Masahiko Ichimura

Current-Induced Magnetization Switching in MgO Barrier Magnetic Tunnel Junctions With CoFeB-Based Synthetic Ferrimagnetic Free Layers

We investigated the effect of using a synthetic ferrimagnetic (SyF) free layer in MgO-based magnetic tunnel junctions (MTJs) on current-induced magnetization switching (CIMS), particularly for application to spin-transfer torque random access memory (SPRAM). The employed SyF free layer had a Co₄₀Fe₄₀B₂₀/Ru/Co₄₀Fe₄₀B₂₀and Co₂₀Fe₆₀B₂₀/Ru/Co₂₀Fe₆₀B₂₀structures, and the MTJs (100 times (150-300) nm²) were annealed at 300 ^degC. The use of SyF free layer resulted in low intrinsic critical current density (<i>J</i> _c0) without degrading the thermal-stability factor (<i>E</i>/<i>k</i> _B <i>T</i>, where <i>E</i>, <i>k</i> _B, and <i>T</i> are the energy potential, the Boltzmann constant, and temperature, respectively). When the two CoFeB layers of a strongly antiferromagnetically coupled SyF free layer had the same thickness, <i>J</i> _c0 was reduced to 2-4 times10⁶ A/cm². This low <i>J</i> _c0 may be due to the decreased effective volume under the large spin accumulation at the CoFeB/Ru. The <i>E</i>/<i>k</i> _B <i>T</i> was over 60, resulting in a retention time of over ten years and suppression of the write current dispersion for SPRAM. The use of the SyF free layer also resulted in a bistable (parallel/antiparallel) magnetization configuration at zero field, enabling the realization of CIMS without the need to apply external fields to compensate for the offset field.

Spin transfer torque in magnetic tunnel junctions with synthetic ferrimagnetic layers

Based on the spin-polarized free-electron model, spin and charge transports are analyzed in the magnetic tunnel junctions with the synthetic ferrimagnetic layers in the ballistic regime, and the spin-transfer torque is derived. In the realistic junctions, the spin torque exerted on the magnetizations of two ferromagnetic layers in the synthetic ferrimagnetic layer shows a trend to rotate the same direction. It is suggested that, through the antiferromagnetic interlayer coupling in the synthetic ferrimagnetic layer, this trend induces the cooperative reversal of magnetizations in two ferromagnetic layers, and expected that this cooperative rotation reduces the critical current for the magnetization reversal in the synthetic ferrimagnetic layer.

Domain-wall displacement triggered by an ac current below threshold

It is theoretically demonstrated that a displacement of a pinned domain wall, typically of order of μm, can be driven by use of an ac current which is below threshold value. The point here is that finite motion around the pinning center by a low current is enhanced significantly by the resonance if the frequency is tuned close to the pinning frequency as demonstrated by recent experiment.

Theoretical Study of Ga Adsorbates around Dangling-Bond Wires on an H-Terminated Si Surface: Possibility of Atomic-Scale Ferromagnets

By using first-principles calculations within the local density functional approach, we have examined the behavior of Ga adsorbates around a dangling-bond wire, which is constructed by extracting H atoms from the H-terminated Si(100)2×1 surface. In addition to clarifying the stable atomic arrangements and the electronic structures of the Ga adsorbates, we found strong evidence of a possible ferromagnetic ground state of the Ga adsorbate wire in specific atomic arrangements. Appearance of the ferromagnetic ground state has been supported by a preliminary calculation of magnetic moments using a simple tight-binding model and the mean-field approximation. The result of this investigation is expected to provide a new way to obtain atomic-scale ferromagnets produced from only nonmagnetic elements.

FERROMAGNETISM IN A HUBBARD MODEL FOR AN ATOMIC QUANTUM WIRE : A REALIZATION OF FLAT-BAND MAGNETISM FROM EVEN-MEMBERED RINGS

We have examined a Hubbard model on a chain of squares, which was proposed by Yajima et al as a model of an atomic quantum wire As/Si(100), to show that the flat-band ferromagnetism according to a kind of Mielke-Tasaki mechanism should be realized for an appropriate band filling in such a non-frustrated lattice. Reflecting the fact that the flat band is not a bottom one, the ferromagnetism vanishes, rather than intensified, as the Hubbard U is increased. The exact diagonalization method is used to show that the critical value of U is in a realistic range. We also discussed the robustness of the magnetism against the degradation of the flatness of the band.

