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The tunnelling magnetoresistance of CoFeB/MgO tunnel junctions is exceptionally high, although the electrodes and the barrier are grown at room temperature in the amorphous state. For their functionality annealing steps up to high temperatures are required. We have analyzed in detail the changes in the chemical and magnetization profile upon annealing up to 360°. The multilayers used for this study are similar to those which are used in magnetic tunnel junctions, however with five repeats. In particular, we have used hard non-resonant and soft resonant magnetic x-ray scattering in order to unravel any changes upon annealing. The multilayers exhibit superior structural quality, which hardly changes with annealing. Surprisingly, only little recrystallization of the CoFeB and the MgO layers can be discerned by x-ray diffraction. [ABSTRACT FROM AUTHOR]

Magnetic properties, magnetoresistance (MR), and spin-transfer switching of magnetic tunnel junctions having a structure of Co60Fe20B20 3 nm/MgO 1 nm/(Co75Fe25)80-XCr(V)XB20 2 nm (X=0–25) were investigated. Magnetization of the (Co–Fe)–(Cr, V)–B free layer decreased from 1.2 T before substitution to 0.6 T at Cr of 10% (0.8 T at V of 10%). The MR ratio and a resistance-area product (RA) before substitution were, respectively, about 130% and about 2 Ω μm2. The MR ratio decreased to 80% at Cr of 10% and 40% at V of 10%. The RA values were almost independent of the composition. The intrinsic switching current density (Jc0) decreased from 15 to 8 MA/cm2 at Cr of 10% and 12 MA/cm2 for V of 10%. Upon the further increase in Cr and V, stable switching was difficult to observe. In summary, Jc0 decreased to half in the case of Cr, but the effect was small for V. [ABSTRACT FROM AUTHOR]

We fabricated a magnetic tunnel junction (MTJ) using an MgO barrier and MnGa electrode, which shows large magnetic anisotropy, and we investigated the spin torque diode effect in the MTJ. The magnetoresistance ratio increased up to approximately 40% by inserting thin FeB/Fe layers at the MnGa/MgO interface. The obtained diode effect was as high as 70 GHz, which is thought to occur due to the coupled precession acoustic mode in the MnGa/Fe-B multi-layer. Numerical simulation suggests that a very high frequency diode signal of more than 150 GHz can be expected in the optical mode precession. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) were fabricated using an Al-O insulating layer prepared on an epitaxially grown Ni[SUB80[Fe[SUB20] bottom electrode and on a polycrystalline Ni[SUB80]Fe[SUB20] bottom electrode. Crystallographic orientations and surface morphology of the films were examined using x-ray diffraction and atomic force microscopy, respectively. The MTJ with an epitaxial bottom electrode showed a tunnel magnetoresistance (TMR) ratio of 51% after annealing at 250 °C. This value was about two times larger than that of the MTJ with a polycrystalline bottom electrode (27%). The applied bias voltage dependences of the TMR ratios were also much different. The V[SUBhalf] values of epitaxial and polycrystalline samples were about 750 and 400 mV, respectively. [ABSTRACT FROM AUTHOR]

We study tunnel magnetoresistance effect in ferromagnet/granular/ferromagnet tunnel junctions in the Coulomb blockade regime. The granular consists of a Al–O/Co/Al–O/Co/Al–O multilayer, which was sputtered sequentially. The magnetoresistance (MR) ratio and the resistance (R[sub s]) of the junction increased with decreasing bias voltage at low temperatures. The MR ratio and R[sub s] were calculated based on the theory proposed by Takahashi and Maekawa [S. Takahashi and S. Mackawa, Phys. Rev. B 90, 1758 (1998)]. Although the variation of these values on bias voltage was different from experimental results, the calculated MR ratio in the Coulomb blockade regime agreed well with the maximum value measured. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigated gain and fan-out in a current-field driven spin transistor that is driven by a current-induced magnetic field. The basic structure of the transistor consisted of a coplanar waveguide for magnetic field application and an MgO-based magnetic tunnel junction with high magnetoresistance and low resistance area product. Under an assisting ac magnetic field and a dc bias voltage of 0.4 V, we demonstrated substantial power gain of 130, current gain of 4.9, and a fan-out value of 5.7. The scaling of the fan-out value in this device is discussed in detail. The current-field driven spin transistor is proven to be a promising candidate as a basic component of a nonvolatile logic device. [ABSTRACT FROM PUBLISHER]

We report a large enhancement of spin-torque diode sensitivity in CoFeB/MgO/CoFeB magnetic tunnel junctions measured under perpendicular magnetic fields and dc bias currents. In the measurement of homodyne detection, a large dc output voltage of 190 \muV was obtained when an RF signal power of -30 dB\cdotm and a dc current of +1.0 mA were applied. This value corresponds to the diode sensitivity of 190 mV/mW (260 mV/mW after impedance matching correction). The main origin of this enhancement is an offset of the damping torque and an increase in the precession angle induced by the spin-transfer torque due to the dc bias current application. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the spin filter effect in tunnel junctions with a Ni1+xFe2−xO4 ferrimagnetic tunnel barrier. For higher T c above room temperature, Ni1+xFe2−xO4 film should be thicker than 4.0nm 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. [Copyright &y& Elsevier]

Spin-momentum transfer between a spin-polarized current and a ferromagnetic layer can induce steady-state magnetization precession, and has recently been proposed as a working principle for ubiquitous radio-frequency devices for radar and telecommunication applications. However, so far, the development of industrially attractive prototypes has been hampered by the inability to identify systems that can provide enough power. Here, we demonstrate that microwave signals with device-compatible output power levels can be generated from a single magnetic tunnel junction with a lateral size of 100 nm, seven orders of magnitude smaller than conventional radio-frequency oscillators. We find that in MgO magnetic tunnel junctions the perpendicular torque induced by the spin-polarized current on the local magnetization can reach 25% of the in-plane spin-torque term, although showing a different bias dependence. Both findings contrast with the results obtained on all-metallic structures, previously investigated, reflecting the fundamentally different transport mechanisms in the two types of structure. [ABSTRACT FROM AUTHOR]

