Katsuhiko Koi

The applicability of CPP-GMR heads for magnetic recording

In this paper, we mainly discuss the extendability of current perpendicular to plane giant magnetoresistive (CPP-GMR) heads beyond 100 Gbpsi by using micromagnetic simulation and simulation techniques. An area map of CPP-GMRs resistance-area product (RA) and magnetoresistive ratio for several areal densities is made, which suggests that downsizing of a CPP-GMR head does not cause a decrease in output voltage differently from a current in plane GMR (CIP-GMR) head. At this point, a CPP-GMR has the advantage of extendability to a higher areal density over CIP-GMR, and is the major candidate for the next generation. A CIP-GMR seems to have a scalability limit around 100-200 Gbpsi. On the other hand, the area map also suggests that the RA of the CPP-GMR is a more important keyfactor for higher areal density than for CIP-GMR. The CPP-GMR, therefore, requires not only higher MR, but also less RA as areal density gets higher. Tunneling MR (TMR) has exactly the same problem, so CPP-GMR also has many advantages over TMR, with too high an RA. Additionally, we present CPP-GMR films with the potential for around 500 Gbpsi and discuss a suitable read-head structure for CPP-GMR. Micromagnetic simulation results indicate that the read track width will be controlled by a magnetic field due to the sense current of CPP. We will discuss overall studies on scalability of CPP-GMR.

MR ratio enhancement by NOL current-confined-path structures in CPP spin valves

We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 m/spl Omega//spl mu/m/sup 2/, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 m/spl Omega//spl mu/m/sup 2/. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 m/spl Omega//spl mu/m/sup 2/. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording.

Output enhancement of spin-valve giant magnetoresistance in current-perpendicular-to-plane geometry

In this work, we present a suitable material for metal-based spin-valve in current-perpendicular-to-plane (CPP) geometry. The AΔR (A is the element size and ΔR is the change in resistance) was investigated for three kinds of free and pinned layers material, that is, Co90Fe10, Fe50Co50, and Fe50Co50 with half-atomic Cu layers. When the free and pinned layers are 5 nm, AΔR is 1.0 mΩ μm2, 1.6 mΩ μm2, and 2.9 mΩ μm2, respectively. Moreover, the dual-type spin valve having Fe50Co50 with half-atomic Cu layers achieved 5.2 mΩ μm2. According to the free and pinned layers thickness dependence of AΔR, the spin-dependent resistance at the interfaces between ferromagnetic layers and spacer Cu is enhanced mainly by changing from Co90Fe10 to Fe50Co50, and the spin-dependent bulk resistance in free and pinned layers is enlarged by inserting half-atomic Cu layers.

Specular spin-valve films with an FeCo nano-oxide layer by ion-assisted oxidation

We compared the specular spin-valve films with an Fe50Co50 nano-oxide layer (NOL) and a Co90Fe10 NOL in a pinned layer, prepared by natural oxidation (NO) and ion-assisted oxidation (IAO). For the IAO, an Ar-ion beam was used for the energy-assist effect during the oxidation, resulting in thermally stable NOL formation. With small oxygen exposures during the oxidation for the Fe50Co50 NOL by IAO, good ferromagnetic coupling through the NOL and high specularity at the NOL interface were concurrently obtained. Moreover, twisted coupling through the NOL was observed for the Fe50Co50 NOL by IAO for higher oxygen exposures. On the other hand, the NO did not cause large magnetoresistance (MR) enhancement for either the Co90Fe10 or Fe50Co50 NOLs, and the Co90Fe10 NOL by IAO caused weak magnetic coupling through the NOL, resulting in a small MR ratio. The Fe50Co50 NOL for small oxygen exposures is a good candidate for a final specular spin-valve film head for 100-Giga-bit per square inch recording.

Giant magnetoresistance in ion beam deposited spin-valve films with specular enhancement

Three different techniques, natural oxidation, remote plasma oxidation and low energy ion beam oxidation, have been proved to be equally effective in forming nano-oxide layers (NOLs) in spin-valve films for specular enhancement of giant magnetoresistance (GMR) effect. GMR values over 12% have been routinely obtained in spin-valve films with NOL, corresponding to a 30% specular enhancement over those without NOL. The consistency and robustness of the oxidation processes has been demonstrated by a very large GMR value ∼19% in a dual spin-valve film with the NOLs formed in both pinned layers, the oscillatory dependence of the interlayer coupling field on Cu layer thickness in specular enhanced spin-valve films and the uniform and repeatable film performance over 5 in. substrates.

Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance

We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA=380mΩμm2, ΔRA=16mΩμm2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous o...

Saturation magnetostriction of an ultrathin CoFe free-layer on double-layered underlayers

We investigated the saturation magnetostriction (λs) of an ultrathin CoFe free-layer on double-layered underlayers of Ru/Cu. By increasing the Ru underlayer thickness, λs becomes positively large; on the other hand, λs becomes negatively large by increasing the Cu underlayer thickness. This tendency can be explained by the model where λs is changed by the film strain. The λs sensitivity on the Cu underlayer thickness becomes large with increasing the Co concentration of the CoFe free-layer. This indicates that CoFe λs becomes sensitive to strain with increasing Co concentration. In order to realize small magnetostriction spin-valve films with an ultrathin CoFe free-layer, we have to take care of the lattice strain of the film.

Spin-filter spin-valve films with an ultrathin CoFe free layer

The concept of spin-filter spin-valve (SFSV) films is reviewed. The dependence of the free layer structure on the spin-filter effect was investigated by using model films and two types of free layers were compared, a single CoFe layer and a conventional NiFe/Co free layer. At the same magnetic thickness of the free layer, the SFSV films with a CoFe free layer showed a larger magnetoresistance (MR) ratio than those with a NiFe/Co free layer. This is partly attributed to the thinner CoFe free layer thickness, which is due to the fact that the CoFe free layer has higher Bs than the NiFe/Co free layer. Moreover, SFSV films with a CoFe free layer still showed a larger MR ratio when the free layer thickness was the same. It suggests that other factors contribute to the high MR performance, such as the quality of the interface between a free layer and a high conductance layer. Film performance of MR 9% to 10%, ΔRs 1.5–2.0 Ω, Hcea∼3 Oe, and λs⩽±0.5 ppm was obtained with a single CoFe free layer and synthetic anti...

Analysis of Effective Field Gradient in Microwave-Assisted Magnetic Recording

Microwave-assisted magnetic recording (MAMR) is one of the promising techniques for future high recording density hard disk drives. In this paper, we estimate the effective field gradient in MAMR using the relation between recorded transition length πa and field gradient by dynamic write process simulation. The effective field gradients reach 600 and 700 Oe/nm on each optimum field generation layer (FGL)-position and FGL oscillation frequency when the microwave field strength (c - Hac) are 700 and 950 Oe, respectively. These results indicate extremely high linear density would be possible by MAMR.

MR enhancement of CPP-GMR by CCP-NOL spacer and Fe/sub 50/Co/sub 50/ magnetic layers

This paper reports that further increase of magnetoresistance (MR) ratio to 6.5/spl sim/8.5% is achieved by using Fe/sub 50/Co/sub 50/ magnetic layers, in addition to current-confined-path(CCP)-nanooxide layer(NOL). The CCP-CPP structure is composed of Ta/Ru/PtMn/Co/sub 90/Fe/sub 10//Ru/pinned layer/NOL spacer/free layer/Cu/Ta cap. Using Fe/sub 50/Co/sub 50//Cu as pinned and free layers, an MR ratio of 6.5/spl sim/8.5% is obtained at area resistance (RA)=300/spl sim/1000 m/spl Omega//spl mu/m/sup 2/ and if Co/sub 90/Fe/sub 10/ is used, MR ratio of 3/spl sim/4% is shown at RA/spl sim/300 m/spl Omega//spl mu/m/sup 2/. The increase of MR ratio is explained by increase of spin dependent scattering at the interface of Fe/sub 50/Co/sub 50/ and Cu metal path where current is concentrated. Conduction examination shows that there is a resistance increased caused by joule heating effect with increasing voltage, indicating that great majority of current flows in metal paths. The origin of MR comes from nano-constricted metal path regions which dimension is less than the mean free path of electron.