The origin of bias-voltage dependence in CoFe/SrTiO3/La0.7Sr0.3MnO3 magnetic tunnel junctions

The relationship between the oxidized state in a barrier and the bias-voltage dependence on tunnel magnetoresistance (TMR) effect was examined in Co90Fe10/SrTiO3(STO)/La0.7Sr0.3MnO3(LSMO) magnetic tunnel junctions using single crystalline LSMO electrode and STO barrier. A TMR junction, whose STO barrier was fabricated stoichiometrically, exhibited an asymmetric dependence with regard to the bias direction. However, when the STO barrier had an oxygen deficiency, the TMR ratio was reduced symmetrically. The oxygen deficiency of the STO barrier is probably the cause of the oxygen deficiency in the interfacial LSMO and the electron scattering in the barrier, and they may reduce the symmetric bias-voltage dependence.

Helical states vs soliton and vortex lattice states in the two-dimensional Hubbard model

We study the spin and charge textures of the two-dimensional Hubbard model for a wide range of electron filling at the ground state both on a square lattice and on a triangular lattice within a mean field approximation. We complete the overall phase diagram of the single-Q helical spin density wave state, which spreads widely in the electron filling and the Coulomb interaction plane, and discuss the relative stability of the soliton and the vortex lattice states on it. It is shown that the imperfect nesting condition, which is the intrinsic feature of two-dimensional systems, induces a rich variety of self-consistent solutions including collinear and noncollinear spin textures.

Voltage-induced magnetization dynamics in CoFeB/MgO/CoFeB magnetic tunnel junctions

Recent progress in magnetic tunnel junctions (MTJs) with a perpendicular easy axis consisting of CoFeB and MgO stacking structures has shown that magnetization dynamics are induced due to voltage-controlled magnetic anisotropy (VCMA), which will potentially lead to future low-power-consumption information technology. For manipulating magnetizations in MTJs by applying voltage, it is necessary to understand the coupled magnetization motion of two magnetic (recording and reference) layers. In this report, we focus on the magnetization motion of two magnetic layers in MTJs consisting of top layers with an in-plane easy axis and bottom layers with a perpendicular easy axis, both having perpendicular magnetic anisotropy. According to rectified voltage (Vrec) measurements, the amplitude of the magnetization motion depends on the initial angles of the magnetizations with respect to the VCMA direction. Our numerical simulations involving the micromagnetic method based on the Landau-Lifshitz-Gilbert equation of motion indicate that the magnetization motion in both layers is induced by a combination of VCMA and transferred angular momentum, even though the magnetic easy axes of the two layers are different. Our study will lead to the development of voltage-controlled MTJs having perpendicular magnetic anisotropy by controlling the initial angle between magnetizations and VCMA directions.

Current-driven magnetization reversal in exchange-biased spin-valve nanopillars

We have investigated the current-driven magnetization reversal of exchange-biased spin-valve giant magnetoresistive nanopillars with a magnetically pinned ferromagnetic layer. Current-driven magnetization reversal of a ferromagnetic layer with a smaller MV (M: magnetization, V: volume) value is found to take place even when the layer is pinned by the exchange bias induced by an antiferromagnet. The critical current density Jc of the spin-valve nanopillar with a MnIr layer adjacent to the current-driven free layer is of the same order as that of a Co/Cu/Co nanopillar (∼107A∕cm2).

Magnetic and electronic structures of finite-size systems of Fe/Au(n)

Abstract Within local-spin density approximation, the electronic and magnetic structures of the finite-size systems of Fe/Au(n) (n = 2–12) are studied. These systems consist of n layers of Au possessing the interface with Fe monolayer on one side and the free surface on the other. The magnetic moment at the surface Au layer rapidly decreases with the increasing number of Au layers. The decreasing behavior of the surface Au layer is quite different from that in the inner Au layers. With increasing number of Au layers, the magnetic moment of Fe monolayer shows non-monotonic change with size of 0.1 μB.