Bottom Spin Valves Research Articles

Exchange-biased spin valves combining a high magnetoresistance ratio with soft-magnetic behavior

We report on the preparation of bottom spin valves combining a high giant magnetoresistance effect (above 10%) with a soft-magnetic behavior (coercivities of a few hundreds of A/m). By optimization of film composition and preparation conditions, it is possible to obtain materials having magnetoresistance values around 16%, which display minor loop coercivities of 0.2 kA/m, and sensitivities of 11%/kA/m.

Study of the Underlayer of a PdPtMn Spin-Valve Film

We investigated the PdPtMn bottom spin-valve, which had an high resistivity NiFeCr underlayer. The MR ratio of the NiFeCr-underlayer spin-valve became 1.14 times higher than that of a NiFe-underlayer, because the shunt loss of the underlayer was reduced, and the Hua of the pinned layer also became higher. We found by X-ray diffraction that a PdPtMn spin-valve with a Ta/NiFeCr underlayer had a strong (111) texture. Cross-sectional TEM observation revealed that PdPtMn on a NiFeCr underlayer had a larger grain than PdPtMn on a Ta underlayer. The initial layer of the PdPtMn with the NiFeCr underlayer had a stronger (111) orientation than the PdPtMn with the Ta underlayer. To achieve a large Hua, PdPtMn with a (111) texture and large grain size is required. We simulated the R-H curves of the spin-valve in the head shape. The shift in the quiescent operating point on the R-H curve was smaller for an NiFeCr underlayer than for an NiFe one.

Oxygen as a surfactant in the growth of giant magnetoresistance spin valves

We have found a novel method for increasing the giant magnetoresistance (GMR) of Co/Cu spin valves with the use of oxygen. Surprisingly, spin valves with the largest GMR are not produced in the best vacuum. Introducing 5×10−9 Torr (7×10−7 Pa) into our ultrahigh vacuum deposition chamber during spin-valve growth increases the GMR, decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of the spin valves. It appears that the oxygen may act as a surfactant during film growth to suppress defects and to create a surface which scatters electrons more specularly. Using this technique, bottom spin valves and symmetric spin valves with GMR values of 19.0% and 24.8%, respectively, have been produced. These are the largest values ever reported for such structures.

Ion beam deposition and structural characterization of GMR spin valves

NiO exchange-biased "bottom" spin valves of the type NiO/NiFe/Co/Cu/Co/NiFe and FeMn exchange-biased "top" spin valves of the type NiFe/Co/Cu/Co/NiFe/FeMn were deposited by ion-beam deposition (except the NiO layer). Their magnetic properties, magneto-transport, and microstructures are characterized and compared with corresponding GMR spin valves deposited by dc magnetron sputtering. High-resolution cross-sectional transmission electron microscopy and X-ray diffraction reveal microstructural differences between ion-beam-deposited and dc magnetron sputtered spin valves. In particular, film texture, surface morphology, GMR ratio, exchange bias, interlayer coupling strength, and coercivity vary widely, but property-structure-processing correlation can be identified. A GMR ratio of /spl sim/9.7% was obtained on random textured NiO exchange-biased bottom spin valves by ion-beam deposition.

Specular electron scattering in giant magnetoresistance spin valves

Specular scattering of electrons at the top surface of bottom spin valves has been found to contribute to the giant magnetoresistance effect. The specular scattering can be increased by the deposition of /spl sim/2 monolayers (ML) of Au, Ag, or Cu, with the result that the GMR increases and the R/sub /spl square// decreases. The deposition of 2 ML of species that disorder the surface, such as Ta, Si, C, or Ni/sub 80/Fe/sub 20/, suppresses the specular scattering. This suppression appears to be useful as a new method for evaluating the importance of specular scattering.

Growth of giant magnetoresistance spin valves using Pb and Au as surfactants

We have investigated the use of Pb and Au as surfactants in an attempt to achieve smoother and sharper interfaces in three types of giant magnetoresistance (GMR) spin valve multilayers: symmetric spin valves, bottom spin valves, and top spin valves. The coupling fields are reduced by a factor of 10 for symmetric and bottom spin valves and by a factor of 3 for top spin valves, presumably by suppressing roughness and interdiffusion at the Co/Cu/Co interfaces, when ∼1 monolayer of Pb is deposited in the early stages of spin valve growth. The Pb has a strong tendency to float out or segregate to the surface during deposition of the spin valve leaving the GMR largely unaltered. Au is almost as effective as Pb, however the Au tends to be left behind in the spin valve, and the GMR is reduced slightly. Attempts to use Hg as a surfactant were unsuccessful. The coupling field increased, and the GMR decreased sharply.

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.

Spin valves with NiO pinning layer

We have studied both top and bottom spin valves with NiO as an antiferromagnetic pinning layer. The largest bias fields of up to /spl sim/130 Oe were seen for NiFe/Cu/NiFe/NiO top spin valves. Larger bias fields appear to be correlated with partially (200)-textured NiO. The NiFe/Cu/NiFe/NiO samples exhibited GMR values in the 3-4% range. Larger GMR values were seen in NiO/Co/Cu/Co and Co/Cu/Co/NiO spin valves; the largest GMR observed was 11.3% for a bottom spin valve sample of this type.