Yasuyoshi Miyamoto

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.

Effects of notch shape on the magnetic domain wall motion in nanowires with in-plane or perpendicular magnetic anisotropy

Magnetic domain walls (DWs) in nanowires have been extensively investigated for potential applications in spintronic devices. For the precise storage of magnetic data, the control of DW pinning and depinning is critical. Here, we report upon the micromagnetic modeling results of the DW motion behaviors in notched or anti-notched nanowires possessing in-plane magnetic anisotropy (IMA) or perpendicular-to-the-plane magnetic anisotropy (PMA). In the nanowires with IMA, the energy of the DW in nanowires with anti-notches was lower compared to that of the nanowires with normal notches. Easier DW depinning motions were observed in the anti-notched nanowires. Unlike in the IMA case, the DW energy in the nanowires with PMA was lower with normal notches. Thus, the DW was able to move faster and easier through the normal notches compared to the anti-notches in the nanowire with the PMA at the same current density.

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.

Investigations of GMR characteristics and crystal structures for Ni/sub 81/Fe/sub 19//Cu multilayers with Ar ion bombardment on interfaces

Ni/sub 81/Fe/sub 19//Cu multilayers with giant magnetoresistance were deposited by dual ion beam sputtering method, which can control the crystallite orientation and interfacial structure by adjusting preparation conditions. In this study, only a few monolayers at interfaces in Ni/sub 81/Fe/sub 19//Cu multilayers were exposed to ion bombardment to change the interfacial conditions, such as sharp interface, the local mixing and the interfacial diffusion. Ion bombardment to interfaces at restricted acceleration voltage of 160 V and reduction of recoiled Ar incidence seems to be effective for attaining preferable GMR characteristics, where clear interfaces and the best crystallinity seems to be constructed in the multilayers.

Yoke-type TMR head with front-stacked gap for perpendicular magnetic recording

This paper describes the design and performance of yoke-type read heads with spin-dependent tunneling magnetoresistive (TMR) elements. Magnetic field calculations have been carried out for a yoke-type TMR head with a front-stacked gap (Front-SG). The head has the advantage of a higher signal-to-noise ratio than that of a shielded TMR head and produces Lorentzian waveforms without any distortion in combination with perpendicular magnetic recording media.

Magnetic Properties of Magnetic Nanowires With Ultra-Small Trap Sites Fabricated by Anodic Oxidation and Nanoindentation Using Scanning Probe Microscopy

We have focused on the magnetic memories using parallel-aligned nanowires without mechanical moving parts, in order to achieve the ultra-high transfer rate of more than 72 Gbps for future ultra-high definition TV. It is important for the storage devices, where the magnetic nanowires are utilized, that the trapping energy of the magnetic domains should be optimized precisely. In order to tune this energy finely, we have fabricated ultra-small notches less than 20 nm in the z-direction of [Co/Pd] nanowires with perpendicular magnetic anisotropy by the nano-scratch, anodic oxidation and nano-indentation method, instead of the conventional notches fabricated in-plane of nanowires. We have succeeded to entrap magnetic domain walls by these trap sites on the nanowire.

Detection of hot electron current with scanning hot electron microscopy

Scanning hot electron microscopy (SHEM) has been proposed as an experimental technique which allows for detection of hot electrons emitted from a subsurface semiconductor structure, thus making it possible to obtain the spatial distribution of hot electrons in a device. Here we present the experimental evidence of SHEM operation. Hot electrons with energies of 3 eV are injected by means of a Si/CaF2/Au heterostructure and subsequently detected at the tip of a scanning tunneling microscope in the SHEM configuration. The measured hot electron current was approximately 4 pA for a tunnel current of 5 nA. These results, although still of a preliminary nature, show the potential of SHEM as a technique suitable for the visualization of electron wave effects in semiconductor structures.

Low resistance spin-valve-type current-perpendicular-to-plane giant magnetoresistance with Co75Fe25

Spin-valve-type current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) read sensors containing Co75Fe25 have been investigated. This type of sensor exhibits a MR ratio of 2.88%, which is a factor of 1.5 times larger than that of the conventional CPP-GMR sensor containing Co90Fe10 (1.98%). Three types of Co75Fe25-CPP-GMR structures were fabricated, with the following sequence of increasing etching depths: through the capping layer only (type A), through the CoFe∕Cu∕CoFe trilayer (type B) and through the IrMn antiferromagnetic layer (type C). The MR ratio increases with the etching depth, and a MR ratio of 3.31% was achieved for the type C sample, which is one of the largest reported MR ratios for low-resistance area product-CPP-GMR sensors.