACT FexCoyPt100-x-y alloy nanoparticles were prepared by the simultaneous reduction of cobalt acetylacetonate and platinum acetylacetonate and the thermal decomposition of iron pentacarbonyl. The relative amounts of iron, cobalt, and platinum in the particles depended on the amount of iron, cobalt and platinum charged to the reaction. As prepared, the particles were superparamagnetic and had a distorted fcc structure. The average particle diameter was 3.5 nm and the size distribution was very narrow. The particles could be dispersed in hydrocarbon solvents and formed films consisting of hexagonal close-packed particles on carbon-coated copper TEM grids. The films were sputter coated with amorphous carbon and then annealed at temperatures ranging from 550 to 700 °C to transform the particles to the tetragonal (L10) phase. The coercivity of the annealed films increased with increasing annealing temperature. For films with a similar degree of transformation to the tetragonal phase, increasing the cobalt content decreased the coercivity of the films.
Magnetic recording technology has made tremendous gains in data storage density, partially by scaling down the grain size of thin film media. However, further scaling to support future increases will bring the grain sizes near the superparamagnetic limit. This has led to a search for new materials and new microstructures for longitudinal recording. The L10 phase of FePt has a very high magnetocrystalline anisotropy (Ku ∼ 6.6 to 10 × 107 erg/cm3).1 Sun et al. have prepared spherical FePt nanoparticles by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl.2 As prepared, the particles were spherical, with a very narrow size distribution, superparamagnetic, and had a disordered face-centered cubic structure. They were dispersed in hexane, the dispersion cast onto a silicon wafer to dry to highly ordered, self-assembled films consisting of close-packed particles. Upon annealing at temperatures above 550 °C, the particles transformed to the L10 (tetragonal) phase. The annealed films were ferromagnetic with coercivities that depended on the annealing conditions. They were able to record on the films and demonstrated that the thermal stability factor (KV/kT > 48) was suitable for a thin film magnetic recording medium. This demonstration has aroused our interest in the synthesis and self-assembly of FePt nanoparticles. We have modified the synthetic procedure reported by Sun et al. to prepare FexCoyPt100-x-y nanoparticles. In a threenecked round-bottom flask under a nitrogen atmosphere, a solution platinum acetylacetonate (0.5 mmol) and cobalt * Corresponding author. E-mail:
[email protected] 10.1021/nl015649w CCC: $22.00 Published on Web 01/23/2002
© 2002 American Chemical Society
acetylacetonate (0.1 mmol) in 10 mL octyl ether was heated to 100 °C. To this was added a solution of 1,2-hexadecanediol (1.5 mmol) in 10 mL dioctyl ether at 80 °C, which gave a purple solution. To this solution was added via syringe oleylamine (0.5 mmol), oleic acid (0.5 mmol), and iron pentacarbonyl (1 mmol). The mixture was heated to reflux and allowed to reflux for 30 min. This gave a black dispersion. The dispersion was allowed to cool to room temperature, and then 40 mL ethanol was added to precipitate the particles. The mixture was centrifuged to isolate the particles from the yellow-brown supernatant. The particles were redispersed in hexane, precipitated with ethanol, and isolated by centrifuging. The particles were dried at room temperature in a vacuum oven to give 100 to 200 mg of particles. The dispersion and precipitation removed impurities. The contents of the supernatant solution were not characterized. The relative amounts of platinum acetylacetonate, cobalt acetylacetonate, and iron pentacarbonyl were varied in order to produce nanoparticles with different compositions, Table 1. The particle composition was determined by energy-dispersive X-ray analysis on a Philips model XL 30 scanning electron microscope. The iron content in the particles was always much lower than the amount charged to the reaction, as iron pentacarbonyl. The fate of the missing iron was not determined. By eliminating the cobalt acetylacetonate, FePt particles were prepared in a manner similar to that reported by Sun et al. As prepared, the particles had a disordered face-centered cubic structure with a unit cell parameter, a ) 390 pm. The average
