Magnetoresistance In Tunnel Junctions Research Articles

Creation of Submicrometer Magnetoresistive Tunnel Junction CoFeB/MgO/CoFeB Using an HSQ/PMMA Resistive Mask

Creation of Submicrometer Magnetoresistive Tunnel Junction CoFeB/MgO/CoFeB Using an HSQ/PMMA Resistive Mask

Self-consistent computation of spin torques and magneto-resistance in tunnel junctions and magnetic read-heads with metallic pinhole defects

A three-dimensional self-consistent spin transport model is developed, which includes both tunnelling transport, leading to tunnelling magneto-resistance, as well as metallic transport, leading to giant magneto-resistance. An explicit solution to the drift-diffusion model is also derived, which allows analysing the effect of both the reference and free layer thickness on the spin-transfer torque polarization and field-like coefficient. It is shown the model developed here can be used to compute the signal-to-noise ratio in realistic magnetic read-heads, where spin torque-induced fluctuations and instabilities limit the maximum operating voltage. The effect of metallic pinhole defects in the insulator layer is also analysed. Increasing the area covered by pinholes results in a rapid degradation of the magneto-resistance, following an inverse dependence. Moreover, the spin torque angular dependence becomes skewed, similar to that obtained in fully metallic spin valves, and the spin-transfer torque polarization decreases. The same results are obtained when considering tunnel junctions with a single pinhole defect, but decreasing cross-sectional area, showing that even a single pinhole defect can significantly degrade the performance of tunnel junctions and magnetic read-heads below the 40 nm node.

The Behavior Analysis of Magnetoresistive Tunnel Junction Devices in State Space

The behavior of magnetoresistive tunnel junction (MTJ) devices in the electronic (digital) circuits is governed by several differential equations. The LLG equation determines the dynamic of the magnetic polarization vector which ultimately affects the (anti)parallel resistance of MTJs. This vector differential equation is decomposed into two scalar differential equations. In the proposed method, the differential equations are expressed in the state space and the corresponding well-developed theories are applied. In fact, the device is considered as a multi-input multi-output control system and the qualitative solutions are presented. The state diagram gives an insight into the device behavior. It can provide additional information about the device performance and can be considered as a complementary technique of other available analysis approaches. With this ability, the designer can engineer the MTJ devices for their optimum performance in the circuit. Using this theory, we also suggested a power and performance improving approach for the spin transfer torque MTJ-based logic circuits. To show the efficiencies offered by the proposed method, a STT-MRAM cell and an array are designed and it is shown that better specifications can be achieved in terms of switching time and power dissipation. The reliability issues are also investigated and we show that the proposed power/performance improvement approach can effectively consider the reliability concerns.

Investigation of the tunnel magnetoresistance in junctions with a strontium stannate barrier

We experimentally investigate the structural, magnetic, and electrical transport properties of La0.67 Sr0.33MnO3 based magnetic tunnel junctions with a SrSnO3 barrier. Our results show that despite the high density of defects in the strontium stannate barrier, due to the large lattice mismatch, the observed tunnel magnetoresistance (TMR) is comparable to tunnel junctions with a better lattice matched SrTiO3 barrier, reaching values of up to 350% at T=5 K. Further analysis of the current-voltage characteristics of the junction and the bias voltage dependence of the observed tunnel magnetoresistance show a decrease of the TMR with increasing bias voltage. In addition, the observed TMR vanishes for T>200 K. Our results suggest that by employing a better lattice matched ferromagnetic electrode, and thus reducing the structural defects in the strontium stannate barrier, even larger TMR ratios might be possible in the future.

Electric field modulation of tunneling anisotropic magnetoresistance in tunnel junctions with antiferromagnetic electrodes

We present electric field modulation of tunneling anisotropic magnetoresistance (TAMR) in MnIr|MgO|Ta tunnel junctions. TAMR enables direct observation of the antiferromagnetic spin direction at the MnIr|MgO interface. We found that the shape of magnetoresistance (MR) curve can be modulated by an electric field, which can be explained by electric field modulation of the interfacial magnetic anisotropy at MnIr|MgO.

