Second order anisotropy contribution in perpendicular magnetic tunnel junctions

Abstract

Hard-axis magnetoresistance loops were measured on perpendicular magnetic tunnel junction pillars of diameter ranging from 50 to 150 nm. By fitting these loops to an analytical model, the effective anisotropy fields in both free and reference layers were derived and their variations in temperature range between 340 K and 5 K were determined. It is found that a second-order anisotropy term of the form −K2cos4θ must be added to the conventional uniaxial –K1cos2θ term to explain the experimental data. This higher order contribution exists both in the free and reference layers. At T = 300 K, the estimated −K2/K1 ratios are 0.1 and 0.24 for the free and reference layers, respectively. The ratio is more than doubled at low temperatures changing the ground state of the reference layer from “easy-axis” to “easy-cone” regime. The easy-cone regime has clear signatures in the shape of the hard-axis magnetoresistance loops. The existence of this higher order anisotropy was also confirmed by ferromagnetic resonance experiments on FeCoB/MgO sheet films. It is of interfacial nature and is believed to be due to spatial fluctuations at the nanoscale of the first order anisotropy parameter at the FeCoB/MgO interface.

Stability phase diagram of a perpendicular magnetic tunnel junction in noncollinear geometry

“…The signature of this higher order anisotropy term is not always visible in the collinear geometry but clearly shows up once the field is applied away from the normal to the plane of the sample. Such second order anisotropy contribution is similar to that reported previously [17,18,[22][23][24][25].…”

“…Additional information on these samples can be found in Ref. [18]. The procedure for VH stability diagram measurements is similar to that described in Ref.…”

“…A geometry which received lot of attention is that of orthogonal polarizer and storage layer (so-called OST-MRAM) in which sub-ns switching was demonstrated [13][14][15]. Recently, it was shown also that non-collinearity between the free layer and the reference layer could be induced by introducing an easy-cone anisotropy in one of the MTJ magnetic electrode, preferably the storage layer [16][17][18]. This easy-cone anisotropy results from higher order anisotropy term which may themselves originate from spatial fluctuations of first order anisotropy [19].…”

State diagram of a perpendicular magnetic tunnel junction driven by spin transfer torque: A power dissipation approach

“…The influence of the second order term was reported in recent experiments and it could result from the atomic structure at the interface and strains due to lattice mismatch [26,27].…”

Increased energy efficiency spin-torque switching of magnetic tunnel junction devices with a higher order perpendicular magnetic anisotropy

“…Subsequently, several experimental studies focused on either this structure, putting in evidence the second order magnetic anisotropy for pMTJs with thin CoFeB layers (1.2-1.35 nm) using FMR, or magnetoresistance for patterned structures. [12][13][14] Other studies explored the influence of this effect on pMTJ state diagrams. [15][16][17][18][19] Reference 19 considered the influence of a second order anisotropy on the switching current.…”