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 size (1000 nm, 500 nm and 300 nm) is presented, which elucidates 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 liftoff 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 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). 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 ~ 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 low-temperature (T < 100 K) frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them.

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One fundamental requirement in the search for novel magnetic materials is the possibility of predicting and controlling their magnetic anisotropy and hence the overall hysteretic behavior. We have studied the magnetism of Au:Co films (~30 nm thick) with concentration ratios of 2:1, 1:1, and 1:2, grown by magnetron sputtering co-deposition on natively oxidized Si substrates. They consist of a AuCo ferromagnetic alloy in which segregated ultrafine Co particles are dispersed (the fractions of Co in the AuCo alloy and of segregated Co increase with decreasing the Au:Co ratio). We have observed an unexpected hysteretic behavior characterized by in-plane anisotropy and crossed branches in the loops measured along the hard magnetization direction. To elucidate this phenomenon, micromagnetic calculations have been performed for a simplified system composed of two exchange-coupled phases: a AuCo matrix surrounding a Co cluster, which represents an aggregate of particles. The hysteretic features are qualitatively well reproduced provided that the two phases have almost orthogonal anisotropy axes. This requirement can be plausibly fulfilled assuming a dominant magnetoelastic character of the anisotropy in both phases. The achieved conclusions expand the fundamental knowledge on nanocomposite magnetic materials, offering general guidelines for tuning the hysteretic properties of future engineered systems.