Comment on “Colossal Reduction in Curie Temperature Due to Finite

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Comment on “Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe2O4 Nanoparticles”

T

extrapolated from lower temperature magnetism as a result of phase and structure changes as TC is approached (typical for the ferrites). SI Figure 5 shows that disproportionation of the CoFe2O4 phase occurs upon heating the nanoparticles above TC and subsequent cooling. The magnetic properties of CoFe2O4 are highly dependent on the stoichiometry and degree of inversion13−16 that can be affected by factors such as the synthesis method and thermal history.17,18 Consequently, a detailed characterization of the structure and composition is essential to understand the magnetism of the sample fully. While the authors acknowledge the effect of the strength of the exchange interaction on TC, they consider only the influence of the interatomic distance (via d311 measured from TEM). The degree of inversion has a significant effect on TC since the superexchange interactions between tetrahedral Fe3+ and octahedral Co2+ are weaker than tetrahedral (A-site) Fe3+ and octahedral (B-site) Fe3+, and JAB interactions are the dominant exchange pathway.17 We have also examined the implications of such a substantial reduction in TC reported by Lopez-Dominguez et al. To produce a TC of 8.5 K would require an exchange interaction strength of the order J ∼ 10−24 J. Such a reduced exchange interaction would require (i) a significant (likely unphysical) reduction in the magnetic ion density, (ii) a weaker Fe/Co magnetic moment, or (iii) an approximately 30-fold increase in the interatomic spacing that is inconsistent with the d-spacing observed by the authors. A possible interpretation for the magnetism observed by Lopez-Dominguez et al. is the formation of a superspin-glass state or superferromagnetic ordering due to dipolar interparticle interactions.19 The aging and critical slowing down associated with superspin glasses are a measure of the energy barrier distribution to magnetization reversal and are characteristics that are observed by nanoparticle systems as well as crystallites that experience both dipolar and exchange interactions that may result in a superspin glass.20 To achieve a superferromagnetic state would require a highly ordered arrangement of nanoparticles,21,22 which is not the case for Lopez-Dominguez et al. In general, it is problematic to characterize the effects of interparticle interactions when polydispersity is a factor, and to identify either superspin glass or superferromagnetic magnetism requires a detailed characterization of the nanoparticle spacial configuration (e.g., separation and dilution experiments). An alternate explanation, supported comprehensively by experiment and theory, of the magnetism reported by LopezDominguez et al.1 is the presence of a noncollinear surface spin population on the nanoparticle. Numerous examples can be found in the literature, most notably in ferrite-based nano-

he recent publication in this journal by Lopez-Dominguez et al.1 describing a size effect on the magnetic ordering temperature of CoFe2O4 nanoparticles prompts us to comment on their observations in light of some of our earlier published work on ferrite-based nanoparticles.2−4 In the first place, we would like remark on the challenges that must be addressed properly to ascertain the structure and composition of CoFe2O4 nanoparticles. Second, we introduce some alternative explanations of the magnetism reported. The X-ray diffraction (XRD) pattern1 using a Cu Kα source presented asymmetric reflections that were attributed to bulk CoFe2O4, assuming that Scherrer broadening of the pattern was solely present. However, fluorescence and instrumental broadening would affect directly the width and shape of the reflections. In addition, any excess surfactant would present a background due to incoherent scattering. A “pattern match” to bulk CoFe2O4 led the authors to determine that their reflections were shifted to lower diffraction anglesindicating a lattice constant, a, that would be larger than expected. The lattice parameter associated with a d311 of 2.49 Å obtained by Lopez-Dominguez et al. is 8.258 Å which is, for a nanoparticle, reasonably close to the bulk value of 8.38 Å; however, the lattice parameter required to obtain a d311 of 3.0 Å reported by Lopez-Dominguez et al. is 9.95 Å (assuming the structure remains cubic), a value inconsistent with a spinel oxide.5 Although a slightly larger lattice parameter has been reported for ferrite nanoparticles relative to the bulk counterpart, an increase of ∼20% appears to be substantially larger than what has been reported for ferrite nanoparticles of similar size6−9 to those of Lopez-Dominguez et al. Consider the XRD pattern of ∼5 nm diameter CoFe2O4 nanoparticles presented in the Supporting Information (SI) (Figure 1) that is quite different from that reported. Additionally, a simulated X-ray diffraction pattern (SI Figures 2 and 3) using the information provided by Lopez-Dominguez et al. is inconsistent with the reported diffraction pattern. Indeed, the structural and compositional characterization presented by the authors is not unique to CoFe2O4 nanoparticles. Relying on XRD to ascertain the structure and composition of a ferrite-based nanoparticle system is problematic (SI Figure 4) since the patterns of mixed phase CoO and Fe-oxide are very similar to that of CoFe2O4. The pattern shown by Lopez-Dominguez et al. is unfortunately also quite similar to that of 2-line ferrihydrite.10,11 High quality XRD patterns (free of instrumental effects) must be refined to ensure confidence in any analysis. Furthermore, complementary experimental techniques such as transmission Mössbauer spectroscopy or X-ray absorption spectroscopy should be adopted for successful phase and compositional analysis of these types of nanoscale materials. For example, consider the results of Mössbauer spectroscopy at temperatures from 5 to 800 K3,4 that permit the unequivocal identification of CoFe2O4 nanoparticles and highlight the challenge of properly ascertaining a Curie temperature (TC). In truth, the TC of CoFe2O4 is still an open question12 since its value must be © 2013 American Chemical Society

