Characterization of magnetic IrMn/NiFe nanostructures

It is now largely demonstrated that magnetic systems structured on a nanometric length scale may exhibit peculiar magnetic properties, generally indicated as disordered magnetism effects, originating from the concomitant presence of topological disorder and competing magnetic interactions, as a consequence of the lack of structural periodicity. It is to be expected that this complex mix of magnetic effects play a relevant role also in the behavior of exchange coupled antiferromagnetic (AFM)/ferromagnetic (FM) systems, where the torque action exerted by the interfacial AFM spins on the FM ones brings about the insurgence of an unidirectional exchange anisotropy for the FM magnetization, and then the exchange bias (EB) effect. In this work the EB properties of the IrMn/NiFe system in form of continuous film and of arrays of square dots with different size (1000, 500 and 300 nm), produced by e-beam lithography and dc-sputtering deposition, have been investigated. The stacking of the final samples was Si (substrate)/Cu[5 nm]/IrMn[10 nm]/NiFe[5 nm] (in square brackets the nominal thickness of each layer is reported). Structural investigations by TEM of the continuous IrMn/NiFe film indicate that the bulk of the AFM layer consists of nanograins with mean size ~ 10 nm, whereas the NiFe phase appears amorphous. In particular, the TEM analysis reveals the existence of a structurally disordered IrMn region (2-3 nm thick) at the interface between the FM phase and the bulk of the AFM layer. This finding, namely the evidence of structural disorder, strongly supports the spin-glass like magnetic character of the interfacial IrMn region, deduced by SQUID measurements on the continuous film, Figure 3. The existence of a low temperature frozen collective regime of the interfacial AFM spins and its break up above a critical temperature, that we have schematically settled at T = 100 K, determine the thermal evolution of the EB properties. In fact, below 100 K, the exchange coupling between IrMn and NiFe is ruled by magnetically correlated, frozen AFM spins, subjected to a strong effective local anisotropy, which results in high Hex (exchange field) and HC (coercivity) values. Above 100 K, the AFM/FM coupling is governed by a fraction of interfacial AFM spins, magnetically polarized by the AFM nanograins. Hence, Hex and HC decrease more and more with rising T, reflecting the progressive thermal instability of the AFM nanograins, assumed as essentially non-interacting. The spatial confinement, namely the passage from the continuous film to the dot structure, affects the exchange coupling mechanism when the dot size enters the sub-micron regime [1]. This research work has been carried out in the framework of the project FIRB 2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices”-NANOREST References 1. F. Spizzo, E. Bonfiglioli, M. Tamisari, A. Gerardino, G. Barucca, A. Notargiacomo , F. Chinni, L. Del Bianco, “Magnetic exchange coupling in IrMn/NiFe nanostructures: from the continuous film to dot arrays”, Physical Review B, 91, pp.064410 1-9, 2015.