Investigation of magnetic hysteresis in biased Ta/Pt/Co/FeMn/Ta antidots: Influence of structural dimensions

Abstract

There exists a controversy in the literature concerning the values of coercive and bias fields in antidots magnetic structures formed by a hexagonal network of nanoholes. The coercive fields (HC) and the exchange bias fields (∣HEXC∣) for antidots (deposited on ultrathin anodic aluminum oxide, namely, AAO) are either increased or diminished by comparison with the same magnetic nanostructures grown on continuous substrates (namely, CML). We propose to elucidate these debates by showing the importance of the easy axis of the magnetization, the direction of the applied magnetic field, the thicknesses of the layers, and the 3D-topology of nanoholes, as well as the magnetic and thermal history of the magnetic measurements. Here, biased Ta(5 nm)/Pt(5 nm)/Co(0.6 nm)/Fe50Mn50(X)/Ta(5 nm) antidots are investigated by extraordinary Hall effect measurements at 5 K, where X varies in the (0–5.5) nm range. The substrate consists in a hexagonal array of holes, described by the pair of (p,d) values, respectively, the period as the distance from center to center of two consecutive holes and the hole diameter. The dimensions of antidots are (p≈100 and d≈40 nm) for X=(2–5.5) nm, (p≈150 and d≈60 nm) for X=3.5 nm, and (p≈100 and d≈60 nm) for X=0. A continuous stack using Si/SiO2(100 nm) is used for comparison. HC and ∣HEXC∣ gradually increase when X is enhanced for both substrates, with nevertheless a weak decrease at high X for the continuous system. Perpendicular magnetic anisotropy is only observed for both unbiased samples, the X=2 nm continuous sample, and both X=5 nm samples that have undergone field cooling treatment from 500 to 5 K under −2 T. Usually, HC(AAO)>HC(CML), ∣HEXC(AAO)∣>∣HEXC(CML)∣, and ∣HA(AAO)∣<∣HA(CML)∣ (HA designating the anisotropy field). However, for certain conditions, as, for instance, for FC-procedures starting from high temperatures and/or strong magnetic field, other situations might be observed. A discussion pertaining to the amplitudes of HC, ∣HEXC∣ and the anisotropy field (∣HA∣) of continuous and discontinuous samples is given for our experimental results as well as for published data in the literature, in the light of structural characteristics (wedge-to-wedge distance, porosity, or coverage ratio). Such biased perpendicular antidots might be particularly used in specific nanomaterials devoted to spintronics.