Single ferromagnetic layer magnetic random access memory

We propose a magnetic random access memory (MRAM) device in which both the writing and reading processes are realized within a single ferromagnetic (FM) layer. The FM layer is sandwiched between layers of heavy element and oxide to enhance the Rashba spin-orbit coupling (RSOC). When the in-plane FM moments are oriented at some intermediate angle to the current direction, the RSOC effect induces a spin accumulation in the FM layer, which in turn generates a Rashba spin torque field via the s-d exchange interaction. This field acts as the writing field of the memory device. The RSOC also induces a charge accumulation in the transverse direction via the inverse spin Hall effect (ISHE), which can be used to realize the memory read-out. The writing and read-out processes of the proposed memory are modeled numerically via the non-equilibrium Green's function technique. Besides the advantages of Rashba spin torque writing, i.e., no spin injection and symmetrical data-writing process, this single FM layer MRAM design does away with having a giant magnetoresistive or magnetic tunnel junction multilayer structure by utilizing the ISHE for the read-out process.

Design and performance of domain wall displacing-type field sensors using a magnetic tunnel junction and a giant magnetoresistive device

We report a new type of field sensor, which utilizes the oscillatory domain wall displacement in the ferromagnetic free layer to detect an external field. The sensor consists of a 1200 × 100 µm2 Al conducting wire placed on a spin-valve giant magnetoresistive (GMR) or magnetic tunnel junction. By transmitting an alternating current through the Al conducting wire, an ac field can be generated to oscillate the domain wall in the free layer. The oscillatory domain wall displacement reduces the influence of wall coercivity and Barkhausen effect, and enables the sensor to detect the magnetic field which is much smaller than the wall coercivity. The field sensitivities of the domain wall displacing-type GMR and tunnel magnetoresistive (TMR) sensors are 2.73 mV V−1 Oe−1 and 5.81 mV V−1 Oe−1, respectively.

Interface and bulk magnetism of Co2Cr0.6Fe0.4Al and Co2CrAl thin films

The interface and bulk magnetic properties of epitaxial thin films of the Heusler compounds Co2Cr0.6Fe0.4Al/Mg/AlOx and CoCr2Al/Mg/AlOx are investigated. We compare the magnetization measured by x-ray magnetic circular dichroism experiments in surface sensitive total electron yield mode (information depth of 2–3 nm) and in bulk sensitive transmission mode. The pronounced temperature dependence of the magnetoresistance of tunneling junctions with Heusler electrodes, which is often related to weakened interface magnetism, is discussed. Evidence is given that this explanation does not apply to the compounds investigated here.

Mn Valency at La0.7Sr0.3MnO3/SrTiO3 (0 0 1) Thin Film Interfaces

This article presents a (scanning) transmission electron microscopy (TEM) study of Mn valency and its structural origin at La 0.7 Sr 0.3 MnO 3/SrTiO3(0 0 1) thin film interfaces. Mn valency deviations can lead to a breakdown of ferromagnetic order and thus lower the tunneling magnetoresistance of tunnel junctions. Here, at the interface, a Mn valency reduction of 0.16 +/- 0.10 compared to the film interior and an additional feature at the low energy-loss flank of the Mn-L3 line have been observed. The latter may be attributed to an elongation of the (0 0 1) plane spacing at the interface detected by geometrical phase analysis of high-resolution images. Regarding the interface geometry, high-resolution high-angle annular dark-field scanning TEM images reveal an atomically sharp interface in some regions whereas the transition appears broadened in others. This can be explained by the presence of steps. The performed measurements indicate that, among the various structure-related influences on the valency, the atomic layer termination and the local oxygen content are most important.

