Co–CoO nanoparticles prepared by reactive gas-phase aggregation
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- González, J.A., Andrés, J.P., De Toro, J.A. et al. J Nanopart Res (2009) 11: 2105. doi:10.1007/s11051-008-9576-8
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The technique of gas-phase aggregation has been used to prepare partially oxidized Co nanoparticles films by allowing a controlled flow of oxygen gas into the aggregation zone. This method differs from those previously reported, that is, the passivation of a beam of preformed particles in a secondary chamber and the conventional (low Ar pressure) reactive sputtering of Co to produce Co–CoO composite films. Transmission electron microscopy shows that the mean size of the particles is about 6 nm. For sufficiently high oxygen pressures, the nanoparticles films become super-paramagnetic at room temperature. X-ray diffraction patterns display reflections corresponding to fcc Co and fcc CoO phases, with an increasing dominance of the latter upon increasing the oxygen pressure in the aggregation zone, which is consistent with the observed reduction in saturation magnetization. The cluster films assembled with particles grown under oxygen in the condensation zone exhibit exchange-bias fields (about 8 kOe at 20 K) systematically higher than those measured for Co–CoO core-shell nanoparticles prepared by oxidizing preformed particles in the deposition chamber, which we attribute, in the light of results from annealing experiments, to a higher ferromagnetic–antiferromagnetic (Co–CoO) interface density.
KeywordsCo nanoparticlesGas-phase aggregationExchange-biasCore-shell particlesAerosolsNanocomposites
A variety of Co–CoO nanostructures have been prepared in the last few decades partly in order to study magnetic perpendicular anisotropy (Yamauchi and Shiiki 2002; Ohkoshi et al. 1984) but mainly due to an interest in the exchange coupling between ferromagnetic (FM) and antiferromagnetic (AFM) phases in close contact (Nogués et al. 2005; Meiklejohn and Bean 1957; Peng et al. 2000; Skumryev et al. 2003; Koch et al. 2005; Morel et al. 2004). The interaction is most clearly manifested by the horizontal shift of the hysteresis loop (by an amount called the exchange-bias (EB) field, HE) when it is measured after cooling in a saturating field from, typically, above the AFM Néel temperature (Nogués et al. 2005). In the case of CoO, this ordering temperature is conveniently close to room temperature (293 K), which, together with its strong exchange coupling with metallic Co, have made Co–CoO the archetypical system for the fundamental study of EB. Exchange coupling is essentially a surface effect and produces significant EB fields only in systems with a sufficiently large FM–AFM surface to FM volume ratio, i.e. in composite nanostructured materials. The discovery of the EB effect itself took place in passivated Co nanoparticles, i.e. core-shell structured particles (Meiklejohn and Bean 1957). Similar particles have been investigated intensively more recently taking advantage of techniques which allow a better control of the core and shell sizes, in particular that of gas-phase aggregation (Peng et al. 2000; Skumryev et al. 2003; Koch et al. 2005; Morel et al. 2004). The industrial exploitation of the EB effect in spin-valve devices (Heim et al. 1994) shifted the research emphasis to layer-structured systems, which, in the case of Co–CoO, are prepared by either natural passivation of the Co layers (Lin et al. 1994; Brems et al. 2007), or by reactively sputtering a CoO layer prior to the Co deposition (Gredig et al. 2002; Hong et al. 2006). Bulk composite Co–CoO structures have also been prepared by reactive sputtering adjusting the oxygen pressure (Yi et al. 1996, 2005) or the sputtering power (De Toro et al. 2006) in order to achieve the necessary nanoscopic size, in some dimension, for the FM regions. Here, we report magnetic and structural characterization of Co–CoO composite nanoparticles prepared by introducing oxygen in the condensation zone of a cluster source, a technique previously employed to produce chromium oxide particles (Hihara et al. 2001), which differs from all methods reported so far for the synthesis of Co–CoO systems.
