Magnetic properties of extremely bimodal magnetite suspensions

In this paper, we investigate the magnetic properties of aqueous suspensions of extremely bimodal magnetite particles, including micro- (size-1450 nm) and nano-(size -9 nm) units. It is found that the addition of increasing concentrations of small particles increases the saturation magnetization, the coercive field, and the low-field susceptibility. The results are explained considering that the nanomagnetite used has a moderately wide size distribution, that embraces both the range of superparamagnetism (the lowest size interval) and of finite coercivity, all being single domain. In addition, the formation of a cloud of small particles surrounding the larger ones favors the chain formation by dipolar magnetic aggregation. It is concluded that the admixture of even small amounts of nanoparticles offers an excellent tool for the control of the magnetic properties of magnetite suspensions. (C) 2007 Elsevier B.V. All rights reserved.