Fully epitaxial Co(001)/MgO(001)/Co(001) magnetic tunnel junctions (MTJs) with metastable bcc Co(001) electrodes were fabricated with molecular beam epitaxy. The MTJs exhibited giant magnetoresistance (MR) ratios up to 410% at room temperature, the highest value reported to date. Temperature dependence of the MR ratio was observed to be very small compared with fully epitaxial Fe/MgO/Fe and textured CoFeB/MgO/CoFeB MTJs. The MR ratio of the Co/MgO/Co MTJ showed larger bias voltage dependence than that of the epitaxial Fe/MgO/Fe MTJs, which probably reflects the band structures of bcc Co and Fe for the k∥=0 direction. [ABSTRACT FROM AUTHOR]

Magnetoresistance (MR) and spin-transfer switching (STS) properties were investigated in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions as a function of free layer thickness (dFree). The MR ratio was about 140% at dFree>=2 nm. It decreased to about 80% at dFree=1.5 nm. Both switching currents and thermal stability were roughly proportional to dFree. The averaged intrinsic switching current density (Jc0av) was 1×107–2×107 A/cm2. The thermal stability of parallel magnetization state was greater than that of antiparallel state. The feasibility of the STS write scheme for nonvolatile magnetic random access memory was discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The bottom shift in a parabolic baseline of resistance-current (R-I) curves in trilayer current perpendicular to plane-giant magnetoresistance (CPP-GMR) elements is often observed in current induced magnetization reversal (CIMR) experiments. We fabricated CPP-metallic junction elements with various metal combinations (Co/Au, Cr/Au, Cr/Co, Cr/Cr), and measured the R-I curves. Our results reveal that the direction of the shift is coincident with the sign of the Peltier coefficient of the CPP structure, and the amount of the shift is proportional to the Peltier coefficient. We estimated the Peltier coefficients by using the current where the Joule heating and the Peltier cooling compensates, and found that the Peltier coefficients are larger than those obtained from bulk materials. The expected cooling power of the CPP-Co/Cr elements was about 100 μW, which results in relatively large areal cooling power (∼105 W/cm²) because of the large current density (∼107 A/cm²). [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions with the structure of Al2O3 (0001)/Pt (111) 20 nm/Ni80Fe20 (111) 50 nm/Al 1.6 nm–O/Co75Fe25 4 nm/Ir22Mn78 10 nm/Ni80Fe20 30 nm were fabricated using UHV sputtering and photolithography process. As the annealing temperature increased up to 250 °C, tunnel magnetoresistance (TMR) ratio at 1 mV bias increased from 28% to 43% for tox=180 s plasma oxidation and the V±1/2, at which the zero bias TMR value is halved, is +640 mV and-650 mV for positive and negative bias voltages, respectively. The bias-voltage dependence of TMR could be explained in terms of the relationship with V±1/2 and the interface of the ferromagnetic electrode and the Al–O insulating layer. V+1/2, which reflects the bottom ferromagnetic electrode-barrier interface state, changes with plasma oxidation time, while V-1/2, which corresponds to top ferromagnetic electrode-barrier interface, hardly changes. [ABSTRACT FROM AUTHOR]

We prepared magnetic tunnel junction films with PtMn/CoFe/Ru/CoFeB/MgO tunnel barrier/FeB free layer/MgO cap layer/Ta multilayers using sputtering and measured magnetic and magnetoresistive properties of the films at room temperature. The magnetization curves of the FeB plane film measured under perpendicular-to-plane magnetic fields showed much smaller saturation fields (Hs) than those expected from the demagnetizing field. Hs decreased from 4 to 0.4 kOe with increasing MgO cap layer thickness. The small Hs is due to the perpendicular magnetic anisotropy (PMA) induced at both MgO barrier-FeB and FeB-MgO cap interfaces. After microfabrication, the small free layer cells having a 1.6 nm thick MgO cap layer showed a magnetization easy axis in the perpendicular-to-plane direction. By inducing PMA from both upper and lower interfaces, we can stabilize the magnetization of the relatively thick (2 nm) FeB free layer in the perpendicular-to-plane direction. [ABSTRACT FROM AUTHOR]

Temperature dependence of tunnel magnetoresistance (TMR) ratio, resistance, and coercivity from 4.2 K to room temperature and applied voltage dependence of the TMR ratio and resistance at room temperature for a tunnel junction, Ta (5 nm)/Ni[sub 79]Fe[sub 21] (3 nm)/Cu (20 nm)/Ni[sub 79]Fe[sub 21] (3 nm)/Ir[sub 22]Mn[sub 78] (10 nm)/Co[sub 75]Fe[sub 25] (4 nm)/Al (0.8 nm)-oxide/Co[sub 75]Fe[sub 25] (4 nm)/Ni[sub 79]Fe[sub 21] (20 nm)/Ta(5 nm), were investigated. TMR ratio, effective barrier height and width, and breakdown voltage of the junction can be remarkably enhanced after annealing at 300 °C for an hour. High TMR ratio of 49.7% at room temperature and 69.1% at 4.2 K were observed. The value of spin polarization of Co[sub 75]Fe[sub 25], P=50.7%, deduced from the TMR ratio at 4.2 K was corresponding well to the experimental data measured at 0.2 K in a spin polarized tunneling experiment using a superconductor/insulator/ferromagnet tunneling junction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]