Magnetic exchange coupling in IrMn/NiFe nanostructures: From the continuous film to dot arrays

A comprehensive description of the exchange bias phenomenon in an antiferromagnetic/ferromagnetic IrMn(10 nm)/NiFe(5 nm) continuous film and in arrays of square dots with different sizes (1000, 500, and 300 nm) is presented, which elucidates the temperature dependence of the exchange field H-ex and coercivity H-C, in conjunction with spatial confinement effects. To achieve this goal, samples prepared by electron beam lithography and lift-off using dc 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 = 300K H-ex decreases by reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at T = 10K (H-ex similar to 1140 Oe in the dots of 300 nm). 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 antiferromagnetic phase: the magnetothermal stability of the nanograins forming the IrMn layer (mean size similar to 10 nm), assumed as essentially noninteracting 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; and the stabilization of a low-temperature (T < 100 K) frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them.