Ion Irradiation-Induced Easy-Cone Anisotropy in Double-MgO Free Layers for Perpendicular Magnetic Tunnel Junctions

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 / Fe 72 Co 8 B 20 / X(0.2 nm) / Fe 72 Co 8 B 20 / MgO layer stack, where X stands for an ultrathin Ta or W spacer. Samples with two different total FeCoB layer thicknesses, t FCB = 3.0 nm and t FCB = 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, B K1 , decreased nearly linearly with the logarithm of the fluence for both FeCoB thicknesses and spacer elements. The decrease in B K1 , 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 t FCB = 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. 24–26 reported similar irradiation-induced modulations of B K1 in Co/Pt multilayers with interfacial PMA, attributing their origin to interfacial mixing. Here we have shown that the PMA in the MgO/FeCoB/MgO system decreases with increasing Ar + ion fluence. The anisotropy modulation resulted in a reorientation of the magnetization direction: while the 3.0-nm-thick free layers were driven deeper into the easy-plane regime, spin reorientations were effectively induced in the free layers with t FCB = 2.6 nm, as highlighted in figure


Introduction
The interfacial perpendicular magnetic anisotropy (PMA) existing at the FeCoB/MgO interface is at the origin of the out-of-plane magnetized magnetic tunnel junctions (pMTJ) which serve as the basic storage elements of spin-transfer-torque magnetic random-access memory (STT-MRAM). These memories offer a higher storage density, higher thermal stability and lower power consumption than their in-plane counterparts 1 .
For a sufficiently thin FeCoB film, the PMA overcomes the shape anisotropy, resulting in a perpendicular orientation of the magnetization. To increase the free(storage)-layer volume, V, and thus to improve the data retention, while keeping a strong PMA, two FeCoB/MgO interfaces may be used 1 . To absorb the boron out of the FeCoB layers upon the post-deposition anneals required to recrystallize the MgO barrier and the FeCoB ferromagnetic (FM) electrodes, a thin metal (e.g., Ta, W) spacer is usually introduced in the middle of the FeCoB storage layer [2][3][4][5][6][7] . Besides improving the data retention, the double-MgO storage layer also exhibits a reduced Gilbert damping, α, as compared to thinner ones sandwiched between a single MgO barrier and a heavymetal layer such as Ta or W. In the latter structures, a damping enhancement is often observed resulting from the spin-pumping effect. The suppression of spin pumping in the double-MgO free layers can result in a more than twofold decrease of the damping (and therefore of the critical current for switching, Ic0) accompanied by an almost doubling of PMA (and consequently, of the thermal stability factor, ∆= K eff V/k B T, where K eff is an effective PMA, k B the Boltzmann constant and T the absolute temperature). The result is an overall improved switching efficiency, ∆/Ic0, which further increases as the pMTJ dimensions are reduced 3,8 . Beside free layers for STT-MRAM, double-MgO free layers with a Ta spacer have also been proposed for the development of synthetic ferrimagnetic bilayers 9 as well as remnant spin injectors onto GaAs-based light emitting diodes 10 .
One issue associated with pMTJ stacks is the stochasticity of the STT switching, as a misalignment between the fixed-and free-layer magnetization is necessary for a transfer of angular momentum by a spin-polarized current to occur. Such a misalignment is introduced by thermal fluctuations, but these cause a broad distribution of the switching times due to their random nature. As shown by analytical calculations 11 and macrospin simulations 12 , setting the free layer in a magnetic easy-cone state, with the cone angle providing the misalignment, would greatly improve the switching characteristics of the pMTJ. Such an easy-cone state is described by phenomenologically including a second-order term, K2, in the PMA energy density of a thin film, in addition to the first-order term, K1, i.e., UPMA = -K1cos 2 θ -K2cos 4 θ, with θ being the angle between the magnetization and the normal to the film plane. An easy-cone equilibrium state emerges when K1 > 0, K2 < 0 and -K2/K1 > 0.5. Beside certain alloys (e.g., NdCo5 (Ref. 13 ) and Mn2RhSn (Ref. 10 )), multi-layers containing the FeCoB/MgO interface can also exhibit easy-cone anisotropy, when the FeCoB thickness is properly adjusted to be close to the reorientation from out-of-plane to in-plane anisotropy [14][15][16][17][18][19] .
Author Accepted Manuscript. This article is published in Appl. Phys. Lett. 112, 202403 (2018), It has been analytically shown that a negative K2 can arise from spatial fluctuations of K1 (Ref. 20 ). In our previous study on granular MgO/FeCoB/Ta (Ref. 17 ) the micromagnetic origin of K2 was demonstrated and it was shown that its magnitude was determined by the ratio between the thickness-dependent magnetic inhomogeneities, such as spatial fluctuations of K1, and the intergrain exchange coupling. Another approach to explain the origin of K2, proposed by J. Sun 21 , considers the simultaneous presence of two effects: weakening of the exchange near the MgO/FM interface and a strong interface-concentrated PMA efficiently acting only on the very first atomic layers of the magnetic film. In this case, the resulting 'exchange-spring'-like effect can also be at the origin of a negative K2.
One should notice that the easy-cone anisotropy can be useful not only for STT-MRAM but also for other spintronic applications such as spin-torque oscillators 22 and magnetic sensors 11,19 .
However, reproducibly controlling the easy-cone anisotropy in FeCoB/MgO systems may be technologically challenging, as this state is only found within a narrow range of layer thicknesses.
It would thus be desirable to find a post-deposition process for reorienting the magnetization direction via the control of the interfacial PMA. In that regard, high-energy particle irradiation appears as a simple and reproducible tool to physically modify the interfaces and thus control interfacial magnetic phenomena. A review of the work published in this field until 2004 is given in Ref. 23 . Light ion (He + ) irradiation has been used to reduce the PMA in Pt/Co multilayers by promoting local intermixing at the interfaces 24 and thus to manipulate the magnetization direction of such systems, from out-of-plane to in-plane and also to oblique orientations 25 . Heavier ions such as Ar + were used to modify the properties of Pt/Co films as well 26 . More recently, He + irradiation was used to manipulate the direction of the exchange-bias field in MTJ stacks 27 , to reduce the annealing temperature for crystallizing CoFeB in MgO-based MTJs 28 , and to control the domain wall velocity 29 . Here, we explore the possibility of inducing an easy-cone anisotropy in the technologically relevant MgO/FeCoB/MgO free layers, with and without inclusion of Ta or W spacers, by 400 keV Ar + irradiation. (3), where X stands for a Ta or W spacer, and FeCoB is a Fe72Co8B20 alloy, were deposited by magnetron sputtering onto a thermally oxidized 4-inch Si wafer and annealed at 300ºC for 2 min. The numbers in parentheses are nominal thicknesses in nanometers. The total thickness of the FeCoB free layer (tFCB = 1.4 + t nm) was varied by growing the topmost FeCoB layer as a wedge (0.4 < t < 1.6 nm). Free layers with equal thickness but without spacer were also prepared. Details regarding the sample preparation can be found in Ref. 30 for similarly prepared structures.