Received: December 3, 2012 Revised: February 7, 2013 Published: March 18, 2013 1998

dx.doi.org/10.1021/cm303893h | Chem. Mater. 2013, 25, 1998−2000

Chemistry of Materials

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particles. For example, Haneda and Morrish23 observed that a noncollinear structure exists within CoFe2O4 particles that is most pronounced for smaller particles. Peddis et al.24 examined the effect of cation distribution on the surface spin canting observed in CoFe2O4 nanoparticles to explain the observed reduction of the saturation magnetization (MS) in a nanoparticle sample relative to bulk. Studies of other nanoscale ferrites have shown clear evidence that a spin-glass-like layer forms at the nanoparticle surface due to broken exchange bonds and that the surface spin layer affects the nanoparticle anisotropy, decreases the saturation magnetization, and results in an enhancement of the coercivity at low temperatures.2,25−27 Tung et al.6 observed a possible reduction of TC in 3.3 nm CoFe2O4 nanoparticles, but only to 668 K (using MS(T)). No other similar “colossal” reduction of the ordering temperature has been reported for like-sized CoFe2O4 nanoparticles. Surface spin-glass freezing has been observed in 3 nm CoFe2O4 nanoparticles by Peddis et al.,28 as characterized by the variation of MS with temperature as related to the degree of surface spin canting. Nearly identical magnetism to what Lopez-Dominguez et al. reported was observed by Winkler et al.29 in 3 nm NiO nanoparticles. Similar AC and DC susceptibility was reported that was weakly field and frequency dependent, and a non-saturating component in the hysteresis loop measurements was also identified from disordered surface spins (also observed in NiFe2O4 nanoparticles25). In addition, Kodama et al. have shown that a model of the nanoparticle magnetism that includes broken exchange bonds that result in a disordered surface layer can be used to reproduce the open hysteresis loop, which is a feature also observed by LopezDominguez et al. Finally, the temperature dependence of the coercivity (HC) reported by Lopez-Dominguez et al. matches exactly that observed by Winkler et al.29 and was described by different nanomagnetism from core and surface spin populations; the observed minimum in HC(T) was also reproduced by Monte Carlo simulations of core/surface spins. In fact, attributing the largest HC(T) to the smallest nanoparticles at temperatures below 10 K, as described by Lopez-Dominguez et al., is not entirely self-consistent. The behaviour of the coercivity as a function of temperature is a consequence of the competition between the core and the surface anisotropies which appear below the maximum near T ∼ 30 K. However, once the surface spins begin to order, these anisotropies reinforce each other as evidenced by the minimum near T ∼ 10 K. Indeed, the HC(T) the authors report is more consistent with magnetic “hardening” through decoupling of a two-phased nanoscale ferromagnet system, with competing magnetocrystalline anisotropies.30 That is, the maximum of HC(T) observed around 30 K is likely from a surface anisotropy being counter-aligned to that of the core and diminishing with warming from 2 K. Since surface spin disorder is due to broken coordination at the nanoparticle surface, some consideration of the surfactant effects should be made (e.g., an oleic acid surfactant is used by Lopez-Dominguez et al.), although conflicting results regarding surfactant effects have been reported.31−34 While the addition of covalently bonded surfactant molecules has been shown to decrease the surface disorder, the symmetry and strength of the exchange interactions in the nanoparticle core are not reproduced at the surface and distinct spin populations can persist. Several examples of nanoparticles with surfactant coatings (including oleic acid) have shown surface spin-glass-

like behavior25,29,35 demonstrating that the surfactant does not necessarily eliminate surface spin disorder. We show χAC(ν,T) on 5 nm diameter CoFe2O4 nanoparticles with an oleic acid surfactant coating in SI Figure 6, where the low temperature frequency (ν) shift of the χAC ′ peak temperature (Tf), i.e., ΔTf/TfΔ(ln 2πν) ∼ 0.01(2), is a value consistent with spin−glass−like behavior.36 The blocked-tosuperparamagnetism behavior presented by the higher temperature χAC(ν,T) can be well described by Néel-type magnetic relaxation and indicates that the CoFe2O4 nanoparticles have a magnetocrystalline anisotropy K ∼ 7 × 106 erg/cm3. In addition, we present χAC ′ (ν,T) and HC(T) for CoO and NiO nanoparticles in the Supporting Information. Both are singlephase antiferromagnets where the measured magnetism is from noncollinear surface spins entirely. For the CoO nanoparticles, measurements with varying interparticle spacing were done in which the spin-glass-like behavior was found to persist. Both nanoparticle systems present similar features in their overall magnetism as reported by Lopez-Dominguez et al. The challenges in determining the structure and composition of CoFe2O4 nanoparticles using XRD pattern matching alone have been discussed. To ensure quality in the analysis, an XRD pattern free of a large amorphous background due to incoherent scattering of an organic surfactant and instrumental effects must be refined. The effects of surface disorder are significant and become more prominent as the particle size is reduced. It is clear by comparison with the literature that the magnetism observed is more consistent with surface spin-glasslike behavior than magnetic ordering.

E. Skoropata R. D. Desautels B. W. Southern J. van Lierop*

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Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, X-ray diffraction patterns and simulations, and magnetic characterization data. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.

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REFERENCES

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