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One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co40Fe40B20/MgO/Co40Fe40B20. We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltage-induced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using Debye-Fröhlich model combined with quartic barrier approximation and spin-dependent drift-diffusion model. Based on our calculation, we predict that the voltage-induced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltage-induced AC spin-dependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., non-volatile memories and spin logic circuits.

We investigated the spin filter effect in tunnel junctions with a Ni1 + xFe2 − xO4 ferrimagnetic tunnel barrier. For higher Tc above room temperature, Ni1 + xFe2 − xO4 film should be thicker than 4.0 nm and grown above 400 °C. The spin filter junctions (SFJs) of Fe3O4(001)/Ni1 + xFe2 − xO4(001)/Al2O3/Fe/Au grown on MgO(001) substrates exhibited an inverse magnetoresistance effect at room temperature, which is consistent with the band calculation of NiFe2O4.

We investigated the annealing temperature dependence of differential tunneling conductance spectra ( d I / d V as a function of V ) in CoFeB/textured MgO(001)/CoFeB magnetic tunnel junctions (MTJs) that exhibit the giant tunneling magnetoresistance (TMR) effect at room temperature. The spectra were strongly affected by annealing the MTJs. A reduction in d I / d V at around ±300 mV was observed only in annealed MTJs in which the CoFeB electrodes were crystallized in a bcc(001) structure. Because the reduction in conductance was observed in both MTJs that have a 1.8-nm-thick MgO barrier and MTJs that have a 3.2-nm-thick MgO barrier, we concluded that Δ 5 and Δ 2 ′ evanescent states, which rapidly decay in the MgO tunneling barrier, are not the cause of the reduction in conductance. We believe the cause of the reduction is either the electronic structure of the interfaces between MgO(001)/bcc CoFeB(001) after annealing or a particular feature of Δ 1 states in MgO(001) or bcc CoFeB(001).

We measured differential tunneling conductance (d I /d V , d 2 I /d V 2 ) spectra of spin-valve-type magnetic tunnel junctions (MTJs) with a MgO(001) tunnel barrier layer and amorphous CoFeB ferromagnetic electrodes that show 315% magnetoresistance (MR) ratio at 4.3 K. The d I /d V spectra showed clear reduction in the conductance at around ±400 mV for a parallel magnetic configuration. Such anomalous spectra have never been observed for MTJs with an amorphous Al–O barrier. The d 2 I /d V 2 spectra showed several distinct peaks between 5 and 100 mV. Magnon excitations are assigned to an origin of those peaks and thought to be a dominant process to reduce MR at finite bias voltage.

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs)1,2 is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices3,4,5. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature6. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.

The magnetoresistance originating from magnetic structures with gradually rotating magnetic moments, like a Bloch wall, was investigated using soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers, whose magnetic structures were well characterized. The magnetoresistance was measured with an electric current in the film plane; the magnetoresistance in this geometry corresponds to that due to a current parallel to a Bloch wall. The main feature of the magnetoresistance curves was ruled by the anisotropic magnetoresistance. It was found that a giant magnetoresistance-type effect coexisted; the effect was very small in comparison with the anisotropic magnetoresistance effect.

A perfectly reversible magnetization process was observed in NiFe/CoSm bilayers. During this process, the magnetic moments in the soft magnetic layer (NiFe) are pinned at the interface with the hard magnetic layer (CoSm), so that the direction of the magnetic moment distributes successively like a Bloch wall. The characteristic reversible magnetization process is explained by an atomic layer model. The magnetoresistance also exhibits a reversible change reflecting the magnetization process. The basic feature of the reversible magnetoresistance curve is understood to be anisotropic magnetoresistance.

The magnetoresistance (MR) originating from a magnetic structure with gradually rotating magnetic moments was studied, using soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers. The main feature of the MR curves was explained as anisotropic magnetoresistance (AMR) effect. It was found that a giant-magnetoresistance- (GMR)-type effect coexisted, although the effect was very small in comparison with the AMR effect.

