Nobuhiko Funabashi

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.

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.

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.

Spin transfer switching of current-perpendicular-to-plane giant magnetoresistance with various Gd-Fe free-layer compositions

We have investigated the spin transfer switching (STS) properties of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with various compositions of Gd-Fe free-layers for magneto-optical light modulator applications, whereby iron (Fe) concentrations are varied from 72.5 to 80.3 at. %. Switching current densities decreased dramatically with increases in Fe concentration within the Gd-Fe compound. The switching current density (Jc0) for a device with composition Gd Fe80.3 at.% was 20.5 MA/cm2, which is more than eight times smaller than that for devices with composition Gd Fe72.5 at. %. This reduction in switching current can be attributed to a decrease in the effective perpendicular anisotropy in Fe-richer Gd-Fe compounds.

Effects of the exchange stiffness constant and the distribution on the recording characteristics of perpendicular media

The relationship between the exchange stiffness constant of a perpendicular medium- and high-density recording characteristics was investigated by micromagnetic simulation. As the exchange stiffness increases, larger magnetic clusters are formed in the ac-erased magnetization state. The shortest recordable bit length of the medium corresponds to the cluster size observed in the ac-erased magnetization state. The slope of the magnetization loop also correlates with the average magnetic cluster size.

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.

Multilevel Transitions of Closely Arranged Spin Valve Pillars Using Spin Transfer Switching

Two closely arranged spin valve pillars sharing a pair of top and bottom electrodes were fabricated and their spin transfer switching (STS) characteristics were investigated. Each pillar was a 300 times 100 nm rectangle, such that the distance d between two pillars was varied up to 1 mum . The STS curves of a single pillar and closely arranged pillars with the distance d of 1 mum showed single-step transitions, but the STS curves of closely arranged pillars with the distance d of 0.3 mum showed clear two-step transitions. We found that the plateau width of the intermediate resistive state of the two-step transitions can be controlled by the distance >d . The basis for this stable intermediate resistive state seemed to be magnetic interactions and the intrinsic switching mode of STS.

BaM/Pt multilayered films with fine grains and large perpendicular magnetic anisotropy for high density recording media

A magnetoplumbite type of Ba ferrite (BaM) thin layer was deposited on a 9 nm thick Pt underlayer, and excellent c-axis orientation was observed even for an 8 nm thick BaM layer, which corresponds to only three or four BaM unit cells. The grain size was almost in the same range of 60–85 nm even when the BaM layer thickness tBaM decreased from 60 to 17 nm, and tBaM should be reduced below 10 nm to make a grain size smaller than 50 nm. However, the perpendicular coercivity Hc⊥ and squareness S⊥ decreased drastically from 2.6 to 0.5 kOe and from 0.6 to 0.2, respectively, with the decrease of tBaM from 60 to 8 nm because of higher demagnetizing field and susceptibility to thermal fluctuation. On the other hand, the [BaM(5–24 nm)/Pt(9 nm)]3 multilayer exhibited higher Hc⊥ and larger S⊥ than the BaM/Pt bilayer for the same BaM layer thickness and Hc⊥ and S⊥ of the [BaM(8 nm)/Pt(9 nm)]3 multilayer was 2.0 kOe and 0.6, respectively. It was clarified that the deposition of the BaM/Pt multilayer was very effective ...

Increase of coercivity and squareness ratio of by ferrite thin films by adding SiO/sub 2/ and substituting Al for Fe

Simple Ba ferrite (BaM) and SiO/sub 2/-added BaM (BaM:SiO/sub 2/) and Al substituted BaM (Al-BaM) films were deposited using the facing targets sputtering apparatus on SiO/sub x//Si wafers with Pt seed layers to enhance the c-axis orientation perpendicular to the film plane. Magnetic characteristics of the three kinds of BaM films were investigated and compared. Perpendicular coercivity H/sub c/spl perp// and squareness ratio S/sub /spl perp// of BaM:SiO/sub 2/ films increased to 4.2 kOe and 0.83, respectively, H/sub c/spl perp// and S/sub /spl perp// of Al-BaM films increased to 3.2 kOe and 0.89, but saturation magnetization M/sub s/ decreased to 2.9 kc. The magnetization reversal mechanism of BaM:SiO/sub 2/ and BaM films is not in coherent rotation mode but seems to be in the curling mode. The reason for high H/sub c/spl perp// of BaM:SiO/sub 2/ films is different from that of Al-BaM ones.

Spin transfer switching of current-perpendicular-to-plane giant magnetoresistance devices with Gd―Fe free layers and Ag-spacer materials for light modulator applications

We investigated how the magneto-optical (MO) and magnetic properties of Gd–Fe-based giant magneto resistance (GMR) films with silver (Ag) spacers are affected using two different free-layer compositions, Gd21.7Fe78.3 and Gd27.5Fe72.5 (at. %). GMR films using Ag spacers exhibited a MO Kerr rotation (θK) that was about 10% larger than that with copper (Cu) spacers. Moreover, GMRs with Gd21.7Fe78.3 free layers exhibited larger θK and saturation magnetization (Ms) values than did those with Gd27.5Fe72.5 free layers. Current-perpendicular-to-plane (CPP) GMR devices using Gd21.7Fe78.3 and Gd27.5Fe72.5 free layers with Ag spacers were fabricated and employed in order to investigate the device’s spin transfer switching characteristics. The switching current density for CPP-GMR devices using Gd21.7Fe78.3 free layers was reduced by 26% compared to Gd27.5Fe72.5 free layers. The larger values of Ms exhibited by Gd21.7Fe78.3 layers may be attributed to a reduction of switching current. Ultimately, we found that CPP-GM...