MgO Barrier Research Articles

Effect of MgO Barrier Insertion on Spin-Dependent Transport Properties of CoFe/n-GaAs Heterojunctions

The effect of MgO barrier insertion on a spin-valve signal in a four-terminal non-local geometry and on tunneling anisotropic magnetoresistance (TAMR) characteristics in a three-terminal geometry was investigated in Co50Fe50/n-GaAs heterojunctions. Inserting a MgO barrier significantly enhanced the spin-valve signal amplitude by a factor of 38, and the sign of spin polarization was opposite that of a sample without a MgO barrier. The TAMR effect was suppressed in the case of a Co50Fe50/MgO/n-GaAs junction. This suppression of the TAMR effect can be explained by the suppression of Fermi-level pinning and the lowering of Schottky barrier height.

The study of conductance in magnetic tunnel junctions with a thin MgO barrier: The effect of Ar pressure on tunnel magnetoresistance and resistance area product

The thickness dependence of tunnel magnetoresistance and resistance area product in Co40Fe40B20/MgO wedge/Co40Fe40B20 magnetic tunnel junctions (MTJs) has been studied for multiple Ar partial pressure (PAr) values during MgO sputtering. The extension of the simple equivalent circuit model [B. Oliver et al., J. Appl. Phys. 91, 4348 (2002)] has been suggested in order to include different transport mechanism contributions to the overall conductance of the MTJ as a function of the MgO barrier thickness. Parameters of the model, used for quantitative description of the conductivity of unpatterned MTJ stacks, have been analyzed as a function of PAr.

Interface Characterization of Epitaxial Fe/MgO/Fe Magnetic Tunnel Junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.

Effect of MgO Barrier Insertion on Spin-Dependent Transport Properties of CoFe/n-GaAs Heterojunctions

Effect of MgO Barrier Insertion on Spin-Dependent Transport Properties of CoFe/n-GaAs Heterojunctions

All-metallic lateral spin valves using Co2Fe(Ge0.5Ga0.5) Heusler alloy with a large spin signal

A large spin signal (ΔRs) of 12.8 mΩ at room temperature is demonstrated in an all-metallic lateral spin valve (LSV) device comprising of ferromagnetic wire of a highly spin-polarized Co2Fe(Ge0.5Ga0.5) Heusler alloy and a Cu wire. From the results on multi-terminal LSV devices, the origin of the large ΔRs was concluded to be the high spin polarization of Co2Fe(Ge0.5Ga0.5) and the resultant small spin absorption. The all-metallic LSV device without MgO barrier has low contact resistance, which is considered to be beneficial for a high frequency ultrathin read sensor for the next generation ultrahigh density magnetic recording.

Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses

The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.

Determination of layer-resolved composition, magnetization, and electronic structure of an Fe/MgO tunnel junction by standing-wave core and valence photoemission

Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of \ensuremath{\sim}8 \AA{} (\ensuremath{\sim}4 monolayers) and that there is a magnetic dead layer \ensuremath{\sim}2--3 \AA{} thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeO${}_{\mathrm{x}}$, with x \ensuremath{\cong} 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at \ensuremath{\sim}3.5 eV below the Fermi level (${E}_{\mathrm{F}}$) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer \ensuremath{\sim}1.3 eV below ${E}_{\mathrm{F}}$, which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap.

Correlation between symmetry-selective transport and spin-dependent resonant tunneling in fully epitaxial Cr/ultrathin-Fe/MgO/Fe(001) magnetic tunnel junctions

Spin-dependent resonant tunneling was investigated in fully epitaxial Cr(001)/ultrathin Fe(001)/MgO(001)/Fe(001) pseudo double-barrier magnetic tunnel junctions (MTJs) by varying structural coherence of the MgO barrier through postdeposition annealing and Mg layer insertion. It was clearly demonstrated that not only the flatness of the Fe quantum-well layer, but also the structural coherence of the MgO barrier caused the appearance of sharp resonant conductance peaks which were not obtained for spin-dependent resonant tunneling in amorphous AlO-barrier MTJs. The symmetry-selective transport via MgO barrier plays a crucial role for the resonant tunneling as well as large tunneling magnetoresistance.

