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Temperature and Concentration Dependence of Magnetic Birefringence of Magnetic Fluids

It was found experimentally that the magnetic birefringence of magnetic fluid thin films obeys a Curie-Weiss like law for all kinds of magnetic fluids except for a water-based one. The birefringence can be described by θ( T , H )=θ a f ( H )/[ T - T a + f ( H )], where θ( T , H ), H and T are the phase difference, the applied field and the temperature of the film, respectively, and θ a is a positive constant. T a and f(H) are also found to be expressed empirically by T a = T 0 +κ ln c , f ( H )=1-exp [-α( c ) H ], respectively, where c is a volume fraction of the colloidal particles, T 0 , κ are positive constants and α( c ) is a function of c . Taking the empirical expression of θ into consideration, the free energy of the magnetic fluid is discussed using Landau's phase transition theory.