Magnetic Tunnel Barrier Research Articles

Phase-tunable Josephson junction and spontaneous mass current in a spin-orbit-coupled Fermi superfluid

Atomtronics has the potential for engineering new types of functional devices, such as Josephson junctions (JJs). Previous studies have mainly focused on JJs whose ground states have 0 or $\pi $ superconducting phase difference across the junctions, while arbitrarily tunable phase JJs may have important applications in superconducting electronics and quantum computation. Here we show that a phase tunable JJ can be implemented in a spin-orbit coupled cold atomic gas with the magnetic tunneling barrier generated by a spin-dependent focused laser beam. We consider the JJ confined in either a linear harmonic trap or a circular ring trap. In the ring trap, the magnetic barrier induces a spontaneous mass current for the ground state of the JJ, demonstrating the magnetoelectric effects of cold atoms.

Nematic versus ferromagnetic spin filtering of triplet Cooper pairs in superconducting spintronics

We consider two types of magnetic Josephson junctions~(JJ). They are formed by two singlet superconductors~S and magnetic layers between them so that the JJ is a heterostructure of the S$_{\text{m}}$/n/S$_{\text{m}}$ type, where~S$_{\text{m}}$ includes two magnetic layers with non-collinear magnetization vectors. One layer is represented by a weak ferromagnet and another one---the spin filter---is either conducting strong ferromagnet (nematic or N\nobreakdash-type JJ) or magnetic tunnel barrier with spin-dependent transparency (magnetic or M\nobreakdash-type JJ). Due to spin filtering only fully polarized triplet component penetrates the normal n~wire and provides the Josephson coupling between the superconductors~S. Although both filters let to pass triplet Cooper pairs with total spin~$\mathbf{S}$ parallel to the filter axes, the behavior of nematic and magnetic JJs is completely different. Whereas in the nematic case the charge and spin currents,~$I_{\text{Q}}$ and~$I_{\text{sp}}$, do not depend on mutual orientation of the filter axes, both currents vanish in magnetic~JJ in case of antiparallel filter axes, and change sign under reversing the filter direction. The obtained expressions for~$I_{\text{Q}}$ and~$I_{\text{sp}}$ show clearly a duality between the superconducting phase~$\varphi$ and the angle~$\alpha$ between the exchange fields in the weak magnetic layers.

Co2Fe6B2/MgO-Based Perpendicular Spin-Transfer-Torque Magnetic-Tunnel-Junction Spin-Valve without [Co/Pt]n lower Synthetic-Antiferromagnetic Layer

In recently years, the research on magnetic tunnel junctions (MTJs) using CoFeB magnetic layer and MgO tunneling barrier layer has been performed for realizing perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT-MRAM). In particular, to integrate a terra-bit-level nonvolatile memory-cell, the p-MTJ needs to achieve critical device performances such as a high tunneling magnetoresistance (TMR) ratio of ~150%, a high thermal stability (°â = E/KBT) of 74, and a low critical current density (Jo ) of 13.4 MA/cm2, where E, Ms , kB , and T are the energy barrier, the saturation magnetization, the Boltzmann constant, and the temperature, respectively.[1] In addition, to achieve CoFeB/MgO based p-MTJs grown on 12-inch TiN/W electrode, a design of [Co/Pt]n synthetic-anti-ferro-magnetic (SyAF) layer being operated at the ex-tu annealing above 350 oC has been intensively studied.[2] However, the [Co/Pt]n SyAF layer essentially has thicker a [Co/Pt]n SyAF layer since a [Co/Pt]n lower SyAF should couple antiferromagnetically with a [Co/Pt]n upper SyAF layer and couple ferromagnetically with a Co2Fe6B2 pinned layer at the same time.[3] Thicker SyAF layer has the problem of a high fabrication cost for p-MTJ spin-valves in the mass production of p-STT MRAM. Thus, we designed a Co2Fe6B2/MgO based p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, which can largely reduce the fabrication cost since the total multi-layer number (n) of [Co/Pt]n for the Co2Fe6B2/MgO-based p-MTJ spin-valve.Figures 1(a) and (b) show the spin-valve structure of a [Co/Pt]n SyAF layer with lower SyAF layer, without lower SyAF layer, respectively. The [Co/Pt]n SyAF layer without a lower SyAF layer is much thinner than that with a lower SyAF layer, as you can see in figure 1. In addition, the upper SyAF layer of p-MTJ without a lower SyAF layer is much thinner than that of p-MTJ with a lower SyAF layer because of fitting for antiferromagnetically coupling between lower and upper SyAF layer. And we studied the TMR ratio dependency on bcc nanoscale capping layer thickness of two structures by using current in-plane tunneling (CIPT) technique. In result, the maximum TMR ratios of two structures were both over 150%. In case of the p-MTJ with a [Co/Pt]n lower SyAF layer, the TMR ratio rapidly increased from 19% to 159% when the bcc nanoscale capping-layer thickness (tbcc ) increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 159% to 2% when tbcc decreased from 0.6 to 0.73 nm. Also, for the p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, the TMR ratio very rapidly increased from 5% to 158% when tbcc increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 158% to 2% when tbcc decreased from 0.6 to 0.73 nm.In the conference, we will present the effect of Co2Fe6B2/MgO-based perpendicular spin-transfer-torque magnetic-tunnel-junction spin-valve without [Co/Pt]n lower synthetic-antiferromagnetic layer and review the advantage of p-MTJ without lower SyAF layer. In addition, we report the physical and magnetic properties of p-MTJ by using vibrating-sampling-magnetometer (VSM), transmission-electron-microscopy (TEM). Furthermore, we review the mechanism why the magnetic properties of p-MTJs are varied by the thickness of SyAF layer. * This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014. Reference [1-3] [1] K. C. Chun, H. Zhao, J. D. Harms, T. H. Kim, J. P. Wang, and C. H. Kim, IEEE J. Solid-State Circuits 48, 598, 2013.[2] S. Ikeda, H. Sato, H. Honjo, E. C. I. Enobio, S. Ishikawa, M. Yamanouchi, S. Fukami, S. Kanai, F. Matsukura, T. Endoh and H. Ohno, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 33.2.1. [3] J. G. Park, T. H. Shim, K. S. Chae, D. Y. Lee, Y. Takemura, S. E. Lee, M. S. Jeon, J. U. Baek, S. O. Park, and J.P. Hong, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 19.2.1. Figure 1

