Ferromagnetic Tunnel Junctions Research Articles

Magnetic transport mechanism in double ferromagnetic tunnel junctions with two-dimensional ferromagnetic particles

Magnetic transport properties of Co/sub 90/Fe/sub 10//AlO/sub x//Co/sub 90/Fe/sub 10//AlO/sub x//Co/sub 90/Fe/sub 10/ ferromagnetic double tunnel junctions have been investigated with respect to the thermal fluctuation. A middle Co/sub 90/Fe/sub 10/ layer is discontinuous by forming Co/sub 90/Fe/sub 10/ particles two-dimensionally, which shows superparamagnetic behavior above 80 K. The average diameter of the Co/sub 90/Fe/sub 10/ particles is estimated to be 4.8-5.0 nm from both a magnetization curve at room temperature and temperature dependence of coercivities deduced from magnetoresistance curves. This finding agrees with the maximum diameter (5.2 nm) evaluated from cross-sectional transmission electron microscope images, revealing that the dominant electron transport process heavily depends on the largest particles.

Significant Magnetoresistance Enhancement due to a Cotunneling Process in a Double Tunnel Junction with Single Discontinuous Ferromagnetic Layer Insertion

We fabricate CoFe/AlOx/CoFe/AlOx/CoFe ferromagnetic double tunnel junctions and observe spin-dependent tunneling phenomena. A middle CoFe layer becomes discontinuous by forming CoFe particles two dimensionally, of which the average diameter is evaluated to be 2.0-4.5 nm from cross-sectional transmission electron microscopy images. Below 50 K, a Coulomb gap is observed in current-voltage curves, and both magnetoresistance ratios and resistances are found to increase significantly with decreasing temperature. This indicates that a cotunneling process is dominant within the gap, which agrees very well with theoretical prediction [Phys. Rev. Lett. 80, 1758 (1998)].

Magnetization Reversal by Electrical Spin Injection in Ferromagnetic (Ga,Mn)As-Based Magnetic Tunnel Junctions

We report current-induced magnetization reversal in a ferromagnetic semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared by molecular beam epitaxy on a p-GaAs(001) substrate. A change in magneto-resistance that is asymmetric with respect to the current direction is found with the excitation current of 106 A/cm2. Contributions of both unpolarized and spin-polarized components are examined, and we conclude that the partial magnetization reversal occurs in the (Ga,Mn)As layer having smaller magnetization with the spin-polarized tunneling current of 105 A/cm2.

強磁性ナノドット層を有する二重トンネル接合におけるＴＭＲの増大

We observed spin-dependent tunneling in Co90Fe10/AlOx/Co90Fe10/AlOx/Co90Fe10 ferromagnetic double tunnel junctions. A middle Co90Fe10 layer, which is inserted between two AlOx layers, is discontinuous due to the difference in the surface energy of these layers. The average diameter of Co90Fe10 particles was estimated to be 2.0-4.5 nm from cross-sectional transmission electron microscopy (TEM) images. At low temperature (< 50 K), a Coulomb gap is observed in current-voltage (I-V) curves, and the tunnel magnetoresistance (TMR) ratio within the gap enhances significantly with decreasing temperature, which indicates that the dominant electron transport is inelastic co-tunneling within the Coulomb gap. These observations in a ferromagnetic tunnel junction with 2-dimensional magnetic nano-particle layer insertion accord well with theoretical predictions by Takahashi and Maekawa [Phys. Rev. Lett. 80, 1758 (1998)].

Radical nitridation of Al films for the barrier formation in ferromagnetic tunnel junctions

Nitridation process of Al films using microwave-excited plasma was investigated to realize highly qualified Al–N barrier for magnetic tunnel junctions (MTJs), in comparison with reactive deposition process of Al–N films. The MTJs fabricated by the plasma nitridation method with Kr+N2 or Ar+N2 mixed gas showed larger values of tunnel magnetoresistance (TMR) ratio and resistance–area product (R×A) than those of the MTJs fabricated with He+N2 plasma. The MTJs with reactive-deposited Al–N showed relatively small values of TMR ratio and R×A. From the optical emission spectroscopy of the respective plasma, we concluded that N2 ion in the plasma is a responsible factor for lowering the barrier height of Al–N and that radical nitridation process is suitable to form the barriers without inducing defects.

Spin-polarized Tunneling in Ultrasmall Vertical Ferromagnetic Tunnel Junctions

We have developed nanometer-scale vertical ferromagnetic tunnel junctions using a Si-based inorganic electron beam resist process, including barrier layer fabrication using metal evaporation in ozone atmosphere. The current–voltage (I–V) characteristics of Ni/NiO/Co multiple junctions with diameters of 20 nm have been measured in a magnetic field to investigate spin-polarized tunneling in the Coulomb blockade regime. The temperature dependence of the I–V curve indicates that Coulomb blockade phenomena occur at temperatures below 40 K, agreeing with the estimation of the single-electron charging energy from the device geometries. The magnetoresistance is strongly enhanced by magnetization reversal of Ni and Co, and the obtained MR ratio is greater than 100% in the Coulomb blockade regime at 15 K.