Figure 2. TEM image of a thick film consisting of Fe49Co7Pt44 particles. The inset shows the low angle electron diffraction. Figure 1. TEM image of a thin film consisting of Fe49Co7Pt44 particles. Table 1: Composition and Size of the FexCoyPt100-x-y Nanoparticles as Prepared, before Annealing batch 1 2 3 4 5
charged found charged found charged found charged found charged found
Fe (mole %)
Co (mole %)
Pt (mole %)
67 48 63 49 57 40 47 34 37.5 23
0 0 6 7 14 17 21 19 25 27
33 52 31 44 29 43 32 47 37.5 50
diam (nm) 3.5 3.4 3.5 3.6 3.6
crystallite size, Table 1, was estimated by Scherrer analysis3 of the line width of the 〈111〉 diffraction peaks, The crystallite size of 3.4 to 3.6 nm agreed with the particle size observed in the TEM images described below. The particles were dispersed in a 50/50 mixture of hexane and octane (2.5 mg particles in 1 mL solvent), containing 0.1 mL of a 50/50 mixture of oleylamine and oleic acid. The dispersion was dropped onto a carbon-coated copper TEM grid (200 mesh from SPI) and the solvent slowly evaporated. Using octane in the solvent mixture slowed the solvent evaporation rate, relative to pure hexane. TEM images for the films (Figures 1 and 2) showed that the particle assembled into hexagonal arrays. In thin regions of the films, Figure 1, the particles formed a honeycomb array consistent with AB stacking of the particles. In thicker regions of the film, Figure 2, the particles assembled into an ABC close-packed structure. The inset in Figure 2 shows the low-angle electron diffraction, indicating a high degree of ordering. The average particle size was 3.5 nm, in agreement with values of crystallite size determined from X-ray diffraction data. The space between the particles was occupied by the surfactants, oleic acid, and oleylamine. 212
Figure 3. TEM image of a self-assembled film of Fe48Pt52 particles sputter coated with amorphous carbon.
As prepared, the particles were superparamagnetic. The films were sputter coated with a 10 nm amorphous carbon layer to inhibit sintering during annealing, Figure 3. The films were annealed in a vacuum annealer or in a tube furnace with flowing 2% hydrogen in argon to transform the particles to the L10 phase. Annealing distorted the quality of the particle ordering, Figure 4, and finding annealing conditions that maintain the particle ordering is the subject of ongoing research. For CoPt and FePt the degree of tetragonal ordering (S) can be calculated by eq 1, where c100 and a100 are the unit cell parameters for the 100% tetragonal phase. 1S) 1-
c a
c100 a100
(1)
With increasing tetragonal ordering, the value of S should increase to a limiting value of 1. However, for the Nano Lett., Vol. 2, No. 3, 2002
Table 2: Effect of Annealing Conditions on the Tetragonal Ordering and Grain Growth of the FexCoyPt100-x-y Films 0.5 h temp (°C) Fe48Pt52
Fe49Co7Pt44
Fe40Co17Pt43
Figure 4. TEM image of a film containing Fe48Fe52 particles after annealing at 600 °C for 30 min.
Fe34Co19Pt47
Fe23Co27Pt50
Figure 5. Effect of annealing temperature on the degree of transformation to the tetragonal phase, as quantified by the quotient c/a, and on the coercivity of the annealed films. The values of c/a are plotted as filled symbols and the values of coercivity are plotted as open symbols. The compositions were Fe48Pt52 (circles), Fe49Co7Pt44 (squares), and Fe40Co17Pt43 (triangles).
