CHANGES IN MAGNETIC ANISOTROPY OF MO FILMS INDUCED BY NITROGEN DIFFUSION FROM PROTECTIVE LAYERS

Abstract

RE/TM films used for magneto-optical (MO) data storage are commonly covered with reactively sputtered SiN protective layers. For stoichiometric Si3N4 cover layers no reaction between the magnetic and the protective layer is expected. But if there is a surplus of nitrogen within the protective layer the magnetic properties and the long-term stability of the magnetic layer may be influenced.To investigate this we prepared ferrimagnetic TbFeCo-films covered with a SiN protective layer containing a surplus of nitrogen. For testing their long-term stability the films were annealed several times. To analyze the magnetic properties measurements of Kerr hysteresis were performed. The magnetic anisotropy was measured by Kerr Torque Magnetometry, a very sensitive magneto-optical method which provides local information about the anisotropy field at the surface region of the magnetic layer.As a result substantial changes of the magnetic properties at the surface region of the TbFeCo layers were detected. These changes were induced by a diffusion of nitrogen from the protective into the magnetic layer leading to a reduced content of magnetically active Tb at the surface. At longer annealing times this led in extreme to a double layer behaviour which makes the medium useless for MO data storage.