ON THE TEMPERATURE-DEPENDENCE OF THE AC SUSCEPTIBILITY OF FE3O4 MAGNETIC FLUIDS

The temperature dependence of the frequency-dependent magnetic susceptibility has been measured for Fe3O4 magnetic fluids with Isopar-M as a liquid carrier. The temperature range was 80 K less-than-or-equal-to T less-than-or-equal-to 300 K, and the volume fractions epsilon ranged from 0.004 to 0.062. With increasing temperature the in-phase magnetic susceptibility chi' increased from its initial value to a maximum at a temperature T(B), then decreased monotonically until around the melting point of the liquid carrier, T(m) is similar to 200 K. For T less-than-or-equal-to T(m), chi' increased rapidly to a second maximum at T = T(B1). This behaviour was qualitatively similar for all studied samples. For the sample with epsilon = 0.047, chi' decreased with increasing frequency at all temperatures. When a static magnetic field was applied, a reduction in chi' was also observed. The effects of interparticle interactions on the static initial susceptibility chi(i), are discussed. Analysis of the data yields information about the effective anisotropy energy, the median magnetic diameter, and relaxation times. The distribution of blocking temperature was calculated from the superparamagnetic blocking model of Wohlfarth. The results are further analysed and discussed within the framework of Lundgren model of spin glasses.