K. Ando

Ultrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctions

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

Influence of perpendicular magnetic anisotropy on spin-transfer switching current in CoFeB∕MgO∕CoFeB magnetic tunnel junctions

We investigated the spin-torque diode effect in submicron-scale Co60Fe20B20∕MgO∕(CoxFe1−x)80B20 (0⩽x⩽0.9) magnetic tunnel junctions (MTJs) under perpendicular magnetic fields Hext up to 10kOe. A single peak was clearly observed in every spin-torque diode spectrum and the dependence of resonant frequency fres on Hext was well explained by using Kittel’s formula. It was found that effective demagnetizing fields in the perpendicular-to-plane direction of the Fe-rich CoFeB free layers obtained from the spectra were considerably smaller than those expected from the magnetizations of the free layers. This suggested that the Fe-rich CoFeB free layers exhibited a perpendicular magnetic anisotropy, which agreed well with the reduced switching current density in the MTJs.

Magneto-optical studies of s,p–d exchange interactions in GaN:Mn with room-temperature ferromagnetism

Magnetic circular dichroism (MCD) spectra of a room-temperature ferromagnetic GaN:Mn film show that the film contains Ga1−xMnxN in which the band s,p-electrons interact with the d-electrons of the manganese ions. Both the magnetic field dependence and the temperature dependence of the MCD intensity show that Ga1−xMnxN is a paramagnetic diluted magnetic semiconductor (DMS). The ferromagnetism of the sample arises from an unidentified material that is not detected by the x-ray diffraction. This ferromagnetic material does not show the MCD signal, indicating that its electronic structure is different from that of Ga1−xMnxN. These results show that magneto-optical spectroscopy is indispensable to confirm the intrinsic ferromagnetism of DMS.

Origin of the anomalous magnetic circular dichroism spectral shape in ferromagnetic Ga(1-x)MnxAs: impurity bands inside the band gap.

The electronic structure of a prototype dilute magnetic semiconductor (DMS), Ga(1-x)MnxAs, is studied by magnetic circular dichroism (MCD) spectroscopy. We prove that the optical transitions originated from impurity bands cause the strong positive MCD background. The MCD signal due to the E0 transition from the valence band to the conduction band is negative indicating that the p-d exchange interactions between the p carriers and d spin is antiferromagnetic. The negative E0 MCD signal also indicates that the hole doping of the valence band is not so large as previously assumed. The impurity bands seem to play important roles for the ferromagnetism of Ga(1-x)MnxAs.

Phase locking of vortex based spin transfer oscillators to a microwave current

Phase locking experiments on vortex based spin transfer oscillators with an external microwave current are realized. We present clear evidence of synchronization, i.e., phase locking, frequency pulling, as well as fractional synchronization in this system with a minimum peak linewidth of only 3 kHz in the locked state. Large locking ranges are achieved (up to 1/3 of the oscillator frequency) allowing us to demonstrate the simultaneous phase locking of two independent oscillators connected in series with the external source.

Spin-dependent tunneling spectroscopy in single-crystal Fe∕MgO∕Fe tunnel junctions

We report a detailed spin-dependent tunneling spectroscopy in single-crystal Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) that show a giant tunnel magnetoresistance effect. Spectra for antiparallel magnetic configurations show asymmetry because of extrinsic electron scatterings caused by structural defects at the barrier/electrode interfaces. Surprisingly, spectra for parallel magnetic configurations exhibit a complex oscillatory structure that has never been observed in conventional MTJs with an aluminum-oxide tunnel barrier. The complex spectra reflect the tunneling process via interface resonant states. These results provide some information that helps to elucidate the physics of spin-dependent electron tunneling and to further enhance magnetoresistance.

Complete magneto-optical waveguide mode conversion in Cd1−xMnxTe waveguide on GaAs substrate

Complete magneto-optical mode conversion was attained in a waveguide of diluted magnetic semiconductor Cd1−xMnxTe grown on GaAs substrate. Mode conversion ratio 98%±2% under a magnetic field of 5 kG was achieved in the waveguide with graded-refractive-index clad layer. The Cd1−xMnxTe waveguide showed an optical loss below 1 dB/cm, and a high magneto-optical figure-of-merit, 200 deg/dB/kG at λ=730 nm. High efficiency magneto-optical mode conversion in a waveguide grown on a semiconductor substrate shows the feasibility of monolithical integration of an optical isolator with semiconductor optoelectronic devices.

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.

Magnetization-dependent loss in an (Al,Ga)As optical waveguide with an embedded Fe micromagnet

The dependence of waveguiding loss on the magnetization of a Fe micromagnet embedded into the (Al,Ga)As optical waveguide was examined as a possible readout method for the spin-photon memory. The optical detection of the magnetization direction of a Fe micromagnet was demonstrated for the micromagnet sizes of 3 microm x 4 microm and 3 microm x 8 microm with signal-to-noise ratios of 4.8 and 6 dB, respectively. In the case of smaller sizes, the use of spin injection from the micromagnet into a semiconductor optical amplifier was proposed for the optical detection of the magnetization.

Isolation effect in ferromagnetic-metal/semiconductor hybrid optical waveguide

The isolation effect in a ferromagnetic-metal/semiconductor hybrid optical waveguide was experimentally studied. Optical transmission in a Ga1−xAlxAs waveguide covered by Co was found to depend on the magnetization of the Co. The isolation direction was different for a waveguide with a SiO2 buffer layer and for a waveguide with a Ga1−xAlxAs buffer layer used between the waveguide core layer and Co layer. The physical origin of the isolation in this isolator structure was clarified.The isolation effect in a ferromagnetic-metal/semiconductor hybrid optical waveguide was experimentally studied. Optical transmission in a Ga1−xAlxAs waveguide covered by Co was found to depend on the magnetization of the Co. The isolation direction was different for a waveguide with a SiO2 buffer layer and for a waveguide with a Ga1−xAlxAs buffer layer used between the waveguide core layer and Co layer. The physical origin of the isolation in this isolator structure was clarified.