Magnetic Dust Aggregation Experiments

Magnetic dust aggregation has been studied numerically [16,13,14], In order to have a means for calibrating and validating these simulations, we investigated the effects of magnetic interactions on the preplanetary grain growth process in analogous laboratory experiments. To this end, we used single domain (SD) ferritic (Ba,Sr)-Fei20i9 powder in the levitation drum setup described by Blum et al. [17]. SD ferritic grains are used as a raw material for the industrial production of sintered permanent magnets [18]. The

Figure 2. Optical and electron microscopic images of magnetic dust aggregates that formed inside the levitation drum. Particles coagulate rapidly and the resulting aggregates are visible to the unaided eye. Left image: chain-like aggregates from the levitation drum. Top; the picture width is approximately 1 mm. The alignment may be due to aerodynamic effects. Bottom; close-up. Right image: a web-like structure containing several elongated aggregates in a spoke-like pattern around a central cluster. These aggregates could act as a "fishing net" for nonmagnetic grains.

Figure 2. Optical and electron microscopic images of magnetic dust aggregates that formed inside the levitation drum. Particles coagulate rapidly and the resulting aggregates are visible to the unaided eye. Left image: chain-like aggregates from the levitation drum. Top; the picture width is approximately 1 mm. The alignment may be due to aerodynamic effects. Bottom; close-up. Right image: a web-like structure containing several elongated aggregates in a spoke-like pattern around a central cluster. These aggregates could act as a "fishing net" for nonmagnetic grains.

grains in our dust sample are micron-sized and strongly magnetized due to the existence of only one magnetic domain. In an approximate way, they can be regarded as magnetic dipoles with a dipole moment |/x| ss 3 • 10-13 Am2. This value can be obtained from direct calculations using the mean particle size and the material parameters. In order to verify this result, we have measured the magnetic moment of individual grains directly by means of a specifically designed magnetic coil target [19]. The magnetic grains are deagglomerated by a rotating cogwheel [20], enter the target and are deflected by a strong gradient of the magnetic field generated by the target coils. The particle motion is imaged by means of a pulsed laser and a CCD-camera, and can be used to calculate the magnetic moment of the deflected particle. The measured distribution of magnetic dipole moments is shown in Figure 1. It depicts a strong maximum around the calculated value. There are rather huge systematic errors due to the loss of 3D information and the finite resolution of the CCD image. In some cases, the deagglomeration may not be perfect. This will produce grain clusters that carry more than one nominal dipole moment and would explain the long tail of the distribution in Figure 1.

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