Giant Tunneling Magnetoresistance Research Articles

Spin Transport in Ferromagnet/Ferromagnet/s-Wave Superconductor Hybrid System

We study the spin transport in ferromagnet/ferromagnet/s-wave superconductor hybrid system with the extended Blander-Tinkham-Klapwijk formalism. We show the subgap spin conductance dominated by Andreev reflection is susceptible to the relative orientation of the two magnetizations. A giant spin tunneling magnetoresistance which is defined with the spin conductance for the collinear and the non-collinear magnetization configurations is obtained. The oscillatory behavior in the conductance spectra and Andreev bound states in the junctions caused by interference effect are also investigated. All these results may possess potential applications in future device design in spintronics.

Large positive spin polarization and giant inverse tunneling magnetoresistance in Fe/PbTiO3/Fe multiferroic tunnel junction

We perform first-principles electronic structure and spin-dependent transport calculations of a multiferroic tunnel junction (MFTJ) with an epitaxial Fe/PbTiO3/Fe heterostructure. We predict a large positive spin-polarization (SP) and an intriguing giant inverse tunneling magnetoresistance (TMR) ratio in this tunnel junction. We demonstrate that the tunneling properties are determined by ferroelectric (FE) polarization screening and electronic reconstruction at the interface with lower electrostatic potential. The intricate complex band structure of PbTiO3, in particular the lowest decay rates concerning Pb 6pz and Ti 3dz2 states near the Γ¯ point, gives rise to the large positive SP of the tunneling current in the parallel magnetic configuration. However, the giant inverse TMR ratio is attributed to the minority-spin electrons of the interfacial Ti 3dxz+3dyz orbitals which have considerably weight in the extended area around the Γ¯ point at the Fermi energy and causes remarkable contributions to the conductance in the antiparallel magnetic configuration.

Giant tunnel magnetoresistance in double quantum well structures under an in-plane magnetic field

The vertical Ohmic magnetotransport in the structures with low but wide barrier Al0.1Ga0.9As between two quantum wells GaAs, in temperature range T=3K – 45K under in-plane magnetic fields B= 0 − 9 T is investigated. At zero-bias voltage, two mechanisms that determine the transitions well-barrier-well observed. The low-temperature regime (3K − 20K) is corresponded to the tunnel transport. The above-barrier ballistic transport with activation energy 50 meV is realized in high-temperature regime (35K − 45K). Magnetoresistance R(B)/R(0) in the high-temperature regime doesn't depend on T and is increased two times at B=0 − 5 T. At low temperatures R(B)/R(0) is increased 10 times at B=0−5 T and 400 times at B=0 − 9 T. The effect is explained by suppression by in-plane magnetic field of resonant tunneling processes conserving the in-plane momentum.

Quantum transport in normal-metal/ferromagnet/spin-triplet superconductor junctions

We study the subgap charge and spin transport in normal-metal/ferromagnet/spin-triplet superconductor (N/F/TS) tunneling junctions induced by bias voltage. Charge and spin conductance are calculated following the extended Blonder–Tinkham–Klapwijk (BTK) theory. We show that the Andreev reflection, which governs the low-energy transport, may be influenced substantially by the exchange field in the ferromagnet giving rise to controllable giant charge and spin tunneling magnetoresistance (TMR) effect. We calculate also the quantum interference effects which lead to oscillatory behavior of charge and spin conductance.

Hard X-ray photoelectron spectroscopy on buried, off-stoichiometric Co x Mn y Ge z (x:z=2:0.38) Heusler thin films

Fully epitaxial magnetic tunnel junctions (MTJs) with off-stoichiometric Co2-based Heusler alloy shows a intense dependency of the tunnel magnetoresistance (TMR) on the Mn composition, demonstrating giant TMR ratios of up to 1995% at 4.2 K for 1. This work reports on the electronic structure of non-stoichiometric CoxMnyGez thin films with a fixed Co/Ge ratio of x : z = 2 : 0.38. The electronic structure was investigated by high energy, hard X-ray photoelectron spectroscopy combined with first-principles calculations. The high-resolution measurements of the valence band of the non-stoichiometric CoxMnyGez films close to the Fermi energy indicate a shift of the spectral weight compared to bulk Co2MnGe. This is in agreement with the changes in the density of states predicted by the calculations. Furthermore it is shown that the co-sputtering of Co2MnGe together with additional Mn is an appropriate technique to adjust the stoichiometry of the CoxMnyGez film composition. The resulting changes of the electronic structure within the valence band will allow to tune the magnetoresistive characteristics of CoxMnyGez based tunnel junctions as verified by the calculations and photoemission experiments.

Coexistance of Giant Tunneling Electroresistance and Magnetoresistance in an All-Oxide Composite Magnetic Tunnel Junction

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

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.

