saturation magnetisation increases with increase in particle size.as temperature increases crystalite size increases.
In respect with your question, The reason for the decrease of Ms when temperature incresases, is because magnetic moments cannot be aligned anymore due to the increase on thermal energy. When temperature is low the magnetic moments do not have enough energy to compete with the magnetic field.
And about the second sentence of your question, could you explain more? I could not understand it
Basically the saturation magnetization means that the complete alignment of magnetic moment vector in the direction of magnetic field. If the temperature is high then the Brownian motion will disturb the alignment of magnetic moment vector towards the field direction. As a result of this the saturation value decreases. At low enough temperature Zeeman energy (MHcos(theta)) dominates over thermal energy resulting in high Ms. With increasing temperature the randomness increases and hence the value of cos(theta) decreases.
The direction of a spin is fixed in the structure of its magnetic carrier (see my article “Ferromagnetic state and phase transitions”). Saturation magnetization is a total projection of all spins in the same direction. Its value decreases with a stronger vibration of the magnetic atoms at higher temperatures.
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