Maximum Tunneling Magnetoresistance Research Articles

Magnetoresistance in tunnel junctions using Co2(Cr,Fe)Al full Heusler alloys

We grew Co2(Cr1−xFex)Al Heusler alloy films using a magnetron sputtering system on thermally oxidized Si substrates at room temperature without any buffer layers. The x-ray diffraction patterns did not show the L21 structure as expected for the bulk but revealed the B2 and A2 structures, depending on the Fe concentration x, in which the structure tends to become the A2 with increasing x. The magnetic moment and the Curie temperature monotonically increased with increasing x. Spin-valve-type tunneling junctions consisting of Co2Cr1−xFexAl (100 nm)/AlOx (1.4 nm)/CoFe (3 nm)/NiFe (5 nm)/IrMn (15 nm)/Ta (10 nm) were fabricated on thermally oxidized Si substrates without any buffer layers using metal masks. The maximum tunneling magnetoresistance at room temperature was obtained as 19% for x=0.4.

Tunnel magnetoresistance using full-Heusler alloys

We grew Co2(Cr1−xFex)Al Heusler alloy films at room temperature (RT) without any buffer layers as well as Co2MnZ (Z=Si and Ge), which were fabricated using a Cr buffer at RT and elevated temperatures, on thermally oxidized Si substrates using a magnetron sputtering system. The X-ray diffraction pattern of the Co2MnZ (Z=Si and Ge) films exhibited L21 structure when deposited at an elevated temperature, while it showed amorphous or nanocrystalline nature for the films deposited at RT. Co2(Cr1−xFex)Al films deposited at RT, on the other hand, revealed the B2 and A2 structures, depending on the Fe concentration x, in which the structure tends to become the A2 with increasing x, which is different from the L21 structure as expected in the bulk. The magnetic moment and the Curie temperature monotonically increased with increasing x. Spin-valve-type tunneling junctions consisting of Co2Cr1−xFexAl (100nm)/AlOx (1.4nm)/CoFe (3nm)/NiFe (5nm)/IrMn (15nm)/Ta (10nm) were fabricated at RT on thermally oxidized Si substrates without any buffer layers using metal masks. The maximum tunneling magnetoresistance obtained is 19% for x=0.4 at room temperature.

ＣＯ２ＭｎＡｌホイスラー合金を用いた強磁性トンネル接合

We investigated the structural and magnetic properties of Co2MnAl thin films with Cr buffer layers, and the tunnel magnetoresistance (TMR) effect of junctions using Co2MnAl Heusler alloy. The saturation magnetization of Co2MnAl film deposited at 350°C was 711 emu/cm3 which was almost the same as that reported for bulk. The maximum TMR ratio of the junctions was 12% at room temperature and 26% at 25K. TMR ratio increased to 17% at room temperature after annealing at 200°C for one hour.

Transport property studies on magnetic tunnel junctions with wedge-shaped barrier

Systematic studies on the transport property were carried out in FM (NiFe, Co, FeCo)/AlO x /NiFe magnetic tunnel junctions (MTJs) by oxidizing a wedge-shaped Al layer, resulting in over-, optimized, and under-oxidized barriers. An interesting resistance peak can be generally observed in the over-oxidized region, which was attributed to the significant change in oxidation level of the bottom electrode. This argument was supported by the barrier thickness and height fittings. Combining the studies of tunnel magnetoresistance (TMR) and the bias dependence as a function of the Al layer thickness, we can conclude that, in the MTJ exhibiting maximum TMR, the Al layer has not been fully oxidized to reach stoichiometric Al 2O 3, and the bottom electrode has already been slightly oxidized. A sizable TMR ratio can still be observed in under-oxidized region, even with very thick Al layer and short oxidation time, which implies that no pure metallic Al exists near the bottom ferromagnet/insulator (FM/I) interface. This is consistent with the grain boundary oxidation model for post-oxidizing the Al layer in MTJs.

Aluminum oxidation by a remote electron cyclotron resonance plasma in magnetic tunnel junctions

The aluminum barrier in exchange biased (Mn–Ir/Co–Fe/AlOx/Py) magnetic tunnel junctions was formed by oxidation with an electron cyclotron resonance plasma source. This technique allows, in contrast to commonly used floating substrate, an independent control of the energy of the ions bombarding the Al film by adjusting a direct current bias voltage at the sample. Here, we show that low energy ions and oxidation times between 50 and 200 s lead to optimum barrier properties and a maximum tunnel magneto resistance (TMR) ratio of 46% at room temperature and 73% at 10 K. Shorter oxidation time gives underoxidized samples with strongly reduced TMR. Ions with energy above 30 eV turn out to strongly overoxidize the sample resulting in large junction resistance and reduced TMR values.

