Tunnel Magnetoresistance Effect in CoFeB/MgAlOx/CoFeB Magnetic Tunnel Junctions (original) (raw)
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Solid State Communications, 2005
We measured differential tunneling conductance (dI/dV, d 2 I/dV 2) spectra of spin-valve-type magnetic tunnel junctions (MTJs) with a MgO(001) tunnel barrier layer and amorphous CoFeB ferromagnetic electrodes that show 315% magnetoresistance (MR) ratio at 4.3 K. The dI/dV spectra showed clear reduction in the conductance at around G400 mV for a parallel magnetic configuration. Such anomalous spectra have never been observed for MTJs with an amorphous Al-O barrier. The d 2 I/dV 2 spectra showed several distinct peaks between 5 and 100 mV. Magnon excitations are assigned to an origin of those peaks and thought to be a dominant process to reduce MR at finite bias voltage.
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.
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.
Physical Review B, 2011
For sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions, it is well known that the tunnel magnetoresistance (TMR) ratio increases with increasing annealing temperature (T a ) up to a critical value (T p ), and then decreases with further increasing T a , resulting in a peak around T p . The improved crystallinity of the MgO barrier and CoFeB electrodes due to annealing has been considered as the main reason for the enhancement of the TMR ratio, especially for T a < T p . In this work, the evidence is provided that the magnon excitation plays a great contribution to the magnetoresistance (MR) behavior in annealed samples based on the measurement of dynamic conductance and inelastic electron tunneling (IET) spectra. The magnon activation energy (E c ) obtained from the fits for IET spectra exhibits a similar temperature dependence with that of the TMR ratio. A detailed analysis shows that the magnon excitation, together with improved crystallinity of the MgO barrier and CoFeB layers, is the main contribution to the annealing-temperature-dependent MR behavior. PACS number(s): 75.70.Cn, 75.30.Hx MgO-based magnetic tunnel junctions (MTJs) have been investigated widely due to their interesting fundamental physics and potential applications. 1-8 Thermal annealing is a critical process for sputtered CoFeB/MgO/CoFeB junctions because the tunnel magnetoresistance (TMR) ratio of junctions annealed at an appropriate temperature increases dramatically. 9-13 Annealing greatly improves the crystallinity of the MgO barrier and ferromagnetic (FM) CoFeB electrodes as well as the interfaces between the MgO and CoFeB layers, 10,12 which enhances spin-dependent tunneling across the MgO barrier. The effects of thermal annealing were studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD), 10,12,14-16 with a focus on the structural characterization.
Applied Physics Letters, 2006
Double-barrier magnetic tunnel junctions with two MgO barriers and three CoFeB layers exhibiting tunneling magnetoresistance ͑TMR͒ values of more than 100% were fabricated. The bias voltage dependence of the TMR ratio is highly asymmetric after annealing at low temperatures, indicating dissimilar CoFeB / MgO interfaces. The TMR effect decays very slowly for positive bias and is only reduced to half of its maximum value at V 1/2 = 1.88 V when the junctions are processed at 200°C. The largest output voltage, 0.62 V, is obtained after annealing at 300°C, a temperature that combines high TMR ratios with a considerable asymmetric bias dependence.
Transport Properties in Sputtered CoFeB/MgAl2O4/CoFeB Magnetic Tunnel Junctions
IEEE Transactions on Magnetics, 2014
CoFeB/MgAl 2 O 4 /CoFeB magnetic tunnel junctions (MTJs) with the barrier sputtered from sintered MgAl 2 O 4 target have been successfully fabricated. Dependence of tunneling magnetoresistance (TMR) ratio on both MgAl 2 O 4 deposition pressure and postannealing temperature has been studied. The TMR ratio of more than 50% at room temperature was obtained with an annealing temperature of 325°C and MgAl 2 O 4 deposition pressure of 1.3 Pa. Temperature dependence of resistance in both parallel and antiparallel configurations can be well fitted by the model based on direct elastic tunneling and magnon-assisted inelastic tunneling. Inelastic electron tunneling spectroscopy (IETS) at low temperature, exhibiting three peaks originating from zero-bias anomaly, interface magnons, and barrier phonons, were measured and compared with the results of AlO x and MgO-based MTJs. The IETS for all three types of MTJs shows quite similar peak positions for all kinds of elementary excitations except barrier phonons.
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.
Physical Review B, 2010
The competition between the interface crystallization and diffusion processes, their influence on the onset of symmetry-filtering coherent tunneling of ⌬ 1 band electrons in the MgO-based magnetic tunnel junctions is investigated. Systematic study of the transport and magnetoresistance during thermal annealing of these junctions shows a unique behavior of the tunneling conductance in the parallel state. The optimal annealing time for achieving giant tunneling magnetoresistance at different temperatures is determined. The evolution of magnetoresistance consists of three distinct regions, responsible by different contributions from CoFeB electrodes and the MgO barrier. The whole phenomenon can be understood through an empirical model based on the Landauer tunneling picture.