CoFe Electrodes Research Articles

High-performance magnetic tunnel junctions based on two-dimensional Bi2O2Se

Magnetic tunnel junctions (MTJs) have attracted tremendous interests recently because of their potential application in magnetoresistive random access high-density memory and magnetic sensor. However, the performance of them is far from satisfying due to the various problems exist in tunnel barrier materials. Here, we propose to use two-dimensional (2D) Bi2O2Se material, which is advantageous in reducing the MTJ size, as the tunnel barrier, and demonstrate that it is able to generate very large tunnel magnetoresistance (TMR) when integrated with CoFe electrodes. The underlying mechanism is elaborated by analyzing the band structures, electron transmission and interface properties. These results provide important guidance for designing high-density and high-performance MTJs.

Large spin signals in n+-Si/MgO/Co2Fe0.4Mn0.6Si lateral spin-valve devices

The spin polarization factor was investigated using electrical spin injection at low temperatures in n+-Si(100)/MgO/ferromagnet lateral spin-valve devices with Co2Fe0.4Mn0.6Si (CFMS) and CoFe electrodes. CFMS films were annealed at different post-annealing temperatures (Ta). Although atomic diffusion of CFMS into the silicon channel was observed at high annealing temperatures, the CFMS device annealed at a Ta of 350 °C, clearly showing a narrow Hanle signal measured using the three-terminal Hanle effect; a consistent spin relaxation time of 7.1 ± 0.4 ns and spin diffusion length of 1.6 ± 0.2 μm were obtained at 10 K. A local three-terminal spin-valve (L-3TSV) signal from the CFMS lateral spin-valve device was obtained at about 370 μV, three times larger than that of the CoFe device. The tunnel spin polarization factor was evaluated from the L-3TSV signals by an analytical equation that considered the spin drift effect. The estimated tunnel spin polarization factor for CFMS was 45% at a Vbias of about 600 mV, while that for CoFe was 18%. This result indicates that the high spin polarization of CFMS is responsible for the large intensity of the L-3TSV signal and that CFMS is a promising FM material for electrical spin injection into silicon.

Cross-shaped nanostructures for the study of spin to charge inter-conversion using spin-orbit coupling in non-magnetic materials

Several spin–orbit effects allow performing spin to charge interconversion due to the spin Hall effects or the spin-momentum locking at Rashba interfaces and topological insulator surface states. Here, we focus on how these interconversions can be made electrically, using three different cross-shaped nanostructures. We apply the measurement configurations to the case of the spin Hall effect in Pt using CoFe electrodes to detect or inject spins. Both the direct and inverse spin Hall effects can be detected, with spin Hall signals up to two orders of magnitude higher than that of nonlocal measurements in metallic lateral spin valves, and with a much simpler fabrication protocol. We compare the respective signal amplitude of the three proposed geometries. Finally, we show that finite element method calculations allow extracting the spin Hall angle and the spin diffusion length of Pt from these measurements.

Efficient thermal spin injection in metallic nanostructures

Thermal spin injection is a unique and fascinating method for generating spin current. If magnetization can be controlled by thermal spin injection, various advantages will be provided in spintronic devices, through its wireless controllability. However, the generation efficiency of thermal spin injection is believed to be lower than that of electrical spin injection. Here, we explore a suitable ferromagnetic metal for an efficient thermal spin injection, via systematic experiments based on diffusive spin transport under temperature gradients. Since a ferromagnetic metal with strong spin splitting is expected to have a large spin-dependent Seebeck coefficient, a lateral spin valve based on CoFe electrodes has been fabricated. However, the superior thermal spin injection property has not been observed, because the CoFe electrode retained its crystalline signature—where s-like electrons dominate the transport property in the ferromagnet. To suppress the crystalline signature, we adopt a CoFeAl electrode, in which the Al impurity significantly reduces the contribution from s-like electrons. Highly efficient thermal spin injection has been demonstrated using this CoFeAl electrode. Further optimization for thermal spin injection has been demonstrated by adjusting the Co and Fe composition.

