High Tunnel Magnetoresistance Ratio Research Articles

Substrate Biasing Effect during MgO Deposition in CoFeB/MgO/CoFeB MTJs

High tunneling magnetoresistance (TMR) ratio and low RA in magnetic tunnel junctions are necessary condition for application in magnetic random access memory. To get high TMR ratio and low RA, good quality of MgO (002) insulating layer is important. To increase crystalline quality of MgO (002) layer, we applied negative bias on the substrate during MgO deposition. We report the results of the tunneling magnetoresistance (TMR) ratio and the resistance-area product (RA) for CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with substrate bias voltage, and showed TMR increase and RA decrease with substrate bias. The effects of substrate bias voltage on the TMR ratio, RA and MgO (002) peak intensity are discussed.

Transport Properties of Magnetic Tunnel Junctions Comprising NiFeSiB/CoFeB Hybrid Free Layers

We report on the magneto-transport measurements of MgO magnetic tunnel junctions (MTJs) composed of NiFeSiB/CoFeB as the free layer for two different structures (top-type and bottom-type pinning). The magneto-transport properties of these MTJs were investigated by varying the thickness of the amorphous NiFeSiB layer for a fixed CoFeB thickness. The tunnel magnetoresistance (TMR), measured in both type of structures, exhibit the same or a higher amplitude (up to 230% measured at room temperature in the case of top-type device), comparing to the case of a single CoFeB free layer. These results suggest that hybrids free layers can be used as good candidates for MTJs with reduced saturation magnetization while keeping a high TMR ratio.

Half-metallic electronic structure of Co2MnSi electrodes in fully epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions investigated by tunneling spectroscopy (invited)

We used tunneling spectroscopy to examine the spin-dependent electronic structure of Co2MnSi (CMS) electrodes facing a MgO barrier and the key tunneling mechanism in fully epitaxial CMS/MgO/CMS magnetic tunnel junctions (MTJs) that showed high tunnel magnetoresistance ratios up to 182% at room temperature and 705% at 4.2 K. Consequently, we developed a model of the spin-dependent electronic structure for CMS electrodes and a tunneling model that can consistently explain the observed tunneling spectra. Here, we show that lower (upper) CMS electrodes possess a half-metal gap of 0.40 eV (0.32 eV) with the Fermi level (EF) near the middle of the half-metal gap for both lower and upper CMS electrodes. Furthermore, we found strong evidence for the existence of interface states in the interfacial region of CMS electrodes facing a MgO barrier, as well as evidence of residual states in the bulk region of upper CMS electrodes, in both cases for minority spins around EF, and we show that interface states play a critical role for spin-dependent tunneling in these half-metallic CMS-based MTJs.

Critical role of interface states for spin-dependent tunneling in half-metallic Co2MnSi-based magnetic tunnel junctions investigated by tunneling spectroscopy

We investigated at 4.2 K the differential conductance (dI/dV) versus V characteristics of fully epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions (MTJs) featuring high tunnel magnetoresistance ratios of about 700% at 4.2 K (about 180% at room temperature). We developed a tunneling model to explain the observed tunneling spectra and showed the critical role played by interface states for minority spins existing around the Fermi level of Co2MnSi electrodes facing a MgO tunnel barrier in the spin-dependent tunneling characteristics of these MTJs with half-metallic electrodes.

Tunnel Magnetoresistance in Full-Heusler Co2FeAl0.5Si0.5-Based Magnetic Tunnel Junctions

We fabricated fully epitaxial magnetic tunnel junctions (MTJs) with full-Heusler Co2FeAl0.5Si0.5 (CFAS) electrodes using the sputtering method. We changed the MgO barrier thickness, the thickness of an Mg layer inserted between the CFAS and MgO layers, and a post-annealing process for CFAS layers in order to obtain conditions necessary for achieving a high tunnel magnetoresistance (TMR) ratio. We found that both the MgO barrier thickness and the post-annealing temperature directly influenced the TMR ratio while the thin Mg layer insertion did not. Post-annealing at higher temperatures above 873 K suppresses the TMR ratio due to degradation of the structure of top CFAS layer during the post-annealing. In addition, a large effective tunnel barrier height was observed in our MTJs because of the poor quality of MgO barrier layer. These facts indicate that the tunneling spin polarization of CFAS/MgO interface is sensitive not only to the degree of ordering of L21 or B2 in CFAS but also to the quality of the MgO barrier and CFAS/MgO interfaces.

