The word ferrofluid was coined by the Ferrofluidics Corporation, a company formed by Dr. Rosensweig who first formulated Ferrohydrodynamics and demonstrated certain novel phenomena exhibited by magnetic fluids composed of single domain ferro or ferri-magnetic particles colloidally dispersed in a liquid carrier. Each single domain particle was coated by a short chain surfactant layer. Since the particles possess a large magnetic moment and are free to rotate in surrounding liquid its magnetization was explained on the basis of Langevin theory of paramagnetism and fluid was also called superparamagnetic fluid. For several years in order to avoid legal problems word magnetic fluids was used. After expiry of patent rights, the word ferrofluid is used as a synonym for magnetic fluids or nanomagnetic fluid or superparamagnetic fluid.

In last decade, superparamagnetic magnetic beads composed of iron oxide nanomagnetic particles uniformly distributed in nonmagnetic matrix like polystyrene or silica are synthesized (SPION). These beads are promising candidate for several biomedical and biotechnological applications like cancer treatment, drug delivery, cell separation etc. Dispersion of such particles in a liquid is also called superparamagnetic fluid.

Thus, a ferrofluid may be called superparamagnetic fluid but vie a versa is not true.

It may be remarked here that originally word superparamagnetism was first used by Bean to explain magnetic aftereffect proposed by Neel. This is due to coherent rotation of spins .To observe this effect the particle should have a critical volume , v0Ku ~ kT/2, with Ku =105 J/m3. In polydispersed ferrofluids up to certain size superparamagnetic nature due to Brownian rotation will be dominant and relaxation time for this rotation is large compared with the Neel relaxation. After freezing the liquid the later may play role in magnetization. Several papers are available in open literature (e.g. Phys. Rev.B, 68,224434,2003 ).

A ferrofluid is a colloidal suspension made of ferromagnetic or ferrimagnetic particles in water or an organic solvent. A superparamagnetic fluid is similar, but it is made of superparamagnetic nanoparticles. The main difference is tha magnetic behavior of the particles. Although the magnetization of both types of fluids may be of the same order, superparamagnetic materials have no hysteresis loops, which is especially important for biomedical applications of these fluids.

Dr Raquel i so much appreciate your answer. i have found researchers using the word ferrofluid most often in place of superparamagnetic fluid.

Raquel, how sharp could one possibly make a distinction along the lines which you depict?

To me, superparamagnetism in a narrow sense applies to magnetic moments fluctuating with respect to the atomic lattice. On the long run and without external field, that results in zero average magnetization. A polarized ensemble of (identical) particles will nevertheless possess a characteristic timescale for relaxing towars the zero average magnetization. Whether or not hysteresis is observed therefore depends on the characteristic timescales of an experiment. (As an example: you will find particles that look perfectly SP in a SQUID M(H) loop but appear blocked under the same conditions in a Mößbauer experiment.) This is Néel-Brownian relaxation.

With Particles in a dispersion, Brownian motion adds to the game. Therefore, also a moment "perfectly locked" to the particle atomic lattice will fluctuate. Again, in the absence of external fields, the ensemble average magnetization will relax to zero for independent particles (relaxation by Brownian motion).

I could imagine that different regimes of behavior have much to do with the relative time scales of the two mechanisms. Trying to translate your arguments to fit here, I'd say that your definition of a ferrofluid applies, when NB-relaxation is very slow compared to other relecant time scales. Would you agree?

In many fluids, the interaction between particles is important. My guess would be, that this the more so, the slower NB relaxation. With particles in sufficiently close proximity, their mutual interactions may actually modify the NB relaxation dynamics, one can then speak of collective blocking. Yet another complication.

So again, what is the defining border between a ferrofluid and a superparamagnetic fluid?

All that you have said is correct. Ferrofluid and superparamagnetic fluid are used as synonyms in most publications and it is very difficult to draw a line between both terms. For me, it is just a matter of particle size, stability of the fluid, and magnetic behavior in conditions of negligible dipolar interactions.

I think that the arguments given and also the distinction between SP particles and FM particles are rather clear-cut in many cases. I just wanted to emphasize that (i) the category of superparamagnetism is not universal but depends on the relevant timescale of observations and (ii) that the dynamic behavior of particles may change qualitatively (SP -> collective blocking) when interactions come into play.

So, there certainly is a fair number of clear-cut cases, nevertheless it is not easy to establish precise terminology unless one introduces it by definition.

The word ferrofluid was coined by the Ferrofluidics Corporation, a company formed by Dr. Rosensweig who first formulated Ferrohydrodynamics and demonstrated certain novel phenomena exhibited by magnetic fluids composed of single domain ferro or ferri-magnetic particles colloidally dispersed in a liquid carrier. Each single domain particle was coated by a short chain surfactant layer. Since the particles possess a large magnetic moment and are free to rotate in surrounding liquid its magnetization was explained on the basis of Langevin theory of paramagnetism and fluid was also called superparamagnetic fluid. For several years in order to avoid legal problems word magnetic fluids was used. After expiry of patent rights, the word ferrofluid is used as a synonym for magnetic fluids or nanomagnetic fluid or superparamagnetic fluid.

In last decade, superparamagnetic magnetic beads composed of iron oxide nanomagnetic particles uniformly distributed in nonmagnetic matrix like polystyrene or silica are synthesized (SPION). These beads are promising candidate for several biomedical and biotechnological applications like cancer treatment, drug delivery, cell separation etc. Dispersion of such particles in a liquid is also called superparamagnetic fluid.

Thus, a ferrofluid may be called superparamagnetic fluid but vie a versa is not true.

It may be remarked here that originally word superparamagnetism was first used by Bean to explain magnetic aftereffect proposed by Neel. This is due to coherent rotation of spins .To observe this effect the particle should have a critical volume , v0Ku ~ kT/2, with Ku =105 J/m3. In polydispersed ferrofluids up to certain size superparamagnetic nature due to Brownian rotation will be dominant and relaxation time for this rotation is large compared with the Neel relaxation. After freezing the liquid the later may play role in magnetization. Several papers are available in open literature (e.g. Phys. Rev.B, 68,224434,2003 ).