Naoki Shimidzu

Spin transfer switching in current-perpendicular-to-plane spin valve observed by magneto-optical Kerr effect using visible light

The authors have succeeded in optically observing the spin transfer switching (STS) in a current-perpendicular-to-plane spin-valve device. The device consists of three spin-valve elements, each of which comprises of a transparent top electrode, free and pinned magnetic layers separated by a copper spacer, and a bottom copper electrode. Despite a relatively large device resistance, spin transfer switching of the free layer was carried out using the spin injection from the transparent top electrode. Magneto-optical Kerr effect measurements performed through the transparent top electrode show clear changes in the signal synchronized with the resistance change as a result of the STS.

Submicron Magneto-Optical Spatial Light Modulation Device for Holographic Displays Driven by Spin-Polarized Electrons

We have proposed a spin transfer switching (STS MO-SLM) device, based on Magneto-optical (MO) spatial light-modulation and driven by spin-polarized current flow, and confirmed its basic operation and characteristics experimentally. The proposed SLM device can be operated without active-matrix devices, has a spatial resolution as small as several hundred nanometers and possesses the potential for being driven at ultra-high speed of several tens of nanoseconds. Unlike existing SLM devices, this device satisfies both the size and speed requirements of SLMs for use in displaying holographic three-dimensional (3D) moving images. To improve the light modulation characteristics of SLM device, we carried out studies on magnetic films with perpendicular magnetic anisotropy to obtain large magneto-optical signals, which enabled us to realize enhanced light modulation performance. In addition, we measured a MO signal that was about twenty-times larger than that possible with in-plane anisotropy. We conclude that the MO-SLM device that we are developing is suitable for displaying future super-high definition, holographic three-dimensional moving images.

Magneto-Optical and Spin-Transfer Switching Properties of Current-Perpendicular-to Plane Spin Valves With Perpendicular Magnetic Anisotropy

We evaluated the magneto-optical properties of spin-valve (SV) stacks with perpendicular magnets comprised of a Gd-Fe (4-30 nm) free layer and a Tb-Fe-Co/CoFe pinned layer, and the spin-transfer switching properties of current-perpendicular-to-plane (CPP) SVs with the perpendicular magnets. The Gd-Fe free layer had polar Kerr rotation thetas_k of 0.12deg at a thickness of 10 nm. The thetas_k reduced with a decrease in GdFe thickness and the sign of thetas_k switched from positive to negative at a Gd-Fe thickness of 6 nm or thinner. Coercivity (H_c) was maximum and saturation magnetization (M_s) was minimum at a thickness of 8 nm. These phenomena may be explained by an inhomogeneous Gd-Fe composition. The CPP spin valves with the Gd-Fe (10 nm) free layer had a magneto-resistance (MR) of 0.038%. The free layer of a CPP SV device was switched by a pulsed current, which exhibited intrinsic switching current densities of J_{c0_P-} _AP= -3.3times10⁷ A/cm² and J_{c0_} _AP-P= 4.3times10⁷ A/cm² with a thermal stability of 75, which is above the required value of 40. Perpendicular magnets are very useful for obtaining large magneto-optical signals from nano-magnets driven by spin-transfer switching.

Direct patterning of solution-processed organic thin-film transistor by selective control of solution wettability of polymer gate dielectric

A simple direct patterning method for solution-processable organic semiconductors (OSCs) is demonstrated. The solution-wettable and nonwettable regions of a polymer gate dielectric layer were selectively controlled by a short tetrafluoromethane gas plasma treatment, and we precisely patterned the OSC film in the desired channel region by lamination coating. The patterned OSC films represent polycrystalline structures consisting of crystalline domains varying from 30 to 60 μm, and the resulting short-channel thin-film transistor (TFT) showed a high mobility of up to 1.3 cm2/Vs, a large on/off ratio over 108, and a negligible hysteresis curve. The proposed method is scalable for patterning TFT arrays with large-area dimensions.

A Magneto-Optical Spatial Light Modulator Driven by Spin Transfer Switching for 3D Holography Applications

We have successfully fabricated a basic magneto-optical spatial light modulator (MO-SLM) device driven by spin-transfer-switching (STS), which we call Spin-SLM technology. The SLM device is comprised of a one dimensional ten-pixel light modulation array (1 ×10 pixels) with a fine pitch of 1 μm. The light modulation pixels were composed of Gd-Fe based giant magneto resistance (GMR) devices, with a free layer of Gd-Fe, an Ag spacer and a Co-Fe/Tb-Fe-Co pinned layer. The GMR devices were sandwiched by a Cu bottom and Indium zinc oxide top electrodes in order to inject current perpendicular to the film plane for switching. Incident light penetrating the transparent top electrode can be modulated due to its magneto-optic Kerr effect with the Gd-Fe free layer. The fabricated device cell size is 220×300 nm2. We confirmed successful switching of the individual free layers, which was controlled by STS. We have also fabricated magnetic hologram patterns with the same magnetic materials used in the light modulation layer of the Spin-SLM device in order to determine its feasibility in display applications. The pixel pitch of the pattern was one micron and a reconstructed image by laser light was successfully observed with a wide viewing zone angle as 38 deg, which is nearly the same value expected from calculations. Although these patterns do not have an electrode to switch the magnetization direction, we confirmed the potential of Spin-SLM technology as a display device for 3D holography applications.

