Electrodes In Magnetic Tunnel Junctions Research Articles

Oxide Spintronics

Concomitant with the development of metal-based spintronics in the late 1980's and 1990's, important advances were made on the growth of high-quality oxide thin films and heterostructures. While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition metal oxides, and notably mixed-valence manganites. The discovery of colossal magnetoresistance in manganite films triggered an intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios one order of magnitude larger than what had been obtained with transition metal electrodes. Since then, the research on oxide spintronics has been intense with the latest developments focused on diluted magnetic oxides and more recently on multiferroics. In this paper, we will review the most important results on oxide spintronics, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices.

Half-metallic {Co 2MnSi/Co 2FeSi} multilayered Heusler electrodes in magnetic tunnel junctions

It is demonstrated that the atomic ordering of Co 2FeSi can significantly be improved in terms of magnetization and tunnel magnetoresistance using multilayered {Co 2MnSi 5 nm /Co 2FeSi 5 nm } 10 full-Heusler magnetic electrodes in magnetic tunnel junctions.

Suppression of orange-peel coupling in magnetic tunnel junctions by preoxidation

We have found that preoxidation of the bottom Co electrode in magnetic tunnel junctions (MTJs) very effectively suppresses orange-peel coupling. The result is a free layer that is much softer. Work by others has demonstrated that preoxidation is compatible with high-quality MTJ fabrication.

Hybrid perovskite-spinel magnetic tunnel junctions based on conductive ferrimagnetic NiFe2O4

Ultrathin epitaxial films of NiFe2O4, when grown under specific conditions, show a large enhancement of their magnetic moment and electrical conductivity. This allowed us to use such layers as electrodes in magnetic tunnel junctions. In this paper, we report on the observation of a large tunnel magnetoresistance at low temperature in La2∕3Sr1∕3MnO3∕SrTiO3∕NiFe2O4 junctions. From these results we infer a spin polarization of up to 45% for NiFe2O4, which stays constant up to ∼300K. NiFe2O4 can therefore be considered as an alternative to Fe3O4 as a high-temperature spin-polarized Fe oxide. Furthermore, our findings provide a way to design materials for spintronics based on the stabilization of nonthermodynamically stable magnetic phases of spinel oxides.

Large tunnel magnetoresistance in magnetic tunnel junctions using Co2MnX (X = Al, Si) Heusler alloys

We fabricated B2-ordered Co2MnAl and L21-ordered Co2MnSi Heusler alloy films by optimizing various fabrication conditions (substrate, composition of sputtering target, substrate and post-annealing temperature, etc) and applied these films to bottom electrodes of magnetic tunnel junctions (MTJs). We used Al-oxide insulating tunnel barriers for our MTJs and varied oxidation times of Al films to control qualities of the Al-oxide insulating layer and Heusler-alloy/Al-oxide interface. Observed tunnel magnetoresistance (TMR) ratios were extremely sensitive to the structure and surface morphology of the prepared Heusler alloy films. Epitaxially grown Heusler alloy films showed good structural quality, very flat surfaces and enhanced TMR ratios. The behaviour of the TMR ratios towards oxidation time for the preparation of the Al-oxide barriers and the measurement temperature dependence of the TMR ratios were quite different between the MTJs with Co2MnAl and Co2MnSi electrodes. The obtained TMR ratio of 83% at 2 K in the MTJ with epitaxially grown B2-ordered Co2MnAl was large among the MTJs with an amorphous Al-oxide tunnel barrier. This result suggests that B2-ordered Co2MnAl is a highly spin-polarized material, as predicted by our theoretical calculation. Moreover, we observed a very large TMR ratio of 159% at 2 K in the MTJ with a high-quality epitaxially grown L21-ordered Co2MnSi electrode. This TMR ratio is the highest value to date in MTJs using an amorphous Al-oxide tunnel barrier. Spin-polarization of the Co2MnSi bottom electrode obtained from Julliere's formula was about 0.89. This value is also the largest achieved to date for a Heusler material and is much larger than those of conventional ferromagnetic materials such as Co–Fe. This large spin-polarization is attributed to a half-metallic band structure, as predicted by theoretical calculations.

Magnetoresistance in magnetic tunnel junctions with amorphous electrodes

Two different types of amorphous material, which includes CoFeB and Co/sub 2/MnSi alloy were examined for use as electrodes in magnetic tunnel junctions (MTJ) to achieve a high tunnel magnetoresistance (TMR) ratio. Films were grown in a Shamrock tool by dc magnetron cosputtering from elemental targets on glass substrates or thermal-oxidized Si substrates. Film compositions were checked by energy dispersive X-ray analysis in a scanning electron microscope. Structural, and magnetic characterization was performed by X-ray diffraction and in a %T superconducting quantum interference device (SQUID) magnetometer, respectively. In the amorphous MTJs, the tunneling spin-polarization does not reflect the magnitude of the net magnetic moment directly.

