Hiroko Arai

Spin-Torque Oscillator Based on Magnetic Tunnel Junction with a Perpendicularly Magnetized Free Layer and In-Plane Magnetized Polarizer

We fabricated a spin-torque oscillator (STO) having a nanopillar-shaped magnetic tunnel junction with perpendicularly magnetized FeB free and in-plane magnetized CoFeB reference layers. The perpendicular magnetization of the FeB was stabilized by strong perpendicular magnetic anisotropy induced at both the MgO tunnel barrier/FeB and FeB/MgO cap interfaces. Under a perpendicular field (3 kOe), the STO exhibited a large emission power (0.55 µW), a high frequency (6.3 GHz) and a high Q factor (135) simultaneously, all of which are the largest to date among nanopillar-shaped STOs. The bias voltage dependence of the oscillation property was well explained by the macrospin model.

Future prospects of MRAM technologies

This paper presents a review and future prospects for the tunnel magnetoresistance (TMR) effect in magnetic tunnel junction (MTJ) and spin manipulation technologies such as spin-transfer torque (STT) for magnetoresistive random access memory (MRAM). Major challenges for ultrahigh-density STT-MRAM with perpendicular magnetization and novel functional devices related to MRAM are discussed.

High Q factor over 3000 due to out-of-plane precession in nano-contact spin-torque oscillator based on magnetic tunnel junctions

We investigated microwave oscillation in a sombrero-shaped nano-contact spin-torque oscillator under an external magnetic field with an out-of-plane component. By increasing the out-of-plane field, we observed an abrupt change from in-plane precession to out-of-plane precession around the effective demagnetization field of the free layer. The out-of-plane precession under optimized conditions yielded not only a fundamental oscillation peak with a very large Q factor (3200) but also higher-order harmonic peaks with frequencies up to at least 45 GHz. A high Q factor and high-frequency oscillations are of great importance for device applications such as telecommunications, radar sensors, and high-speed magnetic field sensors.

High emission power and Q factor in spin torque vortex oscillator consisting of FeB free layer

Microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer were investigated. Because of a high MR ratio and large DC current, a high emission power up to 3.6 µW was attained in the STVO with a thin FeB free layer of 3 nm. In STOs with a thicker FeB layer, e.g., 10 nm thick, we obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW. Such excellent microwave performance is a breakthrough for the mutual phase locking of STVOs by electrical coupling.

Many-body theory of core-level photoemission by Keldysh Green's functions

Abstract The many-body theory of X-ray photoelectron diffraction (XPD) spectra is developed by using Keldysh’s non-equilibrium Green’s functions, which was basically applied by Caroli et al. We have applied skeleton diagram expansion in terms of dressed one-electron Green’s function G and the screened Coulomb interaction W. The damping photoelectron wave functions under the influence of the optical potential play a central role in the present XPD theory. On the basis of the present theoretical framework, we provide detailed discussion on Debye–Waller and Franck–Condon factors in XPD spectra at finite temperature, resonance effects in particular multiatom resonant photoemission (MARPE) effects, and also on the loss effects. The XPD spectra are simply described by a product of the Franck–Condon factor and the XPD intensity including the Debye–Waller factor. This result is in contrast to the X-ray absorption fine structure (XAFS) where the Franck–Condon factor is smeared out by a convolution product. We have discussed the conditions which give rise to prominent MARPE spectra. In particular, symmetry lowering plays an important role giving rise to prominent MARPE spectra. The angular dependence provides us with useful information on near neighbor atomic arrangement. Some model calculations clarify the basic physical processes of MARPE spectra. To discuss the loss spectra, we stress the importance of the interference effects between the intrinsic and extrinsic losses. The explicit formulas, which include both inelastic and elastic scatterings, are newly derived in the present theoretical framework, and enable us to calculate an XPD pattern associated with losses.

High power all-metal spin torque oscillator using full Heusler Co2(Fe,Mn)Si

We showed the high rf power (Pout) emission from an all-metal spin torque oscillator (STO) with a Co2Fe0.4Mn0.6Si (CFMS)/Ag/CFMS giant magnetoresistance (GMR) stack, which was attributable to the large GMR effect thanks to the highly spin-polarized CFMS. The oscillation spectra were measured by varying the magnetic field direction, and the perpendicular magnetic field was effective to increase Pout and the Q factor. We simultaneously achieved a high output efficiency of 0.013%, a high Q of 1124, and large frequency tunability. CFMS-based all-metal STO is promising for overcoming the difficulties that conventional STOs are confronted with.

Importance of radiation field screening in XPS and XAFS spectra

Abstract In this work the importance of radiation field screening is stressed in Keldysh’s Green’s function theory to study X-ray photoelectron spectroscopy (XPS) spectra and X-ray absorption fine structure (XAFS). So far these terms have been supposed to be small, however, we have found that these effects play a crucial role to analyze resonance effects. In particular the importance of these vertex correction is important in the analyses of multiatom resonant photoemission (MARPE) spectra and the resonance effects in XAFS. We also investigate the contribution from polarized part Wp of the screened Coulomb interaction W to the resonance effects. We find that W≈v is quite a good approximation. Off the resonance these radiation field screening diagrams are not important. They have no contribution to loss processes even though they include W> and W

Critical Field of Spin Torque Oscillator with Perpendicularly Magnetized Free Layer

The oscillation properties of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are studied based on an analysis of the energy balance between spin torque and damping. The critical value of an external magnetic field applied normal to the film plane is found, below which the controllable range of the oscillation frequency by the current is suppressed. The value of the critical field depends on the magnetic anisotropy, the saturation magnetization, and the spin torque parameter.

Damping parameter and interfacial perpendicular magnetic anisotropy of FeB nanopillar sandwiched between MgO barrier and cap layers in magnetic tunnel junctions

We systematically investigated the Gilbert damping (α) and interfacial perpendicular magnetic anisotropy (Ki) in magnetic tunnel junctions with a MgO-barrier/FeB/MgO-cap layered structure using the spin-torque diode effect. By increasing the MgO cap thickness, α decreased, whereas Ki increased monotonically. Values down to 0.0054 for α and up to 3.3 erg/cm2 for Ki were obtained for a MgO cap thickness of 0.6 nm. The small α and large Ki suggest that MgO-capped FeB is a suitable free layer for spintronics devices such as spin-torque oscillators and spin-torque magnetic random access memories.

Observations of thermally excited ferromagnetic resonance on spin torque oscillators having a perpendicularly magnetized free layer

Measurements of thermally excited ferromagnetic resonance were performed on spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer (abbreviated as PMF-STO in the following) for the purpose of obtaining magnetic properties in the PMF-STO structure. The measured spectra clearly showed a large main peak and multiple smaller peaks on the high frequency side. A Lorentzian fit on the main peak yielded Gilbert damping factor of 0.0041. The observed peaks moved in proportion to the out-of-plane bias field. From the slope of the main peak frequency as a function of the bias field, Lande g factor was estimated to be about 2.13. The mode intervals showed a clear dependence on the diameter of the PMF-STOs, i.e., intervals are larger for a smaller diameter. These results suggest that the observed peaks should correspond to eigenmodes of lateral spin wave resonance in the perpendicularly magnetized free layer.