Why should a much simpler model - small TM clusters in a ZnO matrix - be eliminated to explain the origin of ferromagnetism in these materials?

Dear Nikolay.....

I could not understand what u want to say. TM clusters if available outside that I can detect. But inside ZnO Matrix, its not so easy to detect. Can u answer my quires bit elaborately?

Thank you...

Clusters would make life complicated and uninteresting. Not only clusters come in different sizes, but the coupling among clusters will not be uniquely ferromagnetic. One can also never saturate such entities magnetically. There will always be an approach to saturation phenomenon which can best be explained in terms of what once we called 'micto-magnetism' championed by Prof. Paul Beck at Illinois.i.e, increasing magnetic saturation at increasing external fields because more and more of the clusters with a distribution of sizes and therefore magnetic moments will fall in line with the external field in a field dependent manner. This is true for Co doped alloys. In the case of Mn with increasing Mn content one should end up with mixed interactions off AF and FM making possible a spin-glass behaviour, In an oxygen atmosphere things become more complicated with spin polarization at the cation defect sites which gives rise to d(o) magnetism in addition to the 3d-Mn related one.

What we can wok with is Mn or Co substituting for Zn in ZnO or Mg in MgO that allows band gap engineering and thus make possible interesting magneto-optic properties and the consequent tunable applications.

To confirm superparamagnetism from M-H curves, you should be able to data collapse all the loops above the blocking temperature into a universal Langevin type function. Ideally, there should be no magnetic coercivity.

Maagnetism of nanoparticles are best studied using SQUID because the moment is small, and what you need is high resolution. One should be aware the SQUID data can be prone to artifacts. However, VSM data is more accurate if we have a larger volume to obtain sufficient moment. But unfortunately monosized nanoparticles are almost impossible, and the size distribution will be wider in larger volumes of the material again resulting in approach to saturation effects, also with a coercivity spread depending on the sizes involved. Don't forget that smaller the particle larger the disorder at the particle surface, which means lower effective moment accounted for by the spin disorder at the particle surface. To complicate matters the spin-disorder at the surface is particle size and size distribution dependent.

Multifeeroics above room temperature is not yet proven.

Are you talking to yourself? What is this statement (not true at all!) on multiferroics to do with dilute magnetic semiconductors?

You can use Electron Spin Resonance *ESR( technique to explain the ferromagnetism in MZnO DMS materials.

Dear Bappaditya Pal,

You want an answer to a question that has no clear answer. So we can offer a variety of experiments that allow only a certain extent to shed light on the nature of room-temperature ferromagnetism in TM doped ZnO.

Good luck,

Yuriy Gnatenko

Dear K V Rao,

I understood your point of view. Even I am not interested in clusters, because the kind of sample that I am trying to synthesize requires development of intrinsic origin of ferromagnetism, means from the doped semiconductor matrix itself, if possible without any TM related secondary phases. Basically I am trying the substitution of TM in ZnO/Tio2 nanostructures. As you said It will also have the spin disorder at the particle surface, the surface-to volume ratio in nanoparticles will have its effects, uncompensated surface spins might play a crucial role on the saturation magnetization. So can you tell me possible orgin RT FM in Co doped ZnO nanoparticles or nanorods??

Thanks..

Bappaditya

Dear Yuriy Gnatenko,

I can understand your notion of reply. I would be pleased if you can provide me some details on the nature of room-temperature ferromagnetism in TM doped ZnO, as you are a lot more experienced person in this field. still I am mentioning few lines as I know a little.

