Magnetic exchange coupling inIrMn/NiFenanostructures: From the continuous film to dot arrays

Abstract

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 ${H}_{\mathrm{ex}}$ 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 $T=300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ ${H}_{\mathrm{ex}}$ decreases by reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at $T=10\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ (${H}_{\mathrm{ex}}\ensuremath{\sim}1140\phantom{\rule{0.16em}{0ex}}\mathrm{Oe}$ in the dots of $300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$). 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 $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$), 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 $(T<100\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them.