Thermally induced changes of magnetic coupling in a pinned artificial antiferromagnet used in magnetic tunnel junctions

The thermally induced changes of the magnetic coupling in an artificial antiferromagnet exchange biased by an antiferromagnetic layer in a magnetic tunnel junction (Mn–Ir/Co–Fe/Ru/Co–Fe/AlOx/Ni–Fe) is investigated for annealing temperatures up to 450 °C. Beside the usual increase of the tunneling magnetoresistance (TMR) by annealing (maximum TMR 43.9% at 325 °C) the degradation of the artificial antiferromagnet causes a well-defined 90° coupling of the Co–Fe layers in a narrow temperature window around 325 °C. At higher temperature, both Co–Fe layers are coupled ferromagnetically but the exchange biasing by the Mn–Ir is still present. This behavior results from an interplay of different coupling mechanisms of the two Co–Fe layers: (1) indirect antiferromagnetic interlayer exchange coupling across the Ru spacer, (2) ferromagnetic coupling by pinholes in the Ru spacer, and (3) exchange coupling between the lower Co–Fe layer and the antiferromagnet Mn–Ir. The importance of each contribution is discussed with respect to thermally activated diffusion processes.

Room temperature tunneling magnetoresistance of electron beam deposited (Co50Fe50)x(Al2O3)1−x cermet granular films

A series of (Co50Fe50)x-(Al2O3)1−x cermet granular thin films deposited on glass substrates by dual electron beam evaporation was studied for their structural, magnetotransport, and magnetic properties. Upon varying the magnetic volume fraction, x, from 0.07 to 0.52 the percolation threshold (xc) was determined from resistivity measurements to be ∼0.17. This value agrees well with the theoretical prediction for a three-dimensional system of spherical particles. Values of the isotropic tunneling magnetoresistance (TMR) as high as 10% at room temperature were found for films with x<0.16. The relationship between magnetotransport (maximum TMR) and the granular film topology (the percolation threshold) is discussed.

Temperature dependence of magnetoresistance for tunnel junctions with high-power plasma-oxidized barriers: Effects of annealing

Magnetic tunnel junctions (MTJ) were fabricated with high oxygen-plasma power and the effects of annealing on the temperature dependence of tunneling magnetoresistance (TMR) were investigated experimentally. As grown, TMR increases, peaks around 160 K, and decreases with increasing temperature from 80 K to 300 K. When MTJs are annealed, Tmax, the temperature at which maximum TMR is obtained, decreases as annealing temperature increases to the optimal point. In order to explain the temperature dependence of TMR, the difference of conductance between parallel and antiparallel alignments of magnetizations as a function of temperature is also analyzed. The shifts of Tmax due to annealing are described phenomenologically with spin-dependent transfer rates of electrons through the barrier.

Tunneling magnetoresistance and electron spin resonance study on Co–Al–O granular films

In this work, a series of Co–Al–O granular films were prepared using an Ar–O 2 reactive rf sputtering system. The optimal magnetoresistance (MR) ratio of 5.1% at room temperature was found in Co 44Al 23O 33 sample. After annealing at 300°C, MR ratio can be raised to 6.1%. We also measured the spectra of electron spin resonance for this series of samples and found an interesting correlation between the magnitude of MR ratio and the width of absorption field (Δ H). Our results show that the maximum tunneling magnetoresistance occurs when Δ H is the largest, indicating the important role of magnetic microstructure in the mechanism of granular tunneling.

Tunnel junctions with AlN barriers and FeTaN electrodes

Tunnel junctions with AlN (AlNxOy) barriers and CoFe and FeTaN electrodes were studied. The AlN barrier was formed by nitridizing a 10 Å thick Al layer using radio frequency N2 plasma. The nitrogen and oxygen content in the barriers was determined by Rutherford backscattering spectroscopy (RBS) on specially prepared barriers deposited on DLC coated substrates. RBS results indicate less than 10% O2 incorporation in the barrier. Top-pinned junctions formed by nitridizing a 10 Å thick Al layer (CoFe electrodes) show resistance×area products from 73 Ω μm2 to 8.5 kΩ μm2 for increasing nitridation times from 30 to 200 s, with corresponding tunneling magnetoresistance (TMR) values from 13% to 33%. Bottom-pinned junctions with FeTaN electrodes were also fabricated. Maximum TMR signal is 17% after anneal at 225 °C for 30 min. In both cases (CoFe or FeTaN electrodes), TMR degrades for anneals above 250 °C.

