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

B M S Teixeira,S Auffret,E Alves,B Dieny,N F F Caçoilo,A A Timopheev,N A Sobolev,R C Sousa

doi:10.1063/1.5026854

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 1012 cm−2 to 1016 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, 1013 cm−2 and 1014 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 memory devices, oscillators, and sensors.

Highlights

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 which serve as the basic storage elements of spin-transfer-torque magnetic random-access memory (STTMRAM)
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
The effective first-order PMA field, BK1, decreased linearly with the logarithm of the fluence, from 1012 cm-2 to 1015 cm-2. We suggest this decrease in BK1 to be a consequence of ioninduced intermixing at the FeCoB/MgO interface

Summary

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 (STTMRAM). To increase the free(storage)-layer volume, V, and to improve the data retention, while keeping a strong PMA, two FeCoB/MgO interfaces may be used[1]. 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.

Results

Conclusion