Tunnel Magnetoresistance Characteristics Research Articles

Bias-dependent two-phase anisotropy in magnetoresistance of a GaMnAs-based magnetic tunnel junction

We report unique two-phase anisotropy in the tunnel magnetoresistance (TMR) of a GaMnAs-based magnetic tunnel junction, obtained by investigating the magnetic-field-direction dependence with various applied bias voltages. The TMR shows two-fold symmetry about the axis at a low bias voltage, while at higher bias voltages, it shows four-fold symmetry about the 〈100〉 axis in the large magnetic field H region. Considering modified Jullière’s model and multiple bands with different anisotropic features, we can well reproduce the experimental TMR characteristics. Our results will lead to novel functionalities using TMRs with different symmetries in multi-orbital materials.

Tunneling magnetoresistance of silicon chains

The tunneling magnetoresistance (TMR) of a silicon chain sandwiched between nickel electrodes was examined by using first-principles density functional theory. The relative orientation of the magnetization in a parallel-alignment (PA) configuration of two nickel electrodes enhanced the current with a bias less than 0.4 V compared with that in an antiparallel-alignment configuration. Consequently, the silicon chain-nickel electrodes yielded good TMR characteristics. In addition, there was polarized spin current in the PA configuration. The spin polarization of sulfur atoms functioning as a linking bridge between the chain and nickel electrode played an important role in the magnetic effects of the electric current. Moreover, the hybridization of the sulfur 3p orbital and σ-conjugated silicon 3p orbital contributed to increasing the total current.

Enhancement of Spin-transfer torque switching via resonant tunneling

We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer devices. Two device designs based on MgO based heterostructures are presented, where the physics of resonant tunneling leads to an enhanced spin transfer torque thereby reducing the critical switching voltage by up to 44%. It is envisioned that the proof-of-concept presented here may lead to practical device designs via rigorous materials and interface studies.

Spin Transport Through a Poly(p-phenylene)-Like Molecular Wire with Multi-noncollinear Polarized Terminals

We numerically investigated the spin transport through a poly(p-phenylene)-like molecular wire connected to three noncollinear polarized terminals. Using the generalized Green’s function technique based on the tight-binding model and Landauer–Buttiker theory, we have calculated the transmission probability, currents, and tunnel magnetoresistance (TMR). It is found that the spin-dependent transport properties are significantly influenced by the relative directions of spin polarized leads, in which one can tune the magnitude of TMR by changing spin direction in the leads. We have also tested the effect of terminal positions as a source of quantum interference. Our results show that different terminal positions have profound effects on the current and TMR characteristics. Different TMR strength and threshold bias voltage Vt are also detected.

Tunnel magnetoresistance in quantum dots in the presence of singlet and tripletstates

We study transport through a two-level quantum dot (QD) weakly coupled toferromagnetic electrodes, the mutual magnetization orientation of which can be changedfrom a parallel to an antiparallel configuration. Calculations are performed in a sequentialtunnelling regime taking into account an inter-level Coulomb and an exchange interactionon the QD with many-body electronic states. Our interests are mainly focused onthe role of singlet and triplet states on the tunnel magnetoresistance (TMR).We have found that TMR characteristics strongly depend on different local spinconfigurations in the QD, which originate from an anti- or ferromagnetic exchange couplingJ as well as its strength. A strong inter-channel Coulomb blockade (which influences theTMR) appears when the ground state is singly occupied. Activation of the singlet or thetriplet states and competition between various tunnelling rates are responsible for the spinaccumulation even in the parallel configuration. We have also found negative TMR andnegative differential resistance (NDR) effects in the system with strong couplingasymmetry.

Effect of O- Ion Beam Irradiation during RF-Magnetron Sputtering on Characteristics of CoFeB–MgO Magnetic Tunnel Junctions

The relevance between electrical properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) and the crystalline orientation of MgO layer was investigated within a large-scale substrate. The tunnel magnetoresistance characteristics and the resistance-area product in CoFeB/MgO/CoFeB MTJs are degraded by deterioration of the crystalline orientation of MgO layer, which depends on sputtering conditions. That deterioration originates from damage caused by high-energy oxygen negative-ion irradiation during RF-magnetron sputtering of the MgO layer.

