  We present a resonant-mode, transverse-field, radio-frequency (rf) neutron
spin flipper design that uses high-temperature superconducting films to ensure
sharp transitions between uniform magnetic field regions. Resonant mode allows
for low power, high frequency operation but requires strict homogeneity of the
magnetic fields inside the device. This design was found to efficiently flip
neutrons at 96.6$\pm 0.6\%$ at an effective frequency of 4 MHz with a beam size
of $2.5~\times~2.5$~cm and a wavelength of 0.4 nm. The high frequency and
efficiency enable this device to perform high-resolution neutron spectroscopy
with comparable performance to currently implemented rf flipper designs. The
limitation of the maximum frequency was found due to the field homogeneity of
the device. We numerically analyze the maximum possible efficiency of this
design using a Bloch solver simulation with magnetic fields generated from
finite-element simulations. We also discuss future improvements of the
efficiency and frequency to the design based on the experimental and simulation
results.