Enhanced magnetic moment and conductive behavior inNiFe2O4spinel ultrathin films

Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that - under appropriate growth conditions - epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (Ms) - about 250% larger - and a metallic character. A systematic study of the thickness dependence of Ms allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the A and B sites of the spinel structure resulting from the off-equilibrium growth conditions and to interface effects. The relevance of these findings for spinel- and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.

Pressure effect on the magnetization of Sr2FeMoO6 thin films grown by pulsed laser deposition

Thin films of Sr2FeMoO6 (SFMO) are grown on SrTiO3 (001) substrates by pulsed laser deposition. The best films provide 3.2μB∕f.u. at 5K, a Curie temperature above 400K, low roughness, high crystallinity, and low splashing. Therefore, the use of such SFMO electrodes in magnetic tunnel junctions patterned with conventional lithography is promising. Pseudomorphic epitaxial growth is obtained for thicknesses under 50nm. Above this thickness the films do not relax homogeneously. A coherent and systematic variation of the magnetization with the deposition conditions is obtained, which highlights a high reproducibility. Under a reasonable O2 partial pressure to avoid parasite phases, the limiting factor for high magnetization is the total pressure or the deposition rate. Therefore, the deposition rate is suspected to have a strong influence on the Fe∕Mo ordering. Highly magnetic samples are obtained under a low gas flow of either a 20% O2+N2 or a 0.3% O2+Ar.

The current potential of Co2MnSi Heusler alloy electrodes in magnetic tunnel junctions

Heusler alloys are ferromagnetic half-metals and candidates for materials with enhanced tunnel magnetoresistance (TMR). In the article [1], Co2MnSi has been used as one electrode in a magnetic tunnel junction. The cover figure shows, together with the unit cell of Co2MnSi, the TMR characteristics at 20 K at 1 mV (red) and 10 mV (blue), respectively. Large spin polarizations of 61% and 66%, respectively, were achieved. The first author, Andreas Hütten, is working in the group of Günter Reiss at the University of Bielefeld. Their research is devoted to magnetism and spin-dependent electronic transport in thin films and nanostructures, the development of optimized magneto- and spin-electronic devices as well as of preparation methods of magnetic nanostructures. This paper is an invited presentation from the Second Seeheim Conference on Magnetism (SCM2004), held in Seeheim, Germany, 27 June–1 July 2004. The full Proceedings of this conference are published in phys. stat. sol. (c) 1, No. 12 (2004).

The current potential of Co2MnSi Heusler alloy electrodes in magnetic tunnel junctions

We have integrated Co2MnSi as a representative of the full-Heusler compound family as one magnetic electrode into technological relevant magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi and (3) homogenizes the AlOx-barrier. The resulting magnetoresistance of the Co2MnSi containing magnetic tunnel junctions has been determined as a function of temperature. At a bias voltage of 1 mV a magnetoresistance of 94.6% has been realized at 20 K leading to a spin polarization of 65.5%. Furthermore, the microstructural as well as magnetic quality of the Co2MnSi/AlOx-barrier interface has been investigated in detail. Additionally, the problem of antisite formation at the Co2MnSi/AlOx-barrier interface is experimentally addressed by intercalating thin Co layers in between the Heusler layer and the AlOx barrier. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Co 2 Mn Si Heusler alloy as magnetic electrodes in magnetic tunnel junctions

As a consequence of the growing theoretical predictions of 100% spin-polarized half- and full-Heusler compounds over the past six years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co2MnSi, as a representative of the full-Heusler compound family, as one magnetic electrode into magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer, subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi, and (3) homogenizes the AlOx barrier. It is shown that the magnetic switching of the ferromagnetic electrodes is well controlled from room temperature down to 10K. The resulting tunnel magnetoresistance-effect amplitude of the Co2MnSi containing magnetic tunnel junctions has been determined as a function of temperature and the spin polarization of the Co2MnSi Heusler compound has been estimated to be 61% at 10K. Thus, the spin polarization of the Co2MnSi layer at 10K exceeds that of conventional transition metals.