40% tunneling magnetoresistance after anneal at 380 °C for tunnel junctions with iron–oxide interface layers

Spin tunnel junctions fabricated with one interposed Fe–FeOx layer between the Al2O3 barrier and the top CoFe pinned electrode show large tunneling magnetoresistance (TMR) (40%) for anneals up to 380 °C. The annealing temperature TTMR*, where maximum TMR occurs, increases with the inserted Fe–FeOx layer thickness. For samples with thicker inserted layer, the pinned layer moment (which usually starts to decay below 300 °C in the normal junctions) increases with annealing temperature up to 380 °C and remains at a maximum until 450 °C. The large TMR at high temperature is related with the diffusion of extra Fe (from the Fe–FeOx layer) into the electrode interfacial region and the as-deposited paramagnetic FeOx decomposition into metallic Fe, and possibly the formation of some Fe3O4, which compensate the interface polarization loss associated with Mn interdiffusion. Rutherford backscattering spectrometry analysis confirms partial Fe diffusion into the top CoFe electrode after anneal. Meanwhile, x-ray photoelectron spectra for the Fe 2p core level show that the FeOx contribution in the upper part of the inserted layer decreases upon annealing, while it increases in the inner part near the barrier, suggesting the FeOx decomposition and the oxygen diffusion toward the inner metallic Fe and Al barrier. The study of R×A values and barrier parameters versus annealing temperature for samples with 7 and 25 Å Fe–FeOx also reflects the above structural changes in the inserted layer.

Tunneling-MR in Fe–Cr–O metal-nonmetal granular systems

Tunneling magnetoresistance (TMR) has been achieved for Fe–Cr–O granular films prepared by RF cosputtering methods. These films are composed of nanoscaled Fe granules with an average diameter of 18–28 nm embedded in the crystalline insulating Cr 2O 3 matrix, and Fe volume fraction f v ranges from 15% to 55%. Ferromagnetic properties exist in all samples. With the decrease of f v, TMR increases from −0.24% for f v=55% to −1.9% for f v=39%, the maximum, then decreases to −0.23% at f v=28%. It is interesting that TMR becomes positive 2% at f v=15%. The relationship between ln ρ(0) and 1/ T 1/2 shows a clear linear relation in the temperature range from 200 to 290 K for the sample with the maximum negative TMR, indicating a tunneling conductive mechanism. Beyond this temperature range (5–200 K) the relationship is obviously nonlinear, implying that some other effects may exist, which needs to be further studied. Our results imply that superparamagnetic state of the metal granules is not the necessary condition for TMR, and it may not be the main factor affecting the TMR, while the microstructure of films, such as the interfaces between the metal and oxide may be important to TMR as the electronic structure of metal-oxide interface can influence the spin polarization.

A study on tunneling magnetoresistance in magnetic tunnel junctions oxidized by ozone

Magnetic tunnel junctions of Ta/NiFe/FeMn/NiFe/CoFe/Al 2O 3/CoFe/NiFe structure were studied. Two different types of oxidation atmosphere were used to form the barrier layer; plasma oxidation and ozone oxidation. The maximum tunneling magnetoresistance ratio of 32% was observed at room temperature for 50 min ozone oxidation. The junction resistance was several hundred ohms at 0.1 mm×0.2 mm junction. This is about one order lower than that of the plasma oxidized junctions. The plasma free oxidation process would eliminate the possible structural damage by an oxygen ion bombardment and give improved control over the tunnel barrier growth.