Large tunnel magnetoresistance in Co2FeAl0.5Si0.5∕MgO∕Co2FeAl0.5Si0.5 magnetic tunnel junctions prepared on thermally oxidized Si substrates with MgO buffer

Magnetic tunnel junctions (MTJs) using polycrystalline Co2FeAl0.5Si0.5 (CFAS) electrodes with a MgO tunnel barrier were fabricated onto thermally oxidized Si substrates. Highly (001)-oriented and B2-ordered CFAS electrodes were obtained by optimizing growth conditions and postannealing temperature. The microfabricated MTJs exhibited relatively high tunnel magnetoresistance (TMR) ratios of 125% at room temperature and 196% at 7K. The large TMR obtained using oxidized Si substrates indicates that CFAS is promising for the practical applications.

Fabrication of Magnetic Tunnel Junctions With Co$_{2}$ FeSi Heusler Alloy and MgO Crystalline Barrier

Tunneling magnetoresistance (TMR) characteristics of magnetic tunnel junctions with a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode and an MgO crystalline barrier have been investigated. Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy electrode grown on Cr-buffered MgO(100) substrate starts to have an <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> 2 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> structure when annealed above 420degC. In the cases of CoFeB/MgO/Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi junctions, a high TMR ratio of 158% has been achieved after annealing at 350degC, which is obtained by coherent tunneling between the electrodes and barrier, not by the half-metallic nature of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy. However, the Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi Heusler alloy electrode degrades the TMR ratio when compared with an amorphous CoFeB electrode and the bottom Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode makes more degradation of the TMR ratio than the top Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrode. The major reason for the low TMR ratio in Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi-based junctions is the broken epitaxial relationship between the bottom Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> FeSi electrodes and the MgO crystalline barrier, which is investigated by a cross-sectional transmission electron micrograph.

Fabrication and Characterization of Fully Epitaxial Magnetic Tunnel Junction Field Sensors Using a ${\rm Co}_{2}{\rm MnSi}$ Thin Film

Fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnSi (CMS) upper free layer, a MgO tunnel barrier, and a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> lower pinned layer were fabricated for sensor application. The fabricated MTJs exhibited clear exchange-biased tunnel magnetoresistance (TMR) characteristics with a relatively high TMR ratios of 94% at room temperature (209% at 4.2 K) with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">half</sub> of approximately - 0.52 and + 0.54 V. Almost hysteresis-free TMR characteristics with sensitivity of approximately 0.4%/Oe were obtained with a cross magnetization configuration, in which the magnetization of the CMS free layer was oriented perpendicularly to that of the pinned layer.

Fabrication of fully epitaxial magnetic tunnel junctions using L21-ordered Co2FeAl0.5Si0.5 electrodes and their tunneling magnetoresistance characteristics

Magnetic tunnel junctions (MTJs) using L21-ordered full-Heusler Co2FeAl0.5Si0.5 (CFAS) electrodes and an MgO tunnel barrier were prepared on MgO-buffered MgO (001) substrates by sputtering method. In situ and ex situ structural characterization confirms that the stacking structure of CFAS/MgO/CFAS is fully epitaxial, with smooth interfaces throughout. The microfabricated MTJs exhibited relatively high tunnel magnetoresistance ratios of 150% at room temperature and 312% at 7K. We observed a symmetrical crossover point from the bias voltage dependence of differential conductance between parallel and antiparallel magnetization configurations, and also a flat behavior in the parallel conductance, which can be explained by considering the characteristic half-metallic band structure of L21-ordered CFAS near the Fermi level.

