Kenichi Aoshima

Spin-valve read heads with NiFe/Co/sub 90/Fe/sub 10/ layers for 5 Gbit/in/sup 2/ density recording

Successful use of a NiFe/Co/sub 90/Fe/sub 10/ bilayer as a soft magnetic free layer in spin-valve films with a GMR enhanced structure comprised of NiFe/Co/sub 90/Fe/sub 10//Cu/Co/sub 90/Fe/sub 10//FeMn is outlined. The GMR ratio of the spin-valve film with Co/sub 90/Fe/sub 10/ is over 7% and the coercivity of the free NiFe/Co/sub 90/Fe/sub 10/ bilayers is less than 5 Oe. A merged inductive/spin-valve head was fabricated with a read track-width of 1.3 /spl mu/m and a read gap-length of 0.26 /spl mu/m using spin-valve film with a Ta(50 /spl Aring/)/NiFe(45 /spl Aring/)/Co/sub 90/Fe/sub 10/(30 /spl Aring/)/Cu(32 /spl Aring/)/Fe/sub 10/(22 /spl Aring/)/FeMn(100 /spl Aring/)/Ta(100 /spl Aring/) structure and 260 /spl Aring/ thick domain control Co/sub 78/Cr/sub 10/Pt/sub 12/ magnet layers. Its read/write performance was tested on a low noise CoCr/sub 17/Pt/sub 5/Ta/sub 4/ thin film disk with an Mr/spl middot/t of 0.41 memu/cm/sup 2/ and a coercivity of 2500 Oe. There is no Barkhausen noise in the readback waveform. The result of the microtrack sensitivity profiles reveals an effective read track-width of 0.8 /spl mu/m. A normalized output for a track-width of 880 /spl mu/Vpp//spl mu/m and D/sub 50/ of 166 kFCI was obtained. Using a PRML channel, a bit error rate of less than 10/sup -8/ was obtained without error correction at a data rate of 3.3 MB/s and at a linear density of 217 kBPI on a thin film disk with an Mr/spl middot/t of 0.72 memu/cm/sup 2/. Thus, 5 Gbit/in/sup 2/ density recording with a linear density of 217 kBPI and a track density of 23 kTPI is possible.

Fe3O4 and its magnetic tunneling junctions grown by ion beam deposition

Magnetic properties of Fe3O4 and magnetic tunnel junctions with Fe3O4 bottom electrode have been investigated. Highly conductive V/Ru layers were used as an underlayer of the Fe3O4 films. The V/Ru/Fe3O4 on [110] out-of-plane oriented MgO single crystal substrate show an anisotropy and high squareness along [110] direction, while the Fe3O4 films with an underlayer of just Ru show isotropic behavior and low squareness. X-ray diffraction shows tensile stress on Fe3O4 for V/Ru/Fe3O4 samples. The anisotropy was shown to be induced by the stress. Finally, magnetic tunnel junction stacks of MgO/V/Ru/Fe3O4/AlO/CoFe/NiFe/Ru were deposited and the magnetic tunnel junctions with a junction size ranging from 2×2 μm2 to 9×9 μm2 were fabricated by optical lithography. The junctions show magnetoresistance ratios of ∼14% and no geometrical effect due to the junction size.

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.

Spin-valve sensors with domain control hard magnet layers

We have fabricated unshielded narrow-track NiFe/Cu/NiFe/FeMn spin-valve sensors with a height of 2 /spl mu/m and a track-width of less than 2 /spl mu/m using CoCrPt hard magnets (4/spl pi/Mr=5500 G, Hc=1000 Oe) as a domain control layer. Barkhausen noise was completely suppressed by a longitudinal biasing field from the CoCrPt layers. The bias state was improved by applying a strong ferromagnetic exchange coupling field of 30 Oe between the free and pinned NiFe layers through a 14 /spl Aring/-thick Cu interlayer. We have also fabricated shielded CoCrPt magnet-biased spin-valve read heads with a track-width of 1.7 /spl mu/m. The heads had no Barkhausen noise and very low crosstalk noise.

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.

NiFe/CoFeB Spin-valve Heads For Over 5 Gbit/in/sub 2/ Density Recording

This paper outlines the successful use of NiFe/(Co/sub 90/Fe/sub 10/)/sub 100-x/Bx (x=5, 10%) for soft magnetic, thermally stabilized spin-valves. The GMR effect in the NiFe/CoFeB spinvalve increases after annealing. The annealing effect of CoFeB on CoFeB/Cu interfaces is investigated by high resolution TEM-EDX analysis. Merged inductive NiFe/CoFeB spin-valve heads with a read gap length of 0.18 /spl mu/m and a write gap length of 0.28 /spl mu/m having NiO(400 /spl Aring/)/NiFe(10 /spl Aring/)/CoFeB/sub 5/(10 /spl Aring/)/Cu(32 /spl Aring/)/CoFeB/sub 5/(20 /spl Aring/)/Ta(100 /spl Aring/) spin-valve film and high Bs laminated FeZrN/NiFe top poles were fabricated. Their read/write performance were tested on a low noise CoCr/sub 19/Pt/sub 5/Ta/sub 2/Nb/sub 2/ thin-film disk with an Mrt of 0.6 memu/cm/sup 2/ and a coercivity of 2600 Oe. A normalized output per track-width of 1000 /spl mu/Vpp//spl mu/m and a 50% rolloff linear density (D/sub 50/) of 182 kFCI is obtained. This performance well exceeds the performance of 5 Gbit/in/sup 2/ spin-valve heads. The measured S/N was 26.8 dB for a spin-valve head with with a narrow read track-width of 0.68 /spl mu/m, and the feasibility of 8 G bit/in/sup 2/ recording density is studied using a 1/7 code Narrow Band PRML channel.

Effect of thin oxide capping on interlayer coupling in spin valves

We controlled interlayer coupling from ferromagnetic to antiferromagnetic by appropriately capping spin valves with thin oxides. The interlayer coupling field was -16.6 Oe at a Cu-spacer thickness of 30 /spl Aring/. The sign of coupling changed at a Cu-spacer thickness of 20 /spl Aring/. The antiferromagnetic coupling achieved in this way allowed a reduction of thickness of the Cu spacer down to 20 /spl Aring/ without loss of good magnetic and electrical properties, and this led to a significant improvement in the MR response of the spin valves. The interlayer coupling field was only +8.6 Oe even at a Cu-spacer thickness of 20 /spl Aring/. We attribute the improvement in MR response to less current shunting through the most conductive Cu layer and to enhanced specular scattering at the interface between the free and the oxide capping layer.

Epitaxial growth and characterization of Fe3O4 on Ru underlayer

The magnetic and transport properties of magnetite (Fe3O4) thin films grown epitaxially with a Ru underlayer on single crystal MgO (110) have been investigated. Epitaxial growth has been confirmed by x-ray diffraction (XRD). It has been shown that addition of a Ru underlayer affects in-plane magnetic anisotropy: Fe3O4 grown directly on MgO exhibits an in-plane anisotropy, while Fe3O4 grown on MgO/Ru does not. XRD results show that presence of Ru underlayer reduces tensile stress in the Fe3O4 film. This difference in the stress explains the difference in the anisotropy of the Fe3O4 films. Resistivity of the Fe3O4 films with and without Ru underlayer both increases at low temperatures, but no clear Verway transition at 120 K was observed.

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.