MgO Barrier Research Articles

Magnetoresistive sensors with hybrid Co/insulator/ZnO:Co junctions

Magnetic tunnel junctions (MTJs), as the seminal spintronic devices, are expected for applications in magnetoresistive sensors due to their large magnetoresistance (MR) and high field sensitivity. Two hybrid Co/insulator/ZnO:Co junctions were fabricated with two different barriers to investigate the magneto-transport properties. Experimental results indicate that, both Co/MgO/ZnO:Co and Co/ZnO/ZnO:Co junctions show the positive and nearly linear MR, and their tunnel magnetoresistances (TMR) are 21.8% and 13.6%, respectively, when the current is applied perpendicular to the film plane under the magnetic field of 2 T at 4 K. The nonlinearity of MR is less than 1% within the magnetic field (H) of 1 kOe < H < 12 kOe at low temperature, making them attractive as magnetoresistive sensors. The higher MR of Co/MgO/ZnO:Co junctions is due to the superior spin filtering effect and larger effective barrier height of the MgO barrier. This linear MR characteristic of Co/insulator/ZnO:Co structures shows a promising future on the applications of diluted magnetic semiconductors in magnetoresistive sensors.

Lateral spin injection and detection through electrodeposited Fe/GaAs contacts

Efforts to achieve efficient injection of spin-polarized electrons into a semiconductor, a key prerequisite for developing electronics that exploit the electron's spin degree of freedom, have so far met with limited success. Here we report experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts. We demonstrate spin injection efficiencies two orders of magnitude higher than for state-of-the-art contacts fabricated via ultra-high-vacuum methods, including those with MgO or Al2O3 tunnel barriers. To account for this enhancement, we propose that an iron oxide layer that forms at the Fe/GaAs interface during electrodeposition, being magnetic acts as a tunnel barrier with a spin-dependent height, presenting quantum spin transport calculations for such systems. This serendipitous discovery of greatly enhanced efficiency of spin injection into GaAs via electrodeposited contacts introduces a promising new direction for the development of practical semiconductor spintronic devices.

Negative spin polarization of Mn2VGa probed by tunnel magnetoresistance

The ferrimagnetic Heusler compound Mn2VGa is predicted to have a pseudogap in the majority spin channel, which should lead to a negative tunnel magnetoresistance (TMR). We synthesized epitaxial Mn2VGa thin films on MgO(001) substrates by dc and rf magnetron co-sputtering, resulting in nearly stoichiometric films. XRD analysis revealed a mostly B2 ordered structure for the films deposited at substrate temperatures of 350, 450, and 550 °C. Magnetic tunnel junctions with MgO barriers and CoFe counter-electrodes were fabricated. After post-annealing at up to Ta = 425 °C negative TMR was obtained around zero bias, providing evidence for inverted spin polarization. The band structures of both electrodes were computed within the coherent potential approximation and used to calculate the TMR(V) characteristics, which were in good agreement with our experimental findings.

Magnetic field sensing characteristics of MgO based tunneling magnetoresistance devices with Co40Fe40B20 and Co60Fe20B20 electrodes

We developed spin valve tunneling magnetoresistance devices based on MgO barrier and two compositions of CoFeB electrodes capable of sensing magnetic field in tunable ranges with high sensitivity and low nonlinearity. The tunable field ranges are due to varying strength of perpendicular anisotropy in a sensing electrode induced by changing its thickness. The sensing field ranges span from ±0.1mT to ±100mT. In the narrowest field range devices showed sensitivity up to 91%/mT and nonlinearity below 1.5% of full scale and in the widest field range sensitivity up to 0.076%/mT and nonlinearity below 2% of full scale. The sensing characteristics and their dependence on the electrode thickness suggest that these device structures are useful for design low to medium magnetic field sensors.