Local transport properties of Co-Fe/Al-O/Co-Fe tunnel junctions with high thermal stability

In order to investigate the annealing temperature dependence of the tunnel magnetoresistance ratio (TMR) for ferromagnetic tunnel junction, the local transport properties were measured for Ta 5nm/Cu 20nm/Ta 5nm/Ni76Fe24 2nm/Cu 5nm/Mn75Ir25 10nm/Co71Fe29 4nm/Al-O junction with metal Al thickness of 0.8nm and oxidization time of 7s. The current histogram statistically calculated from the electrical current image was in good accordance with the fitting result considering the Gaussian distribution of barrier height and Fowler-Nordheim conductance. At high annealing temperature of 340°C, where the TMR ratio of the corresponding MTJ had the maximum value of 44%, the average barrier height φave increased to 1.12eV and the standard deviation σφ decreased to 0.1eV. The increase of TMR ratio after annealing was attributed to the increase of average barrier height and decrease of the barrier height variation. After further annealing at 360°C, the standard deviation σφ increased up to 0.2eV and the TMR decreased to 10%.

Ferromagnetic Ultra-Small Tunnel Junction Devices Fabricated by Scanning Probe Microscope (SPM) Local Oxidation

A novel lithography technique for fabricating ferromagnetic metal/insulator/ferromagnetic metal (FM/I/FM)-based nanoscale devices was investigated using a scanning probe microscope (SPM) local oxidation process. This technique was applied to the surface modification of Ni thin films. Planar-type Ni/Ni oxide/Ni-based diodes and ferromagnetic single-electron transistor (FMSET) structures were fabricated by SPM local oxidation. Ni-based diodes clearly showed nonlinear current-voltage (I-V) characteristics, and capacitively coupled FMSET structures with a Ni-based double ferromagnetic tunnel junction exhibited Coulomb blockade effects.

Spin-polarized transport in diluted GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions

Taking into account the basic physics of diluted ferromagnetic semiconductors (DMS), in which the variation of the splitting energy with temperature is included, we apply a quantum-mechanical approach to studying the spin-polarized transport in GaMnAs/AlAs/GaMnAs DMS tunnel junctions. It is shown that tunneling magnetoresistance first rapidly increases and then decreases with increasing barrier thickness, exhibiting a peak at an optimum value of barrier thickness. We also find that the normalized conductance difference decreases with the enhancement of temperature. The theoretical results can reproduce the main feature of the experiments.

Ferromagnetic tunneling junctions at low voltages: Elastic versus inelastic scattering at T=0 °K

In this article we analyze different contributions to the magnetoresistance of magnetic tunneling junctions at low voltages. A substantial fraction of the resistance drop with voltage can be ascribed to variations of the density of states and the barrier transmission with the bias. However, we found that the anomaly observed at zero bias and the magnetoresistance behavior at very small voltages, point to the contribution of inelastic magnon-assisted tunneling. The latter is described by a transfer parameter TJ, which is one or two orders of magnitude smaller than Td, the direct transmission for elastic currents. Our theory is in excellent agreement with experimental data, yielding estimated values of TJ which are of the order of Td/TJ∼40.

Diffusion and segregation effects in doped manganite/titanate heterostructures

For the properties of devices based on oxide multilayers, interfaces play a crucial role. For ferromagnetic tunnel junctions the sign of the spin polarization depends on the barrier material and the interface termination. Here, we present a transmission electron microscopy study of heteroepitaxial trilayer systems consisting of a SrTiO3 insulating barrier sandwiched between La2/3D1/3MnO3 (D=Ca or Ba) electrodes grown on SrTiO3 substrates. Intermixing during growth depends on the earth alkaline ion: Ca segregates to the surface of the growing epitaxial thin film, leading to a Ca rich interface layer between the bottom electrode and the barrier. In contrast, no Ba segregation could be detected in the well lattice-matched Ba-doped film. In addition, a significant diffusion of both Ca and Ba into the SrTiO3 tunneling barrier is found, whereas the diffusion of Mn is negligibly small. Our results explain the different tunneling magnetoresistance in various manganite based tunnel junctions.

Domain walls unmasked during domain duplication in ferromagnetic tunnel junctions

This paper highlights the role played by domain walls during domain duplication in ferromagnetic junctions. The evolution of the resistance jumps with reversal field when duplication occurs cannot be exclusively explained by parallel tunnel paths of electrons through regions with parallel and antiparallel magnetization alignment. A model of tunnel resistance taking into account the contribution of domain walls was used to extract their density. The extracted values are in agreement with experimental data.