Tunnel magnetoresistance of 604% at 300K by suppression of Ta diffusion in CoFeB∕MgO∕CoFeB pseudo-spin-valves annealed at high temperature

The authors observed tunnel magnetoresistance (TMR) ratio of 604% at 300K in Ta∕Co20Fe60B20∕MgO∕Co20Fe60B20∕Ta pseudo-spin-valve magnetic tunnel junction annealed at 525°C. To obtain high TMR ratio, it was found critical to anneal the structure at high temperature above 500°C, while suppressing the Ta diffusion into CoFeB electrodes and in particular to the CoFeB∕MgO interface. X-ray diffraction measurement of MgO on SiO2 or Co20Fe60B20 shows that an improvement of MgO barrier quality, in terms of the degree of the (001) orientation and stress relaxation, takes place at annealing temperatures above 450°C. The highest TMR ratio observed at 5K was 1144%.

Tunneling magnetoresistance with amorphous electrodes

A detailed first-principles analysis of the transport properties of different magnetic electrode materials for $\mathrm{MgO}$ tunnel junctions is performed to elucidate the microscopic origin of the tunneling magnetoresistance (TMR) effect. The spin-dependent transport properties of the magnetic materials are analyzed separately from the particular interface geometry with the tunneling barrier. We use the bulk properties of the barrier to identify the important tunneling states. For $\mathrm{MgO}$ these are ${\ensuremath{\Delta}}_{1}$-like states. From the analysis of this effective spin polarization we can predict the potential of certain magnetic materials to create a high TMR ratio in a tunnel junction. This polarization is as high as 98 and $86\phantom{\rule{0.3em}{0ex}}%$ for Fe and Co, respectively for only a few monolayers, but is very small and negative, $\ensuremath{-}7\phantom{\rule{0.3em}{0ex}}%$, for amorphous Fe. This explains the finding that for crystalline Co and Fe one monolayer next to the $\mathrm{MgO}$ barrier is sufficient to reach TMR ratios higher than 500 % independent of whether the crystalline monolayer is coupled to a non-magnetic or to an amorphous lead. However, in direct contact with $\mathrm{MgO}$ amorphous Fe reduces the TMR ratio drastically to 44 %.

Fabrication of fully epitaxial Co2MnSi∕MgO∕Co2MnSi magnetic tunnel junctions

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with full-Heusler alloy Co2MnSi thin films as both lower and upper electrodes and with a MgO tunnel barrier. The fabricated MTJs showed clear exchange-biased tunnel magnetoresistance (TMR) characteristics with high TMR ratios of 179% at room temperature (RT) and 683% at 4.2K. In addition, the TMR ratio exhibited oscillations as a function of the MgO tunnel barrier thickness (tMgO) at RT, having a period of 0.28nm, for tMgO ranging from 1.8to3.0nm.

Crystalline structure and magnetic properties of Fe2CrSi Heusler alloy films: New ferromagnetic material for high-performance magnetic random access memory

A new Heusler alloy, Fe2CrSi, which has high spin polarization (P), low saturation magnetization (Ms), and a low Curie temperature (TC), was investigated in order to fabricate high-performance magnetic tunnel junctions (MTJs) with a high tunnel magnetoresistance ratio and with low critical current for the spin-transfer switching method, or a low switching field for the thermally assisted magnetization reversal technique. The main results are as follows: (1) P and the magnetic moment of Fe2CrSi with an L21 structure were 0.98 and 1.98μB∕f.u., respectively, according to density of states calculations. (2) Fe2CrSi films show the (100) orientation with a B2 structure on a MgO substrate upon a thermal treatment with optimum temperature and duration. (3) Fe2CrSi films have Ms and TC values of 385emu∕cm3 and 630K, respectively. (4) The (100) oriented epitaxial MTJs are produced with Fe2CrSi films fabricated with the optimized thermal treatment condition. It is found that the Fe2CrSi Heusler alloy films are a suitable ferromagnetic material for high-performance magnetic random access memory.