Crystal Structures and Spin Injection Signals of Si/Mg/MgO/Co&lt;sub&gt;2&lt;/sub&gt;FeAl&lt;sub&gt;0.5&lt;/sub&gt;Si&lt;sub&gt;0.5&lt;/sub&gt; Junctions

We investigated the crystal structures and the spin injection signals of Si/Mg/MgO/Co2FeAl0.5Si0.5 (CFAS) junctions deposited by molecule beam epitaxy. The (001)-orientation and the structural ordering of CFAS thin films changed by heating deposition of CFAS and the insertion of Mg layer into Si/MgO interface. Spin injection signals observed at 10K for the junctions by 3 terminals Hanle measurements. The maximum voltage change, ¦VMAX, was increased by heating deposition of CFAS and the insertion of Mg layer. It is supposed that the enhancement of spin polarization by the improvement of structural ordering of CFAS or coherent tunneling by (001)-oriented MgO barrier caused the increment of ¦VMAX. [doi:10.2320/matertrans.M2013139]

Si 基板上への(001)配向 MgO 薄膜の作製

We investigated the structural property of MgO thin films on Si(001) substrates by molecular beam epitaxy in order to obtain (001)-oriented MgO barrier for high spin polarization on Si substrate. The MgO layer deposited at 200℃ and deposition rate of 0.30 nm/min grew with (001)-orientation on Si(001) substrate. The (001)-orientation and crystallization of the MgO layer were enhanced by insertion of Mg layer into interface between Si substrate and MgO layer. It is considered that the Mg layer prevented oxidation of Si at interface and functioned as buffer layer.

First-principles Study of Bias Effect on Magnetoresistance of Fe/MgO/Fe Tunnel Junctions

ABSTRACTWe studied the transport properties of the Fe/MgO/Fe and Fe/Ag/MgO/Ag/Fe magnetic tunnel junctions (MTJs) with 13-layer MgO barrier under bias voltage based on first-principles calculations. Our results showed that two features determine the TMR value decreases with bias of Fe/MgO/Fe MTJ: (1) interfacial states lying at 1.06 eV in spin down channel (2) the energy level of the spin down Δ1 band of the Fe electrode. Our results showed that an inserted Ag mono-layer at Fe/MgO interface can remarkably improve the TMR effect at a high bias voltage.

MgO(001) barrier based magnetic tunnel junctions and their device applications

Spintronics has received a great attention and significant interest within the past decades, and provided considerable and remarked applications in industry and electronic information etc. In spintronics, the MgO based magnetic tunnel junction (MTJ) is an important research advancement because of its physical properties and excellent performance, such as the high TMR ratio in MgO based MTJs. We present an overview of more than a decade development in MgO based MTJs. The review contains three main sections. (1) Research of several types of MgO based MTJs, including single-crystal MgO barrier based-MTJs, double barrier MTJs, MgO based MTJs with interlayer, novel electrode material MTJs based on MgO, novel barrier based MTJs, novel barrier MTJs based on MgO, and perpendicular MTJs. (2) Some typical physical effects in MgO based MTJs, which include six observed physical effects in MgO based MTJs, namely spin transfer torque (STT) effect, Coulomb blockade magnetoresistance (CBMR) effect, oscillatory magnetoresistance, quantum-well resonance tunneling effect, electric field assisted magnetization switching effect, and spincaloric effect. (3) In the last section, a brief introduction of some important device applications of MgO based MTJs, such as GMR & TMR read heads and magneto-sensitive sensors, both field and current switching MRAM, spin nano oscillators, and spin logic devices, have been provided.

