  Tunneling spectroscopy is applied to tunnel junctions with only one or no
ferromagnetic electrode to study the excitation of quasi particles in magnetic
tunnel junctions. The bias dependence is investigated with high accuracy by
inelastic electron tunneling spectroscopy. Both types of junctions show a zero
bias anomaly that is different in size and sign compared to magnetic tunnel
junctions, i.e. junctions with two ferromagnetic electrodes. A pronounced
difference is also found depending on the material that is probed by the
tunneling electrons, which might be attributed to the excitation of magnons.
