Comments pubs.acs.org/cm
Reply to Comment on “Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe2O4 Nanoparticles” he comment on our paper1 entitled “Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe2O4” by E. Skoropata et al. is based upon the mixing of the interpretation of results obtained on different materials by different authors. The most curious fact of this comment is the lack of a clear alternate comprehensive interpretation of all the data presented in our paper. We agree with Skoropata et al. that the X-ray spectrum cannot be interpreted solely on the basis of a CoFe2O4 spinel structure and the Scherrer broadening. Calibration spectra recorded under the same conditions and with a reference sample show that the fluorescence and instrumental broadenings of the peaks are below 0.2° in the entire experimental window and cannot be the origin of this discrepancy. The quality of the XRD spectra is sufficient, and the broad peaks we have measured are inherent in the sample and not due to experimental errors. Skoropata et al. argue that a combination of CoO and Feoxide can mimic the CoFe2O4 spectrum. We agree with that point, but the spectrum we reported in our paper is different from both that of bulk CoFe2O4 and the one corresponding to the mixture of CoO and Fe-oxide since in our spectra only two peaks are clearly seen. Moreover, the data commented on by Skoropata et al. suggest that the addition of the CoO in the Feoxide would shift the original peaks to higher angles, but the peaks in our sample are moved toward low angles (that we interpreted as larger interplanar distances). Thus, their simulation data disagrees with our experimental data, both of which rule out the mixture possibility. Skoropata et al. mention that our spectrum is quite similar to that of 2-line ferrohydrite. It is well know that ferrohydrite can be found in two forms, the six-line and the two-line spectra that have been interpreted as a consequence of the variation of the size of the coherent scattering domains of ultradispersed hematite in the range of about 2 nm. But we should have in mind that our particles contain both Fe and Co. Concerning the magnetic data, the authors of the Comment claim the possibility that our data may be explained by the formation of a superspin glass2 or superferromagnetic ordering3,4 due to dipolar interaction between the nanoparticles. This last possibility is ruled out by the authors of the Comment as it would require a highly ordered arrangement of nanoparticles. They also claim, as another possible alternate explanation, the presence of a noncollinear surface spin population on the nanoparticle. For this last case, they are citing various papers.5,6 They are also including the dependence of the coercive field of 3 nm nanoparticles of NiO as an example that some of the data we have observed has also been observed in pure antiferromagnetic particles.7 Moreover, they are also commenting on data for CoO nanoparticles showing a peak in the ac susceptibility at very low temperature as is seen in our case. In conclusion, Skoropata et al. are presenting a collection of data of many different materials showing for each of them one similar behavior to some of the data presented in
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© 2013 American Chemical Society
our paper. That is, our material should be understood, following this Comment, as a “supermaterial” having properties typical of all well-known nanoparticles independently of their crystallographic structure, magnetic ordering, etc. An important fact to remark about before we start with our rebuttal is to be aware that the authors of this comment do not consider all the magnetic data presented in our paper, which, in our opinion, should be the most important fact to be considered. It is clear for all people working in the field of nanomagnetic particles that the variety of observed phenomena is the consequence of the complexity of these systems. Our conclusions are based upon our own experimental observations and knowing most of the published papers in this field. As a summary of what we have said above when discussing the X-ray data, we interpret this sharp low temperature peak in the zero-field-cooled (ZFC) magnetization data and ac susceptibility data as evidence that something is drastically changing in some particles. Due to the small size of the particles and the existence of both kinds of “materials”, small and large particles, we were not able to get a direct observation of the structural differences between them. However, it should be mentioned that we have verified that the large particles in the case of other samples have the usual CoFe2O4 spinel structure. On the other hand, our suggestion is that the smallest particles may present a different crystallographic structure that reduces the Curie temperature of the material down to 10 K. In other words, there is strongly reduced superexchange interaction energy in the smallest particles as a consequence of the enlargement of the interplanar distance experimentally measured. This experimental fact is in agreement with the exponential dependence of the superexchange interaction on the interatomic distances. It is also reasonable to assume that certain modifications in the bonding angles may also contribute to reduce the Curie temperature. That is, small changes in the interatomic distance may produce dramatic changes in the Curie temperature. Consequently we do not agree with what is said in the Comment