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We investigated the effects of sputtering conditions for the deposition of an Iridium (Ir) layer in a [Co/Pt]m/Co/Ir/[Co/Pt]n/Co/W/CoFeB synthetic antiferromagnetic reference layer (Ir-SyF) on the magnetic properties and tunnel magnetoresistance ratio (TMR ratio) of magnetic tunnel junctions (MTJs) stacks annealed at 400 °C for 1 h. The exchange coupling field ($H_{\mathrm {ex}}$) of Ir-SyF was improved by reducing the energy of Ir recoil ions and two times larger than that with Ru-SyF. Energy dispersive X-ray (EDX) spectrometry line analysis revealed greater interlayer diffusion in Ir when Ir was sputtered by using a conditions with large recoiled energy. This could cause the deterioration of the $H_{\mathrm {ex}}$ of the Ir-SyF. Despite the larger $H_{\mathrm {ex}}$ , the TMR ratio of the MTJ with Ir-SyF is smaller than that with Ru-SyF. The $m$ – $H$ curve of MTJ with Ru-SyF showed a large plateau region around zero magnetic field, whereas that with Ir-SyF did not. These results indicated the degradation of perpendicular magnetic anisotropy (PMA) in the top part of the Co/Pt multilayer with CoFeB reference layer and a large biquadratic coupling effect in the thin Ir layer. This causes the deterioration of the TMR ratio of the MTJ with Ir-SyF. TEM image of the Co/Pt layer in the MTJ with Ir shows some lattice defects. The EDX line analysis revealed that a large amount of Pt in the top Co/Pt layer diffused toward CoFeB reference layer in the Ir-SyF, resulting in the degradation of PMA. The structural analysis by X-ray diffraction showed the lattice spacing of CoPt (111) in Ir-SyF to be larger than that in Ru-SyF, indicating the occurrence of strain relaxation at the Co/Pt interface. These crystallographic changes in Ir-SyF might be related to a larger Pt diffusion. Suppression of Pt diffusion as well as low damage Ir deposition in the reference layer is crucial to utilize Ir-SyF. [ABSTRACT FROM AUTHOR]

We investigated the effect of magnetic coupling (the energy constant of unit area $J_{\text {cpl}}$) between CoFeB free layers on thermal stability factor $\Delta $ in magnetic tunnel junction (MTJ) with CoFeB/MgO double interfacial perpendicular magnetic anisotropy (IPMA). We newly introduced $J_{\text {cpl}}$ in models for calculations, which were based on magnetic domain propagation model when MTJ diameter $d$ is larger than critical diameter $d_{\text {c}}$ and magnetization coherent rotation model when $d \le d_{\text {c}}$. With increasing $J_{\text {cpl}}$ , $\Delta $ increases and saturates when $J_{\text {cpl}}$ is over a critical value $J_{\mathrm {cpl\_{}c}}$. Magnetostatic coupling constants ($J_{\text {stat}}$) between the free layers were also calculated. $J_{\text {stat}}$ is much smaller than $J_{\mathrm {cpl\_{}c}}$ and cannot maximize $\Delta $ by itself, so an interlayer exchange coupling (the critical energy constant of unit area $J_{\mathrm {ex\_{}c}}$) is required to cover the difference between $J_{\mathrm {cpl\_{}c}}$ and $J_{\text {stat}}$. $J_{\mathrm {ex\_{}c}}$ also rapidly increases with the decrease in $d$ and reaches a plateau of 0.15 mJ/m2 in $d \le 30$ nm. MTJ devices with the smallest $J_{\text {ex}}\,\,(=0.01 \text {mJ/m}^{2}$) and a moderate PMA (constant per unit area $K_{\text {eff}} t^{\ast }$) were made and $\Delta s$ were evaluated by experiments. The experimental $\Delta s$ were fitted by the calculations with adjusting stiffness constants $A_{s}$. The calculated $\Delta s$ and the experimental $\Delta s$ were in good agreement when $A_{s}$ had a smaller value than that of the blanket film. Since the experimentally used $J_{\text {ex}}$ value was almost 0, $\Delta $ values were estimated when $J_{\text {ex}}$ was a greater value of 0.15 mJ/m2. As a result, 30% enhancement of $\Delta $ was obtained at the smallest $d = 20$ nm. [ABSTRACT FROM AUTHOR]

