In the case of Fe3O4 nanoparticles (8-10 nm), we often see that there are some maghemite layers on the surface. Now, if the nanoparticles have a magnetite phase at the core with very good stoichiometry, and a thin shell of maghemite (2-3 nm), can it show Verwey transition?
Just do resistivity measurements and check instead of speculating. Since you say that your sample has Fe2O3 at the surface then the stoichiometry of your inner Fe3O4 core may not be perfect. In that case you will observe either lowering of Vewey transition temperature or a complete supression of the Verwey transition.
Thank you very much sir for your answer. I did not do the resistivity measurement. But in FC/ZFC verwey transition is seen at 120-122K. But all samples do not show verwey transition. As you suggested earlier, I will do neutron diffraction. But that will take some times.
Dear Arijit, some samples showing signature of VT, others not. This could mean that the preparation conditions are not sufficiently under control. Do you know the fraction of particles participating in the VT transition in the samples where it is observed? If you were my student I'd give you the task to reach reproducible conditions for both cases (no VT, maximum fraction of VT). Then you can compare such specimens with several measurement methods and ultimately hope to find out what makes the difference. I'd consider that a nice achievement.
well, with 3 nm layer of maghemite in a 8 nm particle, you would have 2 nm core of magnetite ie 98 % maghemite, I would not expect any Verwey transition there... if you have even 2 nm layer in 10 nm particles, you still have 78 % maghemite, most probably not sufficient to get a Verwey...
I am wondering how did you get a definite proof that you have maghemite at the surface of such a fine nanoparticle. From my experience neither XPS or Mossbauer spectroscopy can say clearly on this aspect. They just suggest that you might have an intermediate situation between maghemite and maghemite and the most probable speculation is that magnetite is in the core. I have to point here that it has been observed that the Verwey transition decreases for lower size of nanoparticles (this was in the range of more than 30 nm, where the transition was observed). So even from this point, we might expect the lack of the transition in Nps of about 10 nm. Finally, Mossbauer spectra at very low temperature (4.2 K), even of broad lines (typical to distorted Fe positions-hence a well formed magnetite in the core si excluded-) remind rather to the situation of just tetrahedral and octahedral components, more close to the situation at RT, that is, above the Verwey transition. ZFC-FC measurements also do not prove this transition down to 2 K. In conclusion, on my opinion, such Fe oxide NPs can not (in principle) present the Verwey transition down to usually achievable low temperatures.
Victor, I think you have good point in there. Using x-ray absorption spectroscopy, you can achieve something concerning oxidation gradient towards the particle surface (e.g. DOI:10.1063/1.1759792, and the 1999 reference of Pellegrin et al. therein). I do not take "magnetite" and "maghemite" as truly stoichiometric statements in nanomaterials, usually.
Another possibility of Verwey supression might be the presence of some impurities in your samples. Did you check with XRD that its purely iron oxide (magnetite) and/or maghemite? Like Kai and Victor said, it is super difficult to say that particles has these two distinct phases.
I calculated the thickness using Mossbauer spectroscopy, as described in this paper http://pubs.acs.org/doi/abs/10.1021/cm801405n. Is this method gives a true measurement? In my question, the thickness I have written is 2 nm. But It is 1 nm. I am really sorry for the mistake.
In the attached paper the authors find no Verwey transition in particles below 50 nm in diameter, and the transition in the 50 nm particles is at 16 K. The have also done Mossbauer, TEM and XRD characterization of their magnetic nanoparticles. Due to the preparation and storage of the particles used in this publication, it is probable that they have oxidised surfaces (e.g. maghemite rather than magnetite). It is likely that your particles (8-10 nm) are far too small to show a Verwey transition if this is a similar case. It is incredibly difficult to prevent surface oxidization of magnetite nanoparticles when washing and drying them, even when this is done in an anoxic glove box. As you have not detailed how you have prepared your particles, or how they are washed or stored, it is difficult to determine what they are likely to be made of.
Article Static and Dynamic Magnetic Properties of Spherical Magnetit...
I guess that the question of Victor Kuncser is a key one. In bulk form, you can trace the fingerprnts of maghemite vs magnetite, but in NPs, partially oxidized, I cannot think of any single technique that can answer this question. You can always make a combination of XPS, Mossbauer and Neutron diffraction (in a sharp time window if you think that surface is not passivated but oxidation is progressing). At this point, depending on your final interest, your question should be: does my question actually worth the effort?
You know what would be really interesting is whether the lattice change from above to below VT occurs at all. Your saying magnetically it does not show it. But have you performed XRD at progressively lower temperatures? it would be interesting if the lattice change occured, but the resultant magnetic and charge trasport changes didn't. Here is something else. You could perform FC above Curie temps and observe two things. 1) the slope should be linear, look for a change in slope to occur when one Curie temp is reached and not the other. 2) From these slopes, you can determine the magnetic contribution per atom in bohr magneton. Then using rule of mixtures, work out the fraction with valences of 3+ and 2.5+.
Admittedly, no. 2 looks a little sketchy, but if the values fall within the range other methods are indicating then you another means to justify your conclusions.
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