Exchange coupled hard-FM/soft-FM and AFM/soft-FM nanostructures: production, characterization and modeling

The increasing demand for miniaturization of magnetic devices triggers an increasing interest for the study of the magnetic properties of elements confined to the nanoscale. Nonvolatile Magnetic Random Access Memories (MRAMs) are an example of such emerging technologies, consisting of arrays of spin-valve cells, each one representing a bit of stored data. To make progress in this technology, a strict control of magnetic stability in nanostructures (whose size is comparable to magnetic critical lengths) to be employed as electrodes in spin-valves of the MRAM architecture is crucial. This study is aimed at developing a nano-spin-valve architecture, by building both the Reference Layer and the Free Layer in form of magnetic two-phase systems, in which a key role in determining the overall magnetic behaviour is played by the exchange interaction between the two different magnetic phases. To this purpose, two different patterned bilayers have been developed by combining sputtering deposition and e-beam lithography (EBL): exchange-biased in-plane dots (soft-ferromagnetic (FM)/antiferromagnetic (AFM) dots) and Soft/Hard perpendicular dots (soft-FM/hard-FM dots). EBL and lift-off processes have been used to obtain magnetic nanostructures on a large area (up to 5x5 mm2),. In the FM/AFM system, we observed that, in the AFM layer, close to the AFM/FM interface, a structurally disordered layer (thickness ~ 2-3 nm) develops [1]. This disorder is possibly the precursor of a disordered magnetic phase, whose presence is in agreement with our observations, that undergoes a freezing process at temperature (T) lower than 100 K, showing a glassy behavior [1]. To enhance the contribution of this phase, we now investigate an array of circular nanodots with a stacking including a thin AFM layer, Cu[3 nm]/Ir25Mn75[3 nm]/Ni80Fe20[3 nm]; the dots and a reference continuous film were produced by DC magnetron sputtering. The size of the dots is (140 ± 5) nm, and the center-to-center distance is 200 nm; their magnetic properties were studied by SQUID measurements in the 5 K – 300 K range. We will present the results of the magnetic characterization, in particular the HEX vs T (HEX = - (Hright+Hleft)/2, Hright and Hleft being the points where the loop intersects the field axis) dependence showing that the signature of the glassy phase, the rapid HEX increase at low T, is indeed observed. We will also compare the features and T dependence of the exchange interaction with that found on the continuous reference film, along with the model we used to explain the magnetic behavior of the circular nanodots array. In the second system, the soft-FM is CoFe, whereas the [Co/Pt]n multilayer - with perpendicular anisotropy, plays the role of the hard-FM; the anisotropy of the dots is tailored by changing the Co layer thickness and the period n in the multilayer structure and the thickness of the CoFe phase. In order to properly chose the sputtering deposition parameters preliminary ab initio electronic structure investigations have been carried out, directed at providing guidelines for [Co/Pt]n multilayer design. The stability of out-of-plane magnetization with respect to in-plane direction has been found to be critically dependent on the number of Co layers and on the relaxations of the induced strains in the multilayer. Once selected two Co/Pt systems, characterized by different out-of-plane anisotropy, exchange coupled [Co/Pt]n/CoFe bilayers have been deposited in form of thin films and patterned systems. The magnetic properties of the dots have been investigated and will be compared to those of continuous films in order to highlight the effect of nanostructuration on the exchange coupling. This research work has been sponsored by MIUR under project FIRB2010-NANOREST. [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, Phys. Rev. B 91 (2015) 064410