Low temperature magnetization of a ferrofluid of magnetite coated with polysterene

Physical phenomena associated with magnetic fluids have, over the years, attracted considerable attention; we wish to report here on unusual behavior observed in thermomagnetic curves of a suspension of polysterene coated magnetic particles in distilled water. The ferrofluid was produced by Polysciences Inc., USA, the weight fraction of the magnetic particles is approximately 2.5% and the ratio of magnetite to polystyrene is approximately 1 : 1 by weight. The ferrofluid composition was analyzed by an energy dispersive X-ray spectrometer (EDS) and only Fe, 0, and C were found. Using SEM (JEOL Model 6300) we determined that the particles are spherical (diameter 100 to 1200& mean 600 to 700A) so that their magnetic shape anisotropy may be unimportant. The sample magnetization below 300 K was measured by a vibrating sample magnetometer (VSM) with resolution of lop7 A m2. Using a closed cycle refrigerator we carried out thermomagnetic measurements between T = 35 K and room temperature both under zero field cooling (ZFC) and under field cooling (FC) regimes. The rate of temperature change was approximately 2 K/min. At 300 K the ferrofluid has zero remanence ( M I ) and coercivity (H,-), with magnetization reversals taking place by both Brownian rotation of the particles [l] and by NBel’s rotation of the magnetization vector [a] . At 35 K, on the other hand, the carrier liquid is frozen, Brownian rotation is blocked and Nkel’s rotation is slowed down. The sample magnetization thus cannot instantaneously follow the applied field and hysteresis follows, with Mr = 0.26 A m2/kg and H, = 170 Oe (see Fig. 1). In order to avoid unwieldy numerical expressions we use here Oersted and note that 4z Oe = lo3 A/m in SI units. The observed thermomagnetic curves of ZFC and FC runs are shown in Fig. 1 for applied fields between 50 and 1000 Oe, with the sample cooled from its initial state at room temperature. Below 273 K the measured curves resemble the behavior of noninteracting small particles [3] whose magnetization reverses via thermal (NBel) activation [ a ] . In particular, the