In order to partially oxidize the Co particles during its actual formation, a controlled flow of oxygen gas was allowed into the aggregation zone of a cluster source (Mantis Deposition) similar to the original design of Haberland et al. (1994). This method is clearly different from the exposure to oxygen, at ambient conditions or in the deposition chamber, of preformed Co particles (Peng et al. 2000; Morel et al. 2004), which yields core-shell structured particles. In fact, the other method has also been employed in this work to prepare a few samples for comparison. The condensation chamber was evacuated to 2 × 10−7 mbar prior to operation, during which the sputtering gas (Ar) pressure was kept to 0.1 mbar. The nanoparticles were deposited on glass substrates positioned perpendicular to the beam in a secondary chamber, separated from the aggregation zone by a small diameter nozzle. The porous nanoparticle films so grown were then covered by an evaporated Cu layer (thicker than 100 nm). The particle deposition rate, always around 0.4 A/s, was measured with a standard quartz-crystal monitor prior to each deposition, thus disregarding the usually small variations sometimes observed over deposition times of typically 30 min (in any case, the film thickness is not an important parameter in the present study).
Transmission electron microscopy (TEM) images were taken using a JEM 2100 electron microscope in bright field mode. X-ray diffraction (XRD) measurements, using Cu Kα radiation in a Bruker D8 Advance diffractometer were also performed. An extraction MagLab Exa magnetometer was employed to measure EB fields at different temperatures (after cooling in 50 kOe) and also the temperature dependence of the low field magnetization after field- and zero-field-cooling (standard FC and ZFC curves). One of the samples was subjected to moderate annealing for 30 min at progressively higher temperatures in a vacuum chamber where the pressure was kept below 10−6 mbar throughout the treatments, measuring the EB field after each treatment.
Results and discussion
There is also another characteristic temperature, signalled with the symbol TO, in the ZFC curve shown in Fig. 4. The magnetization is constant up to this temperature, but increases rapidly above it. This feature is interpreted as stemming from the disappearance of a unidirectional exchange coupling between the Co and CoO phases, which would pin the moment of the Co regions in their originally frozen directions (Normile et al. 2007), i.e. TO is the EB onset temperature.
The thermally induced aggregation of minute Co regions within the particles would significantly reduce the number of low-moment Co atoms at the Co–CoO interface, which together with the elimination of lattice defects, would render the increase in saturation magnetization manifest in Fig. 5. Thus, considering the above expression for HE, the annealing-induced intra-particle segregation process of an initial mixture of Co and CoO regions will reduce the EB field via a two-fold mechanism: (a) the increase of the effective thickness of the FM component (which yields a reduced FM–AFM interface density) and (b) the increased saturation magnetization, as commented above, of these larger FM regions. It is worth recalling that the EB field of a number of Co–CoO reactively sputtered systems has been observed before to start changing at annealing temperatures of 200–300 °C, including sputtered nano-composite films (De Toro et al. 2006) and Co–CoO particles embedded in a silver matrix (Riveiro et al. 2005; Normile et al. 2006). Thus, the nanoparticle geometry of the composite particles studied here does not seem to significantly affect the onset of the segregation process in comparison with continuous-film nanocomposite systems, suggesting that the relevant segregation length scale is smaller than the particle diameter. Note the possibility of particle coalescence was ruled out, as expected for the moderate annealing temperatures employed here, by the small increase in crystallite size, hardly 1 nm, detected by XRD (not shown).
In short, a method to synthesize metal-oxide composite nanoparticles is proposed: reactive gas-phase aggregation. The nanoparticles prepared using this method exhibit large EB fields, higher than those measured in a series of core-shell particles fabricated at different pressures. This method may also prove to be useful in the synthesis of pure CoO nanoparticles, as shown in Fig. 2.
We thank M. Rivera and E. Prado for their assistance in the synthesis of the samples, and acknowledge financial support from the JCCM (PAI08-0203-1207) and the CICYT (MAT 2006-08398). C. Binns and O. Crisan gratefully acknowledge support from the EC project NANOSPIN (contract number NMP4-CT-2004-013545).