Correlation Between Perpendicular Anisotropy and Magnetoresistance in Magnetic Tunnel Junctions (original) (raw)
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Tunneling magnetoresistance of perpendicular CoFeB-based junctions with exchange bias
Journal of Applied Physics
Recently, magnetic tunnel junctions with perpendicular magnetized electrodes combined with exchange bias films have attracted great 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 dependent on the capping layer, i.e., Hf or Ta. In these stacks, perpendicular exchange bias fields of À500 Oe along with perpendicular magnetic anisotropy are combined. A tunnel magnetoresistance of (47.2 6 1.4)% for the Hf-capped sample was determined compared to the Ta one (42.6 6 0.7)% at room temperature. Interestingly, this observation is correlated with the higher boron absorption of Hf compared to Ta, which prevents the suppression of the D 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 which is generated by the pinned magnetic layer system. Published by AIP Publishing.
Perpendicular Magnetic Tunnel Junctions with CoFe/Pd Multilayer Electrodes and an MgO Barrier
IEEE Transactions on Magnetics, 2000
We studied the magnetic and magnetoresistance characteristics of pseudospin-valve magnetic tunnel junctions (MTJs) based on CoFe/Pd multilayer electrodes with perpendicular magnetic anisotropy and an MgO barrier. The MTJs at annealing temperature ( a ) of 473 K showed a tunnel-magnetoresistance (TMR) ratio of 1.5%. An fcc (111)-oriented texture of the bottom and top Co 90 Fe 10 Pd multilayer electrodes, together with an imperfectly crystallized MgO, were revealed by cross-sectional TEM images. The TMR properties of perpendicular MTJs with a Co 20 Fe 60 B 20 or Co 50 Fe 50 layer inserted between the CoFe/Pd multilayer electrodes and the MgO barrier were also studied. The TMR ratio with Co 20 Fe 60 B 20 insertion was 1.7% at a = 473 K and monotonically decreased at a over 523 K. The TMR ratio with Co 50 Fe 50 insertion increased up to 3% at a = 573 K and then decreased to 0.4% at a = 598 K. The influence of the Pd layer on CoFeB was studied by using the simplified structures of Pd/CoFeB/MgO/CoFeB/Pd and Ta/CoFeB/MgO/CoFeB/Ta with inplane anisotropy. A former structure with Pd resulted in reduced TMR ratio which decreases with increasing a , whereas MTJs with a Ta-based structure showed a monotonic increase of a TMR ratio. The low TMR ratio observed in Pd-containing structures appears to result from crystallization of CoFeB in an unfavorable crystal orientation.
Tunnel Magnetoresistance Effect in CoFeB/MgAlOx/CoFeB Magnetic Tunnel Junctions
IEEE Transactions on Magnetics, 2000
Magnetic tunnel junctions (MTJs) with the core structure of CoFeB MgAlO x CoFeB were fabricated using magnetron sputtering technique. The MgAlO x tunnel barrier was obtained by plasma oxidation of an Mg/Al bilayer in an Ar + O 2 atmosphere. Series of MTJs were fabricated with different Mg layer thicknesses ( Mg ), and Al layer thickness was fixed at 1.3 nm. The annealing effect on the tunneling magnetoresistance (TMR) ratio was investigated, and TMR ratio of 65% at room temperature (RT) was shown when it was annealed at 375 C with the Mg = 0 5 nm. The temperature dependence of conductance can be fit by the magnon-assisted tunneling model by adding spin independent tunneling contribution for the samples investigated here, and the spin independent conductance varies with Mg , possibly due to less oxidation for thicker Mg layer.
We investigated the I-V curves and differential tunneling conductance of two, CoFeB/MgO/CoFeB-based, magnetic tunnel junctions (MTJs): one with a low tunneling magnetoresistance (TMR; 22%) and the other with a high TMR (352%). This huge TMR difference was achieved by different MgO sputter conditions rather than by different annealing or deposition temperature. In addition to the TMR difference, the junction resistances were much higher in the low-TMR MTJ than in the high-TMR MTJ. The low-TMR MTJ showed a clear parabolic behavior in the dI/dV-V curve. This high resistance and parabolic behavior were well explained by the Simmons' simple barrier model. However, the tunneling properties of the high-TMR MTJ could not be explained by this model. The characteristic tunneling properties of the high-TMR MTJ were a relatively low junction resistance, a linear relation in the I-V curve, and conduction dips in the differential tunneling conductance. We explained these features by applying the coherent tunneling model.