Observation of giant magnetoresistance in CoFeN/AlOx/CoFeN magnetic tunneling junctions employing a nitrogen-doped amorphous CoFeN free layer electrode

We examine the crystallographic and magnetic features of nitrogen-doped amorphous CoFe (CoFeN) electrodes for application as ferromagnetic free layers in magnetic tunnel junctions, in which precise control of the nitrogen content is crucial for achieving the desirable magnetic features. Incorporating nitrogen into the CoFe layer during growth provides numerous benefits including a remarkably reduced coercivity of 5 Oe, a phase transition from polycrystalline to amorphous, a low magnetization of 294 emu/cm3, and an enhanced thermal stability up to 400 °C. A high magnetic resistance ratio of about 220% was also obtained for annealed in-plane CoFeN/AlOx/CoFeN magnetic tunneling junctions containing a 1.2-nm-thick amorphous AlOx tunnel barrier. We anticipate that our experimental findings will aid in the development of a variety of future spintronic devices.

Comparison of the use of NiFe and CoFe as electrodes for metallic lateral spin valves

Spin injection and detection in Co60Fe40-based all-metallic lateral spin valves have been studied at both room and low temperatures. The obtained spin signals amplitudes have been compared to those of identical Ni80Fe20-based devices. The replacement of Ni80Fe20 by CoFe allows increasing the spin signal amplitude by up to one order of magnitude, thus reaching 50 mΩ at room temperature. The spin signal dependence with the distance between the ferromagnetic electrodes has been analyzed using both a 1D spin-transport model and finite element method simulations. The enhancement of the spin signal amplitude when using CoFe electrodes can be explained by a higher effective polarization.

Fabrication of Fully Epitaxial CoFe/MgO/CoFe Magnetic Tunnel Junctions on Ge(001) Substrates via a MgO Interlayer

Fully epitaxial magnetic tunnel junctions (MTJs) with Co50Fe50 (CoFe) electrodes and a MgO barrier were fabricated on Ge(001) single-crystal substrates via a MgO interlayer. Microfabricated CoFe/MgO/CoFe MTJs with a 10-nm-thick MgO interlayer showed a high tunnel magnetoresistance (TMR) ratio of 218% at 293 K, which is encouraging for monolithic integration of MTJs and Ge field-effect transistors for constructing future-generation nonvolatile logic circuits featuring ultralow-power consumption. Furthermore, MTJs with even a decreased MgO interlayer thickness of 1.0 nm showed a relatively high TMR ratio of 110% at 293 K, suggesting the promise of heterostructures consisting of MTJ/MgO interlayer/Ge(001) as a key device structure for spin injection into a Ge channel from an MTJ.

The role of Mg interface layer in MgO magnetic tunnel junctions with CoFe and CoFeB electrodes

The tunneling spin polarization (TSP) is directly measured from reactively sputter deposited crystalline MgO tunnel barriers with various CoFe(B) compositions using superconducting tunneling spectroscopy. We find that the Mg interface layer thickness dependence of TSP values for CoFeB/Mg/MgO junctions is substantially different from those for CoFe/Mg/MgO especially in the pre-annealed samples due to the formation of boron oxide at the CoFeB/MgO interface. Annealing depletes boron at the interface thus requiring a finite Mg interface layer to prevent CoFeOx formation at the CoFeB/MgO interface so that the TSP values can be optimized by controlling Mg thickness.

Magnetostructural influences of thin Mg insert layers in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions

It is common to find a thin (∼0.5 nm) layer of Mg deposited prior to the MgO tunnel barrier in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions, due to the improved device performance that results. However, despite their common usage, the reasons why such layers are effective are unclear. We use structures that model the lower electrode of such devices to show that a suitably thick Mg insert layer enhances the crystal quality of both MgO and CoFe(B), permits interfacial oxides to reduce back to a metallic ferromagnetic state, and hence improves magnetic switching of the CoFe(B) electrode, properties which are inextricably linked to device performance.

Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements

Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the major detrimental factors to the performance of a magnetic tunnel junction (MTJ), is difficult to avoid during the fabrication process of the MTJ’s tunnel barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs, overoxidation of the tunnel barrier results in the formation of a CoO antiferromagnetic (AF) interface layer which couples with the bottom FM electrode to form a typical AF/FM exchange bias (EB) system. In this work, surface oxidation of the CoFe and CoFeB bottom electrodes was detected via magnetometry measurements of EB characterizations including the EB field, training effect, uncompensated spin density, and enhanced coercivity. Variations in these parameters were found to be related to the surface oxidation of the bottom electrode, among them the change in coercivity is most sensitive. Annealed samples show evidence for an oxygen migration back to the MgO tunnel barrier by annealing.