that about a 30-fold increase in the interatomic distance is needed to have the observed reduction in the Curie temperature. To even more support the conclusion of both the existence of two sets of CoFe2O4 nanoparticles centered at two different sizes and the magnetic phase transition in the case of small particles, we have introduced in this rebuttal the results of the thermal remnant magnetization (TRM) measurements on our sample (bimodal distribution), Figure 1. The TRM measurements were performed with the following procedure: first we apply a 50 kOe magnetic field at 300 K and then we fieldcooled the sample until 2 K. At this low temperature the field is switched off and the remnant magnetization is measured between 2 and 300 K. In the figure, we have also added the Received: March 4, 2013 Published: March 18, 2013 2001
dx.doi.org/10.1021/cm400724z | Chem. Mater. 2013, 25, 2001−2003
Chemistry of Materials
Comments
The possible existence of surface spin-glass freezing in our small nanoparticles should also be ruled out as the low temperature peak is still present even when the applied external field is 5 T. This also reinforces our suggestion that the most plausible interpretation for the low temperature peak corresponds to a magnetic phase transition. That is, we interpret the 10 K peak as the transition between the ferrimagnetic ordering to the paramagnetic disorder of the smallest CoFe2O4 particles. As we did mention above, this is in full agreement with the frequency independent behavior of the ac susceptibility. Below 10 K, the smallest particles are both ordered and blocked, most of them, and the magnetic relaxation is due to the net magnetic moment of the nanoparticles. Note also that the maxima in the ZFC and viscosity occur at different temperatures because of the different time scales of these two experiments. In Figure 8 of our paper,1 we show the magnetic hysteresis curves recorded at different temperatures. From these curves it is clear that the average anisotropy field of all particles at low temperatures is higher than 5 T, in agreement with the existence of high anisotropic CoFe2O4 nanoparticles, while the coercive field, even at the lowest temperature of 2 K, is only 0.3 T. This small value of the coercive field is the consequence that, even at this low temperature, there are still very small nanoparticles that behave superparamagnetically, having, consequently, zero magnetization at zero field. The Langevin temperature dependence of these very small superparamagnetic nanoparticles is deforming the entire magnetization curve, reducing, therefore, the value of the coercive field. It is also important to remark at this stage of the discussion that any attempt to fit the high temperature magnetization data M(H,T) above the blocking temperature (T > 100 K) of the large particles, assuming that all particles are superparamagnetic, fails. However, we have obtained a good fit of the magnetization curve at these high temperatures with the expression
Figure 1. TRM magnetization data for the bimodal sample: our sample (black points) and the reference unimodal sample (red points).
TRM magnetization data for a CoFe2O4 sample having a single distribution of sizes and a very similar blocking temperature. This unimodal size distribution sample does not show the peak at low temperature. The data for the bimodal sample, our sample, could agree well with two possible interpretations: (a) the existence of two blocking temperatures corresponding to each size of particles or (b) the existence of a blocking temperature for the large particles at high temperature and the loss of magnetic ordering of the small ones. The first interpretation does not fit with the rest of our magnetic data. To go deeper in this question, we have also measured the ac susceptibility for the unimodal size distribution in order to compare the data of both samples. In Figure 2 we show the real
M (H , T ) =
∫0
∞
⎛ μH ⎞ L⎜ ⎟f (μ) dμ ⎝ kBT ⎠
when assuming that the large particles are superparamagnetic and that there is a paramagnetic signal due to the smallest particles. In this case we have determined that the value of the magnetic moment of the “paramagnetic” smallest particles is about 2 ± 1 Bohr magnetons. This number is about 2 orders of magnitude smaller than the value expected for 2−3 nm superparamagnetic CoFe2O4 particles. Our interpretation, therefore, agrees well with this result: the magnetization of the smallest particles at 100 K is not due to a coherent rotation of all spins in the particles (superparamagnetism) but to the paramagnetic signal of individual atomic spins. That is, at temperatures higher than 10 K (the low temperature peak), the smallest particles behave paramagnetically, not superparamagnetically. The peak at low temperature is almost frequency independent, and fitting it to an Arrhenius law produces an unphysical value for the attempt time, τ0, of about 10−20 s. This value of the attempt time is totally unrealistic. There are also some papers commenting that CoFe2O4 nanoparticles larger than 5 nm show a tendency toward cubic anisotropy. On the other hand, particles with diameter lower than 5 nm show uniaxial anisotropy.8,9 Therefore it is reasonable to think that for samples with bimodal distribution centered at 3 and 6 nm, a coexistence of uniaxial and cubic
Figure 2. Temperature dependent real component of the ac susceptibility for both bimodal (solid squares) and unimodal (open circles) size distributions at different frequencies.