Journal of Applied Physics, 2009
The main focus of improving the tunneling magnetoresistance ͑TMR͒ of magnetic tunnel junctions ͑MTJs͒ has been on optimizing the structure and thickness of the MgO barrier layer ͓Moriyama et al., Appl. Phys. Lett. 88, 222503 ͑2006͒; Yuasa et al., Nat. Mater. 3, 868 ͑2004͔͒. However, in this paper, we found that the thicknesses of the capping layers also play an important role in TMR. We studied the influence of the capping layers above the CoFeB/MgO/CoFeB. It was intuitively believed that these capping layers did not affect the TMR because they were deposited after the critical CoFeB/MgO/CoFeB structure. Surprisingly, we found that the thicknesses of the capping Ta and Ru layers have significant influence on the TMR. The stress or strain applied onto the MgO barrier by these capping layers appear to be responsible. The results in this paper shed light on optimizing TMR of MgO MTJs.
High-field anisotropy of the tunnelling magnetoresistance of CoFeB/MgO/CoFeB junctions
Journal of Magnetism and Magnetic Materials, 2010
Differential resistance data is gathered on both as-deposited and annealed tunnel junctions based on CoFeB/MgO interfaces, where one of the layers is pinned via exchange bias. When sufficiently strong magnetic field is rotated out of the plane of the junctions, a characteristic bias dependence of the anisotropic term in the differential resistance is observed at low temperature and all fields (from 5-14 T), which may be represented by a voltage dependent base function AðVÞ together with an angular sin 2 ðy þ jÞ dependence, where y is the angle between the normal to the plane of the junction and j is an offset phase. The base function reflects the anisotropy in the spin-polarized density of states of crystallized CoFeB and its spin-resolved Fermi surfaces.
Co 40 Fe 40 B 20 / MgO single and double barrier magnetic tunnel junctions ͑MTJs͒ were grown using target-facing-target sputtering for MgO barriers and conventional dc magnetron sputtering for Co 40 Fe 40 B 20 ferromagnetic electrodes. Large tunnel magnetoresistance ͑TMR͒ ratios, 230% for single barrier MTJs and 120% for the double barrier MTJs, were obtained after postdeposition annealing in a field of 800 mT. The lower TMR ratio for double barrier MTJs can be attributed to the amorphous nature of the middle Co 40 Fe 40 B 20 free layer, which could not be crystallized during postannealing. A highly asymmetric bias voltage dependence of the TMR can be observed for both single and double barrier MTJs in the as-deposited states and after field annealing at low temperature. The asymmetry decreases with increasing annealing temperature and the bias dependence becomes almost symmetric after annealing at 350°C. Maximum output voltages of 0.65 and 0.85 V were obtained for both single and double barrier MTJs, respectively, after annealing at 300°C, a temperature which is high enough for large TMR ratios but insufficient to completely remove asymmetry from the TMR bias dependence.
Effect of annealing and applied bias on barrier shape in CoFe/MgO/CoFe tunnel junctions
Physical Review B, 2011
Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions as a function of annealing and in-situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the asgrown sample. In-situ off-axis electron holography was employed to image the barrier potential profile of an MTJ directly, with the specimen under electrical bias. Varying the bias voltage from −1.5 V to +1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface. Introduction Metal-oxide interfaces are the subject of extensive experimental and theoretical research for next generation nano-scale spintronic devices that exploit spin as a degree of freedom for charged electrons. 1 They play a key role in metal-oxide based science and engineering, with applications including magnetic tunnel junctions (MTJs) 2 and other heterogeneous structures such as resistance switching oxides 3 with uses or potentials uses in low-power non-volatile memories. In its simplest form, the MTJ is a trilayer structure consisting of two ferromagnetic (FM) electrode layers separated by an ultra-thin dielectric layer. The electrical resistance across the insulating tunnel barrier is dependent upon the relative orientation of the magnetizations of the two ferromagnetic electrodes. In most cases, the electrical resistance is lower when the magnetization of the two ferromagnetic layers is
Applied Physics Letters, 2011
Three-dimensional elemental distributions in magnetic tunnel junctions containing naturally oxidized MgO tunnel barriers are characterized using atom-probe tomography. Replacing the CoFeB free layer ͑reference layer͒ with a CoFe/CoFeB ͑CoFeB/CoFe͒ bilayer increases the magnetoresistance from 105% to 192% and decreases the resistance-area product from 14.5 to 3.4 ⍀ m 2 . The CoFe/CoFeB bilayer improves the compositional uniformity within the free layer by nucleating CoFeB crystals across the entire layer, resulting in a homogeneous barrier/free layer interface. In contrast, the simple CoFeB free layer partially crystallizes with composition differences from grain to grain ͑5-30 nm͒, degrading the tunnel junction performance.