Low-frequency and shot noises in CoFeB/MgO/CoFeB magnetic tunneling junctions

The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic tunneling junctions were studied at low temperature. The measured $1/f$ noise around the magnetic hysteresis loops of the free layer indicates that the main origin of the $1/f$ noise is the magnetic fluctuation, which is discussed in terms of a fluctuation-dissipation relation. Random telegraph noise (RTN) is observed to be symmetrically enhanced in the hysteresis loop with regard to the two magnetic configurations. We found that this enhancement is caused by the fluctuation between two magnetic states in the free layer. Although the $1/f$ noise is almost independent of the magnetic configuration, the RTN is enhanced in the antiparallel configuration. These findings indicate the presence of spin-dependent activation of RTN. Shot noise reveals the spin-dependent coherent tunneling process via a crystalline MgO barrier.

Very low 1/f barrier noise in sputtered MgO magnetic tunnel junctions with high tunneling magnetoresistance

Low frequency 1/f barrier noise has been investigated in sputtered MgO magnetic tunnel junctions (MTJs) with a tunneling magnetoresistance ratio of up to 330% at room temperature. The lowest normalized noise parameter α of the tunnel barrier reaches 2.5 × 10−12–2.1 × 10−11 μm2, which is comparable to that found in MTJs with the MgO barrier grown by MBE or electron–beam evaporation. This normalized barrier noise is almost bias independent in the voltage range of up to ±1.2 V. The low noise level and high voltage stability may reflect the high quality of the sputtered MgO with a uniform distribution of defects in the MgO layer.

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 &amp;gt; 3 T.

Magnetotransport properties of dual MgO barrier magnetic tunnel junctions consisting of CoFeB/FeNiSiB/CoFeB free layers

We report the transport properties of a dual MgO barrier magnetic tunnel junction (DMTJ) where a FeNiSiB layer was inserted in a CoFeB free layer. Upon post-deposition annealing at 330 °C, the tunneling magnetoresistance (TMR) ratio of the DMTJ with a hybrid CoFeB/FeNiSiB/CoFeB free layer reached 195% which is higher than the TMR ratio of 121% from the DMTJ with the single CoFeB free layer. The bias voltage dependence profile was more symmetric for the hybrid case. Boron depth profiling result suggests that the FeNiSiB layer dragged boron atoms more to it rather than letting them diffuse toward CoFeB/MgO interfaces, resulting in improved MTJ performances.

Radiation Tolerance of Magnetic Tunnel Junctions With MgO Tunnel Barriers

The gamma and neutron radiation tolerance of magnetic tunnel junctions with MgO barriers were investigated. The electrical transport and magnetic properties were measured in this effort. The characterization results show no statistically significant change in either tunneling magnetoresistance or coercivity from the radiation.

Magnetization dynamics of a MgO-based spin-torque oscillator with a perpendicular polarizer layer and a planar free layer

Current-induced magnetization excitations are studied for a spin-torque oscillator (STO) composed of a nanopillar with a perpendicular polarizer layer (PL), a MgO barrier layer, and a planar free layer (FL). By applying direct current and perpendicular-to-plane magnetic field, we measure resistance and radio-frequency electrical signal of the STO, which reflect magnetization motions of both PL and FL. Examination of the experimental results reveals that large-cone-angle magnetization oscillation occurs in the FL regardless of the current direction, whereas the PL magnetization shows principally either synchronized excitation with the FL oscillation or thermal-induced ferromagnetic resonance (FMR), depending on the current direction. Utilizing macrospin simulations, we show that hybridization of the excitation modes of the PL and FL through mutual dipolar field explains the magnetization dynamics. When the current flows from the PL to the FL, large-cone-angle oscillation of the FL magnetization occurs with the same rotation direction as that of FMR of the PL magnetization, leading to emergence of the synchronized excitation modes. On the other hand, when the current flows from the FL to the PL, the magnetization motions of the two layers have opposite rotation directions, and consequently, the PL and FL show their respective intrinsic excitation modes.

