Static and dynamic magnetic behavior of exchange-coupled IrMn/NiFe films and nanodots

Nowadays, the pressing demand for miniaturization of magnetic devices triggers an increasing interest for the study of the magnetic properties of elements size-confined to the nanometric scale (nanodots). Going from the continuous film to single-phase dots, strong changes occur both in the magnetic configuration and in the reversal process as well as on the spin-dynamics. Further changes are expected when passing from single-phase systems to antiferromagnetic (AF)/ferromagnetic (FM) structures. In fact, the exchange coupling (EC) at the interface between the two magnetic phases brings about the appearance of an extra source of magnetic anisotropy (exchange anisotropy) for the FM layer, which determines the onset of the exchange bias effect and significantly affects the magnetic behavior of the coupled system as a whole. In this context, we have studied the static and dynamic magnetic properties of AF/FM continuous films and nanodots. As AF we used a 10 nm thick Ir25Mn75 layer, whilst as FM we used a 5 nm thick Ni80Fe20 layer. The nanodot patterns were produced using electron beam lithography and the lift-off method. Both the continuous and the patterned samples were grown by dc-magnetron sputtering in Ar atmosphere on a 5 nm thick Cu underlayer, in a static magnetic field Hdep = 400 Oe. Both square (SQ) and stadium-shaped (ST) dots were produced: SQ dots’ size ranges from 500 nm down to 300 nm; ST dots’ major axis ranges from 1 µm down to 300 nm, and eccentricity from 0.75 to 0.5. ST dots’ major axis was both parallel and perpendicular to Hdep, to sense the interplay between exchange and shape anisotropy, as well. The interdot distance was nearly twice the dot size, to avoid interdot dipolar interactions. The patterned samples have been characterized by Scanning Electron Microscopy and by Atomic/Magnetic Force Microscopy. For the study of the magnetic properties, we have carried out Magneto-Optic Kerr Effect and Brillouin Light Scattering measurements at room temperature, and SQUID and Kerr microscopy measurements at low temperature (5-300 K). The obtained results have allowed us to gain a good insight into the EC mechanism and its thermal evolution in the investigated system and to highlight the effects of size-confinement on the magnetic behavior. This research work has been partially carried out in the framework of the project FIRB2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices” – NANOREST.