  We report microwave spectroscopy of co-trapped $^9\text{Be}^+$ and
$^{40}\text{Ca}^+$ within a compact permanent-magnet-based Penning ion trap.
The trap is constructed with a reconfigurable array of NdFeB rings providing a
0.654 T magnetic field that is near the 0.6774-T magnetic-field-insensitive
hyperfine transition in $^9\text{Be}^+$. Performing Ramsey spectroscopy on this
hyperfine transition, we demonstrate nuclear spin coherence with a contrast
decay time of >1 s. The $^9\text{Be}^+$ is sympathetically cooled by a Coulomb
crystal of $^{40}\text{Ca}^+$, which minimizes $^9\text{Be}^+$ illumination and
thus mitigates reactive loss. Introducing a unique high-magnetic-field optical
detection scheme for $^{40}\text{Ca}^+$, we perform spin state readout without
a 729~nm shelving laser. We record a fractional trap magnetic field instability
below 20 ppb (<13 nT) at 43 s of averaging time with no magnetic shielding and
only passive thermal isolation. We discuss potential applications of this
compact, reconfigurable Penning trap.