We developed perpendicular magnetic tunnel junctions (MTJs) with four synthetic anti-ferromagnetically coupled Co/Pt layers (quad-SyF) and investigated their magnetic and transport properties. The quad-SyF comprised four Co/Pt layers and three 0.9 nm-thick Ru coupling layers, which consisted of Co/[Co/Pt] ${}_{a}$ /Ru/Co/[Co/Pt] ${}_{b}$ /Ru/Co/[Co/Pt] ${}_{c}$ /Ru/Co/[Co/Pt] ${}_{d}$ from top to bottom. The exchange coupling field ($H_{\mathrm {ex}}$) reached a maximum of 1 T when the values of $a$ , $b$ , $c$ , and $d$ were 1, 2, 2, and 1, respectively. The tunnel magnetoresistance ratio of the MTJ with the quad-SyF and the second-peak conventional double-SyF increased as the annealing temperature was increased up to 400 °C, whereas that of the MTJ with the first-peak conventional double-SyF degraded at temperatures of more than 350 °C in blanket films. A 55 nm diameter MTJ with quad-SyF was found to be stable even against an external magnetic field up to 300 mT. On the contrary, in the conventional double-SyF, the reference-layer (RL) magnetization direction flips at around 250 mT. The shift magnetic field of the MTJ with quad-SyF becomes approximately zero when the values of $a$ , $b$ , $c$ , and $d$ are 1, 4, 1, and 2, respectively. No back-hopping of the MTJ with quad-SyF was observed even for the write pulsewidth ($t_{W}$) down to 10 ns. On the contrary, an MTJ with conventional double-SyF exhibited back-hopping. In the patterned MTJ with conventional double-SyF, as the MTJ size decreases, the coercive field of Co/Pt significantly increases and $H_{\mathrm {ex}}$ decreases, causing the $m$ – $H$ curve of the RL to cross the zero magnetic field. This enables both parallel and antiparallel configurations for the top and bottom Co/Pt layers in double-SyF at the zero magnetic field, which could induce back-hopping. However, the $m$ – $H$ curves of the RL in the patterned MTJ with quad-SyF are far from the zero magnetic field owing to the high $H_{\mathrm {ex}}$ and low $H_{c}$ , which could lead to the suppression of back-hopping. [ABSTRACT FROM AUTHOR]

We successfully developed 25-nm quad CoFeB/MgO-interfaces perpendicular magnetic tunnel junction (quad-MTJ) with enough thermal stability. To fabricate the quad-MTJ, a physical vapor deposition (PVD) process for depositing novel free layer and low resistance-area (RA) product MgO layer and low-damage fabrication processes were developed. The developed quad-MTJ technology and advanced process bring better tunnel magneto resistance (TMR) ratio and RA to quad-MTJ than those of double-interface MTJ (double-MTJ), even though quad-MTJ has an additional MgO layer. Scaling down the MTJ size to 25 nm, we demonstrated the advantages of quad-MTJ compared with double-MTJ as follows: 1) two times larger thermal stability factor (Δ), which results in over ten years retention; 2) superiority of large Δ in the measuring temperature range up to 200 °C; 3) ~1.5 times higher write efficiency; 4) lower write current at short write pulse regions at less than 100 ns; and e) excellent endurance of over 1011 thanks to higher write efficiency, which results from the reduced voltage owing to low RA and the low damage integration process technology. As a result, the developed quad-MTJ technologies will open the way for the realization of high-density STT-MRAM with low power, high speed, high reliability, and excellent scalability down to 2 × nm node. [ABSTRACT FROM AUTHOR]

We have successfully observed the process-induced microstructure of the ultrafine CoFeB–MgO-based magnetic tunnel junctions with a perpendicular easy axis (p-MTJs) down to junction size of less than 20 nm using scanning transmission electron microscope (STEM) tomography combined with energy-dispersive X-ray spectroscopy (EDX). This made it possible to examine 3-dimensionally the precise structural information inside the entire MTJ. By this, the root cause of the change in the resistance of the MTJ was found to be the altered region consisting of Fe-dipping attached to the edge of the MgO and ultrathin TaO layer uniformly formed on the MTJ surface. This altered region is caused by etching during patterning of the MTJ, and the resistance value of the MTJ changes depending on the degree of adhesion of insulating TaO and Fe atoms at the edge of the MgO barrier layer. Furthermore, it was found that the unevenness of the MgO/CoFeB interface inside the MTJ was not a factor that caused the change in the thickness of the MgO barrier layer in the current process. Our results show that STEM tomography is an effective tool for failure analysis to clarify the relationship between the detailed structure of fine MTJs and magnetic properties. Unlike the conventional cross-sectional STEM observation, since the STEM tomographic observation includes the whole view of the MTJ, this enables higher resolution observation as the MTJ size decreases without overlooking the cause of failure inside the MTJ. [ABSTRACT FROM AUTHOR]

