Switching Field Variation in MgO Magnetic Tunnel Junction Nanopillars: Experimental Results and Micromagnetic Simulations (original) (raw)
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Physica B: Condensed Matter, 2013
This work provides a systematic simulation study of magnetic tunnel junction (MTJ) nanodevices behavior, consisting of a multilayered stack incorporating an in-plane CoFeB free layer and a synthetic antiferromagnetic CoFe-based pinned layer, and including exchange and interlayer couplings. A finite element tool is used to simulate both the magnetic and magneto-transport behaviors of these MTJ nanopillars with distinct geometries, namely circles with diameter ranging from 20 nm up to 250 nm and ellipses with aspect ratios of 1/2, 1/3 and 1/5, corresponding to sizes from 20 Â 40 nm 2 up to 50 Â 250 nm 2 . This study envisages two clear applications for nanopillars: memory and sensor devices. We address the impact of the nanopillar size on the coercivity and saturation field, as figures of merit for device performance. In particular a competitive sensitivity of 0.15%/Oe is envisaged for sensors with a size of 50 Â 100 nm 2 . Our results provide a validation of this simulation method as a expedite tool to assist the nanofabrication process.
Journal of Applied Physics - J APPL PHYS, 2010
Current induced magnetization switching and interlayer exchange coupling ͑IEC͒ in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin ͑0.96-0.62 nm͒ MgO wedge barrier is investigated. The IEC is found to be ferromagnetic for all samples and the associated energy increases exponentially down to a barrier thickness of 0.7 nm. Nanopillars with resistance area product ranging from 1.8 to 10 ⍀ m 2 and sizes of 0.13 m 2 down to 0.03 m 2 and tunneling magnetoresistance values of up to 170% were prepared. We found, that the critical current density increases with decreasing MgO barrier thickness. The experimental data and theoretical estimations show that the barrier thickness dependence of the spin transfer torque can largely be explained by a reduction in the tunnel current polarization at very small barrier thickness.
Journal of Magnetism and Magnetic Materials, 2010
We have studied the effects of the initial stages of the annealing on magnetic tunnel junctions with MgO barriers and CoFeB electrodes. We report changes in the resistance-voltage characteristics and tunneling magnetoresistance for patterned transport junctions, and correlate these with the observed changes in the structural and magnetic interface morphologies determined by soft X-ray resonant magnetic scattering from sheet films from the same wafer. An important feature of our experiment was that all measurements were carried out within the soft X-ray diffractometer on samples from the same wafer subjected to simultaneous annealing cycles, so that our magnetotransport and scattering data are directly comparable. The as-grown junction showed a tunneling magnetoresistance ratio of 5.5%, and a specific barrier resistance of 85:6 kO mm 2 . A 200 3 C anneal for 1 h resulted in a small rise in barrier resistance and magnetoresistance coupled with a smoothing of the magnetic interfaces, consistent with the healing of barrier defects and removal of tunneling hot-spots. A subsequent 300 3 C anneal for a further hour resulted in further smoothing, and a rise in the magnetoresistance ratio to 72%, and a much weaker dependence of the parallel state resistance upon voltage bias, indicating the development of ð0 0 1Þ crystallographic texture in the electrodes. Annealing to 325 3 C yielded a further decrease in magnetic interface width (the quadrature sum of roughness and intermixing length scales). The reduction in interface width for Co species occurred at higher temperatures than for Fe throughout the experiments.
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.
Effects of elemental distributions on the behavior of MgO-based magnetic tunnel junctions
Journal of Applied Physics, 2011
Three-dimensional atom-probe tomography and transmission electron microscopy have been utilized to study the effects of Ta getter presputtering and either a Mg or Ru free-layer cap on the elemental distributions and properties of MgO-based magnetic tunnel junctions after annealing. Annealing the samples resulted in crystallization of the amorphous CoFeB layer and diffusion of the majority of the boron away from the crystallized CoFeB layers. The Ta getter presputter is found to reduce the segregation of boron at the MgO/CoFeB interface after annealing, improving the tunneling magnetoresistance of the tunnel junction. This effect is observed for samples with either a Ru free-layer cap or a Mg free-layer cap and is thought to be a result of a reduced oxygen concentration within the MgO due to the effect of Ta getter presputtering. A Ru free-layer cap provides superior magnetic and magnetotransport properties compared to a Mg free-layer cap. Mg from the Mg free-layer cap is observed to diffuse toward the MgO tunnel barrier upon annealing, degrading both the crystalline quality of the CoFeB and magnetic isolation of the CoFeB free-layer from the CoFeB reference-layer. Lateral variations in the B distribution within the CoFeB free-layer are observed in the samples with a Ru free-layer cap, which are associated with crystalline and amorphous grains. The B-rich, amorphous grains are found to be depleted in Fe, while the B-poor crystalline grains are slightly enriched in Fe. V
The effect of annealing on the junction profile of CoFeB/MgO tunnel junctions
Journal of Applied Physics, 2010
The tunnelling magnetoresistance of CoFeB/MgO tunnel junctions is exceptionally high, although the electrodes and the barrier are grown at room temperature in the amorphous state. For their functionality annealing steps up to high temperatures are required. We have analyzed in detail the changes in the chemical and magnetization profile upon annealing up to 360°. The multilayers used for this study are similar to those which are used in magnetic tunnel junctions, however with five repeats. In particular, we have used hard non-resonant and soft resonant magnetic x-ray scattering in order to unravel any changes upon annealing. The multilayers exhibit superior structural quality, which hardly changes with annealing. Surprisingly, only little recrystallization of the CoFeB and the MgO layers can be discerned by x-ray diffraction.
We study nanopillar spin-torque oscillators processed from low-resistance-area product MgO-based magnetic tunnel junctions. The influence of spin torque can be seen in quasistatic experiments as a strong astroid distortion, consistent with a pure Slonczewski-type spin torque. At microwave frequencies, the spin-torque results in pronounced magnetization auto-oscillations with a clear threshold behavior, though only evidenced in the antiparallel configuration. Two kinds of auto-oscillations are seen depending on the polarity of the voltage applied to the junction. Free layer oscillations require a large easy axis applied field and electrons flowing from the reference layers to the free layer. Acoustic excitation of the reference synthetic ferrimagnet can be seen, provided the electron flow is reversed, indicating a clear similarity with the behavior of conventional allmetallic spin valves. The analysis of the threshold indicates that the amplitude of the spin torque is neither proportional to the current nor to the voltage and it is not accompanied by any significant fieldlike torque.
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
2006
Annealing effects on the structural and transport properties of sputtered CoFeB / MgO / CoFeB magnetic tunnel junctions deposited on SiO 2 / Si were investigated. At the as-deposited state, the CoFeB was amorphous at the CoFeB / MgO interface. High-resolution transmission electron microscope image clearly shows that after annealing at 270°C for 1 h, crystallization of amorphous CoFeB ͑three to four monolayers͒ with lattice matching to MgO ͑100͒ occurred locally at the interface between MgO and CoFeB, producing a magnetoresistance ͑MR͒ around 35%-40%. After annealing at 360°C for 40 min, the MR increased to 102%. The increase in the MR with annealing is attributed to the complete formation of ͑100͒ crystalline structure of CoFeB well lattice matched with the ͑100͒-oriented MgO barrier. The bias voltage dependence of the MR shows a consistent correlation with each CoFeB / MgO interface.
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.