Probing multilayer spintronic structures with photoelectron and x-ray emission spectroscopies excited by x-ray standing waves (invited)

We discuss a newly developed x-ray standing-wave∕wedge (swedge) method for probing the composition, magnetization, and electronic densities of states in buried interfaces and layers in spintronic nanostructures. In work based on photoemission, this method has permitted determining concentration and magnetization profiles through giant magnetoresistive (GMR) and magnetic tunnel junctions structures, as well as individual layer densities of states near the Fermi level in a tunnel junction. Using x-ray emission and resonant inelastic x-ray scattering for detection has permitted probing deeper layers and interfaces in a GMR structure. Various future applications of this method in nanomagnetism are suggested, including using more energetic hard x-ray standing waves so as to probe more deeply below a surface and standing-wave excitation in spectromicroscopy to provide depth sensitivity.

Large room-temperature inverse magnetoresistance in tunnel junctions with a Fe3O4 electrode

Magnetic tunnel junctions (MTJs) consisting of Fe3O4 and three-dimensional ferromagnetic metal electrodes with MgO or MgO∕Al2O3 as a barrier layer have been fabricated. Fe3O4∕MgO(5nm)∕Al2O3(1nm)∕CoFe MTJs had room-temperature (RT) magnetoresistance (MR) ratios ranging from −26% to 18%, and Fe3O4∕MgO(6nm)∕CoFe MTJs with a mildly heat-treated Fe3O4∕MgO interface had only inverse tunneling MR ratios that were as large as −8% at RT. The sign of the MR ratio changed when the absolute value of the bias voltage V was about 1V, and normal MR was evident when ∣V∣>1V because the nontunneling current was dominant.

Oscillatory tunneling magnetoresistance caused by antiferromagnetic Mn layers

The ballistic magnetoresistance of tunnel junctions that comprise Mn films is found to exhibit oscillations with increasing Mn-film thickness, as is investigated by means of first-principles electronic-structure and transport calculations. The period of two monolayers is directly related to the layer-wise antiferromagnetic structure of the Mn films, in particular to the alternating magnetization at the interfaces. These findings substantiate unequivocally the effect of the electronic and magnetic structure of interfaces on the conductance of tunnel junctions.

Theory of Tunnel Magnetoresistance

We present a review of theories of tunnel magnetoresistance (TMR) putting an emphasis on the role of electron scattering due to randomness. We adopt the linear response theory and generalize the conductance formula to calculate the electrical conductance in layered structures. The effect of randomness is treated in the coherent potential approximation and⁄or direct numerical simulation. Because the momentum conservation parallel to the junction planes does not hold in the presence of randomness, number of tunneling path through the barrier increases. Using a simple tight-binding model, we demonstrate first how the tunnel conductance and TMR are influenced by the randomness and by the shape of the Fermi surface of the metallic electrodes. The theory is applied to epitaxial tunnel junctions of Fe⁄MgO⁄Fe. It is shown that the basic concept of tunneling in the presence of randomness is also hold in realistic junctions. Magnetoresistance of tunnel junctions with semimetals and those with manganites is studied to clarify the effect of their electronic structures and interaction on TMR.

Tunneling magnetoresistance studies ofSr3Ru2O7

We have performed planar tunneling measurements on ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$, which has a field-tuned metamagnetic quantum phase transition. Our previously reported work revealed an unusual oscillation in the tunneling magnetoresistance of junctions made on ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ single crystals [J. Hooper et al., Phys. Rev. Lett. 92, 257206 (2004)]. We here report further characterization of this intriguing phenomenon, revealing that the oscillation has a systematic dependence on the tunnel barrier, field orientation, and temperature. We discuss possible origins of this unusual oscillation phenomenon in light of these new experimental results.

Tunnel Magnetoresistance of Junctions with Twisted Magnetization

The tunnel magnetoresistance (TMR) of ferromagnetic junctions including twisted magnetization near the insulator barrier has been theoretically studied using linear response theory and a single orbital tight-binding model. The region of the twisted magnetization has been found to have a large effect on TMR only when the twisted magnetization region is several atomic layers. This agrees with experimental results.