Influence of film composition in Co2MnSi electrodes on tunnel magnetoresistance characteristics of Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions

Fully epitaxial Co2MnSi (CMS)/MgO/CMS magnetic tunnel junctions (MTJs) with various values of Mn composition α in Co2MnαSi electrodes were fabricated and the influence of α on the tunnel magnetoresistance (TMR) characteristics of these MTJs was investigated. MTJs with Mn-rich CMS electrodes showed TMR ratios, up to 1135% at 4.2 K and 236% at room temperature for α=1.29, exceeding those of MTJs with CMS electrodes having an almost stoichiometric composition. A possible origin of the higher TMR ratios for α beyond 1.0 is a decreased density of gap states existing around the Fermi level in the half-metal gap caused by the suppression of CoMn antisites, where a Mn site is replaced by a Co atom, for Mn-rich CMS electrodes.

Fabrication and Characterization of Fully Epitaxial Magnetic Tunnel Junction Field Sensors Using a ${\rm Co}_{2}{\rm MnSi}$ Thin Film

Fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> MnSi (CMS) upper free layer, a MgO tunnel barrier, and a Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> lower pinned layer were fabricated for sensor application. The fabricated MTJs exhibited clear exchange-biased tunnel magnetoresistance (TMR) characteristics with a relatively high TMR ratios of 94% at room temperature (209% at 4.2 K) with V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">half</sub> of approximately - 0.52 and + 0.54 V. Almost hysteresis-free TMR characteristics with sensitivity of approximately 0.4%/Oe were obtained with a cross magnetization configuration, in which the magnetization of the CMS free layer was oriented perpendicularly to that of the pinned layer.

Fabrication of fully epitaxial magnetic tunnel junctions using L21-ordered Co2FeAl0.5Si0.5 electrodes and their tunneling magnetoresistance characteristics

Magnetic tunnel junctions (MTJs) using L21-ordered full-Heusler Co2FeAl0.5Si0.5 (CFAS) electrodes and an MgO tunnel barrier were prepared on MgO-buffered MgO (001) substrates by sputtering method. In situ and ex situ structural characterization confirms that the stacking structure of CFAS/MgO/CFAS is fully epitaxial, with smooth interfaces throughout. The microfabricated MTJs exhibited relatively high tunnel magnetoresistance ratios of 150% at room temperature and 312% at 7K. We observed a symmetrical crossover point from the bias voltage dependence of differential conductance between parallel and antiparallel magnetization configurations, and also a flat behavior in the parallel conductance, which can be explained by considering the characteristic half-metallic band structure of L21-ordered CFAS near the Fermi level.

Layer-thickness dependence in tunneling magnetoresistance and current-driven magnetization reversal using GaMnAs-based double-barrier structures

We have investigated the layer-thickness dependence on tunneling magnetoresistance (TMR) in double-barrier structures using GaMnAs-based magnetic tunneling junctions in order to clarify the origin of low threshold current density for current-driven magnetization orientation reversal. The TMR characteristics are well explained by assuming the spin configurations due to the difference in coercive force in each magnetic layer. Using the thin middle magnetic layer, current-driven magnetization reversal in low current density and spin accumulation are realized.

Fabrication of Fully Epitaxial Co$_{2}$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/Co$_{2}$Cr$_{0.6}$Fe$_{0.4}$Al Magnetic Tunnel Junctions

Fully epitaxial magnetic tunnel junctions (MTJs) of Co2Cr0.6Fe0.4Al (CCFA)/MgO/CCFA with exchange biasing were fabricated. The fabricated MTJs showed clear exchange-biased tunnel magnetoresistance (TMR) characteristics due to the CCFA/Ru/Co90Fe10/IrMn exchange-biased synthetic ferrimagnetic layer. The TMR characteristics were investigated as a function of in situ annealing temperature (Ta) for the upper CCFA layer. We obtained TMR ratios of 60% at room temperature (RT) and 238% at 4.2 K for MTJs with Ta of 400 degC, while those for MTJs with Ta of RT (i.e., having an as-deposited upper CCFA layer) were 17% at RT and 80% at 4.2 K. These results clearly suggest that the spin polarization of the as-deposited upper CCFA layer was significantly increased by in situ annealing

Improved tunnel magnetoresistance characteristics of magnetic tunnel junctions with a Heusler alloy thin film of Co2MnGe and a MgO tunnel barrier

We fabricated magnetic tunnel junctions (MTJs) with a Co-based full-Heusler alloy thin film of Co2MnGe (CMG) and a MgO tunnel barrier. The microfabricated MTJs with a Co-rich CMG film showed relatively high tunnel magnetoresistance ratios of 83% at room temperature and 185% at 4.2K. These values are much higher than those previously obtained for CMG/MgO MTJs with a Co-deficient CMG film.