For the general description of FM in DMSs various physical models exist. The first proposed model of ferromagnetism was the Zener model (Phys. Rev., 1951, 81, 440–444; 83, 299-301) driven by the exchange interactions between carriers and localized spins (s–d interaction). Later Dietl et al.( Science, 2000, 287, 1019–1022.) used the Zener description to realize possible origin of RT FM in ZnO and GaN based DMSs as hole-mediated exchange interactions. Based on ab initio calculations K Sato (Jpn. J. Appl. Phys., 2001, 40, L334–L336.) shows that the ferromagnetic state is stabilized by additional electron doping in the case of Fe, Co and Ni doped ZnO. Coey model (Nat. Mater., 2005, 4, 173–179.) deals with shallow donor impurity bands. The sources of shallow donor in ZnO are defects which provide n-type conduction like oxygen vacancies and/or Zn interstitials. Also there is special consideration on account of the nanoparticulate nature of Co-doped ZnO. The surface-to volume ratio in nanoparticles is high in comparison to the corresponding bulk material, and therefore the surface effects are more pronounced with decreasing particle size. Sanchez et al. investigated the local magnetic order at the polar (0001) oriented surface of ZnO.50 using first principles calculations. They showed that the influence of uncompensated surface spins plays a crucial role on the saturation magnetization

So the main common feature of these contributions is the importance of an exchange interaction between the charge carriers introduced in the ZnO host through specific point defects or by additional doping and magnetic cations.

Now my question is that suppose I have synthesized various TM (Co,Ni, Fe etc.) doped ZnO Nanostructures (NPs, NWs, NRd) of different shape and sizes, using different route and they are showing RT FM with Curie temperature (Tc) far above room temperature. Some with very low saturation moment and some with high moment for the different synthesis conditions. Now on what kind of experimental basis I can support some of the above models?? How to link some of the experimental findings with above models?? Please provide some one to one correspondence to support ‘a model for some experimental findings’. I want a kind of general answer for these particular nanostructures cases. So can anybody help me with bit detailed explanation??

Thanks

Bappaditya Pal

Bappaditya: Look at the recent article of Dietl and coworkers in arXiv:1308.4517

The above article does not discuss Co,Ni doped ZnO but discusses mainly Mn doped GaN. But you may still get some idea there.

Dear Tapan Sir.

Thanks a lot.

Bappaditya: You have chosen a tough problem of nanostructed DMS. You have to be very careful and first experimentally understand which preparation method produces what type of nanoparticles (size, dispersion, Co/Ni concentration). They you have to understand what determines your moment and Tc. Not an easy job. Measure all these parameters very carefully and try to correlate them first purely experimentally. All existing theories have a lot of loopholes and do not be misguided by them. First do careful experiments with varying conditions and repeat them to check the reproducibility of the results. You should only try to correlate only if you have reproducible results. Theoretical understanding of your experimental results should come at last.

And also do not publish (even if you are pressurized to do so) unless you can reproduce your results. There are already many published papers which cannot be reproduced!

Dear Tapan Sir,

Actually I follow pretty much same systematic process and analysis to produce good quality DMSs materials, as you prescribed. I execute all the possible measurements like structural(XRD, Raman, TEM), optical, magnetic properties and try to find possible good correlation between them. Ofcourse I try to make sure that FM is not appearing from secondary phases, outside ZnO matrix. But I have some limitation with the availability of more specific instruments, so in that sense sometimes I used to face kind of incomplete characterization. Still I try to optimize with the availability. Then finally I try to observe changes on the moment value with the variation of present defects(studied from optical measurements) in the as grown materials. As many times carriers contributed from the defect sources have important roles in the FM interaction. So in that way I try to correlate with some physical model as well as intrinsic nature of FM can be established. I have published few data earlier, right now I am trying following different synthesis route.

J. Appl. Phys. 108, 084322 (2010); J. Nanosci. Nanotechnol. 11, 9215 (2011)

(http://www.iitg.ac.in/physics/fac/giri/peer_j/peer_j.html)

Thanks

Bappaditya

Maybe this paper comes in handy for you:

Advanced spectroscopic synchrotron techniques to unravel the intrinsic properties of dilute magnetic oxides: the case of Co:ZnO

A Ney et al 2010 New J. Phys. 12 013020

Link

http://iopscience.iop.org/1367-2630/12/1/013020