Magnetic and nonmagnetic tunnel barriers in Sr2FeMoO6

Tunnelling magnetoresistance (TMR) in polycrystalline Sr2FeMoO6 has been a matter of intense interest for more than a decade now, where the nature of the insulating tunnel barrier turned out to be the core issue of debate. Other than the nonmagnetic grain boundaries as conventional tunnel barriers, intragrain magnetic anti-phase boundaries (APB) as well as magnetically frustrated grain surfaces have also been proposed to act as tunnel barriers and influence the TMR response of Sr2FeMoO6 in different ways. In this paper, it is shown that depending upon the physical state of the sample, at least three different types of tunnelling mechanism may persist in polycrystalline Sr2FeMoO6 and a purely conventional TMR response, seldom reported in this important magnetoresistive material, is also possible.

Cover Picture: Electronic structure of EuO spin filter tunnel contacts directly on silicon (Phys. Status Solidi RRL 12/2011)

Establishing magnetic oxides as spin-selective tunnel barriers on mainstream semiconductors opens up a promising route towards highly efficient spin injection and detection. Europium oxide (EuO) is the only magnetic oxide predicted to be stable in direct contact with silicon. A throughout hard X-ray photoemission spectroscopy study (see the Letter by Caspers et al. on pp. 441–443) reveals the nearly ideal stoichiometry of EuO “spin filter” tunnel barriers grown by Oxide-MBE, with no silicon oxide or silicide formation being observed at the EuO/Si interface. This result clearly demonstrates the successful integration of a magnetic oxide tunnel barrier with silicon, opening up the future integration of magnetic oxides into functional spintronics devices.

Electronic structure of EuO spin filter tunnel contacts directly on silicon

AbstractWe present an electronic structure study of a magnetic oxide/ semiconductor model system, EuO on silicon, which is dedicated for efficient spin injection and spin detection in silicon‐based spintronics devices.A combined electronic structure analysis of Eu core levels and valence bands using hard X‐ray photoemission spectroscopy was performed to quantify the nearly ideal stoichiometry of EuO “spin filter” tunnel barriers directly on silicon, and the absence of silicon oxide at the EuO/Si interface. These results provide evidence for the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices.magnified imageHard X‐ray photoemission spectroscopy of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface.(© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial growth and ferrimagnetic behavior of MnFe2O4(111) ultrathin layers for room-temperature spin filtering

We report on the epitaxial growth and physical properties of spinel MnFe2O4(111) thin films with thicknesses down to 2 nm. The thin films, grown on alpha-Al2O3(0001) single crystals or Pt(111) buffer layers by oxygen-assisted molecular beam epitaxy, exhibit high structural order with sharp interfaces and low roughness. The electrical and magnetic properties are carefully investigated and it is shown that MnFe2O4(111) ultrathin films keep an insulating and ferrimagnetic behavior at room temperature. Special attention is given to the iron/manganese valence state and the cationic ordering. X-ray absorption spectroscopy and magnetic circular dichroism measurements reveal that thin films contain mainly Fe3+ and Mn2+ cations, distributed predominantly in a normal spinel structure. This study proves the high potential of MnFe2O4 to be used as a magnetic tunnel barrier for spin filtering applications at room temperature.