Spin transfer switching of closely arranged multiple pillars with current-perpendicular-to-plane spin valves

Spin transfer switching (STS) characteristics of two closely arranged spin valve (SV) pillars sharing a pair of top and bottom electrodes were investigated. Each pillar had a 300×100nm2 rectangular shape, which was fabricated by electron beam lithography. The separation between the pillars was 300nm or 1μm. The STS curves clearly show the two-step switching of the free layer for the device with a separation of 300nm. The first switching occurred at a switching current density of a single SV pillar or below. The second switching occurred at a switching current density approximately 1.2 times the first one. Furthermore, the STS characteristics of the paired free layers were estimated by a micromagnetic simulation using the Landau–Lifshitz–Gilbert–Slonczewski equation, which showed similar switching behavior to the experimental result of the free layers switched first.

Compensation of Interference Fringe Distortion Due to Temperature Variation in Holographic Data Storage

Photopolymer materials are feasible for holographic recording media. However, these materials shrink owing to photopolymerization and interference fringes recorded in them distort. In addition, temperature variation causes shrinkage and expansion of these materials and thus distorts recorded interference fringes. This distortion degrades reconstructed image quality and decreases the signal-to-noise ratio of the reproduced data. We applied adaptive optics controlled by a genetic algorithm to compensate for the distortion and improved the reconstructed image quality at 25 and 30 ?C ambient temperature. Under these conditions, the signal-to-noise ratio of reproduced data was more than 4 dB. Furthermore, we evaluated the distortion due to the temperature variation by using a medium angle and the wavefront of the reference beam. We found that the distortion caused by anisotropic shrinkage is slight; consequently, an optimised wavefront at 25 ?C can compensate for the interference fringe distortion and increase the signal-to-noise ratio by adjusting only the medium angle even if a temperature variation occurs. Adaptive optics can thus be used to compensate for interference fringe distortion caused by shrinkage and expansion due to temperature variation.

Low-current-density spin-transfer switching in Gd22Fe78-MgO magnetic tunnel junction

Magnetization switching of a relatively thick (9 nm) Gd-Fe free layer was achieved with a low spin injection current density of 1.0 × 106 A/cm2 using MgO based magnetic tunnel junction devices, fabricated for light modulators. At about 560 × 560 nm2 in size, the devices exhibited a tunneling magnetoresistance ratio of 7%. This low-current switching is mainly attributed to thermally assisted spin-transfer switching in consequence of its thermal magnetic behavior arising from Joule heating.

Half-data-page insertion method for increasing recording density in angular multiplexing holographic memory

We have developed a method to use a half-size data page between two full-size data pages to increase the recording density in angular multiplexing holographic memory up to 1.5× as much as the conventional angular multiplexing sequence. In our recording sequence, the full- and half-size data pages are alternately multiplexed. This is because each plane wave from various points in a data page has different angular selectivity. A half-size data page has higher angular selectivity than a full-size data page. The required angular intervals were estimated by numerical simulation taking holographic medium tilt into account. Also, an angular multiplexing experiment using the half-data-page insertion method resulted in a low bit error rate of the order of 10(-3), which is sufficient for practical use.

Spin Transfer Switching and MR Properties of Co/Pt Multilayered Free Layers for Submicron Sized Magneto-Optical Light Modulation Device

Co/Pt multilayered films show strong perpendicular magnetic anisotropy and have a large magneto-optical Kerr effect in the short wavelength side. To use these films with submicron spatial light modulators driven by spin transfer switching (STS), we fabricated current perpendicular to plane giant magnetoresistance (CPP-GMR) and tunnel magnetoresistance (TMR) devices with Co/Pt multilayers for free layers and investigated the MR properties, the STS characteristics, and the Kerr effects. A Kerr hysteresis loop was clearly observed in the CPP-GMR device, which was 125 × 180 nm2. Full magnetization reversal of the Co/Pt multilayered film by spin transfer torque was demonstrated for a CPP-GMR device with a Cu-based top electrode. On the other hand, there was hardly any resistance change in a CPP-GMR with a transparent top electrode due to the low MR ratio of the device. A TMR stack with a Ag buffer layer showed a strong perpendicular magnetic anisotropy. An MR curve was detected for a TMR device with a transparent top electrode.