  Magnetic insulators are a key resource for next-generation spintronic and
topological devices. The family of layered metal halides promises ultrathin
insulating multiferroics, spin liquids, and ferromagnets, but new
characterization methods are required to unlock their potential. Here, we
report tunneling through the layered magnetic insulator CrI3 as a function of
temperature and applied magnetic field. We electrically detect the magnetic
ground state and inter-layer coupling and observe a field-induced metamagnetic
transition. The metamagnetic transition results in magnetoresistances of 95%,
300%, and 550% for bilayer, trilayer, and tetralayer CrI3 barriers,
respectively. We further measure inelastic tunneling spectra for our junctions,
unveiling a rich spectrum of collective magnetic excitations (magnons) in CrI3.
Our results establish vertical tunneling as a versatile probe of magnetism in
atomically thin insulators.
