Tailoring the exchange coupling in IrMn/NiFe films and nanodots by interface confinement

The scientific interest towards antiferromagnetic (AFM) materials has been increasing continuously mainly because of their crucial role in the operating principle of modern, miniaturized spintronic devices. In spin valves and tunnel junctions, a fine control of the magnetization reversal process in the ferromagnetic (FM) electrodes is usually achieved through the interface exchange coupling with an AFM layer [1]: 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 of the exchange bias (EB) effect. Moreover, since the strategic importance of the EB effect in the technology of magnetoresistive spin valves and tunnel junctions and the increasing demand of miniaturization of modern devices (magnetic sensors, high-density data storage media), it is admittedly crucial to expand the description of the EB mechanism so as to include the effects of spatial confinement. The exchange interaction between AFM and FM interfacial spins depends, in polycrystalline systems, on the magnetic anisotropy of the bulk AFM phase and on the size distribution of the crystalline AFM grains [2]. Recent investigations have proposed the presence of disordered AFM spins at the AFM/FM interface, with spin-glass-like magnetic properties [3]. With this respect, we have recently observed that, at low temperature, these disordered AFM spins are frozen in a magnetic disordered state and are collectively involved in the exchange coupling with the FM moments, showing a magnetic correlation length, lambda [4]. With increasing temperature, lambda progressively shortens (we have established that at T ~ 100 K the frozen collective regime breaks up) even if the AFM spins do not enter the full paramagnetic regime due to the polarizing action of adjacent FM and AFM spins. Due to that, when interface confinement is observed, namely passing from a continuous film to a nanodot or when the morphology of the AFM/FM interface is modulated at the nanoscale, lambda is expected to play a role in the EB effect. In this contribution, we present our study on the mechanism of the magnetic exchange coupling in the Ir25Mn75/Ni20Fe80 system. The interface confinement has been accomplished in different ways: by producing that system in form of arrays of dots with different size D = 1000, 500, 300, 140 nm and by inserting, in the continuous films, a Cu spacer with a nominal thickness, tCu, of the order of 1Å. Due to the small tCu value, Cu islands, whose presence was confirmed by X-Ray Absorption Fine Structure investigations, are obtained at the AFM/FM interface. The EB properties of the samples, i.e. exchange field HEX and coercivity HC, and their thermal dependance, were investigated by SQUID and MOKE magnetometers in the 5-300 K temperature range. The role of  in the dots arrays was reflected by the strong dependence of HEX on D. In more detail, at 5 K HEX ~ 750 Oe when D = 1000 nm, whilst HEX ~ 1100 Oe when D = 1000 nm; when D = 140 nm, HEX decreases down to ~ 100 Oe [5]. In the continuous films with the Cu insertion at the interface, we observed that, at high temperature, the change in the HEX value may be explained just in terms of a dilution effect, namely in terms of the reduction of the extension of the AFM/FM interface. Differently, at low temperature, i.e. when lambda approaches the interdistance between Cu islands, the HEX values strongly depend on tCu, namely on Cu islands size/interdistance. These findings will be presented and discussed, taking also into account the results of micromagnetic calculations. [Research sponsored by MIUR Italy, project RBFR10E61T-NANOREST.]1. C. Chappert, A. Fert, F.N.V. Dau, Nature Mater. 6, 813 (2007) 2. G. Lhoutellier et al. J. Appl. Phys. 120, 193902 (2016) 3. V. Baltz et al. Phys. Rev. B 81, 052404 (2010). 4. F. Spizzo et al., Phys. Rev. B. 91, 064410 (2015) 5. F. Spizzo et al., J. Magn. Magn. Mater. 400, 242 (2016)