FexCoyPt100-x-y particles, the values a and c for the L10 phase are not well-established and will vary with composition. We were not comfortable with using eq 1 to quantify the degree of tetragonal ordering. Accordingly, the degree of transformation to the tetragonal phase was quantified by the quotient c/a, where a and c are the unit cell parameters determined by X-ray diffraction. For the face-centered cubic phases, a ) c, and c/a ) 1. When transformed to the tetragonal phase, the value of c/a decreased to a limiting value. For the FePt L10 phase, the unit cell parameters are a ) 385.23 pm and c ) 371.33 pm and the quotient c/a is 0.96392. For the L10 phase of CoPt, a ) 380.3 pm, c ) 370.1 pm, and c/a ) 0.9732. In this work for any given composition with increasing annealing temperature or increasing annealing time, the value of c/a decreased, Table 2 and Figure 5, indicating a more complete transformation to the tetragonal phase. For the films annealed at 600 °C for 30 min, small increases in the amount of Co had little effect on the ability Nano Lett., Vol. 2, No. 3, 2002
a (pm) c (pm) c/a d (nm) a (pm) c (pm) c/a d (nm) a (pm) c (pm) c/a d (nm) a (pm) c (pm) c/a d (nm) a (pm) c (pm) c/a d (nm)
1.0 h
3.0h 700
550
600
700
700
3.85 3.79 0.984 4.7 3.85 3.80 0.988 4.5
3.87 3.76 0.971 7.9 3.85 3.75 0.976 6.4 3.82 3.71 0.971 6.1 3.81 3.76 0.987 4.9 3.83 1.00 3.6
3.86 3.73 0.966 16.4 3.85 3.72 0.966 16.2 3.83 3.69 0.964 15.7
3.86 3.72 0.965 21.2 3.72 3.85 0.966 21.6
3.81 3.70 0.970 16.4 3.85 3.84 0.998 6.0
3.81 3.69 0.967 20.2 3.84 3.73 0.971 18.9
of the particles to transform to the tetragonal phase. Values of c/a were in the range 0.971 to 0.976 for Fe48Pt52, Fe49Co7Pt44, and Fe40Co17Pt43. Annealing at 700 °C for 30 min decreased the values of c/a to 0.964 to 0.966. However, further increasing the Co content inhibited the transformation. Fe34Co19Pt47 or Fe23Co27Pt50 were only completely transformed after annealing at 700 °C for 3 h. Annealing also increased the average crystallite size, as measured by Scherrer analysis of the 〈111〉 peak in the X-ray diffraction. There was a slight increase in average crystallite size upon annealing at 550 °C. The crystallite size increased greatly when annealing at temperatures beyond 600 °C. These results are in agreement with a recent paper by Dai, et al., which reported that FePt nanocrystals coalesce to form larger grains when annealed at temperatures above 600 °C.4 For films having small amounts of Co and annealed at 600 °C, the average crystallite size decreased with increasing Co content. However the films annealed at other temperatures showed little effect of composition on crystallite size. The films containing higher amounts of Co (Fe34Co19Pt47 and Fe23Co27Pt50) were not as susceptible to coalescing. Annealing at 600 °C for 30 min resulted in smaller increases in average crystallite size. Even after 1 h annealing at 700 °C, the average crystallite size was still smaller than that for the films containing lower amounts of Co. Magnetic hysteresis curves were measured on a Princeton Micromag 2900 alternating gradient magnetometer using a 18 kOe saturating field. After annealing, the films were ferromagnetic with an in-plane coercivity that depended on the annealing temperature and the composition, Table 3. We were not able to determine values of Ms, because of the unknown amount of surfactants in the films. The coercivity for the film containing Fe48Pt52 and annealed at 700 °C was so high that the AGM magnet (maximum field 18 kOe) could not saturate the sample and only a minor loop was obtained. 213
Table 3: Effect of Annealing Conditions on the Coercivity of the FexCoyPt100-x-y Films 0.5 h
1.0 h
3.0 h 700
temp (°C)
550
600
700
700
Fe48Pt52 Fe49Co7Pt44 Fe40Co17Pt43 Fe34Co19Pt47 Fe23Co27Pt50
3970 2430
6500 4500 3800 2180 242
>11 600a 8700 6500
>12 300a 9170
a
6630 4590
6990 9090
Minor loop.
In Figure 5 are plots of the quotient c/a and the coercivity as a function of annealing temperature for films containing Fe48Pt52, Fe49Co7Pt44, and Fe40Co17Pt43. Values of c/a decrease with increasing annealing temperature and appear to be reaching some limiting value as the phase transition becomes complete. However, the coercivity continues to increase with increasing annealing temperature because of particle grain growth due to coalescence. For the films annealed at 700 °C having similar values of c/a and similar average crystallite sizes, the coercivity decreased as the cobalt content increased. This can be qualitatively explained by the lower value of magnetocrystalline anisotropy for CoPt (Ku ) 4.9 × 107 erg/cm3) relative to FePt (6.6 to 10 × 107 erg/ cm3).1 However, when completely transformed by annealing at 700 °C for 3 h the film with the highest Co content, Fe23Co27Pt50, had a high coercivity (Hc ) 9090 Oe). This could not be explained by grain size, since the average crystallite size was comparable (18.9 nm) to that for the samples annealed at 700 °C for 1 h. The magnetic properties of the films containing self-assembled FexCoyPt100-x-y particles were consistent with the properties of thin alloy films prepared by rf sputtering5 or by electron beam evaporation.6 Films prepared by rf sputtering at low temperatures (
