Stability Of Magnetic Tunnel Junctions Research Articles

Annealing stability of magnetic tunnel junctions based on dual MgO free layers and [Co/Ni] based thin synthetic antiferromagnet fixed system

We study the annealing stability of bottom-pinned perpendicularly magnetized magnetic tunnel junctions based on dual MgO free layers and thin fixed systems comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co reference layer and a FeCoB polarizing layer. Using conventional magnetometry and advanced broadband ferromagnetic resonance, we identify the properties of each sub-unit of the magnetic tunnel junction and demonstrate that this material option can ensure a satisfactory resilience to the 400 °C thermal annealing needed in solid-state magnetic memory applications. The dual MgO free layer possesses an anneal-robust 0.4 T effective anisotropy and suffers only a minor increase of its Gilbert damping from 0.007 to 0.010 for the toughest annealing conditions. Within the fixed system, the ferro-coupler and texture-breaking TaFeCoB layer keeps an interlayer exchange above 0.8 mJ/m2, while the Ru antiferrocoupler layer within the synthetic antiferromagnet maintains a coupling above −0.5 mJ/m2. These two strong couplings maintain the overall functionality of the tunnel junction upon the toughest annealing despite the gradual degradation of the thin Co layer anisotropy that may reduce the operation margin in spin torque memory applications. Based on these findings, we propose further optimization routes for the next generation magnetic tunnel junctions.

Modeling of switching energy of magnetic tunnel junction devices with tilted magnetization

Abstract For spin transfer torque (STT), the switching energy and thermal stability of magnetic tunnel junctions (MTJ) bits utilized in memory devices are important factors that have to be considered simultaneously. In this article, we examined the minimum energy for STT induced magnetization switching in MTJ devices for different in-plane angles of the magnetization in the free layer and the pinned layer with respect to the major axis of the elliptical cylinder of the cell. Simulations were performed by comparing the analytical solution with macrospin and full micromagnetic calculations. The results show good agreement of the switching energy calculated by using the three approaches for different initial angles of the magnetization of the free layer. Also, the low-energy location specifies the suitable value of both time and current in order to reduce the heat effect during the switching process.

Determination of Thermal Stability of Magnetic Tunnel Junction Using Time-Resolved Single-Shot Measurement

Time-resolved single-shot measurement has been investigated to characterize switching times of the free layer in magnetic tunneling junctions (MTJs). In this study, a stacked MTJ film is patterned into an elongated, nano-scale device using standard electron beam lithography in conjunction with ion beam etching. Subsequently, current-induced magnetization switching is initiated using a single-shot measurement based on the time-domain transmission method. A current pulse with a duration of 50 ns is utilized to switch the magnetization configuration between parallel and anti-parallel states. After the pulse rises, the transmission profile shows a transition in which resistance varies. That is, the incubation time between pulse onset and resistance change represents the switching time of the free layer. The statistical distribution results of the switching times are obtained. The switching times are strongly dependent on current density and the external magnetic field. By fitting the data points using the Neel-Arrhenius law, the thermal stability factor JCo and intrinsic current density JCo are also determined.

Critical switching current and thermal stability of magnetic tunnel junctions with uncompensated CoFeB/Ru/CoFeB synthetic free layers

The critical switching current and thermal stability parameter are investigated for magnetic tunnel junctions with uncompensated synthetic ferrimagnetic free layers. The parameters are obtained by analyzing the experimental results for the thermally activated magnetization switching probability as functions of both a bias current and an applied magnetic field. The analysis is greatly facilitated by the use of an analytical equation for the applied magnetic field dependence of the energy barrier. A figure of merit given by the ratio of the two parameters differs substantially depending on the direction of the magnetization switching.

Stability of Magnetic Tunnel Junction Integrated on CMOS Circuit

Stability of Magnetic Tunnel Junction Integrated on CMOS Circuit

A TEM structural study of thermal stability of magnetic tunnel junctions integrated with CMOS devices

We present a TEM/STEM study of the structure and thermal stability of Co/AlOx/Si, CoFe(B)/MgO/Si and FeNi/SiOx/Si magnetic junctions deposited directly on patterned Si. In all three types of junctions the films are uniform and continuous. Annealing at 300 at 550 °C does not change their structure. CoFe and CoFeB layers remained amorphous after the annealing process.