Tailoring the growth of L10‐FePt for spintronics applications

AbstractThe effect of seed layers on the magnetic and structural properties of FePt thin films was investigated. For the case of Cr(200) seed layer, no ordered L10‐FePt phase in (001) orientation was observed. The insertion of a thin MgO barrier between the Cr and FePt layers improved the perpendicular magnetization orientation of L10‐FePt by limiting diffusion of Cr into the FePt magnetic layer. Ordered L10‐FePt thin films can be developed on Pd[100] at high substrate temperature with the lattice mismatch of below 1% and therefore low roughness compared to MgO or Cr seed layers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In-plane uniaxial magnetic anisotropy in epitaxial Fe3O4-based hybrid structures on GaAs(100)

The evolution of the in-plane magnetic anisotropies in Fe${}_{3}$O${}_{4}$/GaAs(100) and Fe${}_{3}$O${}_{4}$/MgO/GaAs(100) hybrid spintronic structures has been studied by magneto-optical Kerr effect and ferromagnetic resonance (FMR). The surface and volume contributions to the in-plane cubic and uniaxial anisotropies have been distinguished in Fe${}_{3}$O${}_{4}$/GaAs by fitting the anisotropy constants, measured by FMR, as a function of the magnetic film thickness. It was found that interfacial chemical bonding rather than strain relaxation plays the dominant role in causing the unexpected uniaxial magnetic anisotropy (UMA) in Fe${}_{3}$O${}_{4}$ films grown directly on the GaAs surfaces. In contrast, after MgO barrier insertion, FMR results show that the UMA is greatly reduced, and strain relaxation is found to be the main origin of the UMA in Fe${}_{3}$O${}_{4}$/MgO/GaAs structures.

Spin control by application of electric current and voltage in FeCo–MgO junctions

Efficient control and detection of spins are the most important tasks in spintronics. The current and voltage applied to a magnetic tunnel junction may exert a torque on the magnetic thin layer in the junction and cause its reversal or continuous precession. The discovery of the giant tunnelling magnetoresistance effect in ferromagnetic tunnelling junctions using an MgO barrier enabled us to obtain a large signal output from the magnetization reversal and precession. Also, the interplay of large spin configuration-electric conduction coupling provides highly nonlinear effects like the spin-torque diode effect. The negative resistance effect and amplification using it are predicted. A new discovery about a voltage-induced magnetic anisotropy change in Fe ultrathin films is also discussed.

Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d = 1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm2 have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.

Inductive determination of the optimum tunnel barrier thickness in magnetic tunneling junction stacks for spin torque memory applications

We use pulsed inductive microwave magnetometry to study the precessional magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic tunneling junction stacks with varying MgO barrier thickness. From the field dependence of the precession frequency we are able to derive the uniaxial anisotropy energy of the free layer and the exchange coupling between the free and the pinned layer. Furthermore the field dependence of the effective damping parameter is derived. Below a certain threshold barrier thickness we observe an increased effective damping for antiparallel orientation of free and pinned layer which would inhibit reversible low current density spin torque magnetization reversal. Such inductive measurements, in combination with wafer probe station based magneto transport experiments, allow a fast determination of the optimum tunnel barrier thickness range for spin torque memory applications in a lithography free process.

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.

Simulation of tunneling magneto-resistance in Fe/MgO/Fe junction

The conductivity asymmetry of junctions with insulating MgO barriers embedded between ferromagnetic Fe electrodes has been investigated by the first principles calculation program of Atomistix ToolKit 2008. The conductivity in up-spin electron is larger than that in down-spin electron for the ideal structure. It leads to the large TMR ratio. The defect of O or Fe atoms hardly varies the conductivity in both up and down-spin electrons. The defect of Mg atoms increases the conductivity in down-spin electron, although the up-spin conductivity does not change very much. So the defect of Mg decreases the TMR ratio. The defect of Mg near the interface especially shows this degradation. The derivative conductivity dI/dV vs. V characteristics are asymmetric for the polarity of the bias voltage V according to the location of the atom defect. The quality of the junction in devices can be evaluated by the asymmetry.

