We present detailed measurements of the discrete electron-tunneling level s
pectrum within nanometer-scale cobalt particles as a function of magnetic f
ield and gate voltage, in this way probing individual quantum many-body eig
enstates inside ferromagnetic samples. Variations among the observed levels
indicate that different quantum states within one particle are subject to
different magnetic anisotropy energies. Gate-voltage studies demonstrate th
at the low-energy tunneling spectrum is affected dramatically by the presen
ce of nonequilibrium spin excitations.