Thermal stability of magnetic tunneling junctions with MgO barriers for high temperature spintronics

Thermal stability of MgO-based magnetic tunnel junctions has been investigated from room temperature up to 500°C, in both the memory and sensor configurations. Junctions showed magnetoresistances of over 200% at room temperature and over 100% at 300°C. Below 375°C, the resistance of the parallel state remains constant, while the antiparallel state resistance linearly decreases with temperature. Above that, a rapid increase in the resistance of both states was observed, along with an irreversible loss of magnetoresistance. Junctions in the sensor configuration exhibited a constant sensitivity of 1.0%/Oe at temperatures up to 300°C before getting degraded.

Switching and reliability issues of magnetic tunnel junctions for high-density memory device

Abstract Magnetoresistive random access memory (MRAM) is one of the promising universal memories, which combines the high speed of static RAM, high-density of dynamic RAM, and non-volatility of FLASH memory. However, MRAM faces several challenges prior to the appearance at the commercial market on a large scale. The most important challenge among them will be an issue, related to the magnetic switching. The conventional MRAM structures and writing methods cause several issues to be overcome for high packing density without cross-talk. Furthermore, small ferromagnetic elements will require high magnetic field, which will increase the power consumption of the devices. This article will review new writing schemes, such as thermal-assisted switching and spin-transfer switching methods. In addition, reliability characteristics and thermal stability of magnetic tunnel junctions will be presented, because ensuring the reliability of magnetic tunnel junctions emerges as an another challenging problem for the successful application of the new writing schemes to the high-density MRAM devices in the next generation.

Thermal stability of Ir-Mn∕Co-Fe-B∕Al-O∕Co-Fe-B tunnel junctions

The thermal stability of magnetic tunnel junctions with structures of Ta(5)∕Cu(30)∕Ta(5)∕Ni79Fe21(5)∕Ir22Mn78(12)∕Co62Fe20B18(4)∕Al(0.8)-oxide∕Co62Fe20B18(4)∕Cu(30)∕Ta(5) (thicknesses unit in nanometers) has been investigated. The tunnel magnetoresistance (TMR) shows a large increase up to 54.4% after annealing at 265 °C due to the improved characteristic properties of the barrier and the interface between the barrier and the ferromagnetic electrodes. The TMR was observed to decrease drastically above the annealing temperature of 310 °C accompanied by a notable increase of junction resistance and coercivity of the free layer. The amorphous Co62Fe20B18 layers seem to behave as a barrier of diffusion, preventing the migration of Mn or Cu atoms into the interface between the barrier and the ferromagnetic layers. This may cause the drastic decrease of TMR due to the deterioration of the barrier and its interface with Co62Fe20B18 layers. The observed crystallization in the amorphous Co62Fe20B18 layers is considered to contribute to the increase in coercivity of the free layer.

Thermal stability of magnetic tunnel junctions with new amorphous ZrAl-alloy films as the under and capping layers

We studied the thermal stability of new amorphous ZrAl-based magnetic tunnel junctions (MTJs) with a ZrAl-oxide barrier replacing the Ta layers traditionally used for the under and capping layers. The MTJs were compared with similar conventional MTJs (Ta-based MTJ with Al-oxide). After annealing at various temperatures up to 450/spl deg/C, the ZrAl-based MTJs still had a significant tunnel magnetoresistance signal of nearly 21%. The thermal stability of amorphous ZrAl-based and conventional Ta-based MTJs differs dramatically, mainly because of the different microstructural evolution. The noncrystalline ZrAl-alloy film had superior surface uniformity and an induced microstructure that resisted interdiffusion, with dense, equiaxed grains making up the upper stacks' films. By contrast, the conventional Ta-based MTJ had a broad columnar structure with less dense boundaries, which act as a source of interdiffusion, resulting in barrier deformation at elevated temperatures.