Magnetostriction and tunneling magnetoresistance of CoFeB∕AlOx∕Co∕IrMn junctions

Cross-strip magnetic tunnel junctions (MTJs) were fabricated following this sequence: Si(100)∕Ta(30Å)∕CoFeB(80Å)∕AlOx(to)∕Co(70Å)∕IrMn(90Å)∕Ta(100Å), under an in-plane deposition field (h)=500Oe, where to=12, 17, 22, and 26Å is the thickness of the AlOx layer. From the resistance (Ro) versus to plot, Ro increases from 255to1138Ω exponentially, as to increases from 12to26Å. This indicates that the quantum tunneling mechanism is valid in our case. Furthermore, the tunneling magnetoresistance (TMR) increases initially from 49% to 63% and then decreases to 33%, as to increases monotonically. This shows that with the increase of the barrier thickness, to⩾17Å, the spin polarization in the current after tunneling is weakened eventually. The microstructure of the to=17Å MTJ observed under high-resolution cross-sectional transmission electron microscope shows that the interfaces between CoFeB∕AlOx and AlOx∕Co are smooth, resulting in the high TMR ratio observed. Saturation magnetostriction (λs) of this series of CoFeB∕AlOx∕Co MTJs ranges from −5to−2ppm. The λs versus to plot also shows the concave-up feature. Finally, we find that the optimal MTJ has the following properties: TMR=63% and λs=−5ppm.

Effect of Nb concentration on the microstructure of Al and the magnetoresistive properties of the magnetic tunnel junction with a Nb‐doped Al‐oxide barrier

AbstractWe have investigated the composition dependence of magnetic tunnel junctions (MTJs) with Nb‐alloyed Al‐oxide (NbAlOx) and analyzed the microstructure changes and electrical property of Nb alloyed Al‐oxide layer. After annealing, tunnel magnetoresistance (TMR) ratio of MTJ with Nb‐alloyed Al‐oxide barrier increased up to 38.5% at 9.26 at.% Nb. As the Nb concentration increases, the grain size decreases and the microstructure becomes a dense, fine equiaxed‐type structure with fine, continuous selected area diffraction (SAD) patterns, until Nb concentration reaches 9 at.%. The microstructural changes of Nb‐alloyed Al layer results in smooth interface roughness, so that we could achieve a high TMR ratio. Resistance decreased from 900 Ω to 220 Ω and barrier height decreased from 1.62 eV to 0.847 eV at 9.26 at.% Nb. We speculated that the reduction of junction resistance of the MTJ with Nb‐doped Al‐oxide barrier was due to Nb d states formation in the band gap. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Patterned nanoring magnetic tunnel junctions

Patterned nanoring (NR) magnetic tunnel junctions (MTJs) with outer diameters between 100 and 400nm and narrow ring widths between 25 and 30nm were fabricated. The NR structure consists of CoFeB electrodes and Al-oxide barrier. The tunneling magnetoresistance (TMR) ratio in the range of 20%–50% for the NR-MTJs with the resistance-area product lower than 50Ωμm2 were observed at room temperature. These NR-MTJs allow current-induced magnetization switching with a low switching current density of around 9×106A∕cm2. Due to the small stray field and high TMR ratio, NR-MTJs offer superior prospects for very high density magnetic random access memory, recording medium, and other spintronic devices.

Improved tunnel magnetoresistance characteristics of magnetic tunnel junctions with a Heusler alloy thin film of Co2MnGe and a MgO tunnel barrier

We fabricated magnetic tunnel junctions (MTJs) with a Co-based full-Heusler alloy thin film of Co2MnGe (CMG) and a MgO tunnel barrier. The microfabricated MTJs with a Co-rich CMG film showed relatively high tunnel magnetoresistance ratios of 83% at room temperature and 185% at 4.2K. These values are much higher than those previously obtained for CMG/MgO MTJs with a Co-deficient CMG film.

