Exchange-biased Magnetic Tunnel Junctions Research Articles

Exceeding 400% tunnel magnetoresistance at room temperature in epitaxial Fe/MgO/Fe(001) spin-valve-type magnetic tunnel junctions

Giant tunnel magnetoresistance (TMR) ratios of 417% at room temperature (RT) and 914% at 3 K were demonstrated in epitaxial Fe/MgO/Fe(001) exchange-biased spin-valve magnetic tunnel junctions (MTJs) by tuning growth conditions for each layer, combining sputter deposition for the Fe layers, electron-beam evaporation of the MgO barrier, and barrier interface tuning. Clear TMR oscillation as a function of the MgO thickness with a large peak-to-valley difference of ∼80% was observed when the layers were grown on a highly (001)-oriented Cr buffer layer. Specific features of the observed MTJs are symmetric differential conductance (dI/dV) spectra for the bias polarity and plateau-like deep local minima in dI/dV (parallel configuration) at |V| = 0.2–0.5 V. At 3 K, fine structures with two dips emerge in the plateau-like dI/dV, reflecting highly coherent tunneling through the Fe/MgO/Fe. We also observed a 496% TMR ratio at RT by a 2.24 nm-thick-CoFe insertion at the bottom-Fe/MgO interface.

Backhopping in magnetic tunnel junctions: Micromagnetic approach and experiment

Abstract Micromagnetic simulations of Current Induced Magnetization Switching (CIMS) loops in CoFeB/MgO/CoFeB exchange-biased Magnetic Tunnel Junctions (MTJ) are discussed. Our model uses the Landau–Lifshitz–Gilbert equation with the Slonczewski׳s Spin-Transfer-Torque (STT) component. The current density for STT is calculated from the applied bias voltage and tunnel magnetoresistance which depends on the local magnetization vectors arrangement. We take into account the change in the anti-parallel state resistance with increasing bias voltage. Using such model we investigate influence of the interlayer exchange coupling, between free and reference layers across the barrier, on the backhopping effect in anti-parallel to parallel switching. We compare our simulated CIMS loops with the experimental data obtained from MTJs with different MgO barrier thicknesses.

Exchange-biased magnetic tunnel junctions with antiferromagnetic ε-Mn3Ga

Oriented c-axis films of the hexagonal triangular antiferromagnetic ε-Mn3Ga have been used in bottom-pinned synthetic antiferromagnet magnetic tunnel junctions with MgO barriers, which show up to 150% tunneling magnetoresistance at room temperature. Exchange bias fields as high as 150 mT can be achieved for samples field-cooled from 100 °C. Thin films of the antiferromagnet have a Néel temperature in excess of 650 K and provide an interface exchange energy with CoFe of 0.09 mJ m−2. They show an isotropic uncompensated magnetization of Ms = 48 kA m−1, with a coercivity μ0Hc > 3 T.

Annealing effect on low frequency noise in MgO-based magnetic tunnel junctions

The annealing temperature (Ta) dependence of the low frequency noise in exchange-biased magnetic tunnel junctions (MTJs) with an MgO barrier and CoFeB electrodes has been investigated for 200 < Ta < 425 °C. A 1/f noise spectrum is observed for the parallel and antiparallel states, where the Hooge parameter (α) for the parallel state remains unchanged with Ta after the CoFeB electrodes is fully crystallized, while α for the antiparallel state increases over the range of Ta from 275 to 425 °C. The 1/f noise for the antiparallel state is largely of magnetic origin, and it can be attributed to magnetization fluctuations of ferromagnetic electrodes due to the loss of the exchange bias, whereas there is little magnetic noise in the parallel state. An annealing temperature of 325 °C yields an optimized signal-to-noise ratio, as the CoFeB electrodes become fully crystallized. The bias dependence of α is discussed based on the Glazman-Matveev model, which considers both elastic and inelastic tunneling.

Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

Current induced magnetization switching and interlayer exchange coupling (IEC) in sputtered CoFeB/MgO/CoFeB exchange-biased magnetic tunnel junctions with an extremely thin (0.96–0.62 nm) 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 Ω μm2 and sizes of 0.13 μm2 down to 0.03 μm2 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.

Effects of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by in situ annealing

We study the effect of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by high-temperature in situ annealing above 400 °C, circumventing Mn diffusion conventionally caused by postannealing process. The high temperature in situ annealing leads to Ru diffusion at the CoFeB/Ru interfaces, and thereby results in a reduction in tunnel magnetoresistance (TMR). The minimization of Ru diffusion during the in situ annealing provides a large TMR of 294% at room temperature with an exchange-bias field of 280 Oe. In addition, the temperature and voltage dependence of TMR reveals that there is neither significant spin-exchange scattering nor severe impurity-assisted scattering in the MgO barrier.

