  We demonstrate a sensitive optically-pumped magnetometer using rubidium vapor
and 0.75 amg of nitrogen buffer gas in a sub-mm-width sensing channel excavated
by femtosecond laser writing followed by chemical etching. The channel is
buried less than 1 mm below the surface of its fused silica host material,
which also includes reservoir chambers and micro-strainer connections, to
preserve a clean optical environment. Using a zero-field-resonance magnetometry
strategy and a sensing volume of 2.25 mm$^3$, we demonstrate a sensitivity of
$\approx$ 1 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at $10$ Hz. The device can be
integrated with photonic structures and microfluidic channels with 3D
versatility. Its sensitivity, bandwidth and stand-off distance will enable
detection of localized fields from magnetic nanoparticles and \mul NMR samples.