Magnetic Tunnel Junctions for Spintronic Memories and Beyond

In this paper, recent developments in magnetic tunnel junctions (MTJs) are reported with their potential impacts on integrated circuits. MTJs consist of two metal ferromagnets separated by a thin insulator and exhibit two resistances, low (Rp) or high (Rap) depending on the relative direction of ferromagnet magnetizations, parallel (P) or antiparallel (AP), respectively. Tunnel magnetoresistance (TMR) ratios, defined as (Rap $Rp)/Rp as high as 361%, have been obtained in MTJs with Co40Fe40B20 fixed and free layers made by sputtering with an industry-standard exchange-bias structure and post deposition annealing at Ta = 400 degC. The corresponding output voltage swing DeltaV is over 500 mV, which is five times greater than that of the conventional amorphous Al-O-barrier MTJs. The highest TMR ratio obtained so far is 500% in a pseudospin-valve MTJ annealed at Ta = 475 degC, showing a high potential of the current material system. In addition to this high-output voltage swing, current-induced magnetization switching (CIMS) takes place at the critical current densities (JCO) on the order of 106 A/cm2 in these MgO-barrier MTJs. Furthermore, high antiferromagnetic coupling between the two CoFeB layers in a synthetic ferrimagnetic free layer has been shown to result in a high thermal-stability factor with a reduced JCO compared to single free-layer MTJs. The high TMR ratio enabled by the MgO-barrier MTJs, together with the demonstration of CIMS at a low JCO, allows development of not only scalable magnetoresistive random-access memory with feature sizes below 90 nm but also new memory-in-logic CMOS circuits that can overcome a number of bottlenecks in the current integrated-circuit architecture

Tunnel magnetoresistance in fully epitaxial magnetic tunnel junctions with a full-Heusler alloy thin film of Co2Cr0.6Fe0.4Al and a MgO tunnel barrier

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film and a MgO tunnel barrier. Pseudo-spin-valve-type, fully epitaxial CCFA∕MgO∕Co50Fe50 MTJs with a CCFA film composition close to the stoichiometric one demonstrated high tunnel magnetoresistance (TMR) ratios of 90% at room temperature (RT) and 240% at 4.2K. These high TMR ratios were attributed to the increased spin polarization at the Fermi level due to the increased ratio of the B2 region to the A2 region in CCFA films achieved by bringing the CCFA film composition close to the stoichiometric one. Fully epitaxial CCFA∕MgO∕Co50Fe50 MTJs with exchange biasing were also fabricated, where a Co50Fe50 upper electrode was used in a synthetic ferrimagnetic Co50Fe50∕Ru∕Co90Fe10 trilayer exchange-biased with an IrMn layer through the Co90Fe10∕IrMn interface. These exchange-biased CCFA∕MgO∕Co50Fe50 MTJs exhibited further enhanced TMR ratios of 109% at RT and 317% at 4.2K. We suggested enhancement of the TMR ratio by a coherent tunneling contribution for fully epitaxial CCFA∕MgO∕Co50Fe50 MTJs.

Ｃｏ２Ｃｒ０．６Ｆｅ０．４Ａｌ薄膜を用いたエピタキシャル強磁性トンネル接合の製作とトンネル磁気抵抗特性

Fully epitaxial magnetic tunnel junctions (MTJs) were fabricated with a full-Heusler alloy Co2Cr0.6Fe0.4Al (CCFA) thin film and a MgO tunnel barrier. Cross-sectional high-resolution transmission electron microscope observations clearly showed that all layers of CCFA/MgO/Co50Fe50 MTJ layer structures were grown epitaxially and were single-crystalline. The fully epitaxial CCFA/MgO/Co50Fe50 MTJs, where we used the difference in the coercive forces to form the antiparallel magnetization configurations between the lower and upper electrodes, demonstrated high tunnel magnetoresistance (TMR) ratios of 90% at room temperature (RT) and 240% at 4.2 K. This result suggested that a CCFA film composition close to the stoichiometric one is essential for obtaining high spin polarizations in CCFA thin films. The fully epitaxial CCFA/MgO/Co50Fe50 MTJs with exchange-biasing, where a Co50Fe50 upper electrode was used in a synthetic ferrimagnetic Co50Fe50/Ru/Co90Fe10 trilayer exchange-biased with an IrMn layer through the Co90Fe10/IrMn interface, exhibited high TMR ratios of 109% at RT and 317% at 4.2 K. From the obtained TMR ratio of 317% at 4.2 K, a high tunneling spin polarization of 0.88 at 4.2 K was estimated for epitaxial CCFA films with the B2 structure. These results confirm the promise of an epitaxial MTJ using a Co-based full-Heusler alloy as a key device structure for utilizing the potentially high spin polarization of this material system.