Thermal and spatial confinement effects in exchange coupled IrMn/NiFe dot arrays

We present a comprehensive study of the exchange bias phenomenon (EB) in an antiferromagnetic (AF)/ferromagnetic (FM) continuous film and in arrays of square dots with different size (D), aimed at elucidating thermal and spatial confinement effects on the AF/FM exchange coupling and their correlation with the AF structural and magnetic properties. For this purpose, an AF/FM Ir25Mn75[10 nm]/Ni80Fe20[5 nm] continuous film and arrays of square dots (D = 1000 nm, 500 nm and 300 nm) were prepared by electron beam lithography and lift-off using dc-sputtering. Structural investigations by electron microscopy techniques indicated that the AF layer consists of nanograins (mean size ~ 10 nm), but also clearly revealed the existence of a structurally disordered IrMn region (2-3 nm thick) at the interface with the NiFe phase. The magnetic properties, in particular the temperature dependence of the exchange field Hex and coercivity HC, were studied by SQUID and MOKE measurements. At room temperature, 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 for D = 300 nm). The EB mechanism and its thermal evolution have been explained through a phenomenological model [1] that combines spatial confinement effects with other crucial items concerning the AF phase: the magnetothermal stability of the IrMn nanograins, the glassy magnetic nature of the structurally disordered IrMn region, 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. The model predictions have been supported by micromagnetic calculations, satisfactorily reproducing the experimental findings. This research work has been sponsored by MIUR under project FIRB2010-NANOREST. [1] F. Spizzo et al., Phys. Rev. B 91 (2015) 06441