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.
Highlights
Hard-axis magnetoresistance loops were measured on perpendicular magnetic tunnel junction pillars of diameter ranging from 50 to 150 nm
This work was followed by experiments carried out on ultrathin NiFe films grown on Cu(111)[2] which confirmed the interfacial nature of the perpendicular magnetic anisotropy (PMA) observed in this system
While easy-axis magnetoresistance loops allow for determination of switching current and coupling fields, hard axis magnetoresistance loops provide additional information about the magnetic anisotropy in pMTJ pillars
Summary
Configuration or in the initial AP configuration giving two hard-axis hysteresis branches. In this regime, an component of the reference layer magnetization around its easy cone thereby skipping the parabolic part of the R(H) curve, resulting in the observed triangular shape of the R(H) response at low temperature. One cannot rule out a priori that certain types of micromagnetic distortions in the ferromagnetic electrodes could be responsible for the observed hard-axis MR(H) curve deformations in the studied pillars at low temperatures To exclude this possibility, experiments were conducted at sheet film level in order to check whether the second order anisotropy is evidenced in this case. Knowing that the “real” films are in multidomain state, we did not try to match the macrospin simulations and experiments exactly
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