Evidence for magnon excitation contribution to the magnetoresistance behavior during thermal annealing in CoFeB/MgO/CoFeB magnetic tunnel junctions

For sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions, it is well known that the tunnel magnetoresistance (TMR) ratio increases with increasing annealing temperature (${T}_{a}$) up to a critical value (${T}_{p}$), and then decreases with further increasing ${T}_{a}$, resulting in a peak around ${T}_{p}$. The improved crystallinity of the MgO barrier and CoFeB electrodes due to annealing has been considered as the main reason for the enhancement of the TMR ratio, especially for ${T}_{a}&lt;{T}_{p}$. In this work, the evidence is provided that the magnon excitation plays a great contribution to the magnetoresistance (MR) behavior in annealed samples based on the measurement of dynamic conductance and inelastic electron tunneling (IET) spectra. The magnon activation energy (${E}_{c}$) obtained from the fits for IET spectra exhibits a similar temperature dependence with that of the TMR ratio. A detailed analysis shows that the magnon excitation, together with improved crystallinity of the MgO barrier and CoFeB layers, is the main contribution to the annealing-temperature-dependent MR behavior.

Suppression of in-plane tunneling anisotropic magnetoresistance effect in Co2MnSi/MgO/n-GaAs and CoFe/MgO/n-GaAs junctions by inserting a MgO barrier

The effects of MgO tunnel barriers on both junction resistance and tunneling anisotropic magnetoresistance (TAMR) characteristics of Co2MnSi(CMS)/MgO/n-GaAs junctions and Co50Fe50(CoFe)/MgO/n-GaAs junctions were investigated. The resistance-area (RA) product of the CMS/MgO/n-GaAs junctions showed an exponential dependence on MgO thickness (tMgO), indicating that the MgO layer acts as a tunneling barrier. The RA product of CMS/MgO/n-GaAs with tMgO&amp;lt;1 nm was smaller than that of the sample without MgO. The observed spin-valvelike magnetoresistance of CMS/n-GaAs and CoFe/n-GaAs Schottky tunnel junctions attributed to the TAMR effect did not appear in the cases of CMS/MgO/n-GaAs and CoFe/MgO/n-GaAs tunnel junctions. The lowering of the RA product and the suppression of the TAMR effect caused by inserting a thin MgO layer between CMS and n-GaAs were both possibly due to suppression of the Fermi-level pinning of GaAs and lowering of the Schottky barrier height.

Characterization of Field Penetration in Superconducting Multilayers Samples

The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (HC ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having HC higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality model samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field BC1. For comparison, BC1 was also mea sured with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field.

&lt;jats:formula formulatype="inline"&gt;&lt;jats:tex Notation="TeX"&gt;${\rm MgB}_{2}/{\rm MgO/MgB}_{2}$&lt;/jats:tex&gt;&lt;/jats:formula&gt; Josephson Junctions for High-Speed Circuits

MgB2/MgO/MgB2sandwich-type Josephson junctions have been fabricated using MgB2films grown by hybrid physical-chemical vapor deposition (HPCVD). The MgO barrier and the insulating layer between wiring layers were made by RF magnetron sputtering. The junctions exhibit non-vanishingJcup to 40 K, theTcof MgB2. MgB2wiring is achieved without significantly changing the junction properties. The fabrication and properties of the junctions are described in detail. TheI-Vcurves of the junctions are hysteretic at temperatures below 25 K and are RSJ-like above 25 K. Multiple Andreev reflection was observed, manifested by subgap peaks in the dI/dV-Vcurves and in good agreement with the theory. TheJcspread (1&amp;#x03C3;) of 10 junctions on one chip is better than 10%, which is likely due to the defects inside the MgO barrier as identified by transmission electron microscopy. Superconducting circuits made of these junctions may be expected to work at frequencies up to 1 THz or at temperatures over 20 K.

Improvement of the low-frequency sensitivity of MgO-based magnetic tunnel junctions by annealing

Magnetic tunnel junctions can serve as ultrasensitive low-frequency magnetic sensors, however, their low-frequency performance is limited by low-frequency noise, i.e., 1/f noise. In this paper, we investigate the 1/f noise in MgO magnetic tunnel junctions (MTJs) with a tunneling magnetoresistance (TMR) of 160%, and examine the influence of annealing and MTJ size. The results show that the annealing process can not only dramatically improve the TMR, but can also strongly decrease the MTJ noise. The effect is discussed in terms of the structure of MgO barriers and tunneling probabilities. Increasing the MTJ area to 6400 μm2 yields a voltage spectral density as low as 11 nV/Hz1/2 at 1000 Hz. The possible reasons for the area dependence are discussed.