component of the ac susceptibility for both, bimodal and the unimodal distributions. This figure shows clearly that the effect of the blocking of the large particles does not avoid the detection of the contribution of the small particles. This, therefore, is another proof that the low temperature peak is fully related to the existence of the very small particles, which are blocked just below the low temperature peak. Neither the appearance of a superspin-glass state nor of superferromagnetic ordering, due to dipolar interactions between particles, nor the existence of surface effects may be the reason for the observation of the low temperature peak. If this were the case, the low temperature peak should also be observed for the unimodal size distribution of particles, and this is not the case. 2002
dx.doi.org/10.1021/cm400724z | Chem. Mater. 2013, 25, 2001−2003
Chemistry of Materials
Comments
anisotropy should be present. We think, however, that this change in the type of anisotropy cannot explain the presence of such a narrow (and nearly field and frequency independent) peak in the low temperature regime, despite the broadness of the size distribution. In conclusion, it would be nice to perform more studies, using other techniques, to achieve a full elucidation of the crystallographic structure of the small particles presented in our paper. However, our interpretation is the most plausible, taking into account both the XRD characterization and all the magnetic data.
Javier Tejada*,† Joan Manel Hernàndez† Victor López-Domínguez† Ronald F. Ziolo‡ †
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Dept. de Física Fonamental, Universitat de Barcelona, C. Martí i Franqués 1, Barcelona 08028, Spain ‡ Centro de Investigación en Química Aplicada, Boulevard Enrique Reyna 140, Saltillo, 25253 México.
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Lopez-Dominguez, V.; Hernàndez, J. M.; Tejada, J.; Ziolo, R. F. Chem. Mater. 2013, 25, 6. (2) Southern, P.; Robinson, A. P.; Kasyutich, O. I.; Warne, B.; Bewick, A.; Schwarzacher, W. J. Phys.: Condens. Matter 2007, 19, 456204. (3) Imry, Y.; Ma, S.-K. Phys. Rev. Lett. 1975, 35, 1399. (4) Binns, C.; Howes, P. B.; Baker, S. H.; Marchetto, H.; Potenza, A.; Steadman, P.; Dhesi, S. S.; Roy, M.; Everard, M. J.; Rushforth, A. J. Phys.: Condens. Matter 2008, 20, 055213. (5) Haneda, K.; Morrish, A. H. J. Appl. Phys. 1988, 63, 4258. (6) Peddis, D.; Yaacoub, N.; Ferretti, M.; Martinelli, A.; Piccaluga, G.; Musinu, A.; Cannas, C.; Navarra, G.; Grenèche y, J. M.; Fiorani, D. J. Phys.: Condens. Matter 2011, 23, 426004. (7) Winkler, E.; Zysler, R. D.; Vasquez Mansilla, M.; Fiorani, D.; Rinaldi, D.; Vasilakaki, M.; Trohidou, K. N. Nanotechnology 2008, 19, 185702. (8) Peddis, D.; Orrù, F.; Ardu, A.; Cannas, C.; Musinu, A.; Piccaluga, G. Chem. Mater. 2012, 24, 1062−1071. (9) Moumen, N.; Bonville, P.; Pileni, M. P. J. Phys. Chem. 1996, 100, 14410.
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