Klokkenburg M, Erné B H, Mendelev V, Ivanov A O
Van' t Hoff Laboratory for Physical and Colloid Chemistry, Science Faculty, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
J Phys Condens Matter. 2008 May 21;20(20):204113. doi: 10.1088/0953-8984/20/20/204113. Epub 2008 May 1.
Theories and simulations have demonstrated that field-induced dipolar chains affect the static magnetic properties of ferrofluids. Experimental verification, however, has been complicated by the high polydispersity of the available ferrofluids, and the morphology of the dipolar chains was left to the imagination. We now present the concentration- and field-dependent magnetization of particularly well-defined ferrofluids, with a low polydispersity, three different average particle sizes, and with dipolar chains that were imaged with and without magnetic field using cryogenic transmission electron microscopy. At low concentrations, the magnetization curves obey the Langevin equation for noninteracting dipoles. Magnetization curves for the largest particles strongly deviate from the Langevin equation but quantitatively agree with a recently developed mean-field model that incorporates the field-dependent formation and alignment of flexible dipolar chains. The combination of magnetic results and in situ electron microscopy images provides original new evidence for the effect of dipolar chains on the field-dependent magnetization of ferrofluids.
理论和模拟表明，场致偶极链会影响铁磁流体的静态磁性能。然而，由于现有铁磁流体的高多分散性，实验验证变得复杂，偶极链的形态只能靠想象。我们现在展示了具有特别明确的铁磁流体的浓度和场依赖磁化强度，这种铁磁流体具有低多分散性、三种不同的平均粒径，并且使用低温透射电子显微镜对有磁场和无磁场情况下的偶极链进行了成像。在低浓度下，磁化曲线遵循非相互作用偶极子的朗之万方程。最大颗粒的磁化曲线与朗之万方程有很大偏差，但在定量上与最近开发的平均场模型一致，该模型考虑了柔性偶极链的场依赖形成和排列。磁性结果和原位电子显微镜图像的结合为偶极链对铁磁流体场依赖磁化强度的影响提供了全新的证据。
