New bounds and future prospects for axion force searches at Penning trap experiments

In this note we consider Penning trap experiments as probes of axion-mediated forces. We show that the current measurement of electron's $g$-factor already sets a new exclusion limit for monopole-dipole axion forces acting on the electron spin. We also show that the Penning trap's capability of switching an electron and a positron can isolate the effect of an axion force and suppress systematic effects.