Characteristics of GaMnAs‐based double‐barrier TMR structures

We have investigated the tunneling magnetoresistance (TMR) in GaMnAs-based double barrier magnetic tunneling junctions (DB-MTJs) to clarify the origin of extremely low threshold current density for current-driven magnetic orientation reversal achieved in the systems. The TMR characteristics including minor loop behavior are well explained by assuming that three magnetic layers in the DB-MTJ have different coercive forces and that the uniaxial magnetic anisotropy in the middle magnetic layer is enhanced with decreasing the layer thickness. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

La 0.6Sr 0.4MnO 3 ramp-edge junctions using (LaAlO 3) 0.3–(SrAl 0.5Ta 0.5O 3) 0.7 barrier

We have fabricated La 0.6Sr 0.4MnO 3 (LSMO)/(LaAlO 3) 0.3–(SrAl 0.5Ta 0.5O 3) 0.7 (LSAT)/LSMO spin tunnel junctions with a small area by using a ramp-edge geometry. The ramp angle was estimated to be about 25° from measurements of an atomic force microscope. The thickness of a counter LSMO electrode, a bottom LSMO electrode and an insulating LSAT barrier were 100, 200, and 4 nm, respectively. The 1-μm-wide junction showed a tunnel magneto-resistance (TMR) behavior with the TMR ratio of 550% at 100 K. Depending on the directions of the external magnetic field, different TMR characteristics were observed. It may reflect domain structures of the counter and the bottom electrodes, since the ramp-edge junctions are fabricated at the film edges of the base electrode.

Fabrication of fully epitaxial magnetic tunnel junctions using cobalt-based full-Heusler alloy thin film and their tunnel magnetoresistance characteristics

Cobalt-based full-Heusler alloy thin films have recently attracted much interest as highly desirable ferromagnetic electrodes for spintronic devices because of the half-metallic ferromagnetic nature theoretically predicted for some of these alloys and because of their high Curie temperatures, which are well above room temperature (RT). In this study, Co-based full-Heusler alloy thin films of Co2Cr0.6Fe0.4Al (CCFA) and Co2MnGe (CMG) were epitaxially grown on MgO-buffered MgO (001) substrates using magnetron sputtering. The films were deposited at RT and subsequently annealed in situ at temperatures ranging from 400 to 600 °C. X-ray pole figure measurements of the CCFA films indicated that these films were epitaxial and crystallized in the B2 structure. X-ray pole figure measurements of the annealed CMG films showed (111) peaks with four-fold symmetry, which provides direct evidence that these films were epitaxial and crystallized in the L21 structure. Furthermore, cross-sectional transmission electron microscope images of a fabricated CMG film indicated that it was single-crystalline. The annealed films of CCFA and CMG had sufficiently flat surface morphologies with roughness of about 0.23 nm rms for 100 nm thick CCFA films and 0.26 nm rms for 45 nm thick CMG films. Using these epitaxially grown thin films, we fabricated fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Co-based full-Heusler thin film of either CCFA or CMG as a lower electrode, a MgO tunnel barrier and a Co50Fe50 (CoFe) upper electrode. All layers were successively deposited in an ultrahigh vacuum chamber through the combined use of magnetron sputtering and electron beam evaporation. Reflection high-energy electron diffraction patterns observed in situ for each layer during preparation clearly indicated that all layers grew epitaxially in both the CCFA/MgO/CoFe and CMG/MgO/CoFe MTJ layer structures. The microfabricated epitaxial CCFA/MgO/CoFe MTJs demonstrated relatively high tunnel magnetoresistance (TMR) ratios, for MTJs using a full-Heusler alloy, of 42% at RT and 74% at 55 K. On the other hand, the microfabricated epitaxial CMG/MgO/CoFe MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 14% at RT and 70% at 7 K. These results confirm the promise of epitaxial MTJs as a key device structure for clarifying and utilizing the potentially high spin-polarization of Co-based full-Heusler alloy thin films.