Double tunnel junctions for magnetic random access memory devices

Optimum junction resistance and minimum tunnel magnetoresistance (TMR) ratio required for high density and high performance magnetoresistive random access memory (MRAM) devices with a TMR cell plus field effect transistor (FET) switch architecture are discussed by taking into account the variation of FET resistance causing noise. This implies that a TMR ratio over 25% at a 400 mV bias voltage and junction resistance of several tens of kilo-ohms for TMR cells are required with a signal voltage of 30 mV and a sense current of 10 μA, which leads to about 10 ns read time. This large magnetoresistance ratio at the elevated bias voltage requires low bias voltage dependence of TMR for the MRAM devices. In order to try to meet this requirement, double tunnel junctions were fabricated which possess the central ferromagnetic layer consisting of a thin discontinuous layer of hard ferromagnetic Co80Pt20 nanoparticles and insulating Al2O3 prepared by alternate sputtering of Co80Pt20 and Al2O3 targets. The maximum TMR obtained was 20.5% at room temperature for FeCo top and bottom electrodes without annealing. Bias voltage dependence of the (NiFe/CoFe)/1.5 nm Al2O3/discontinuous CoPt/2.6 nm Al2O3/(CoFe/NiFe) double tunnel junctions were revealed to be small compared to that of single junctions, the barrier of which was also fabricated by sputtering of an Al2O3 target.

Superconducting and spin-dependent tunneling using native oxide barriers on CoFe thin films

Superconducting tunneling and magnetoresistance were exhibited in junctions with native oxide barriers grown by exposing thin films of ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ to ambient atmosphere. When Pb was used as a counterelectrode, $I\ensuremath{-}V$ curves at 4.2 K reflected the presence of the superconducting energy gap of Pb. Conductance measurements at 1.5 K revealed clear longitudinal and transverse phonon structure from the superconducting Pb as evidence of single-step, elastic tunneling. Junctions with Co counterelectrodes showed a maximum tunneling magnetoresistance of 2.5% at room temperature and 6% at 77 K. These are the largest values reported to date in spin-dependent tunnel junctions using a native oxide barrier. Junction performance depended strongly on the deposition conditions for the top ferromagnetic layer.

Inverse Tunnel Magnetoresistance inCo/SrTiO3/La0.7Sr0.3MnO3: New Ideas on Spin-Polarized Tunneling

We report tunnel magnetoresistance (TMR) measurements on $\mathrm{Co}/{\mathrm{SrTiO}}_{3}/{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ junctions. The half-metallic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ electrode is used as a spin analyzer. The large $(\ensuremath{-}50%)$ inverse TMR indicates a negative spin polarization of Co, in agreement with the density of states (DOS) of the $d$ band in Co. The bias dependence of the TMR, with a maximum inverse TMR at $\ensuremath{-}0.4\mathrm{V}$ and a crossover to normal TMR above $+0.8\mathrm{V}$, reflects the structure of this DOS. Our results demonstrate that the choice of the insulating barrier can strongly influence and even reverse the spin polarization of tunneling electrons.

Magnetoresistance of Single and Double Tunnel Junctions Formed by Direct Sputtering Using Al2O3 Target

Tunnel magnetoresistance (TMR) has been investigated in single (Ni-Fe/Al2O3/Co) and double (Ni-Fe/Al2O3/Co/ Al2O3/Co) tunnel junctions. The Al2O3 layer was formed by a direct sputtering method with an Al2O3 target. Metal masks were used to form a cross pattern geometry. In the single junctions the dependence of the tunnel resistance and the TMR ratio on the Al2O3 thickness was investigated. The tunnel resistance increased exponentially with increasing the Al2O3 thickness. A maximum TMR ratio was 4-5 %. In the double junctions the dependence of the tunnel resistance and the TMR ratio on the thickness of central Co layer was investigated. The tunnel resistance decreased with increasing the Co thickness. The maximum TMR ratio was 2%, which was smaller than the value of the single junctions.

Ａｌ自然酸化膜を用いた強磁性トンネル接合の磁気抵抗

Ferromagnetic tunnel junctions with oxidized Al layers were prepared by electron-beam (EB) evaporation and dc magnetron sputtering (SP). The tunneling magnetoresistance (TMR) ratio observed in an EB-Co/Al(5.0 nm)-oxide/EB-Fe sample was around 3%. When the first ferromagnetic layer was fabricated by sputtering, the measured TMR ratio increased to 10% and the Al thickness at the maximum TMR ratio shifted to a thinner one. According to the results of surface observation with an atomic force microscope (AFM), the surface morphology of the first ferromagnetic layer strongly influenced the oxidized Al layer, and the smooth surface of the first ferromagnetic layer increased the TMR ratio.