Yoshishige Suzuki

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.

Micromagnetic understanding of current-driven domain wall motion in patterned nanowires

In order to explain recent experiments reporting a motion of magnetic domain walls (DW) in nanowires carrying a current, we propose a modification of the spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that it explains, with reasonable parameters, the measured DW velocities as well as the variation of DW propagation field under current. We also introduce coercivity by considering rough wires. This leads to a finite DW propagation field and finite threshold current for DW propagation, hence we conclude that threshold currents are extrinsic. Some possible models that support this new term are discussed.

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 100u2009nm, 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 up to 410% at room temperature in fully epitaxial Co∕MgO∕Co magnetic tunnel junctions with bcc Co(001) electrodes

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.

High Tunnel Magnetoresistance at Room Temperature in Fully Epitaxial Fe/MgO/Fe Tunnel Junctions due to Coherent Spin-Polarized Tunneling

We fabricated fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) and observed a magneto-resistance (MR) ratio of 88% at T = 293 K (146% at T = 20 K), the highest value yet reported. The origin of the high MR ratio is not the diffusive tunneling of Jullieres model but the coherent spin-polarized tunneling in epitaxial MTJs, in which only the electrons with totally symmetric wave functions with respect to the barrier-normal axis can tunnel. The bias-voltage dependence of the MR was very small, resulting in a high output voltage of 380 mV. This high voltage will help overcome problems in the development of high-density magnetoresistive random-access-memory (MRAM).

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.

Characterization of growth and crystallization processes in CoFeB∕MgO∕CoFeB magnetic tunnel junction structure by reflective high-energy electron diffraction

We performed reflective high-energy electron diffraction observations to investigate the growth and crystallization processes of Co60Fe20B20∕MgO∕Co60Fe20B20 magnetic tunnel junction structures. A MgO layer grown on an amorphous CoFeB layer has an amorphous structure up to the MgO thickness (tMgO) of 4 monoatomic layers (ML) and begins to crystallize with (001) preferred orientation when tMgO⩾5ML. By annealing, an amorphous CoFeB layer grown on MgO(001) crystallizes in a body-centered-cubic structure with (001) orientation because MgO(001) acts as a template to crystallize CoFeB. The results give important information for understanding the mechanism of giant tunneling magnetoresistance effect in CoFeB∕MgO∕CoFeB MTJs.

Subnanosecond magnetization reversal in magnetic nanopillars by spin angular momentum transfer

Sub-ns magnetization switching has been triggered by spin momentum transfer in pulsed current in pillar shaped CoFe∕Cu∕CoFe trilayers. By analyzing the change in magneto-resistance induced after the application of individual short current pulses (100ps–10ns), we measured the probability of magnetization reversal as a function of the current pulse magnitude, polarity and duration, at various temperatures between 150 and 300K. At all studied temperatures, the reversal process can take place within a few 100ps. The energy cost of the reversal scales favorably with the switching speed and decreases in the 1pJ range when using 100ps current pulses at 300K. Significantly higher switching speeds are obtained at lower temperatures, which is opposite to a thermal activation of the reversal.

Giant tunneling magnetoresistance in fully epitaxial body-centered-cubic Co∕MgO∕Fe magnetic tunnel junctions

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.

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.