We have studied the magnetic and electrical properties of double CoFeB–MgO interface magnetic tunnel junctions (MTJs) with various W insertion layer thicknesses $t_{\mathrm {W}}$ annealed at 400 °C and evaluated $t_{\mathrm {W}}$ dependence of thermal stability factor $\Delta $ , intrinsic critical current $I_{\mathrm {C0}}$ and damping constant $\alpha $. It was found that spin-transfer torque efficiency ($\Delta /I_{\mathrm {C0}}$) values increased with decreasing $t_{\mathrm {W}}$ as compared with the same MTJ size $D$. $\alpha $ measured by ferromagnetic resonance increased with an increase in $t_{\mathrm {W}}$ , which is consistent with those calculated from $\Delta /I_{\mathrm {C0}}$. This result indicates that lower damping constant of MTJ with thin W insertion layer contributes to higher $\Delta /I_{\mathrm {C0}}$. [ABSTRACT FROM AUTHOR]

We investigated an effect of sputtering gas species (Ar, Kr, and Xe) for deposition of a W insertion layer in the CoFeB/W/CoFeB free layer on magnetic properties of the free layer and tunnel magnetoresistance (TMR) ratio of magnetic tunnel junctions (MTJs) stacks using the free layer annealed at 400 °C for 1 h. As the W insertion layer thickness tW increased, we found the degradation of perpendicular anisotropy and larger reduction of saturation magnetic moment per unit area mS in the free layer using Ar compared to those using Kr and Xe. We also found a smaller TMR ratio for the MTJ stack using Ar compared to those using Kr and Xe. Energy-dispersive X-ray spectrometry line analysis revealed more significant interdiffusion between W and CoFeB layers in the free layer using Ar than those using Kr and Xe, that could result in the smaller mS and perpendicular anisotropy in the free layer and smaller TMR ratio for the MTJ stack using Ar than those using Kr and Xe. We also investigated concentration of Ar, Kr, and Xe in W layers deposited using Ar, Kr, and Xe, respectively, by high-resolution Rutherford backscattering spectrometry, revealing that 0.2 at% Ar was detected in the W layer using Ar, while Kr and Xe were not detected in W layers using Kr and Xe. Such a difference in concentration of inert gas atoms in the W layer could be one possible reason for the difference about degree of interdiffusion between W and CoFeB layers. [ABSTRACT FROM AUTHOR]

We investigated annealing temperature Ta dependence of tunnel magnetoresistance (TMR) ratio and magnetic properties for perpendicular-anisotropy (CoFe)100–XBX/MgO magnetic tunnel junctions (MTJs) with single (CoFe)100–XBX/MgO interface (s-MTJ) and double CoFeB–MgO interface (d-MTJ) structures with various boron compositions X. High TMR ratio over 100% was observed in the s-MTJ with $X= 35$ at.% after annealing at 360 °C–400 °C, whereas the s-MTJ with X = 30$ at.% showed the degradation of TMR ratio with the increase of X = 25$ at.% maintained high TMR ratio up to owing to its higher perpendicular anisotropy compared with the s-MTJ. The difference of perpendicular anisotropy between the s-MTJ and the d-MTJ can be attributed to higher interfacial anisotropy together with lower saturation magnetization of the d-MTJs. The lower saturation magnetization is attributable to two MgO layers that suppress boron diffusion from CoFeB layers, which was verified by cross-sectional line analysis using electron energy-loss spectroscopy. [ABSTRACT FROM PUBLISHER]

Diffusion and redistribution of constituent elements on the CoFeB surface after natural oxidation and annealing were investigated by means of X-ray photoelectron spectroscopy with argon beam sputtering. Boron and iron atoms were found to redistribute into the $\sim 3$ nm-thick oxidized surface by the natural oxidation even at room temperature. After annealing up to 400 °C in vacuum, boron atoms drastically diffused into the oxidized surface to reduce the oxidized iron/cobalt. The migration of boron atoms was observed within the depth of $\sim 6$ nm. In contrast, iron atoms diffused little. The diffusion and redistribution behaviors during the redox reactions, such as the natural oxidation and annealing, are speculated to be related to the values of the standard free energy of formation of oxides. In other words, the reduction nature of elements on the CoFeB surface is speculated to be the driving force in the diffusion and redistribution of iron and boron on the surface during the redox reaction. [ABSTRACT FROM AUTHOR]