I got coercivity 101 Oe for manganese ferrite, but I also got a central doublet in mossbauer spectrum. What I will say that, my sample is superparamagnetic or paramagnetic material. What is the explanation behind this phenomenon. Sample's crystallite size is around 44 nm. After chitosan coating, the size becomes almost 50 nm, whose hydrodynamic diameter is 177 nm.
I think the criteria whether the particles are superpara or not is M-T curve and specifying the Tb. The low coercivity is just a good indicator and evidence but there is no a certain value. So it is possible for particles to be suoerpara at coercivity 101 Oe. Besides that, you must check your particle size distribution, because you may some particle sizes lower than critical ones.
Sadeq H. Lafta many many thanks for your response. Can you tell me the physical reason behind this. Why I can labeled this material as superparamagnetic material though it possess high coercivity. Currently, due to some lab limitation, I'm not able to measure temperature dependent magnetization. How can I strongly establish this manganese ferrite as superparamagnetic material.
Before everything, you must do particle size distribution, then one can how population of each size range. The physical reason behind this , as I said, belong to particle size distribution, besides, I think, 101 Oe can not be considered as a high value. There are some indications refer to supepara state is formed, the best one the FC-ZFC curves (i.e. M-T measurements). The 2nd indication is low Hc value. the 3d one is M-H curve, where usually superpara curve suffering from unsaturation state (.i.e. low tilt in the curve with no clear saturation) that is happened when measuring temperature is higher than TB. The 4th indication is particle size, where if the particle size is lower than certain critical size (one domain size), this size is different from material to another and it is composition dependent, it can be found in literature.
Two points should be checked; the presence of just single phase (XRD) and the presence of any degree of saturation in the M-H curve (VSM).
Adel Maher Wahba thanks for your suggestion. Yes there are single phase in XRD and clear saturation is present in the M-H curve. But Mossbauer spectrum shows central doublet.
I think your sample could be safely considered as a superparamagnetic one. The observed coercivity is simply due to the size value as compared to the critical one.
Dear Shamima Nasrin,
You must consider that the blocking temperature (that is, the temperature at which you 'see' the blocking/unblocking transition) depends on the measuring time of your technique. Roughly you have a relation for the blocking temperature Tb with the magnetic anisotropy K and the volume of your particle V that is
Tb=K*V/kB*ln(tau_m/tau_0)
where tau_m is your measuring time and tau_0 =10^-9 s (approx).
Therefore if your measuring time tau_m=100 s (an approximation for magnetization measurements) you will get Tb=K*V/25*kB.
But when you use Mossbauer spectroscopy the measuring time is much shorter Tau_m=10^-8 s, and now you will find that the blocking temperature as seen from Mossbauer data is Tb=K*V/2*kB.
Hope it helps.
Dear Shamima Nasrin,
I agree with professor Goya. His response is very complete and comprehensive.
I agree with professor Goya, and also I add some possible reasons such high coercivity in systems might be due to (i) Size distribution of the nanoparticles, (ii) Some particles's size is may be larger than the SPM critical size, (iii) The effect of inter-particle interactions. However one can do a detailed analysis of (a) memory effect, and/or (b) ACS studies to confirm the SPM behavior in the system.
for MnFe2O4 the size 44 nm is higher than SPM limit. and the obtained 101 Oe coercivity shows that the nanoparticles are not in SPM region and even possibility of multidomain particles in the powders. But if there are particles with different size(polydisperse), use TEM to find this, you may hope to have some SPM particles at RT. Also you should not that the relaxation time will affect the SPM property and the blocking temperature. you can find this in the above answers.
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