Demonstrated Reliability of 4-Mb MRAM

The successful commercialization of MRAM will rely on providing customers with a robust and reliable memory product. The intrinsic reliability of magnetoresistive tunnel junction (MTJ) memory bits and the metal interconnect system of MRAM are two areas of great interest due to the new materials involved in this emerging technology. Time dependent dielectric breakdown (TDDB) and resistance drift were the two main failure mechanisms identified for intrinsic memory bit reliability. Results indicated that a lifetime over 10 years is achievable under the operating condition. For metal interconnect system, the initial results of Cu with magnetic cladding have met the reliability performance of typical nonclad Cu backend process in electromigration (EM) and iso-thermal annealing (ITA). Finally data retention is demonstrated over times orders of magnitude longer than 10 years.

Reliability of 4-Mbit Toggle MRAM

ABSTRACTA 4Mb magnetoresistive random access memory (MRAM) with a novel magnetic free layer and toggle switching mode is presented. The new free layer uses a balanced synthetic-antiferromagnet trilayer structure and a novel write pulse sequence to provide robust switching performance with immunity from ½-select disturbs. This new mode greatly improves the switching performance of the MRAM as compared to conventional MRAM. The intrinsic reliability of the magnetoresistive tunnel junction (MTJ) and the metal interconnect system of MRAM are two other areas of great interest due to the new materials involved. Time dependent dielectric breakdown (TDDB) and resistance drift were the two main failure mechanisms identified for intrinsic memory bit reliability. Results indicated that a lifetime over 10 years is achievable under the operating conditions. Finally data retention is demonstrated over times that are orders of magnitude longer than 10 years.

Reliability of 4-Mbit Toggle MRAM

ABSTRACTA 4Mb magnetoresistive random access memory (MRAM) with a novel magnetic free layer and toggle switching mode is presented. The new free layer uses a balanced synthetic-antiferromagnet trilayer structure and a novel write pulse sequence to provide robust switching performance with immunity from ½-select disturbs. This new mode greatly improves the switching performance of the MRAM as compared to conventional MRAM. The intrinsic reliability of the magnetoresistive tunnel junction (MTJ) and the metal interconnect system of MRAM are two other areas of great interest due to the new materials involved. Time dependent dielectric breakdown (TDDB) and resistance drift were the two main failure mechanisms identified for intrinsic memory bit reliability. Results indicated that a lifetime over 10 years is achievable under the operating conditions. Finally data retention is demonstrated over times that are orders of magnitude longer than 10 years.

Spectroscopy of bosonic excitations in manganite-based heterostructures

The tunneling current transport is investigated in layered structures based on lanthanum manganite. The spectral function of the electron–boson interaction is reconstructed over a wide energy range. High-energy excitations localized in the barrier and near-barrier regions are revealed. The inelastic scattering of charge carriers on these states can lead to suppression of the magnetoresistance of tunnel junctions with increasing applied voltage.

Fabrication and magnetoresistance of tunnel junctions using half-metallic Fe3O4

Magnetite (Fe3O4) is believed to be half metal, providing 100% spin-polarized conduction electrons. The half-metallic nature of magnetic electrodes for tunneling junction devices is expected to induce a large magnetoresistance. We investigated the structural and chemical properties of interfaces in ferromagnet–insulator–ferromagnet (Fe3O4/MgO/Fe) tunnel junctions. Al/Ag/Fe3O4/MgO multilayers for magnetic tunnel junction have been fabricated on α-Al2O3 (001) and MgO (100) substrates by a molecular beam epitaxy system. The Fe3O4 quality was examined by reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), superconducting quantum interference device magnetometry, atomic force microscopy (AFM), and in situ x-ray photoelectron spectroscopy. RHEED and XRD results showed that the epitaxial Fe3O4 layer with a smooth surface was successfully grown on substrates. The stoichiometric Fe3O4 was confirmed by Verway transition in temperature dependence of magnetization. AFM data showed relatively smooth surface for Fe3O4 prepared at Ts=250 °C and P(O2)=3×10−3 Pa. The Fe 2p3/2 and Fe 2p1/2 peak profiles for Fe3O4 layer are little changed by overlaying MgO in the XPS measurements. These results suggest the Al/Ag/Fe3O4/MgO multilayers available for spin-dependent tunnel junction.