Magnetic properties of EuS spin filter tunnel contacts to silicon

We investigate the magnetic properties of the ferromagnetic insulator EuS in view of its potential in spin-filter tunnel contacts to silicon. We prepared thin polycrystalline EuS films directly on (001) oriented Si substrates that show well-defined magnetic properties down to the monolayer regime. Addressing the question of magnetic coupling between a EuS magnetic tunnel barrier and a CoO/Co magnetic electrode, we succeeded in realizing an independent magnetic switching behavior in this spin-valve-type system. These results are important prerequisites for future spin-dependent transport experiments.

Spin-dependent resonant quantum tunneling between magnetic nanoparticles on a macroscopic length scale

Macroscopic quantum phenomena are common features observed in superconductors, superfluid helium, and Bose-Einstein condensates. However, most of quantum transport studies are based on a small number of dots and are not in long-range electron transport length scale. Here we show that spin-dependent resonant quantum tunneling is achieved in the macroscopic length scale (a few millimeters) corresponding to an array of up to ${10}^{4}$ junctions in a series consisting of Co nanoparticles embedded in an oxygen-deficient TiO${}_{2}$ matrix. This phenomenon is observed by magnetoresistance measurements at 5 K in a Coulomb blockade regime. We further present a model based on resonant spin-polarized quantum tunneling of electrons of Co particles. It occurs through resonant continuous spin-polarized defect band states located near the Fermi level of the defective TiO${}_{2}$, which acts as a magnetic tunnel barrier. These results might be potentially useful for future designs of spintronic quantum devices.

Coulomb oscillations of indium-doped ZnO nanowire transistors in a magnetic field

We report on the observation of Coulomb oscillations from localized quantum dots superimposed on the normal hopping current in ZnO nanowire transistors. The Coulomb oscillations can be resolved up to 20 K. Positive anisotropic magnetoresistance has been observed due to the Lorentz force on the carrier motion. Magnetic field-induced tunneling barrier transparency results in an increase in oscillation amplitude with increasing magnetic field. The energy shift as a function of magnetic field indicates electron wave function modification in the quantum dots.

Large Magnetoresistance Based on Double Spin Filter Tunnel Barriers

We propose and theoretically analyse a double magnetic tunnel device that takes advantages of the spin filter effect. Two magnetic tunnel barriers are formed by different spin filters which have different barrier heights. The magnetoresistance of the device is low (high) when the magnetic moments of the two spin filters are parallel (antiparallel). We present a theoretical calculation of the magnetoresistance based on electric tunnel effect. In addition, the effect of the difference barrier heights and exchange splitting energies between the two spin filters are also analysed in detail. The numerical results show that the spin filter in this configuration gives a magnetoresistance larger than that with standard magnetic tunnel junctions.

Interface stability of magnetic tunnel barriers and electrodes

AbstractGrazing incidence X‐ray scattering during in‐situ annealing has been used to study the interface stability in MgO‐based electrodes and barriers in magnetic tunnel junctions (MTJs). In MTJs based on Co60Fe20B20/MgO, low temperature annealing at about 200 °C can result in a drop in the tunnelling magnetoresistance (TMR). Specular reflectivity measurements show a change in fringe structure at high scattering vector, at the temperature at which the TMR drops. Also, a change in diffuse scatter relative to the specular scatter is observed. Annealing of [Co60Fe60B20/MgO]×14 multilayers prepared by three different techniques shows no change in the specular Bragg peaks with no loss of Kiessig fringe structure on crystallization and no change in the diffuse scatter up to 200 °C. We suggest that diffusion of Mn from the IrMn pinning layer, present in the MTJs but not in the multilayers, may be responsible for the differences in TMR behaviour. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of a metallic or oxide top layer in epitaxial magnetic bilayers containingCoFe2O4(111)tunnel barriers

We investigate the interaction of $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}(111)$ tunnel barriers with Co and ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ electrodes in light of their potential as room temperature spin filters. The question of the exchange coupling that often prohibits the independent switching between a magnetic tunnel barrier and its magnetic electrode is addressed, as is the difference between an oxide/metal and oxide/oxide system. Our study of the magnetic reversal in the $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕\mathrm{Co}$ and $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ bilayers, supported by a detailed structural and chemical analysis of the samples and their interfaces, clearly evidences the effect of a metallic or an oxide interface. An unusual exchange spring magnet behavior arises in the case of the $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ samples due to the superexchange interactions found in these ferrimagnetic oxides. This unique exchange phenomenon at the oxide-oxide interface ultimately leads to a bilayer system displaying a two-phase magnetization loop without the insertion of a nonmagnetic spacer.