Magnetic properties of NiFe/Ru(V)/NiFe synthetic ferrimagnetic layers

We fabricated synthetic ferrimagnetic layers (SyFL) based on NiFe/Ru/NiFe or NiFe/V/NiFe for the free electrode of magnetic tunnel junctions (MTJ) and characterized their spin flopping, coercive and saturation fields. NiFe/Ru/NiFe SyFL exhibited a coercive field of 10 Oe, which would produce a coercive field of less than 50 Oe when incorporated into sub-micrometer sized MTJ. In comparison, NiFe/V/NiFe SyFL showed extremely low exchange coupling; only when the thickness of the V layer was -0.8 nm, appreciable level of exchange coupling was observed. The coercive field of the NiFe/V/NiFe SyFL, however, did not have a large dependence on the thickness of V and remained at ∼2 Oe. The coercive field of the V-SyFL is expected to maintain below 10 Oe even after patterned into a tunnel junction. Both NiFe/Ru/NiFe and NiFe/V/NiFe SyFLs were stable up to 525 K without showing any gross changes in microstructure and magnetic properties when thermally annealed.

High resolution depth profiling of thin STO in high- k oxide material

High k-dielectrics have become an important material in the development of ultra shallow junctions. In addition, strontium titanium oxide (STO) has been used as an interposer material to grow GaAs on Si for MESFETs [RF devices implemented on GaAs on Si substrates using a SrTiO 3 buffer layer, Motorola knowledge base, 14 September 2001]. Also, SrTiO 3 has a potential to be used as insulating material between ferromagnetic electrodes in magnetic tunnel junctions for magnetic random access memory devices (M-RAM). In this study we have investigated a multilayer system of STO with lanthanum barium manganese oxide (LBMO). The layer thickness was down to 0.8 nm, corresponding to two unit cells of SrTiO 3. With low energy Cs bombardment a depth resolution down to 2 nm per decade and a peak width of 2 nm FWHM was achieved. Since the erosion rates of STO and LBMO differ greatly, we will discuss a depth calibration procedure based on actual material composition.

On the use of exchange biased top electrodes in magnetic tunnel junctions

We have investigated the magnetic behavior and the structural properties of ferromagnetic–antiferromagnetic systems (NiFe–IrMn and Co–IrMn) deposited directly on a thin tantalum buffer layer (bottom configuration) or above a thin Al 2O 3 tunnel barrier layer (top configuration). In the bottom configuration, the bilayer system exhibits higher magnetic performances than in the top configuration in terms of thermal stability. We have performed a detailed structural study by X-ray diffraction and high-resolution transmission electron microscopy which allow us to establish a clear correlation between the situation of the bilayer with respect to the tunnel barrier, its texture and its magnetic properties.

Effect of diffusion barrier in the thermally annealed exchange-biased IrMn-CoFe electrode in magnetic tunnel junctions

Exchange-biased electrodes IrMn/Ta(2 /spl Aring/)/CoFe/AlO/sub x/ and IrMn/CoFe/Ta(2 /spl Aring/)-AlO/sub x/ used in a magnetic tunnel junction were annealed at 300/spl deg/C to study the Mn diffusion characteristics. Auger electron spectroscopy and X-ray photoelectron spectroscopy analysis of the annealed electrodes show that the Ta diffusion barrier effectively blocked the Mn migration, regardless of the barrier location. Also concluded from the study was that the Mn migration is largely enhanced by the preferential oxidation of Mn.

Diffusion study of the exchange-biased NiFe/MnIr/CoFe electrode in magnetic tunnel junctions

The exchange-biased electrode of magnetic tunneling junctions with under- and overoxidized AlOx insulator layers were annealed at 300 °C to study the extent of the Mn diffusion. A significant amount of Mn diffusion into the AlOx layer was observed in the overoxidized junction sample from Auger electron spectroscopy and the extent of Mn diffusion increased with increasing oxidation time. X-ray photoelectron spectroscopy (XPS) proved that Mn found at the CoFe/AlOx interface in the overoxidized junction sample was in the form of Mn oxide. The XPS data suggest that the Mn diffusion was accelerated by the preferential oxidation of Mn at the CoFe/AlOx interface.

Magnetostatic interactions in artificial ferrimagnet based magnetic tunnel junctions

Magnetostatic interactions between the soft and the hard magnetic electrodes in magnetic tunnel junctions (MTJs) using artificial ferrimagnets (AFis) are analyzed. We attribute these interactions to the dispersion fields associated to magnetic inhomogeneities arising from domain walls due to local anisotropic ordering. These magnetostatic interactions can be controlled by adjusting the net magnetic moment of the AFi to optimize the magnetotransport response of the MTJ devices.