Free layers composed of Ta
Samples with an area of 4  4 mm 2 were cut from the wafer ensuring a negligible (± 0.18 Å) nominal thickness gradient. Two thicknesses of the FeCoB layer were selected for two irradiation runs: tFCB = 3.0 nm and tFCB = 2.6 nm. The irradiation with 400 keV Ar + ions was performed at five Author Accepted Manuscript. This article is published in Appl. Phys. Lett. 112, 202403 (2018), different fluences, Φ, ranging from 10 12 cm -2 to 10 16 cm -2 . The irradiation conditions guarantee that elastic and inelastic interactions occur within the multilayers, with expected intermixing of elements across the interfaces (changes in the element concentration profiles smaller than 7  10 -14 in units of %Φ -1 ), whereas the Ar + ions are deposited inside the Si substrate (not shown), as simulated by the TRIM (Transport of Ions in Matter) package included in the SRIM software 31 .
The magnetic properties of the free layers were investigated using angle-dependent ferromagnetic resonance (FMR) measurements performed at room temperature at a microwave frequency of 9.37 GHz in an EPR spectrometer. To account for small variability of magnetic properties of the free layers cut from the same wafer, the FMR spectra were acquired for all samples before and after irradiation. From the out-of-plane angular dependence of the resonance field, Bres (see, e.g., figure 3), we estimated the first-and second-order anisotropy contributions, starting from the description of the magnetic free-energy density: where B K1 = (2k s1 /(t FCB M S )) − 4πM S is the effective first-order anisotropy field resulting from the competing contributions of the interfacial PMA (2k s1 /t FCB ) and the thin-film shape anisotropy

