A soft ferromagnetic plate uniformizes tesla-level fields generated by attached permanent magnets, producing a smooth and electronically tunable surface field on the opposite side. By arranging $n$ precisely fabricated rectangular plates, a nearly ideal magnetic quadrupole field with a substantial gradient can be created at the center. This robust and rapidly tunable field configuration is well suited for two-dimensional magneto-optical trapping (2D MOT) and magnetic guiding of cold atoms. By aligning an optical nanofiber (ONF) along the zero-field line of a planar 2D MOT in a two-plate assembly, we demonstrate quasicontinuous, field-free operation of the quantum optical interface without switching off the magnetic field. Transient transmission spectroscopy with nanosecond laser pulses is performed on the ${}^{87}\mathrm{Rb}D2$ line at a measurement repetition rate as high as 250 kHz. The observed line broadening, while not yet fully understood, is partially attributed to residual magnetic fields in the $n=2$ assembly. Through additional measurements and simulations, we verify that these residual fields can be fully eliminated in an $n=4$ assembly, resulting in an ultrastraight 2D trap that supports uniform light-atom interaction over exceptionally long, field-free distances. We extend our discussion to $n=6$, $n=8$ designs with similar uniformity but multiple zero-field lines. With its strong gradient for magnetic trapping, the ferromagnetic devices also enable quantum optical scenarios featuring interactions between coguided atoms and photons at ONF interfaces.

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• Atom & ion trapping & guiding
• Cooling & trapping
• Magneto-optics
• Devices
• Ferromagnets
• Nanofibers
• Permanent magnets
• Trapped atoms