Enhanced spin-dependent shot noise in magnetic tunnel barriers

We report the observation of enhanced spin-dependent shot noise in magnetic tunnel barriers, suggesting transport through localized states within the barrier. This is supported by the existence of negative magnetoresistance and structure in the differential conductance curves. A simple model of tunneling through two interacting localized states with spin-dependent tunneling rates is used to explain our observations.

The phenomena of spin-filter tunnelling

The spin filtering phenomenon allows one to obtain highly spin-polarized charge carriersgenerated from nonmagnetic electrodes using magnetic tunnel barriers. The exponentialdependence of tunnel current on the tunnel barrier height is operative here. The magnetic,semiconducting europium chalcogenide compounds have strikingly demonstrated this effect.The possibility of employing ferrites and other methods opens the potential for display ofthis phenomenon at room temperature, which can be expected to lead to hugeprogress in spin injection and detection in semiconductors. But first, extremelychallenging material-related issues have to be addressed. This review covers the field.

Oxidation of Thin Al Layers for Magnetic Tunnel Barriers: a Three-dimensional Atom Probe Study

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Evidence for positive spin polarization in Co with SrTiO3 barriers

Recent negative tunneling magnetoresistance results with epitaxial SrTiO3 and TiO2 tunnel barriers were attributed to tunneling that favor d-like states from Co, resulting in negatively polarized electron tunneling. In order to confirm this, we carried out direct measurements of spin polarization of Co with SrTiO3 barriers using a superconductor as a spin detector. The polarization was observed to be positive (+31%), in contrast to published results. Furthermore, studies conducted on magnetic tunnel junctionswith SrTiO3 and TiO2 tunnel barriers are consistent with the direct measurements. The implications of our results, for the case of amorphous barriers, in comparison to crystalline barriers is discussed.

Spin-polarized transport properties in magnetic tunnel junctions with spin–orbit coupling

Effects on spin-polarized transport properties of interfacial scattering and random substitutional non-magnetic disorder in magnetic tunnel barrier are investigated numerically taking into account spin–orbit (SO) coupling and local s–d exchange interaction under finite bias voltage. The method implemented with calculation of real space Green's function is employed in tight-binding model. Tunnel conductance is analyzed in terms of spin-conserving and spin–flip contributions due to mixing of spin states induced by the SO coupling. Spin polarization can even be inverted due to modification of spin-dependent density of states and shifted Fermi level with bias voltage. Interfacial spin–flip scattering plays a crucial role in the depression of spin-injection rate.

Fabrication of niobium SIS junctions with a magnetic tunneling barrier NiO x

We report on the fabrication of a new type of superconductor–insulator–superconductor junctions with intrinsically vanishing Josephson current. We have fabricated junctions with a tunneling barrier containing oxidized 3d transition metal Ni, i.e., Nb/NiO x /Nb and Nb/NiO x /Al/Nb stacked junctions. In the junction fabrication process, we used a conventional RIE technique called selective niobium etch process. The junction size was varied from 2.5×2.5 to 10×10 μm 2. The Josephson current of these junctions was suppressed to zero or nearly zero, while a sharp gap structure was also observed. This new-type device will be promising in realizing a highly integrated mixer.

Fabrication of niobium superconductor–insulator–superconductor junctions with a magnetic tunneling barrier NiOx for millimeter and submillimeter mixer applications

A thin-film superconductor-insulator-superconductor junction with a magnetic barrier, NiOx, and superconducting electrode, Nb, was fabricated. It is shown that the Josephson current was strongly suppressed without external magnetic field. The tunneling barrier was formed by plasma oxidation and or thermal oxidation. The junction with thermal oxidation showed low specific resistance and current–voltage characteristics with a clear energy gap structure. It is shown that this junction has the potential for quasiparticle mixer working at higher submillimeter wave frequency region above ∼700 GHz, especially suited for making an imaging array.