  The different types of magnetism arise mainly from how electrons move and
interact with each other. In this work, we show how protons (H$^+$) also
exhibit magnetic behavior. We measured the magnetic susceptibility of the
ammonium halides and identified pronounced increases at 232 K, 233 K and 243 K
for NH$_4$I, NH$_4$Br and NH$_4$Cl, respectively, which all coincide to the
geometric ordering of its ammonium cations. With extensive literature
establishing the fact that the ammonium cations exhibit rotational motion even
towards the lowest temperatures, we take into account that the orbital motion
of the protons carries a magnetic moment and find it to be larger than that of
the paired electrons. Consequently, the structural phase transitions are
magnetically-driven as the system attempts to lift 8-fold energy degeneracies
of the proton orbitals via Jahn-Teller distortions. Our findings identify that
NH$_4$$^+$ cations are capable of comprising magnetism which appears to be
ubiquitous in ammonia-based molecular solids.