Magnetoresistance in magnetic tunnel junctions with an organic barrier and an MgO spin filter

Exchange-biased magnetic tunnel junctions with a CoFeB/MgO pinned layer, a Co free layer and an aluminum tris-(8-hydroxyquinoline) spacer layer of thickness ranging from 0–8 nm have been prepared by photolithography. The devices show a room-temperature, zero-bias magnetoresistance of 12.5±0.3%, which is unchanged after the crossover from tunneling to hopping transport at a barrier thickness of about 4 nm. The spin-diffusion length in the hopping regime is much greater than 10 nm. The magnetoresistance in the tunneling regime changes sign at a positive bias of 250 mV, and it is maximum at −100 mV.

Study on Exchange-Biased Perpendicular Magnetic Tunnel Junction Based on Pd/Co Multilayers

We investigated top and bottom exchange biased perpendicularly magnetized magnetic tunnel junctions (pMTJs) with IrMn and the effects of the multilayer structure on the magnetoresistance and perpendicular exchange bias. The TMR ratio of the top exchange biased (TEB)-pMTJ was significantly deteriorated compared with that of the pseudo-pMTJ, due to the high junction resistance and the in-plane anisotropy induced in the Co/IrMn interface. The TMR ratio and perpendicular exchange bias were improved in the bottom exchange biased (BEB)-pMTJ as compared with those of the TEB-pMTJ. Perpendicularly magnetized buffer layers, such as (Pd/Co) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> , should be used to induce perpendicular exchange coupling with antiferromagnets such as IrMn in BEB-pMTJs.

Effects of Field Cooling Direction on Magnetoresistance of Exchange-Biased Magnetic Tunnel Junction Rings

We report an effect of field cooling direction on the magnetoresistance of ring-shaped magnetic tunnel junctions consisting of Ta 5/Cu 20/Ta 5/NiFe 2/Cu 5/MnIr 10/CoFe 4/Al–N 1.5/CoFe 4/NiFe 20/Ta 5-cap (thickness in nm). The magnetoresistances were both enhanced, in comparison to the as-fabricated ring device, with field-cooling directions applied in the film plane as well as perpendicular to the film plane but with greater increase in the case of perpendicular-field-cooled. Many cycles of planar- and perpendicular-field-cooled alternately gave the same results. The greater enhancement of magnetoresistance is shown to be due to the formation of an onion like magnetization configuration in the pinned layer after perpendicular-field-cooled, resulting in a better relative magnetization alignment between the pinned layer and the free layer in the minor loop region. Magnetic force microscopy (MFM) was undertaken to reveal the uniform and onion like magnetization configurations in the pinned layer of ring devices with the same dimensions, but with layer structures of Ta 5/NiFe 2/Cu 5/MnIr 10/CoFe 4/Cu 1/Ta 1 (thickness in nm) for the planar- and perpendicular-field-cooled, respectively. In addition, size-dependent behavior was explored and the results show that the formation of onion like magnetization after perpendicular-field-cooled is associated with the strong shape anisotropy in the narrower linewidths of ring devices.

Fabrication of exchange-biased epitaxial magnetic tunnel junctions with a Heusler alloy Co 2Cr 0.6Fe 0.4Al thin film

Exchange-biased epitaxial magnetic tunnel junctions (MTJs) with a Heusler alloy Co 2Cr 0.6Fe 0.4Al (CCFA) lower electrode, a MgO tunnel barrier, and a Co 50Fe 50 upper electrode were fabricated. Reflection high-energy electron diffraction observations indicated that all layers of the CCFA/MgO/Co 50Fe 50 MTJ layer structure were grown epitaxially. The fabricated epitaxial MTJs exhibited clear exchange-biased tunnel magnetoresistance (TMR) characteristics with high TMR ratios of 102% at room temperature and 291% at 4.2 K. From the TMR ratios, a high effective spin polarization of 0.86 at 4.2 K was obtained for the epitaxial CCFA films with the B2 structure.

Spin-dependent tunneling characteristics of fully epitaxial magnetic tunneling junctions with a full-Heusler alloy Co2MnSi thin film and a MgO tunnel barrier

Fully epitaxial, exchange-biased magnetic tunnel junctions (MTJs) were fabricated with a Co-based full-Heusler alloy Co2MnSi (CMS) thin film as a lower electrode, a MgO tunnel barrier, and a Co50Fe50 upper electrode. The microfabricated CMS∕MgO∕Co50Fe50 MTJs exhibited relatively high tunnel magnetoresistance ratios of 90% at room temperature and 192% at 4.2K. The bias voltage dependence of differential conductance (dI∕dV) for the parallel and antiparallel magnetization configurations suggested the existence of a basic energy gap structure for the minority-spin band of the CMS electrode with an energy difference of about 0.4eV between the bottom of the vacant minority-spin conduction band and the Fermi level.

