All materials experience a force when placed in a region of magnetic field and field gradient. The magnitude of this force depends upon the magnetic susceptibility of the material and this varies over a wide range depending on the type of material. Ferromagnetic materials have large susceptibilities and can generate large forces even in modest fields and gradients. Force interactions between ferromagnetic materials and magnetic fields are evident in everyday life and even more so when working with high magnetic fields.
The physical principle behind magnetic levitation is that all diamagnetic objects magnetically polarize when immersed in a strong magnetic field. In the presence of a strong magnetic field gradient, parallel to the polarizing field, the diamagnetic sample experiences a magnetic force. Orienting this force opposite to the gravitational force can lead to no net force on the object; thus, levitation occurs (Fig 1). The force per unit mass is:
where cr is specific magnetic susceptibility (cm3/g), B is the field-gradient product Bz is the field, ¶Bz/¶z is the field gradient and g is the constant for gravitational acceleration. Note that cr is a molecular property and the value of the field-gradient product necessary for levitation is independent of mass. For levitation to occur in most diamagnetic materials we must satisfy the relation 1300 T2/m < (Bz)*(¶Bz/¶z)<1600 T2/m, e.g. for 1400 T2/m either Bz = 10 T, ¶Bz/¶z = 140 T/m or .Bz = 8 T, ¶Bz/¶z = 150 T/m satisfies the criteria for levitation.
A 17 Tesla/ 50 mm warm bore superconductive magnet was delivered to the CIA-MR in September 2004.
This magnet is similar to high-field NMR spectroscopic magnets, however, the magnet design incorporates a very strong field gradient (~ 100 T/m) near the top of the magnet which is necessary for magnetic levitation. Levitation occurs when the magnetic force counterbalances the gravitational force and thus simulates orbital freefall. This instrument will provide a unique environment to process organic and inorganic materials and investigate biological materials, living and non-living, in a simulated microgravity environment. The levitation aspect of the magnet will be used by a consortium of academic and industrial scientists for the implementation of new types of materials processing and biotechnology applications.
Top view of 17 Tesla superconducting magnet used for magnetic levitation. The magnet was funded by the National Institutes of Health (NIH grant number 1S10RR016783-01).
To download video of levitating box elder bug click on link:
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