u can use Fe 2+ and Fe 3+ salts in 2:1 ratio and followed by addition of NH4OH  fine magnetic particlees can be obtained 

Can you provide me the detail of the process or any reference?

Google is your friend:

http://nano.iphy.ac.cn/n04/papers/NO4_papers%20all%20pdf/HJ_Gao.0826.pdf

http://file.scirp.org/pdf/JWARP_2015013015444805.pdf

Remember that once they're in powder form, they are no longer nano (< 100 nm) - at least there are no free, independent particles of this size.  A powder is a collection of fused sub- and post-micron aggregates.  See:

Nov 11th, 2008. Dispersion and nanotechnology

http://www.brainshark.com/brainshark/vu/view.asp?pi=106756756&text=M021507

http://tinyurl.com/hpywsge

According to my knowledge, functionalized nanoparticles can maintain their size and shape..

In suspension, yes.  In a powder, no.

How to keep the nanoparticles in suspension? which liquid is used for this purpose?

If the particles are 10 - 20 nm they'll keep in suspension in water through Brownian motion.  If larger they'll settle quickly. Carry out a Stokes' Law calculation...or take a look at ASTM E2490-09 (2015) where such a table is displayed. If you have settling in the system then the particles are in the micron not nano size range.

Yes, that's the thing which I required.. Thanks you very much..