Current-Induced Crystallisation in Heusler Alloy Films

Abstract

The current information technology has been relied on von Neumann type computation [1]. In order to manage the recent digital universe and big data, it is essential to investigate alternative technologies to sustain the development in computation. Among them, neuromorphic computation has been attracting intensive studies using the current complementary metal oxide semiconductor (CMOS) architecture and beyond in the last decade to mimic the functionality and operation of a synapse in a brain. A promising synapses operation has been demonstrated via the stochastic operation of a magnetic tunnel junction (MTJ) [2]. However another important feature of a synapse, memory potentiation, has been overlooked to date. In this study we have demonstrated memory functionality via current-induced annealing in a giant magnetoresistive (GMR) junction with ferromagnetic Heusler alloy films.A GMR junction consisting of Cr (3)/W (10)/Co2FeAl0.5Si0.5 (CFAS) (10)/Ag (3)/CFAS (2.5)/Ru (3) (thickness in nm) was fabricated as an artificial synapse with achieving a low resistance-area product for low power consumption. Here we recently reported the reduction in the crystallisation temperature by promoting layer-by-layer growth on a (110) surface [3], of which is comparable with Joule heating induced by a controlled current introduction. The multilayered stack was grown on a thermally oxidised Si substrate using a high target utilisation sputtering system (PlasmaQuest, HiTUS) at room temperature. The stack was patterned into a nanopillar junction using a combination of electron-beam lithography and Ar-ion milling. The seed layer, Cr/W, was patterned into a bottom electrode with a width of 100-200 nm. Just above the bottom Heusler-alloy layer was patterned into a nanopillar with a diameter between 80 and 200 nm, followed by the patterning of a top electrode with Ti/Au.The fabricated nanopillars were measured using a probe station (HiSOL, HMP-400 SMS) with a conventional four-terminal method with a constant current source (Keithley, 2400) and a nanovoltmeter (Keithley, 2182a). The resistance changes were measured after a series of current pulse applications of 500 µA up to 5 mA for 100 µs up to 500 µs. Little change was observed at 500 μA, which was used for GMR measurements, while monotonic resistance change was measured above 1 mA with the repetition of 100 µs pulse introduction, confirming the current-induced crystallisation of CFAS(110) and the associated change in the resistivity.The Heusler-alloy crystallisation was then observed using transmission electron microscope (TEM; JEOL, JEM-2200FS) by thinning with focused ion beam (FIB, FEI, Nova 200 Dual Beam). The crystallinity of the CoFeAl0.5Si0.5 has been confirmed using nanobeam, which shows some CFAS(220) diffraction spots within polycrystalline ring feature. TEM imaging confirmed CFAS was partially crystallised with maintaining a smooth interface of < 1 nm roughness in the GMR junction. Due to the nature of a simple electrical current introduction employed in this study, the GMR junction does not require post-annealing processes but it can stores the operation cycle permanently as the change in the resistivity. Hence such current-induced crystallisation can be used in a neuromorphic node network, which can revolutionalise solid state memory. **