The effect of interfaces on magnetic activation volumes in single crystal Co2FeSi Heusler alloy thin films

Structural and magnetization reversal studies have been carried out on single crystal Co2FeSi thin films grown on MgO (001) substrates. The films are highly L21 ordered after annealing above 500 °C. Magnetization reversal has been investigated by measurements of the activation volumes (Vact) within the films. This volume represents the unit of reversal in a magnetic material. Vact (∼4 × 103 nm3) has been found to be independent of the physical structure. Vact is found to correspond to an array of periodic misfit dislocations at the Co2FeSi/MgO interface. Such a small Vact potentially prevents coherent magnetization reversal as required for giant magnetoresistance or tunnel magnetoresistance devices.

Breakdown by magnetic field in a La0.7Sr0.3MnO3/MgO/Fe spin valve

A La0.7Sr0.3MnO3/MgO/Fe spin valve with inverse tunneling magnetoresistance (TMR) was fabricated on a (100) SrTiO3 substrate by radio frequency magnetron sputtering. Giant TMR ratios up to 540% were obtained. The breakdown of the spin valve was observed at high magnetic field, which was attributed to the joint action of the invalidation of MgO barrier and the shift of Fermi energy in La0.7Sr0.3MnO3 at high magnetic field.

Effect of Electronic Correlations on Magnetotransport through a Parallel Double Quantum Dot

We theoretically investigate the effect of electronic correlations (including spin and Coulomb correlations) on magnetotransport through a parallel double quantum dot coupled to ferromagnetic leads. Within the framework of the generalized master equation, we analyze the current, differential conductance and tunnel magnetoresistance versus bias for different electron correlations. Our results reveal that spin correlations can induce a giant tunnel magnetoresistance, while Coulomb correlations can lead to the occurrence of negative tunnel magetoresistance and negative differential conductance, and the relevant underlying physics of this problem is discussed.

Spin-polarized tunneling in MgO-based tunnel junctions with superconducting electrodes

We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al–Si electrode. Pure cobalt electrodes were compared with a Co–Fe–B alloy and the Heusler compound Co2FeAl. The polarization of the tunneling electrons was determined using the Maki–Fulde model and is discussed along with the spin–orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co–Fe–B/MgO/Co–Fe–B junctions.

Giant tunneling magnetoresistance and quantum interference effect in ferromagnet/ferromagnet/ s-wave superconductor junctions

We study the tunneling magnetoresistance (TMR) effect in asymmetric ferromagnet/ferromagnet/ s-wave superconductor junctions based on the extended Blonder–Tinkham–Klapwijk theory. When the left ferromagnet (FM) is in the half-metallic limit, the low-energy charge transport in this system is governed by the so-called same-spin-band Andreev reflection, where the incident electron and the Andreev reflected hole belong to the same spin subband. It is demonstrated that the same-spin-band Andreev reflection depends sensitively on the relative orientation of the magnetizations in the two FMs leading to giant TMR effect (TMR > 10 6% for T/ T c ≪ 1). Distinctive oscillatory variation of TMR with the thickness or the spin polarization of the intermediate FM is also found.

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.

Supramolecular spin valves

Magnetic molecules are potential building blocks for the design of spintronic devices. Moreover, molecular materials enable the combination of bottom-up processing techniques, for example with conventional top-down nanofabrication. The development of solid-state spintronic devices based on the giant magnetoresistance, tunnel magnetoresistance and spin-valve effects has revolutionized magnetic memory applications. Recently, a significant improvement of the spin-relaxation time has been observed in organic semiconductor tunnel junctions, single non-magnetic molecules coupled to magnetic electrodes have shown giant magnetoresistance and hybrid devices exploiting the quantum tunnelling properties of single-molecule magnets have been proposed. Herein, we present an original spin-valve device in which a non-magnetic molecular quantum dot, made of a single-walled carbon nanotube contacted with non-magnetic electrodes, is laterally coupled through supramolecular interactions to TbPc(2) single-molecule magnets (Pc=phthalocyanine). Their localized magnetic moments lead to a magnetic field dependence of the electrical transport through the single-walled carbon nanotube, resulting in magnetoresistance ratios up to 300% at temperatures less than 1 K. We thus demonstrate the functionality of a supramolecular spin valve without magnetic leads. Our results open up prospects of new spintronic devices with quantum properties.

Microscopic and electronic roles of B in CoFeB-based magnetic tunnel junctions

The giant tunneling magnetoresistance (TMR) in lattice-matched crystalline magnetic tunnel junctions (MTJs) strongly depends on the majority-spin Δ1 bands of ferromagnetic electrodes. Our synchrotron radiation X-ray photoelectron spectroscopy and first-principles calculations show that B atoms suppress the formation of CoFe–O during CoFeB deposition in CoFeB/MgO/CoFeB MTJs, thereby lessening an effect that significantly degrades the majority-spin Δ1 bands, and that CoFe–B has properties superior to CoFe–O in electron conduction in the majority-spin Δ1 bands. During annealing, some of the B in CoFeB diffuses out, enhancing the valence bands of metallic CoFe, which improves the TMR value even further. Our present work elucidates the microscopic and electronic roles of B in MgO MTJs with CoFeB electrodes.

