Consider a common bar magnet, magnet 1 resting on a surface with its North pole facing up. Suspended distance $y$ above it (supported side-to-side by a plastic tube) is a second, smaller bar magnet, magnet 2, with its North pole facing down. The magnetic forces between them exceed the force of gravity, and keep magnet 2 suspended. Consider some material, material-X that is moving towards the gap between the two magnets at an initial velocity $v$.
Does a material, material-X exist that would reduce the distance $y$ between the two magnets, and pass through the gap without changing velocity $v$?
Answer
The material you are looking for could be a super conductor. These material have zero resistance for current and thus can compensate penetrating field lines within the first layers of the material. This phenomenon is called the Meissner effect and is the very definition of the supra-conductive state.
In your case of a plate between two magnets, this would definitively reduce $y$.
For the velocity:
Here, normally the eddy currents induced by a magnetic field lead to a loss of power, given by:
$$P = \frac{\pi^2 B_\text{p}^{\,2} d^2 f^2 }{6k \rho D},$$
since, however, a super conductor has zero resistance and thus, de facto,
$$\rho = \infty$$
no kinetic energy should be lost, and thus the velocity will remain unchanged.
There is only one problem:
Superconductor can only exist under very low temperature, so this might not be realizable in the case of your machine...you would at least need a cooling system working with liquid nitrogen to cool it.
Other than superconductors, I do not see any possible material because, either the material is a conductor, then you always have losses due to the eddy currents (thus reducing $v$) or the material is not a conductor (then $y$ will not decrease).
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