Enhanced thermal stability of magnetic tunnel junctions formed by in situ radiation annealing process on AlOx insulating barriers

An in situ direct radiation annealing (IDRA) technique on the AlOx insulating barrier was performed to enhance the thermal stability of magnetic tunneling junctions (MTJs). Our method was found to improve the dielectric and structural properties of AlOx insulating barrier in MTJs. After the proper IDRA process, the conventional ex situ annealing process exhibited an additional enhancement of thermal stability by significantly reducing the interdiffusion process between Co80Fe20 electrode and AlOx insulating barriers. Experimentally observed tunneling magnetoresistance of MTJs after the IDRA process showed about 55% up to 350°C. Finally, a surface-plamon resonance spectroscope measurement observed that the dielectric constant of insulating barriers was increased from 2.7 to 3.2 after the IDRA process.

Thermal stability of magnetic tunnel junctions investigated by x-ray photoelectron spectroscopy

We have studied the thermal stability of magnetic tunnel junctions with and without nano-oxide layer (NOL) using x-ray hpotoelectron spectroscopy (XPS). The con centration and chemical states of elements,in particular Mn,have been obtained b y angel-resolved XPS and peak decomposition technique.It is confirmed that Mn in the antiferromagnetic layer can diffuse into the pinned ferromagnetic layer and the insulating barrier layer when a magnetic tunnel junction without NOL is ann ealed at high temperature.However,the interdiffusion of Mn during the annealing process is suppressed by inserting a NOL between the antiferromagnetic and pinne d ferromagnetic layer,and then the thermal stability is improved.

Magnetotransport properties and thermal stability of magnetic tunnel junctions with wave resonance plasma oxidized barrier

AlO x tunnel barriers prepared by oxidizing ultra-thin Al films of various thickness by means of a Rf wave resonance plasma beam source were investigated to understand the influence of the plasma oxidation conditions on the junction resistance R j, the magnetoresistance ratio (MR) and the switching characteristics of exchange-biased magnetic tunnel junctions (MTJs) with NiMn pinning layer. The junction properties were characterized as a function of oxidation time, plasma power and distance between plasma source and sample. The MR of the as-deposited junction was about 15%. The highest exchange bias fields (17 mT) and pinned layer coercivities (23 mT) can be achieved with extended annealing at low temperatures T ≅ 32 0 ∘ C or with a rapid annealing at T ≅ 40 0 ∘ C , respectively. Short-time annealing (1 min) at intermediate temperatures (350 °C) and field cooling at 1 T leads to the highest MR effect of 35% at room temperature and 55% at 4.2 K.

Thermal stability of magnetic tunnel junctions with FeO/sub x/ doped tunnel barrier

Magnetic tunnel junctions were fabricated with tunnel barriers that had a thin film of Fe(0-8 /spl Aring/ thick) inserted in the middle to study the effect of FeO/sub x/ doping on the magneto-electron transport properties of the junction. Tunneling magnetoresistance (TMR) ratio of 24% was realized with the tunnel junction with a 2 /spl Aring/-thick Fe interlayer, whereas the junction without doping did not exhibit the TMR effect. The FeO/sub x/ doping also considerably improved the thermal stability of the junction, retaining the TMR ratio of 15% at 350/spl deg/C.

High thermal stability of magnetic tunnel junctions with oxide diffusion barrier layers

We developed two types of magnetic tunnel junctions (MTJs) that showed high thermal stability. One is a PtMn exchange-biased spin-valve MTJ with a CoFe/Al-oxide (AlOx)/NiFe free layer and a CoFeTaOx/CoFe pinned layer, and the other is a pseudo-spin-valve (PSV) MTJ with a CoFe/AlOx/NiFe soft layer, where AlOx and CoFeTaOx act as barriers for Ni and Mn diffusion toward the tunnel barrier, respectively. After 390 °C-1H annealing, the PSV MTJs maintained 28% and the SV MTJs 39% of tunnel magnetoresistance. Transmission electron microscopy observation of the SV MTJs after 380 °C-1H annealing revealed that the migrated Mn atoms were trapped at the CoFeTaOx layer.