Manganese diffusion in annealed magnetic tunnel junctions with MgO tunnel barriers

Atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterize MgO-based magnetic tunnel junctions in as-deposited and annealed states. The annealing produced giant tunneling magnetoresistance (TMR) values of ∼350%. Upon annealing, Mn diffuses from an IrMn exchange bias layer within the device to one interface of the tunnel barrier layer, while the TMR initially increases. TEM and APT show that Mn does not appear to diffuse into the MgO barrier, but segregates at the CoFe/MgO interface.

A perpendicular-anisotropy CoFeB–MgO magnetic tunnel junction

Magnetic tunnel junctions (MTJs) with ferromagnetic electrodes possessing a perpendicular magnetic easy axis are of great interest as they have a potential for realizing next-generation high-density non-volatile memory and logic chips with high thermal stability and low critical current for current-induced magnetization switching. To attain perpendicular anisotropy, a number of material systems have been explored as electrodes, which include rare-earth/transition-metal alloys, L1(0)-ordered (Co, Fe)-Pt alloys and Co/(Pd, Pt) multilayers. However, none of them so far satisfy high thermal stability at reduced dimension, low-current current-induced magnetization switching and high tunnel magnetoresistance ratio all at the same time. Here, we use interfacial perpendicular anisotropy between the ferromagnetic electrodes and the tunnel barrier of the MTJ by employing the material combination of CoFeB-MgO, a system widely adopted to produce a giant tunnel magnetoresistance ratio in MTJs with in-plane anisotropy. This approach requires no material other than those used in conventional in-plane-anisotropy MTJs. The perpendicular MTJs consisting of Ta/CoFeB/MgO/CoFeB/Ta show a high tunnel magnetoresistance ratio, over 120%, high thermal stability at dimension as low as 40 nm diameter and a low switching current of 49 microA.

Magnetic tunnel junctions with Co-based perpendicular magnetic anisotropy multilayers

Magnetic CoFeB/MgO/CoFeB-based tunnel junctions with perpendicular magnetic anisotropy Co/M multilayers (M=Ni, Pd, Pt) have been investigated as a function of structural and magnetic properties. Magnetometry, ferromagnetic resonance, x-ray diffraction, stress tests, and local electrode atom probe tomography were carried out primarily on Co/Ni multilayers. A statistical design of experiments was conducted to optimize the perpendicular magnetic anisotropy and damping parameter α of these multilayers. Seed layers, thickness, and thickness ratios are all critical to achieve perpendicular behavior. Perpendicular MgO-based magnetic tunnel junctions with Co/Ni and Co/Pd reference and free layers were fabricated and tested. Sharp MR-H switching characteristics were observed for the Co/Pd multilayers, and a somewhat softer transition was observed for the Co/Ni multilayer with a Cu seed, which did not have as high a perpendicular anisotropy. Tunneling magnetoresistance (TMR) values were limited to about 10%, primarily because the fcc-bcc-fcc transition does not promote the “MgO giant TMR” symmetry filtering effect.

Unravelling the role of the interface for spin injection into organic semiconductors

Whereas spintronics brings the spin degree of freedom to electronic devices, molecular/organic electronics adds the opportunity to play with the chemical versatility. Here we show how, as a contender to commonly used inorganic materials, organic/molecular based spintronics devices can exhibit very large magnetoresistance and lead to tailored spin polarizations. We report on giant tunnel magnetoresistance of up to 300% in a (La,Sr)MnO3/Alq3/Co nanometer size magnetic tunnel junction. Moreover, we propose a spin dependent transport model giving a new understanding of spin injection into organic materials/molecules. Our findings bring a new insight on how one could tune spin injection by molecular engineering and paves the way to chemical tailoring of the properties of spintronics devices.

Spin Transfer Switching and MR Properties of Co/Pt Multilayered Free Layers for Submicron Sized Magneto-Optical Light Modulation Device

Co/Pt multilayered films show strong perpendicular magnetic anisotropy and have a large magneto-optical Kerr effect in the short wavelength side. To use these films with submicron spatial light modulators driven by spin transfer switching (STS), we fabricated current perpendicular to plane giant magnetoresistance (CPP-GMR) and tunnel magnetoresistance (TMR) devices with Co/Pt multilayers for free layers and investigated the MR properties, the STS characteristics, and the Kerr effects. A Kerr hysteresis loop was clearly observed in the CPP-GMR device, which was 125 × 180 nm2. Full magnetization reversal of the Co/Pt multilayered film by spin transfer torque was demonstrated for a CPP-GMR device with a Cu-based top electrode. On the other hand, there was hardly any resistance change in a CPP-GMR with a transparent top electrode due to the low MR ratio of the device. A TMR stack with a Ag buffer layer showed a strong perpendicular magnetic anisotropy. An MR curve was detected for a TMR device with a transparent top electrode.