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Register a new userCorrelation of tunnel magnetoresistance with the magnetic properties in perpendicular CoFeB-based junctions with exchange bias
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We investigate the dependence of magnetic properties on the post-annealing temperature/time, the thickness of soft ferromagnetic electrode and Ta dusting layer in the pinned electrode as well as their correlation with the tunnel magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions of materials sequence Ta/Pd/IrMn/CoFe/Ta$(textit{x})$/CoFeB/MgO$(textit{y})$/CoFeB$(textit{z})$/Ta/Pd. We obtain a large perpendicular exchange bias of 79.6$,$kA/m for $x=0.3,$nm. For stacks with $z=1.05,$nm, the magnetic properties of the soft electrode resemble the characteristics of superparamagnetism. For stacks with $x=0.4,$nm, $y=2,$nm, and $z=1.20,$nm, the exchange bias presents a significant decrease at post annealing temperature $T_textrm{ann}=330,^{circ}$C for 60 min, while the interlayer exchange coupling and the saturation magnetization per unit area sharply decay at $T_textrm{ann}=340,^{circ}$C for 60 min. Simultaneously, the tunnel magnetoresistance ratio shows a peak of $65.5%$ after being annealed at $T_textrm{ann}=300,^{circ}$C for 60 min, with a significant reduction down to $10%$ for higher annealing temperatures ($T_textrm{ann}geq330,^{circ}$C) and down to $14%$ for longer annealing times ($T_textrm{ann}=300,^{circ}$C for 90 min). We attribute the large decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the pinned electrode.
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Recently, magnetic tunnel junctions with perpendicular magnetized electrodes combined with exchange bias films have attracted large interest. In this paper we examine the tunnel magnetoresistance of Ta/Pd/IrMn/Co-Fe/Ta/Co-Fe-B/MgO/Co-Fe-B/capping/Pd magnetic tunnel junctions in dependence on the capping layer, i.e., Hf or Ta. In these stacks perpendicular exchange bias fields of -500,Oe along with perpendicular magnetic anisotropy are combined. A tunnel magnetoresistance of $(47.2pm 1.4)%$ for the Hf-capped sample was determined compared to the Ta one $(42.6pm 0.7)%$ at room temperature. Interestingly, this observation is correlated to the higher boron absorption of Hf compared to Ta which prevents the suppression of $Delta_{textrm{1}}$ channel and leads to higher tunnel magnetoresistance values. Furthermore, the temperature dependent coercivities of the soft electrodes of both samples are mainly described by the Stoner-Wohlfarth model including thermal fluctuations. Slight deviations at low temperatures can be attributed to a torque on the soft electrode that is generated by the pinned magnetic layer system.
Thermal stability factor (delta) of recording layer was studied in perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with various CoFeB recording layer thicknesses and junction sizes. In all series of p-MTJs with different thicknesses, delta is virtually independent of the junction sizes of 48-81 nm in diameter. The values of delta increase linearly with increasing the recording layer thickness. The slope of the linear fit is explained well by a model based on nucleation type magnetization reversal.
The transport properties of magnetic tunnel junctions (MTJs) are very sensitive to interface modifications. In this work we investigate both experimentally and theoretically the effect of asymmetric barrier modifications on the bias dependence of tunneling magnetoresistance (TMR) in single crystal Fe/MgO-based MTJs with (i) one crystalline and one rough interface and (ii) with a monolayer of O deposited at the crystalline interface. In both cases we observe an asymmetric bias dependence of TMR and a reversal of its sign at large bias. We propose a general model to explain the bias dependence in these and similar systems reported earlier. The model predicts the existence of two distinct TMR regimes: (i) tunneling regime when the interface is modified with layers of a different insulator and (ii) resonant regime when thin metallic layers are inserted at the interface. We demonstrate that in the tunneling regime negative TMR is due to the high voltage which overcomes the exchange splitting in the electrodes, while the asymmetric bias dependence of TMR is due to the interface transmission probabilities. In the resonant regime inversion of TMR could happen at zero voltage depending on the alignment of the resonance levels with the Fermi surfaces of the electrodes. Moreover, the model predicts a regime in which TMR has different sign at positive and negative bias suggesting possibilities of combining memory with logic functions.
Current-driven magnetization switching in low-resistance Co40Fe40B20/MgO/Co40Fe40B20 magnetic tunnel junctions (MTJs) is reported. The critical-current densities Jc required for current-driven switching in samples annealed at 270C and 300C are found to be as low as 7.8 x 10^5 A/cm^2 and 8.8 x 10^5 A/cm^2 with accompanying tunnel magnetoresistance (TMR) ratios of 49% and 73 %, respectively. Further annealing of the samples at 350C increases TMR ratio to 160 %, while accompanying Jc increases to 2.5 x 10^6 A/cm^2. We attribute the low Jc to the high spin-polarization of tunnel current and small MsV product of the CoFeB single free layer, where Ms is the saturation magnetization and V the volume of the free layer.
We report the intrinsic critical current density (Jc0) in current-induced magnetization switching and the thermal stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature, respectively) in MgO based magnetic tunnel junctions with a Co40Fe40B20(2nm)/Ru(0.7-2.4nm)/Co40Fe40B20(2nm) synthetic ferrimagnetic (SyF) free layer. We show that Jc0 and E/kBT can be determined by analyzing the average critical current density as a function of coercivity using the Slonczewskis model taking into account thermal fluctuation. We find that high antiferromagnetic coupling between the two CoFeB layers in a SyF free layer results in reduced Jc0 without reducing high E/kBT.
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