X-Ray Absorption and Magnetic Circular Dichroism Studies of Co$_{2}$ FeAl in Magnetic Tunnel Junctions

The bulk magnetic moment and the element specific magnetic moment of Co2FeAl thin films were examined as a function of annealing temperature by alternating gradient magnetometer (AGM) and X-ray absorption spectroscopy (XAS)/X-ray magnetic circular dichroism (XMCD), respectively. A high magnetic moment can be achieved for all annealing temperatures and the predicted bulk and interface magnetic moment of about 5 ?B are reached via heating. We will also present tunnel magnetoresistance (TMR) values of up to 153% at room temperature and 260% at 13 K for MgO based magnetic tunnel junctions (MTJs) with Co2FeAl and Co-Fe electrodes.

All nitride magnetic tunnel junctions based on nitrogen-doped CoFe electrode and AlNx insulating barrier

The dependence of the structural and magnetic properties of nitrogen-doped CoFe (CoFeN) films as a function of the nitrogen gas flow rate was analyzed to investigate its potential use as ferromagnetic electrodes in magnetic tunnel junctions (MTJs). The addition of a small amount of nitrogen into the CoFe film was strongly associated with the formation of low coercivity and low magnetization in the CoFeN film. We also discuss the electrical and microstructural properties of the as-grown and postannealed CoFeN/AlN/CoFeN MTJs devices developed by using only nitride materials of CoFeN electrodes and AlNx tunneling barriers.

Effect of Plasma Oxidation on Pre-Oxidized Magnetic Tunnel Junctions

The technique of pre-oxidation of the bottom CoFe electrode prior to deposition of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was previously shown to be capable of suppressing intermixing at the ferromagnet/Al interface and removing partial shorts near Al grain boundaries. In this paper, we studied the influence of the plasma oxidation on the pre-oxidized magnetic tunnel junctions (MTJs). In general, the tunneling magnetoresistance (TMR) is independent of the plasma oxidation time whereas the resistance-area product (RA) increases with it. However, for the pre-oxidized MTJs with very thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Al thickness < 0.7 nm prior to plasma oxidation), the TMR decreases with plasma oxidation while the RA increases with it.

Structural Properties of Co and CoFe Electrodes Forming a Magnetic Tunnel Junction

The M1/MgO/M2 trilayer tunnel magnetoresistance systems are studied by means of X-ray diffraction, NMR, and transmission electron microscopy techniques. As M1 and M2 electrodes we used Co, Fe, and CoFe layers. The growth mechanism and structural quality of both electrodes and of the epitaxial MgO barrier forming the magnetic tunnel junctions are experimentally examined. It is shown that the crystallographic coherence of magnetic tunnel junctions across the MgO barrier is significantly disturbed by imperfect crystal structure of magnetic electrodes. The NMR results indicate a difference in short-range order between bottom and top electrodes.

Conversion from Al to Al2O3 of subnanometer barriers for magnetic tunnel junctions monitored in real time

In situ differential ellipsometry measurements are used to investigate the plasma oxidation dynamics of subnanometer Al layers in real time. An analysis of the measured oxidation rate allows us to monitor the conversion from Al to Al2O3 in real time and mark the onset of the CoFe electrode oxidation. In situ x-ray photoelectron spectroscopy (XPS) measurements of identical samples that were plasma oxidized for 3s are used to confirm the ellipsometry results. A linear relationship was found between the amount of Al2O3 determined from the XPS data and the amount of oxidized Al derived from the ellipsometry data. From this, we conclude that the conversion from Al to Al2O3 of subnanometer thin barriers for magnetic tunnel junctions can be observed by using the derivative of the ellipsometer signal.

Microstructure and exchange coupling parameters of MTJ with CoFeB bottom electrode

AbstractIn this study, we report the relationship between interface roughness, texture and the exchange bias and the interlayer‐Néel coupling fields of the spin‐valve magnetic tunnel junction (SV‐MTJ). SV‐MTJs with the structure: Si(100)/SiO2/buffer/IrMn/CoFeB/AlOx /NiFe/Ta were deposited on two buffers: Cu and Ta/Cu/Ta/Cu, and next the samples were annealed in vacuum and in a magnetic field. XRD analysis reveals that the texture degree of MTJ with Cu buffer is significantly lower than that of the sample with Ta/Cu/Ta/Cu buffer. The enhancement of IrMn and CoFeB texture leads to a higher amplitude of roughness and an increase of exchange bias and Néel coupling fields. The comparison of junctions with CoFe and CoFeB bottom electrodes shows that addition of 5% B in a CoFe layer makes the surface of the pinned layer smoother. This leads to a lower Néel coupling field, which is desirable for application of MTJ in MRAM technology. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Low resistance AL2O3 magnetic tunnel junctions optimized through in situ conductance measurements

