Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions

Abstract

We have used the ferromagnetic resonance in the X-band (9.37 GHz) to investigate the effect of 400 keV Ar + irradiation on the perpendicular magnetic anisotropy (PMA) and Gilbert damping parameter, α, of double-MgO free layers designed for application in perpendicular magnetic tunnel junctions. The samples comprised a MgO / Fe72Co8B20 / X(0.2 nm) / Fe72Co8B20 / MgO layer stack, where X stands for an ultrathin Ta or W spacer. Samples with two different total FeCoB layer thicknesses, tFCB = 3.0 nm and tFCB = 2.6 nm, were irradiated with ion fluences ranging from 10 12 cm-2 to 10 16 cm-2. The effective first-order PMA field, BK1, decreased nearly linearly with the logarithm of the fluence for both FeCoB thicknesses and spacer elements. The decrease in BK1, which is likely caused by an ion-induced intermixing at the FeCoB/MgO interfaces, resulted in a reorientation of the magnetization of the free layers with tFCB = 2.6 nm, initially exhibiting a perpendicular easy-axis anisotropy. For intermediate fluences, 10 13 cm-2 and 10 14 cm-2 , easy-cone states with different cone angles could be induced in the free layer with a W spacer. Importantly, no corresponding increase in the Gilbert damping was observed. This study shows that ion irradiation can be used to tune the easy-cone anisotropy in perpendicular magnetic tunnel junctions, which is interesting for spintronic applications such as spin-torque magnetic memories, oscillators and sensors.

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

“…There are at least two reasons for that: First, K2 B in these Ta-capped films is smaller in comparison with the double-MgO free layers of Ref. 26 , as discussed above. Starting with a smaller K2 B restricts the range of K1eff B values for which the easy cone can be obtained.…”

“…In a previous work 26 we demonstrated that 400 keV Ar + ion irradiation can induce the easycone anisotropy in initially perpendicularly magnetized MgO/FeCoB/X/FeCoB/MgO stacks (X = Ta or W spacer). Such an easy-cone anisotropy is sought after as it can lead to faster and lower-energy spin-transfer torque (STT) switching, thanks to the intrinsic tilt in magnetization direction provided by the easy cone [27][28][29] .…”

“…In the double-MgO layer, ion-induced intermixing of the FeCoB/MgO interface produces a more rapid decrease of s1 k than of S M , resulting in a FL K1eff B evolving into the negative values. Consequently, in double-MgO free layers, with increasing ion fluence, it is possible to reorient the magnetization from easy axis to easy cone and then to easy plane 26 . In contrast, in a Ta-capped free layer, the ion irradiation promotes a stronger decrease of MS due to intermixing at the top FL/Ta interface than of ks1 at the MgO/FeCoB and, thus, FL By extrapolating the trend observed for the anisotropy of the Ta-capped free layer, the easycone anisotropy is expected to be reached by irradiating an initially in-plane magnetized film (see figure 6(b)).…”

“…The use of ion irradiation in Ref. 26 was motivated by the fact that it can be technologically challenging to reproducibly set the easy-cone ground state in multilayer stacks containing a FeCoB/MgO interface, as it is only accessible within a narrow range of FeCoB thicknesses (see e.g. Refs.…”

Unveiling temperature dependence mechanisms of perpendicular magnetic anisotropy at Fe/MgO interfaces

“…Typically, the first order term K 1 dominates the higher order ones in structures with interfacial anisotropy. However, experiments have observed the influence of the second order K 2 , namely in the formation of an easy-cone state [ [24,25]]. The origin of significant K 2 values is attributed to spatial fluctuations of the film thickness [ [23,26]], strongly interface-concentrated PMA combined to a moderate exchange coupling of the interface moment to the rest of the film [ [27]] or strong magnetic inhomogeneities [ [28]].…”