Ferromagnetic tunnel junctions with high thermal stability

The junctions exposed to pure oxygen at the interface between the bottom CoFe layer and Al-oxide insulator were prepared. The junctions showed high thermal stability of tunnel magnetoresistance (TMR) ratio up to 375°C in comparison with the standard junction oxidized with plasma. Auger electron spectroscopy (AES) profiles revealed that the Mn diffusion occurred even for the junction with high thermal stability after annealing over 375°C. A large amount of oxygen existing at the interface might oxidize the Mn diffusing at the interface.

Nanofabrication of magnetic tunnel junctions by using electron beam lithography

Abstract Nanometer-sized ferromagnetic tunnel junctions (MTJs) are suitable to investigate the enhancement of tunnel magnetoresistive effect (TMR) by Coulomb blockade effect. A new fabrication process that enables us to shrink the size of MTJs down to the order of 10 nm has been developed. MTJs structured by using this process exhibited large TMR ratio of 38%.

Electronic transport through quantum dots coupled to ferromagnetic leads: from Coulomb blockade to Kondo effect

Recent experiments on magnetic nanostructured materials revealed new phenomena related to the interplay of ferromagnetism and discrete charging effects. Typical example of such systems is a double ferromagnetic tunnel junction with a quantum dot as the central electrode . Interplay of charge and spin degrees of freedom in such systems is analyzed theoretically in the sequential tunneling, cotunneling and strong coupling regimes.

Spin-dependent single electron tunneling in double tunnel junctions with a nano-granular layer

We have examined the spin-dependent tunneling properties of ferromagnetic double tunnel junctions with a nano-granular ferromagnetic intermediate layer, whose average grain size is about 2 nm. At low temperatures, samples show a linear current–voltage characteristic at low voltages, which indicate elastic coherent tunneling. In this regime, the conductances for both parallel and anti-parallel configurations of the magnetizations of the grains fluctuate to the applied bias voltage, with spin-dependent periods. The tunnel magnetoresistance ratio can be as large as 100%. We can conclude that we have observed a spin-dependent single electron tunneling via a single discrete level for the first time in a metallic system.

A theoretical investigation of ferromagnetic tunnel junctions with 4-valued conductances

In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet(FM)/barrier/FM junctions, we have theoretically investigated the multiple-valued (ormulti-level) cell property, which is in principle realized by sensing the conductances of fourstates recorded with magnetization configurations of two FMs; that is, (up, up),(up, down), (down, up), (down, down). To obtain such 4-valued conductances, wepropose FM1/spin-polarized barrier/FM2 junctions, where FM1 and FM2 aredifferent ferromagnets, and the barrier has spin dependence. The proposed ideais applied to the case of the barrier having localized spins. Assuming that allthe localized spins are pinned parallel to the magnetization axes of FM1 andFM2, 4-valued conductances are explicitly obtained for the case of many localizedspins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.

Improved thermal stability of ferromagnetic tunnel junctions with a CoFe/CoFeO/sub X/CoFe pinned layer

Annealing temperature dependence of the tunnel magnetoresistance (TMR) ratio for magnetic tunnel junctions with CoFeO/sub x/ inserted in the pinned layer was investigated. A junction with a CoFe/CoFeO/sub x//CoFe pinned layer exhibited TMR ratios of 46.5% and 42.5% after annealing at 350/spl deg/C and 375/spl deg/C, respectively. The reason for the improvement of the thermal stability is related to oxygen diffusion from the CoFeO/sub x/ layer, and there is a possibility that CoFeO/sub x/ plays the role of a Mn (in MnIr exchange layer) diffusion barrier.

22pWA-15 強磁性ナノ粒子を介したスピン依存共鳴トンネル効果の観測

22pWA-15 強磁性ナノ粒子を介したスピン依存共鳴トンネル効果の観測

Junction area scaling and statistical description of dc breakdown of ferromagnetic tunnel junctions

Magnetoresistive random access memory (MRAM) with ferromagnetic tunnel junctions (MTJs) has been recognized as a successful candidate for next generation solid-state memory. There are, however, still significant problems to realize MRAM. In particular, reliability of an ultrathin dielectric layer composing tunnel junctions is of great importance for MRAM developments. In this article, we performed systematical studies of the dielectric breakdown of MTJs. The samples have a double tunnel barrier structure with IrMn/CoFe/AlOx/CoFe/AlOx/CoFe/IrMn. Its width is ranging from 0.4 to 30 μm. Different from previous reports, the breakdown voltage gradually decrease with decreasing junction area. The breakdown voltage only shows weak dependence on junction area in the sub-μm region. In addition, the breakdown voltage is inversely proportional to the junction perimeter. Results of time-dependent failure measurements demonstrate that, within the limit of this experiment, the failure function of the MTJs obeys a Weibull distribution. The shape parameter of the Weibull distribution is less than unity, indicating early device to failure. We consider that process-induced defects at the sidewall of the junctions may serve a certain role in their early failure and lead the failure of the junction area scaling of the breakdown voltage.