The magnetization process and magnetoresistance were studied for soft-magnetic (NiFe)/hard-magnetic (CoSm) bilayers. In the course of the magnetization reversal, the magnetic moments in the soft magnetic layer rotate reversibly, while they are pinned by the hard magnetic layer at the interface; consequently, the direction of the magnetic moment distributes successively as in a Bloch wall. The magnetoresistance also shows a reversible change, reflecting the magnetization process. The change is explained by means of the anisotropic magnetoresistance due to angle distributed magnetic moments.

We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with ${\ensuremath{\Delta}}_{1}$ symmetry at the Fermi energy $({E}_{F})$, ${\ensuremath{\Delta}}_{1}$ evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness $({t}_{\text{Cr}})$, indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of ${\ensuremath{\Delta}}_{1}$ states at ${E}_{F}$. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of ${t}_{\text{Cr}}$ with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-${\ensuremath{\Delta}}_{1}$ Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing ${t}_{\text{Cr}}$. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch ${T}^{3/2}$ law, the conductance due to nonspecular scatterings deviates from the Bloch ${T}^{3/2}$ law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.

The surface morphology of epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions, which show the giant tunneling magnetoresistance effect, was investigated by in situ scanning tunneling microscopy. It was observed that an epitaxial MgO barrier layer forms flat surface structures. The surface was flatter with distinct steps and terraces after annealing, which would lead to an increase of the tunneling magnetoresistance ratio. Examination of the local electronic structures of 1.05-nm-thick MgO barrier layers by scanning tunneling spectroscopy revealed no pinholes in the layers, so they would be perfect barriers in magnetic tunnel junctions.

In quasi-magnetic tunnel junctions (QMTJs) with a EuS spin filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions (MTJs). This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory shows excellent agreement with experiment., 4pages, 4Postscript figures, RevTeX4

The combination of magnetic tunnel junctions (MTJs) and magnetic insulating (MI) layers has attracted much attention because of its potential as novel spintronic devices. To realize such devices, the epitaxial growth and magnetoresistance of MTJs with spinel ferrite were investigated. Non-magnetic (NM) layers were inserted between the MTJs and MI layers as magnetic decoupling layers, the epitaxial growth of which was important to obtain high-quality epitaxial multilayers. Multilayers of MTJ/NM/MI and MI/NM/MTJ were fabricated and tunnel magnetoresistance (TMR) values of 70 and 50% at room temperature, respectively, were observed. The shape of the magnetoresistance curve depended on the sample structure.

The frequency dependence of tunneling magnetocapacitance (TMC) in magnetic tunnel junctions (MTJs) is investigated theoretically and experimentally. According to the calculation based on Debye-Frohlich model combined with Julliere formula, the TMC ratio strongly depends on the frequency and it has the maximum peak at a specific frequency. The calculated frequency dependence of TMC is in good agreement with the experimental results obtained in MgO-based MTJs with a tunneling magnetoresistance (TMR) ratio of 108%, which exhibit a large TMC ratio of 155% at room temperature. This calculation also predicts that the TMC ratio can be as large as about 1000% for a spin polarization of 87%, while the TMR ratio is 623% for the same spin polarization. These theoretical and experimental findings provide a deeper understanding on AC spin-dependent transport in the MTJs and will open up wider opportunities for device applications, such as highly sensitive magnetic sensors and impedance-tunable devices.

First-principle theories predicted an extremely high magnetoresistance (MR) ratio over 1000% in epitaxial Fe(001)/MgO(001)/Fe(001) MTJs. We have fabricated fully epitaxial Fe-Co(001)/MgO(001)/Fe-Co(001) MTJs and textured CoFeB/MgO(001)/CoFeB MTJs and achieved giant MR ratios above 400% at room temperature. An ultra-low resistance-area (RA) product indispensable for magnetic sensor application has also been achieved in CoFeB/MgO(001)/CoFeB MTJs. The giant TMR effect in MgO-based MTJs is the key for next-generation spintronic devices.

