Is it possible to measure magnetic properties of nanoparticles in a liquid medium as a suspension by VSM? I think if the magnetic properties of nanoparticles will be depended on the individual nanoparticle's interactions, so which we study in powder form could differ from what happens in the suspension form?
The main difference originates from the fact that superparamagnetic particles in a powder don't move. Only the magnetizaation can be changed while the particle is stationary. In a liquid you can rotate the particle physically and change the magnetization this way as well. All this has been studied in detail. In the original Neel Brown theory it has already been considered what happens if the particles can move and reverse the magnetic moment through simple motion.
To perform such type of measurement you have to encapsulate carefully the sample in order to (i) avoid contamination with liquid of the measurement chamber and (ii) to don't have air inside of the capsule because the sample will not follow accurately the vibrator movement. The last issue can introduce noise in the measured signal.
In addition, it is possible to have a strong diamagnetic contribution because of the liquid. This contribution can be subtracted easily.
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I agree with Prof.Christian's statement "In a liquid you can rotate the particle physically and change the magnetization" and the behavior is well explained by Neel relaxation theory , also to attain stable suspension of nanoprticles , suitable surfactant should be used to avoid agglomeration of nanoparticles in fluid . Only during such case each nanoparticle will experience overall magnetic field , else the magnetization will be the effect of bulk / agglomerated nano spheres , where the overall particle size will be increased and reduce the effect of magnetic field. so it is essential to use a proper surfactant into nanoparticles before magnetic measurements .
Also , during VSM measurements , the exciter moves the sample up and down at a given frequency [PFA for reference] , in such case , there wont be much disturbance in solid samples [ powder/pellet] , but in case of liquid samples the movement will have an effect over the naoparticles suspended in the liquid and may cause error in output data , u can chk this link http://www.microsense.net/products-vsm.htm , here they have mentioned that the instrument can measure liquid samples , i think this is possible only if it was in pure liquid form rather than particles suspended in liquid . Hence , in if we need to measure magnetic properties of liquid sample then the particle size should be very small [ properly treated with surfactant ], [i.e] the sample should be a perfect ferrofluid . Also , the saturation magnetization of the nanoparticles in solid form should be analyzed before dispersion into liquid for magnetic measurements.
One can measure magnetic properties of nano-particles in stable suspensions. If one lower the temperature of the suspension then surrounding liquid will freeze and both rotational and translation of the particles will be blocked and magnetic moment will be able to flip. One may measure then pure Neel's superparamagnetic behavior. Above that temperature ,particle can rotate freely and one can have both Brownian as well as Neel rotation. This is also some times called as extrinsic superparamagnetism.
For superparamagnetic nanoparticles, most of the interesting physics occur at cryogenic temperatures. So if the particles are suspended in liquid, freeze them first, and then there shouldn't be problems with liquid sloshing around in VSM mode. Once the liquid is frozen, it's a lot easier to handle and measure. So you might want to freeze the liquid first (without measuring), and then start your analysis in the frozen state.
If you absolutely require data above the freezing point of the liquid, then you can use DC scan extraction or ACMS mode, where the liquid is stationary or does not experience large accelerations, you can use those methods too.
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