Hiroki Maehara

230% room-temperature magnetoresistance in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

The magnetoresistance ratio of 230% at room temperature is reported. This was achieved in spin-valve type magnetic tunnel junctions using MgO barrier layer and amorphous CoFeB ferromagnetic electrodes fabricated on thermally oxidized Si substrates. The amorphous CoFeB electrodes are of great advantage to the polycrystalline FeCo electrodes in achieving a high homogeneity in small 100 nm-sized MTJs.

Bias-driven high-power microwave emission from MgO-based tunnel magnetoresistance devices

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. Improvements in the microwave output efficiency of MgO-based magnetic tunnel junctions brings them a step closer to practical applications and enables greater insight into the physics of spin transfer in such devices.

Giant tunneling magnetoresistance effect in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions for read-head applications

The giant tunneling magnetoresistance effect has been achieved in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions (MTJs) at room temperature. A magnetoresistance (MR) ratio as high as 138%, seven times that of state-of-the-art MTJs for magnetic sensor application, was obtained at room temperature in MTJs with a resistance-area product (RA) as low as 2.4Ωμm2. Such a high MR ratio at such a low resistance was made possible by introducing an ultrathin Mg metal layer with a thickness of 4 A between the CoFeB bottom electrode layer and the MgO(001) tunnel barrier layer. The Mg layer was slightly but not fully oxidized, which resulted in a reduction in MR for a thicker MgO barrier (high RA) region and in an increase in MR for a thinner barrier (low RA) region. The Mg layer improves the crystalline orientation of the MgO(001) layer when the MgO(001) layer is thin. These MTJs will accelerate the realization of highly sensitive read heads for ultrahigh-density hard-disk drives.

Evaluation of Spin-Transfer Switching in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

Current-induced magnetization switching was demonstrated on Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions (MTJs), which exhibited giant tunnel magnetoresistance ratios of about 100%. Switching current density at a pulse duration of 100 ms was about 6×106 A/cm2 at room temperature. The switching current density was reduced to one-third of the smallest value for the MgO-based MTJs reported to date. Dependence of the switching current on pulse duration and on the external magnetic field was discussed based on a theoretical model incorporating thermally activated spin-transfer switching. The spin-transfer switching in the MgO-based MTJs realizes low writing power consumption and a high read-out signal in high-density magnetoresistive random access memory.

Vertical-current-induced domain-wall motion in MgO-based magnetic tunnel junctions with low current densities

In the past few years, there have been a number of proposals for fabricating magnetic memories based on the current-induced motion of magnetic domain walls. A device that uses a novel geometry for injecting electrical currents into the sample is shown to work with current densities that are two orders of magnitude lower than in previous approaches.

Ultralow resistance-area product of 0.4Ω(μm)2 and high magnetoresistance above 50% in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

An ultralow resistance-area (RA) product of 0.4Ω(μm)2 was achieved in CoFeB∕MgO∕CoFeB magnetic tunnel junctions with a high magnetoresistance ratio of 57% at room temperature. Various growth conditions for polycrystalline MgO(001) tunneling barrier were optimized to improve the crystalline orientation of the MgO(001) layer, which resulted in a significant enhancement of magnetoresistance in an ultralow RA region below 1Ω(μm)2. Removal of residual H2O molecules from a growth chamber was especially effective in improving the crystalline orientation. The present achievements will enable the development of highly sensitive read heads for ultrahigh-density hard disk drives.

Large Diode Sensitivity of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

We report on rf current-induced excitation of the ferromagnetic resonance in CoFeB/MgO/CoFeB magnetic tunnel junctions under a perpendicular magnetic field. By choosing an appropriate external field and using an Fe-rich CoFeB free layer, the effective precession of the free layer could be excited. In a measurement of homodyne detection, a large dc output voltage of 180 µV was obtained when an rf signal power of -25 dBm was applied. The sensitivity of this junction, as an rf rectifier, reaches about 170 mV/mW (280 mV/mW after impedance matching correction), which is the same order compared with that of a Schottky diode operated at room temperature.

Dependence of spin-transfer switching current on free layer thickness in Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions

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⩾2nm. It decreased to about 80% at dFree=1.5nm. Both switching currents and thermal stability were roughly proportional to dFree. The averaged intrinsic switching current density (Jc0av) was 1×107–2×107A∕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.

Effect of Ta getter on the quality of MgO tunnel barrier in the polycrystalline CoFeB∕MgO∕CoFeB magnetic tunnel junction

X-ray photoelectron spectroscopy and high-resolution Rutherford backscattering reveal that Ta getter presputtering enhances the stoichiometry and lowers the interstitial defect density of MgO barrier. This results in higher magnetoresistance ratio, 205%, of magnetic tunnel junction, compared to 46% for no Ta getter, at 1.2nm MgO thickness. Fitting yields the corresponding barrier height of the MgO of 3.0eV, which is higher compared to 2.3eV for without Ta getter. However, the tunnel junction prepared with Ta getter shows lower resistance-area product by an order of magnitude. Microstructure of MgO barrier and oxidation of bottom electrode can be attributed to the contradictory results.

Huge magnetoresistance and low junction resistance in magnetic tunnel junctions with crystalline MgO barrier

Inserting a 4 /spl Aring/-Mg metal layer between the amorphous CoFeB bottom electrode layer and the MgO barrier layer was found to be effective in realizing huge magnetoresistance effect in low-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). As a result, magnetoresistance (MR) ratio as high as 138% at resistance-area product (RA) of about 2.4 /spl Omega//spl middot//spl mu/m/sup 2/ was obtained. This value is about seven times that of state-of-the-art MTJs for magnetic sensor application. X-ray diffraction analysis clarified that crystal orientation of the poly-crystalline MgO(001) barrier layer was improved by the Mg layer. It is suggested that the higher crystalline orientation of the MgO(001) barrier layer could have enhanced the coherent tunneling of /spl Delta//sub 1/ electrons, resulting in an increase of MR ratio at the low RA (thin MgO thickness) region. The annealing temperature and free layer materials have also been optimized to satisfy the requirements for practical read head application. Although this optimization resulted in a reduction in the MR ratio to about 45%-53%, this value is still more than twice the highest MR ratio of conventional MTJs. The currently developed fabrication process will accelerate the development of highly sensitive read heads for ultrahigh-density hard-disk drives.