Antiferromagnetic spin correlation and exchange coupling in IrMn/NiFe films and nanodots

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, 2]: 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, as an alternative to conventional spintronics based on ferromagnets, recent studies have disclosed magnetoresistive phenomena fully governed by antiferromagnets [3]. In order to make progress in this field, achieving a fundamental comprehension of magnetism of nanostructured AFM elements is mandatory. In this context, we have studied the mechanism of the magnetic exchange coupling in samples of IrMn[10 nm]/NiFe[5 nm], in form both of continuous thin films and dot arrays. The EB properties and their thermal dependence (investigated by SQUID and MOKE magnetometers in the 5-300 K temperature range) have been explained considering the glassy magnetic nature of a structurally disordered IrMn region located between the FM phase and the ‘bulk’ of the AFM layer, detected by high-resolution transmission electron microscopy analyses (Fig. 1a). At low temperature, the spins of the disordered AFM region are frozen in a spin-glass-like state, characterized by the appearance of a magnetic correlation length (L) among those spins, and are collectively involved in the exchange coupling with the FM moments. With increasing temperature, L 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 bulk AFM spins [4]. Hence, interfacial AFM spins exhibit a complex magnetic dynamics, governed by intertwined parameters: temperature, anisotropy energy barriers distribution and correlation length L. L is seen to play a key role when the AFM/FM interface undergoes lateral confinement at the nanoscale. In one case, we accomplished that by modulating the AFM/FM interface through the insertion of a Cu spacer (nominal thickness ~ 1Å), resulting in the formation of Cu islands, which strongly affect both the exchange coupling strength and its thermal dependence as their interdistance approaches L. In another case, we studied the magnetic behavior of arrays of dots with different size D = 1000, 500, 300 nm (Fig. 1b) [4]. At T < 100 K, the exchange field Hex increases up to ~ 1000 Oe with reducing D (Fig. 1c). On the other hand, Hex falls down to ~ 100 Oe with further decreasing D down to ~140 nm [5]. These results have been explained, also with the support of micromagnetic calculations, in terms of a strict dependence of the low-temperature value of Hex on the D/L ratio. [Research sponsored by MIUR Italy, project FIRB2010-NANOREST.] 1. C. Chappert, A. Fert, F.N.V. Dau, Nature Mater. 6, 813 (2007) 2. B. Dieny et al., Phys. Rev. B 43,1297 (1991) 3. B. G. Park et al., Nature Mater. 10, 347 (2011) 4. F. Spizzo et al., Phys. Rev. B. 91, 064410 (2015) 5. F. Spizzo et al., J. Magn. Magn. Mater. 400, 242 (2016