Giant tunneling magnetoresistance in epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions by half-metallicity of Co2MnSi and coherent tunneling

Giant tunnel magnetoresistance (TMR) ratios of up to 1995% at 4.2 K and up to 354% at 290 K were obtained for epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions (MTJs) featuring a reduced lattice mismatch in the MTJ trilayer by introducing a thin Co2MnSi lower electrode deposited on a Co50Fe50 buffer layer. The obtained giant TMR ratios can be explained by the enhanced contribution of coherent tunneling originating from the increased misfit dislocation spacing at the lower and upper interfaces with a MgO barrier along with the half-metallicity of Co2MnSi electrodes.

Perpendicular magnetization of Co20Fe50Ge30 films induced by MgO interface

Epitaxial growth of Co20Fe50Ge30 films on single crystal MgO (001) substrate is reported. Structure characterization revealed (001)-oriented B2 order of Co20Fe50Ge30, well lattice matched with the MgO barrier. Perpendicular magnetic anisotropy was achieved in the MgO/Co20Fe50Ge30/MgO structure with an optimized magnetic anisotropy energy density of 2 × 106 erg/cm3. The magnetic anisotropy is found to depend strongly on the thickness of the MgO and Co20Fe50Ge30 layers, indicating that the perpendicular magnetic anisotropy of Co20Fe50Ge30 is contributed by the interfacial anisotropy between Co20Fe50Ge30 and MgO. With reported low damping constant, Co20Fe50Ge30 films are promising spintronic materials for achieving low switching current.

Highly Spin-Polarized Tunneling in Epitaxial Magnetic Tunnel Junctions with a Co2MnSi Electrode and a MgO Barrier with Improved Interfacial Structural Properties

Highly Spin-Polarized Tunneling in Epitaxial Magnetic Tunnel Junctions with a Co2MnSi Electrode and a MgO Barrier with Improved Interfacial Structural Properties

Magnetic Instability in Tunneling Magnetoresistive Heads Due to Temperature Increase During Electrostatic Discharge

Recently, there has been a growing interest in the effects of electrostatic discharge (ESD) failure on tunneling magnetoresistive (TMR) recording heads because it directly affects reliability in manufacturing of these heads. Therefore, we study the magnetic degradation in TMR junctions caused by the temperature increase using three different ESD models. A 3-D finite-element method is used for analyzing the spatial and temporal profiles of the temperature during the discharge. The results from the three models show that, although the highest temperature occurs in the MgO barrier layer, the initial magnetic modification likely arises in the IrMn antiferromagnetic layer due to its low Néel temperature. We also found that the increase in temperature is proportional to the square of the ESD voltage. The magnetic instability of the antiferromagnetic layer due to the ESD effect is the important parameter realized for development of the future TMR devices.

Highly Spin-Polarized Tunneling in Epitaxial Magnetic Tunnel Junctions with a Co2MnSi Electrode and a MgO Barrier with Improved Interfacial Structural Properties

Fully epitaxial magnetic tunnel junctions (MTJs) with a Heusler alloy Co2MnSi electrode and a MgO barrier were prepared on MgO-buffered MgO(001) substrates with various layer structures to elucidate the contribution of coherent tunneling to the spin-dependent tunneling characteristics of these MTJs, which potentially feature both the half-metallicity of Co2MnSi and coherent tunneling. MTJs consisting of (from the lower side) Co50Fe50 (CoFe)/MgO/Co2MnSi or CoFe-buffered Co2MnSi/MgO/CoFe showed almost identical, high tunnel magnetoresistance (TMR) ratios of 335% at 290 K (1049% at 4.2 K) and 340% at 290 K (879% at 4.2 K), respectively. In contrast, MTJs consisting of MgO-buffered Co2MnSi/MgO/CoFe showed a lower TMR ratio of 173% at 290 K (448% at 4.2 K). The higher TMR ratios obtained for CoFe/MgO/Co2MnSi MTJs and CoFe-buffered Co2MnSi/MgO/CoFe MTJs can be ascribed to the enhanced contribution of coherent tunneling that originated from decreased misfit dislocation densities at the lower and upper interfaces with a MgO barrier.

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.