Enhanced tunneling magnetoresistance and thermal stability of magnetic tunnel junction by rapid thermal anneal

The effect of rapid thermal anneal (RTA) has been investigated on the properties of an FeMn exchange-biased magnetic tunnel junction (MTJ) using magnetoresistance and I– V measurements and transmission electron microscopy (TEM). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be ∼27%, while the TMR in MTJs annealed by RTA increases with annealing temperature up to 300°C, reaching ∼46%. TEM images reveal that the interface of Al 2O 3 layer for the annealed MTJ has changed into a relatively clear morphology, as compared to that for the as-grown MTJ. The oxide barrier parameters are found to vary abruptly with annealing time within a few ten seconds. Our results demonstrate that the present RTA enhances the thermal stability of MTJs.

Stability of magnetic tunnel junctions

When magnetic tunnel junctions (MTJ) are built into a memory device they will be arranged in a matrix; therefore some of the not addressed elements will be exposed to a significant field during the switching of one element. It has to be avoided that the state of not selected MTJ is changed during this process. Here we present data on the stability of MTJ against small fields which occur during a writing process.

Electrode roughness and interfacial mixing effects on the tunnel junction thermal stability

The thermal stability of magnetic tunnel junctions with ultrathin (<8 Å) Al2O3 barriers was studied and compared with 15 Å barriers. The tunnel magnetoresistance (TMR) decay cannot be explained only by Mn diffusion into the pinned CoFe layer since this diffusion starts above 300 °C independently of the barrier thickness, while the TMR degradation already occurs at 250–270 °C for the thinner barriers. The thermal stability is probably controlled by changes at the CoFe/Al2O3 interfaces and/or barrier structure. Structural analysis of 15 Å barriers after annealing at 435 °C, shows the existence of an interface region (8–12 Å thick) where CoFe and Al2O3 are found. This interfacial region can be explained by the increased roughness in the bottom electrode after annealing, as measured by atomic-force microscopy (from 1.5 to 4 Å). Ultrathin barriers show a similar trend. The use of low-resistance junctions using thin barriers requires good control of the roughness of the low-resistance bottom electrodes. This is done by preannealing and low-angle ion-beam smoothing 500-Å-thick Cu or Al films, which will then keep a roughness <2 Å during processing temperatures up to 400 °C. Low-resistance junctions (R×A∼40–60 Ω μm2) with 7 Å barriers grown on 600 Å Al buffers after the surface treatment show 25% TMR after annealing at 270 °C.

Thermal stability of magnetic tunnel junctions studied by x-ray photoelectron spectroscopy

We have studied the evolution of chemical state of the metallic layers in NiFe/Al oxide/NiFe tunnel junction structures in as-deposited films and after postdeposition annealing. Both top and bottom NiFe layers in as-deposited films show significant Fe oxidation, but no Ni oxidation. This Fe is reduced in annealed samples, implying that oxygen migrates from the FeNi layers, possibly into the Al oxide layer. We also find that both top and bottom electrodes are significantly oxidized even in optimally annealed films.

Evolution of the dielectric breakdown in Co/Al2O3/Co junctions by annealing

The temperature and dielectric stability of magnetic tunnel junctions are important requirements for magnetic memory devices and their integration in the semiconductor process technology. We have investigated the changes of the tunneling magnetoresistance (TMR), the barrier properties (height, thickness, and asymmetry) and the dielectric stability upon isochronal annealing up to 410 °C in Co/Al2O3/Co junctions with an artificial antiferromagnet as a pinning layer. Besides a small decrease of the TMR signal after annealing up to 230 °C, a strong decrease between 300 and 350 °C is found. According to Auger and transmission electron microscopy investigations, this decrease is mainly due to interdiffusion of the metallic layers. The dielectric breakdown is characterized by voltage ramp experiments. The size-averaged breakdown voltage improves from 1.35 V for the as prepared junctions to 1.55 V by annealing at 300 °C. At higher temperatures the breakdown voltage decreases strongly to 0.8 V (at 380 °C). Simultaneously, the typical breakdown process changes from few sudden current jumps to a large number of small steps. The breakdown properties are discussed within a statistical model and related to structural changes of the barrier.