Koji Tsunekawa

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.

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.

Inelastic tunneling spectroscopy of magnetic tunnel junctions based on CoFeB∕MgO∕CoFeB with Mg insertion layer

Magnetic tunnel junctions (MTJs) based on textured MgO barriers have thus far shown the highest tunneling magnetoresistance (TMR) at room temperature. In contrast to traditional magnetic tunnel junctions, it appears that the large TMR observed in these systems arises from a type of coherent tunneling in which the symmetry of the Bloch state wave functions plays a critical role. We have fabricated MTJs with artificial asymmetric barriers by depositing a thin layer of Mg of varying thickness (0–10 A) prior to the growth of the MgO barrier into otherwise identical CoFeB∕MgO∕CoFeB MTJs. The inelastic tunnel spectrum shows magnon and phonon excitation peaks similar to traditional Al2O3 barriers, and an additional peak at about 300 meV. The conventional interpretation that this peak corresponds to density of states of the s electrons in the ferromagnetic electrodes, however, does not apply in the MgO system.

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.

Transmission electron microscopy study on the polycrystalline CoFeB/MgO/CoFeB based magnetic tunnel junction showing a high tunneling magnetoresistance, predicted in single crystal magnetic tunnel junction

We carried out a detailed transmission electron microscopy study on the polycrystalline CoFeB∕MgO∕CoFeB based magnetic tunnel junction, which shows high magnetoresistance predicted theoretically only in single crystal magnetic tunnel junction. Previously it has been suggested that the crystallinity and degree of texture of MgO tunnel barrier play a crucial role in the polycrystalline system. Here we suggest, on top of the crystallinity issue of MgO, that a grain-to-grain epitaxy of CoFeB∕MgO∕CoFeB satisfies the prerequisite for the coherent tunneling for the giant tunneling magnetoresistance theoretically predicted.

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.

Frequency-dependent magnetoresistance and magnetocapacitance properties of magnetic tunnel junctions with MgO tunnel barrier

The frequency-dependent impedance of magnetic tunnel junctions (MTJs) with MgO barriers was investigated. The capacitance of the MTJs switches from high to low when the relative electrode magnetizations change from parallel to antiparallel, opposite the resistance change. Additionally, for parallel magnetizations, the capacitance varies with temperature though resistance remains approximately constant. The low frequency resistance and the tunneling magnetoresistance are in agreement with dc values. The capacitance is found to be larger than the expected (geometrical) capacitance, in contrast to MTJs with Al2O3 barriers. These results are explained by screening due to charge and spin accumulation at the interfaces.