Results
The 3.0-nm-thick FeCoB layers originally exhibited an easy-plane anisotropy, while the 2.6nm-thick layers with a Ta or W spacer were initially in the easy-axis regime, with the magnetization oriented along the film normal. Table I    also occurs (the 10 12 cm -2 point is treated as an outlier, and the value for 10 15 cm -2 and the W spacer may be explained by an increased inhomogeneity of the initial sample inherent to these thinner layers). Both irradiation runs showed a higher decrease in BK1 when the Ta spacer was Author Accepted Manuscript. This article is published in Appl. Phys. Lett. 112, 202403 (2018), used instead of the W one. A slight decrease of |BK2| with the ion fluence increasing up to 10 14 cm -2 was also observed (bottom panels of figures 1a) and 1b)). At 10 16 cm -2 the FMR line disappeared in all cases, probably due to a strong sample intermixing inducing a ferromagneticparamagnetic transition. Both the nearly unchanged BK1 for Φ = 10 12 cm -2 and the lack of FMR at Φ = 10 16 cm -2 are consistent with the order of magnitude found in the TRIM simulations for the maximum profile concentration changes (around 7  10 -14 %Φ -1 ).
Looking at the definition of BK1, its decrease can be explained either by an increase of MS or by a decrease of ks1. Reports on Co thin films (0.5 nm) have shown MS to decrease upon a 10 16 cm -2 He + irradiation at 30 keV (Ref. 25 ). In Ref. 33  spacer for different ion fluences: 0 cm -2 (squares), 10 13 cm -2 (dots) and 10 14 cm -2 (triangles). The solid lines are fits to the data.
The free layer with a W spacer initially exhibits a uniaxial perpendicular magnetic anisotropy.
After an Ar + irradiation with 10 13 cm -2 and 10 14 cm -2 , the magnetization is driven into the easycone state with the cone angle increasing to 30º and 45º, respectively. In the free layer with a Ta spacer, the stronger decrease of BK1 leads to a complete reorientation of the magnetization from easy axis (before irradiation) to easy cone at 10 13 cm -2 and then to easy plane at 10 14 cm -2 .
Author Accepted Manuscript. This article is published in Appl. Phys. Lett. 112, 202403 (2018), The effect of the irradiation on the peak-to-peak linewidth, ∆BPP, was also assessed for those free layers whose magnetization direction was reoriented (figures 3a) and 3b)). We modelled the ∆BPP(θ M ) dependence using two contributions to the linewidth: the intrinsic one, proportional to the Gilbert damping, α, and the inhomogeneous one caused by fluctuations of the anisotropy fields, ∆(BKi). Fluctuations of the anisotropy direction, ∆θ B , were found to be negligible in our layers. Following the method of Ref. 34 where γ and ω are the gyromagnetic ratio and the magnetization precession frequency, respectively.
We found the linewidth to be dominated by the inhomogeneous broadening caused by spatial  The intrinsic broadening, on the other hand, has a different angular dependence, with equal minima found at θB = 0º and θB = 90º and can therefore be distinguished from the inhomogeneous one during the fitting. From the fittings of figures 3a) and 3b), we made the estimation of α reported in figure 3c).

Conclusions
We irradiated MgO / FeCoB(tFCB) / MgO free layers, with and without a 0.2-nm-thick W or Ta spacer, and tFCB = 3.0 nm or tFCB = 2.6 nm, with 400 keV Ar + ions at fluences ranging from 10 12 cm -2 to 10 16 cm -2 . The effective first-order PMA field, BK1, decreased linearly with the logarithm of the fluence, from 10 12 cm -2 to 10 15 cm -2 . We suggest this decrease in BK1 to be a consequence of ioninduced intermixing at the FeCoB/MgO interface.
We have shown that the decrease in PMA caused by the ion irradiation induces spin reorientations in these free layers. While the 3.0-nm-layer remains in its original easy-plane regime, we effectively produce easy-cone anisotropy, with different cone angles, in the thinner layer (2.6 nm) with a W spacer and obtain a complete spin reorientation from easy axis to easy cone and then to easy plane in the free layer with a Ta spacer. Importantly, the irradiation, at fluences for which these transitions occur, did not increase the Gilbert damping parameter, which is vital, from an application viewpoint, to keep a low current density for STT-switching.