Fully, epitaxially-grown Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions were prepared by using electron beam evaporation under ultrahigh vacuum. The dynamic resistance curve and the inelastic electron tunnelling (IET) spectrum were measured simultaneously at 4.2 K by a lock-in technique using a homemade electrical circuit. Both curves show asymmetry with the bias. Peaks observed in the IET spectra for antiparallel (AP) magnetization around /spl plusmn/25 mV are considered to be due to magnon-assisted inelastic excitation. The strong intensity of the IET spectra in AP magnetization causes the decrease of tunneling magnetoresistance (TMR) ratio around the voltage of the peak position. The existence of spin-flip scattering sites at the interface between the bottom electrode and the MgO barrier can be attributed to the obvious shoulder observed around 150 mV for negative bias. These results imply that the strong asymmetry of the bias dependence of TMR could be improved by reducing the dislocations.

Double barrier magnetic tunnel transistors were prepared and the magnetoresistance (MR) and magnetocurrent effect (MC)were measured. The films were deposited on SiO/sub 2/ substrate by sputtering. The structure of the films is SiO-substrate/buffer/IrMn/CoFe(2 nm)/Ru(0.85 nm)/CoFe(2 nm:drain)/Al-O(1.2 nm)/NiFe(5 nm:base)/Al-O(1.2 nm)/CoFe(3 nm:source)/IrMn(1.5 nm)/Ru(5 nm). (Hereafter "drain layer" is denoted as "D", "base layer" is "B" and "source layer" is "S".) Al-O tunnel barriers were formed by plasma oxidation of Al. The films were fabricated into 3 terminal device using photolithography, Ar ion etching for metallic layers and wet etching for Al-O layers. The area of D/B junction is 40/spl times/90 /spl mu/m/sup 2/, and B/S junction area is 10/spl times/10 /spl mu/m/sup 2/. The product of resistance and area (RA) is 1.8 M/spl Omega//spl mu/m/sup 2/ for D/B junction and 0.2 M/spl Omega//spl mu/m for B/S junction, respectively. MR measurements were done by conventional 4-probe method at 10 mV. MC measurements were done by using two Keithley 2400 source meters.

A fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions were fabricated using electron beam evaporation under ultrahigh vacuum and achieved a giant tunneling magnetoresistance (TMR) effect over 180% at room temperature. The magnetoresistance (MR) ratio for the crystalline MgO(001) tunnel barrier is about three times that for the amorphous Al-O barrier indicating an importance of the crystalline symmetry of tunnel barrier for the coherent tunneling of /spl Delta//sub 1/ electrons. The MTJs with crystalline MgO(001) tunnel barrier also have high thermal stability and reproducibility of TMR. An oscillation of the TMR effect was also observed as a function of the thickness of MgO tunnel barrier, which could be a direct evidence of coherent spin-dependent tunneling. Such conservation of the coherency of spin polarized electron across the tunnel barrier will enable us to develop a variety of novel spintronics devices with quantum mechanical functions.

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

We fabricated perpendicularly magnetized MgO-based magnetic tunnel junctions (p-MgO-MTJs) with a [Co/Pt]n/CoFeB/CoFe bottom electrode layer (free layer) and a CoFe/CoFeB/TbFeCo top electrode layer (reference layer). The insertion of thin CoFeB/CoFe layers at the barrier/electrode interfaces and post-annealing at a relatively low temperature of 225 °C simultaneously yielded high magnetoresistance (MR) ratios of up to 85% at room temperature and a low resistance–area (RA) product of 4.4 Ω µm2. Such a high MR ratio in low-RA p-MgO-MTJs is the key to developing ultrahigh-density spin-transfer-torque magnetoresistive random access memories (MRAMs).

