A strong acid would usually do the job. I'd suggest sulfuric acid will certainly help you dissolve it, but the question is what are you planning to do with the solution? Are you just trying to remove the oxide layer on your samples?
I would suggest attack to the sample with sodium tetraborate (Na2B4O7) or borax (Na2B4O7•10H2O) by ~745 ºC (slightly above the melting point of the mixture of the flux with the sample), followed by attack of the cooled solid residue with concentrated aqueous HCl. The dissolution of the residue will be possibly complete. If not, as that may depend on the actual sample, it may be necessary to add more HCl and/or heat the acid solution with the undissolved residue while stirring. The spinel ferrite sample can be expected to dissolve accompanied by excess HCl, NaCl, and boric acid, which results from reaction of Na2B4O7 with the added HCl: Na2B4O7 + 2 HCl + 5H2O → 4 H3BO3 + 2 NaCl.
No i want to subject the material to measure the particle size using paritcle size analyzer. For that purpose i want to dissolve the ferrite.
Are your samples in powder or bulk form? I presume your samples are powders and what you are actually trying to do is disperse powder in liquid to do the particle size measurement.
In that case you need a solution that will hold up the dispersed particles in space i.e. form a colloid solution. Now, in order to create a colloid solution you would have to adjust the pH close to the value of the point of zero charge (PZC) of your ferrites, since colloid solution will only be stable around that pH point. Higher or lower pH than PZC will result in sedimentation of your solid phase from your solution or some chemical reaction, depending on the chemical properties and composition of your solution.
Depending on your ferrite powder PZC will be different and you will have a problem if you have a mixture of more phases. PZC for hematite (alpha-Fe2O3) is around 8.5 and for Fe3O4 it is 6.6, so one is more alkali and the other one more in acidic area.
So I would suggest to try with distilled water which has pH around 5.5. and then change the pH value by adding some alkali media to bring it up to the PZC point and then disperse your powder sample by means of ultrasonic bath in the solution and check again the pH of your solution and adjust again. This should give you stable colloid solution for your measurements.
I hope this will help you out.
You can use ethanol with ultrasound dispersion or use a tensoactive to stabilize your suspension.
You can use some surfactants Such as SDS,PDDA,PVP to stablize the collodial dispersion.
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