Effect of stray field on local spin modes in exchange-biased magnetic tunnel junction elements

We report on the detection of localized spin modes in a multilayered spintronic device by means of time-resolved scanning Kerr microscopy. Measurements on this 13×9μm2 exchange-biased magnetic tunnel junction element at different applied bias fields indicate a strong effect of the stray field from the pinned CoFe layer on the magnetization dynamics in the free NiFe layer. This view is supported by micromagnetic simulations, which also show that the dynamics can be attributed to the specific shape of the internal magnetic field in the element.

Probing local spin modes in exchange-biased MTJ elements

We have studied the local magnetization dynamics in exchange-biased magnetic tunnel junction (MTJ) elements, by time-resolved magneto-optical Kerr effect measurements. We have observed a strong spatial dependence of the frequency and damping of the precessional motion in the free layer. A local edge mode has been detected, as well as increased damping at the edges. The experimental results are in good agreement with micromagnetic simulations, in which the stray field from the pinned layer of the MTJ is included.

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.

Thermally Assisted Switching in Exchange-Biased Storage Layer Magnetic Tunnel Junctions

A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field.

Temperature and bias voltage dependence of Co-Fe-AlO/sub X/-Py-AlO/sub X/-Co-Fe double-barrier junctions

Exchange-biased magnetic tunnel junctions with single and double barriers are investigated with respect to the temperature and bias voltage dependence of the tunneling magnetoresistance. The single-barrier junctions show a tunneling magnetoresistance ratio of up to 49% at room temperature (72% at 10 K), the double-barrier systems up to 38% (57%), respectively, with three clearly separated magnetic states. The temperature and bias voltage dependence of the double-barrier junctions is explained as a serial connection of two single junctions. Theoretical studies of the tunneling magnetoresistance exhibit that the magnetoresistance ratio can be enhanced by ballistic electrons in double-barrier junctions, but only if the potential of the middle electrode can be shifted.

Optimization of tunneling magnetotransport and thermal properties in magnetic tunnel junctions by rapid thermal anneal

We report on a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased magnetic tunnel junctions (MTJs). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be ∼27%, whereas the TMR in MTJs annealed by RTA increases with annealing temperature up to 300 °C, reaching ∼46%. A significant change in the effective barrier thickness ( t eff) and height ( Φ eff) occurs within 10 s during RTA. Transmission electron microscopy and X-ray reflectivity studies demonstrate that the interface of the alumina tunnel barrier for the MTJ annealed by RTA became sharper and clearer, giving rise to the enhanced TMR.

Thermally stable exchange-biased magnetic tunnel junctions over 400 °C

Exchange-biased magnetic tunnel junctions (MTJs) with interposed Fe1−xPtx metal alloy layers between the Al oxide barrier and the ferromagnetic electrodes maintain large tunneling magnetoresistance (TMR) after thermal treatment in excess of 400 °C, owing to an improved barrier interface. After 400 °C annealing, TMRs of MTJs with Fe1−xPtx (x=0.1–0.2) exhibit over 40% and retain 30% TMR after 420 °C annealing. The tunnel barrier height derived from the current–voltage curve fitted to the Simmons equation increases with richer Pt content. Secondary ion mass spectroscopy depth profiles and cross-section transmission electron micrographs of MTJs with Fe0.85Pt0.15 show a clear interface around the Al oxide barrier even after annealing at 400 °C.

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.

デュアルスピンバルブタイプ強磁性二重トンネル接合の熱処理による特性変化

Exchange-biased dual spin-valve type magnetic double tunnel junctions (DSV-MTJs) of Ir-Mn/Co-Fe/AlOx/Co-Fe/AlOx/Co-Fe/Ir-Mn were annealed at 175-400°C. After annealing at 325°C, the magnetoresistance (MR) ratio and bias voltage Vh, at which the MR ratio is reduced by 50%, in the DSV-MTJs were greatly increased to 42% and 930 mV, respectively. After annealing above 325°C, the MR ratio and Vh decreased. This degradation of DSV-MTJs annealed above 325°C was investigated using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS spectra and TEM images showed that an AlOx/MnOx composite tunnel barrier was formed due to the diffusion of Mn through the Co-Fe layer and oxygen redistribution above 300°C. Taking into account the spin-independent two-step tunneling via defect states in the barrier, we considered that this AlOx/MnOx composite barrier leads to decreasing the MR ratio and Vh and increasing the tunnel resistance.