In situ electrical conductance is used to monitor the growth and natural oxidation of aluminum on top of a CoFe electrode. Light oxidation is found to enhance the electron specular scattering of the CoFe/vacuum interface. Aluminum deposited onto CoFe intermixes to a depth of a few atomic layers, however, subsequent natural oxidation tends to reverse this interdiffusion through oxygen-driven A1 segregation. At the right A1 thickness, natural oxidation creates a clean and specular CoFe∕AlOx interface very similar to the best achievable CoFe/vacuum interface. For thicker A1, natural oxidation leaves behind underoxidized AlOx and most importantly an interdiffused CoFe∕Al interface. Using 2Torr×150-s natural oxidation, we have fabricated magnetic tunnel junctions (MTJs) with a peak tunnel magnetoresistance (TMR) of 18% for a resistance area product of 7Ωμm2, at the A1 metal thickness of 6 Å. With the same oxidation process TMR drops to only 8% when A1 is increased to 9 Å. Contrary to the accepted view, we do not attribute this TMR drop to A1 underoxidation, but primarily to the interdiffusion at the CoFe∕Al interface. This assertion is strongly supported by a second set of MTJs differing only by the insertion of a nano-oxide layer (NOL) on top of CoFe. In this case when Al is increased from 6 to 9 Å, the TMR does not drop but increases from 16% to 27%. This is significant because NOL is found to effectively prevent Al diffusion into CoFe. NOL is also found to act as a reservoir of oxygen, which is subsequently consumed by Al.

Towards All Electrical Spin Injection and Detection in GaAs in a Lateral Geometry

Herein we discuss our approach to realizing all electrical spin injection and detection in GaAs. We propose a lateral geometry, with two ferromagnetic electrodes crossing an n-doped GaAs channel. AlOx tunnel barriers are to be used in order to overcome the impedance mismatch and different widths of the two electrodes ensure different coercive fields. We present a detailed theoretical analysis of the expected magnetoresistance. Differences in behavior between lateral and vertical devices, the influence of the applied bias (electric field), and opportunities offered by different measurement geometries were explored. The MBE grown wafer consisted of 100 nm Al0.3Ga0.7As, acting as confinement layer, 100 nm n-doped (4 × l017 cm−3) GaAs, 3 nm n++ GaAs (1021 cm−3), to suppress Schottky barrier formation, and 1.5 nm Al. The Al was oxidized naturally in order to obtain tunnel barriers. By making use of in-situ shadow masks, a 0.1 mm wide channel is defined by covering the rest of the sample by insulating SiO2, followed by deposition of Ta bonding pads. Finally, 500 and 1000 nm wide CoFe electrodes crossing the GaAs channel are obtained by e-beam lithography and sputtering. We show that the I–V characteristics of the CoFe/AlOx/GaAs interface are consistent with tunneling as the main injection mechanism. However, the resistance-area (5 × 109 Ω μm2) of our barriers is too high compared to the GaAs conductance (50 Ω square resistance) leading to a strong suppression of magnetoresistance. Further experiments are in progress toward optimizing barrier and channel impedance matching.

Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers

Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM). The performance of these devices is currently limited by the modest (< approximately 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to approximately 220% at room temperature and approximately 300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of approximately 85%, which rivals that previously observed only using half-metallic ferromagnets. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 degrees C) will accelerate the development of new families of spintronic devices.

Transmission Electron Microscopy evidence of the growth of a magnetite layer acting as a spin filter in CoFe∕Al2O3∕CoFe magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) in which an iron oxide was deposited between the Al2O3 barrier and the pinned CoFe electrode were shown to exhibit a large tunneling magnetoresistance (TMR) value of 39% after annealing at 380°C. Local transmission electron microscopy (TEM) analyses were performed on the MTJs before and after the thermal treatments. In the as-deposited state, the high-resolution transmission electron microscopy (HRTEM) and the electron-energy-loss spectroscopy (EELS) demonstrate that the pure Fe exists at the Al2O3 interface, followed by the half-metallic ferromagnetic compound Fe3O4 that is formed when oxidizing the Fe layer. Similar structural studies show that the 380°C annealing achieves the oxidation of the iron into an extra Fe3O4 layer, now in contact with the Al2O3 barrier that leads to the high TMR values. These local TEM studies indicate that further annealing at higher temperature (400°C) induces the oxidation of the pinned CoFe electrode and the diffusion of Mn in the whole junction, resulting in a large decrease of the TMR.