Exchange coupling and spatial confinement in IrMn/NiFe films and dot arrays

Nowadays, the exchange coupling at the interface between antiferromagnetic (AF) and ferromagnetic (FM) phases plays a crucial role in the technology of magnetoresistive spin-valves and tunnel junctions and, due to the increasing demand for the miniaturization of magnetic devices, a large interest is devoted to the investigation of confinement effects on this phenomenon. In this context, we present a comprehensive study of the exchange bias phenomenon in an AF/FM IrMn[10 nm]/NiFe[5 nm] (IrMn = Ir25Mn75, NiFe = Ni80Fe20) continuous film and in arrays of square dots with different size (1000 nm, 500 nm and 300 nm), aimed at elucidating the temperature dependence of the exchange field Hex and coercivity HC, in conjunction with spatial confinement effects. To achieve this goal, samples prepared by electron beam lithography and lift-off using dcmagnetron sputtering were subjected to structural investigations by electron microscopy techniques and to magnetic study, through SQUID and magneto-optic magnetometry measurements coupled to micromagnetic calculations. In particular, we have observed that at T = 300 K Hex decreases with reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at T = 10 K (Hex ~ 1140 Oe in the dots of 300 nm and Hex ~ 750 Oe for the continuous film). The exchange bias mechanism and its thermal evolution have been explained through an exhaustive phenomenological model, which joins spatial confinement effects with other crucial items concerning the pinning AF phase: the magnetothermal stability of the nanograins forming the IrMn layer (mean size ~ 10 nm), assumed as essentially non-interacting from the magnetic point of view; the proven existence of a structurally disordered IrMn region at the interface between the NiFe phase and the bulk of the IrMn layer, with a magnetic glassy nature; the stabilization of a lowtemperature (T < 100 K) frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them. This research work has been carried out in the framework of the project FIRB2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices” – NANOREST.