  The axion is a hypothetical particle considered to be the most economical
solution to the strong CP problem. It can also be formulated as a compelling
component of dark matter. The haloscope, a leading axion detection scheme,
relies on the conversion of galactic halo axions into real photons inside a
resonant cavity structure in the presence of a static magnetic field, where the
generated photon frequency corresponds to the mass of the axion. For maximum
sensitivity it is key that the central frequency of the cavity mode structure
coincides with the frequency of the generated photon. As the mass of the axion
is unknown, it is necessary to perform searches over a wide range of
frequencies. Currently there are substantial regions of the promising
pre-inflationary low mass axion range without any viable proposals for
experimental searches. We show that 3D resonant LC circuits with separated
magnetic and electric fields, commonly known as re-entrant cavities, can be
sensitive dark matter haloscopes in this region, with frequencies inherently
lower than those achievable in the equivalent size of empty resonant cavity. We
calculate the sensitivity and accessible axion mass range of these experiments,
designing geometries to exploit and maximize the separated magnetic and
electric coupling of the axion to the cavity mode.