The current potential of Co2MnSi Heusler alloy electrodes in magnetic tunnel junctions

Heusler alloys are ferromagnetic half-metals and candidates for materials with enhanced tunnel magnetoresistance (TMR). In the article [1], Co2MnSi has been used as one electrode in a magnetic tunnel junction. The cover figure shows, together with the unit cell of Co2MnSi, the TMR characteristics at 20 K at 1 mV (red) and 10 mV (blue), respectively. Large spin polarizations of 61% and 66%, respectively, were achieved. The first author, Andreas Hütten, is working in the group of Günter Reiss at the University of Bielefeld. Their research is devoted to magnetism and spin-dependent electronic transport in thin films and nanostructures, the development of optimized magneto- and spin-electronic devices as well as of preparation methods of magnetic nanostructures. This paper is an invited presentation from the Second Seeheim Conference on Magnetism (SCM2004), held in Seeheim, Germany, 27 June–1 July 2004. The full Proceedings of this conference are published in phys. stat. sol. (c) 1, No. 12 (2004).

Effect of Zr concentration on the microstructure of Al and the magnetoresistance properties of the magnetic tunnel junction with a Zr-alloyed Al–oxide barrier

We studied the composition dependence of the microstructure of Al–Zr alloy films and the tunneling magnetoresistance (TMR) behavior of magnetic tunnel junctions (MTJ) with a Zr-alloyed Al–oxide barrier. A highly stable MTJ with a superior-quality 9.89 at. % Zr alloyed Al–oxide barrier was achieved with 39.5% TMR and a bias voltage of 711 mV at half (Vh) TMR. The microstructure of Al changed systematically from polycrystalline to single phase amorphous to duplex phases composed of an amorphous Al–Zr phase and a crystalline AlZr compound, probably Al3Zr, as the Zr concentration increased. The microstructure of the Al–Zr alloy before oxidation has a dominant effect on the TMR characteristics and MTJ stability. A single amorphous Al–Zr alloy (∼10 at. % Zr) film has superior surface uniformity; consequently, an ultrahigh-quality oxide barrier was formed after oxidation.

Theoretical Study on Tunnel Magnetoresistance Oscillation Due to Coulomb Blockade in Nanoscale Magnetic Tunnel Junction

AbstractIn a small magnetic tunnel junction (MTJ) whose charging energy is larger than a thermal energy, the tunnel magnetoresistance (TMR) is enhanced by the Coulomb blockade. Transport properties in a double MTJ were theoretically studied by using the Monte Carlo method. The temperature dependence of current-voltage and TMR characteristics indicated that a small junction with 5 nm electrodes exhibited the Coulomb blockade at 300 K. It was also found that the background charge changed the threshold voltage of the Coulomb blockade. In asymmetric double tunnel junctions, the current-voltage characteristic exhibited a Coulomb staircase and the voltage dependence of the TMR ratio oscillated.

Tunneling magnetoresistance in GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor heterostructures

We have observed tunneling magnetoresistance (TMR) in epitaxially grown GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions. The TMR ratio, which is the change in tunnel resistance due to the change of magnetization configuration from parallel to anti-parallel, was more than 70% (maximum 75%) in junctions with a very thin (⩽1.6 nm) AlAs tunnel barrier, when the magnetic field was applied along the [1 0 0] axis in the film plane. Peculiar TMR characteristics were observed due to the magneto-crystalline anisotropy of GaMnAs. The TMR ratio was found to rapidly decrease with increasing barrier thickness, which is explained by calculations assuming that the parallel wave vector of carriers is conserved in tunneling.

Magnetoresistance in Tunnel Junctions Made of Epitaxially Grown Manganate Films with 1.6-nm-Thick Barriers

Magnetic tunnel junctions with barrier thicknesses of either 1.6 or 2.4 nm have been fabricated from epitaxially grown La0.8Sr0.2MnO3/SrTiO3/La0.8Sr0.2MnO3 trilayers. For the junctions with 1.6-nm-thick SrTiO3, tunneling magnetoresistance (TMR) as large as 150% was observed at 5 K. A small TMR was observed even at 270 K, which is close to the ferromagnetic Curie temperature (290 K) of the La0.8Sr0.2MnO3 film. Besides tunneling conduction, parallel semiconduction through the SrTiO3 barrier appeared to exist, and became dominant at high temperatures, reducing the TMR ratio and operating temperature, especially for thicker SrTiO3 barriers. The SrTiO3 barrier thickness is the key to improving TMR characteristics, and fabricating a sufficiently thin and uniform barrier layer is essential for achieving a large TMR and a high operating temperature.