We investigated bias-voltage and temperature dependence of conductivity arising from the magnon contribution in MgO-based magnetic tunneling junctions (MTJs) with different ferromagnetic electrodes. Second derivative conductance curves showed broad peak structure, which extends from 5 to 200 mV, accompanied by additional peaks at around 23, 54, and 85 mV. The peak intensities were larger for antiparallel configuration than for parallel configuration except that at 85 mV. This difference in the peak intensity was observed to be larger for the MTJs having higher tunneling magnetoresistance ratio, indicating a magnetic origin of these peaks. Abrupt increase in the second derivative conductance at very low biasing voltage in the antiparallel configuration suggests the important role of the surface magnon excitation.

The authors fabricated fully epitaxial Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (tMgO) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (RP) and that in the antiparallel magnetic state (RAP) exhibited short-period oscillations as functions of tMgO with the same period of 3.2A and different phases. RAP also showed a long-period oscillation with a period of 9.9A. As a result, tMgO dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory tMgO dependence of the tunneling magnetoresistance effect.

The magnetoresistance (MR) originating from a magnetic structure with continuous rotation of magnetic moments was studied using soft-magnetic/hard-magnetic bilayers. The feature of the MR curves was explained with anisotropic magnetoresistance (AMR) applying to twisted magnetic structures. The giant magnetoresistance (GMR)-type effect was found to be very small compared with the AMR effect.

The microscopic structures of MgO(001) barrier layers in magnetic tunnel junctions showing giant tunneling magnetoresistance were characterized by in situ scanning tunneling microscopy. The MgO thin films formed exceedingly flat surfaces, and their terraces were made even flatter by annealing after deposition. This flattening of MgO surfaces apparently promotes coherent transport of electrons, which should enhance the tunneling magnetoresistance ratio. Local tunneling spectroscopy revealed that an annealed MgO layer has a critical thickness between 3 and 5 ML (monolayer), and a continuous film without pinholes can be formed over the thickness.

Fully epitaxial bcc Fe1−xCox(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (x=0, 0.5, 1) were fabricated with molecular-beam epitaxy and microfabrication techniques. While the bcc Fe(001) and Fe0.5Co0.5(001) electrodes had similar magnetoresistance (MR) ratios of about 180% at room temperature, the bcc Co(001) electrode exhibited a higher MR ratio up to 271% at room temperature (353% at 20 K). The fact that the MR ratio for a bcc Co electrode is much higher than that for a bcc Fe electrode is consistent with first-principle calculations, indicating the importance of electrode band structure in the k‖=0 direction.

Cobalt-samarium alloy films were prepared by alternate deposition of Co and Sm in the presence of an external magnetic field to obtain thin magnetic layers with large in-plane coercive fields. These anisotropic CoSm layers were used as hard magnetic layers of noncoupled-type magnetoresistance multilayers or as pinning layers of spin-valve structures. Large magnetoresistance changes up to 4.6% were attained in the CoSm/Co/Cu/NiFe system.

We fabricated fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) and observed a magneto-resistance (MR) ratio of 88% at T = 293 K (146% at T = 20 K), the highest value yet reported. The origin of the high MR ratio is not the diffusive tunneling of Julliere's model but the coherent spin-polarized tunneling in epitaxial MTJs, in which only the electrons with totally symmetric wave functions with respect to the barrier-normal axis can tunnel. The bias-voltage dependence of the MR was very small, resulting in a high output voltage of 380 mV. This high voltage will help overcome problems in the development of high-density magnetoresistive random-access-memory (MRAM).

The transport properties of magnetic tunnel junctions with single-crystalline ultrathin Fe(001) electrodes are studied. The tunnel spectra and the bias dependence of the differential magnetoresistance show quantum-well oscillations. This effect provides evidence of ballistic transport through the Al–O barrier and shows that magnetic tunnel junctions with quantum-well states can be a tool for studying spin-dependent transport mechanisms in magnetic tunnel junctions.