Soft a-CoZr-based spin valves (abstract)

Abstract

The materials options to combine high giant magnetoresistance (GMR) ratios with soft magnetic properties have been relatively few, with NiFe providing low-field switching in the great majority of studies. These NiFe layers tend to be hardened appreciably when directly exchange coupled to Co to provide higher GMR ratios, as in (Ta)/NiFe/Co/Cu/Co/NiFe/FeMn spin valves. We propose magnetically soft, amorphous CoZr as a replacement couple to Co in these and other structures. While the high resistivity of amorphous films discourages their use as a single switching element, it favors their use as the drive layer for Co in a spin valve. Our transport simulations, using the Camley–Barnas model modified by Dieny,1 indicate that GMR ratios fall off much less steeply with increasing a-CoZr drive layer thickness than with NiFe drive layer thickness. Since lower coercivities are expected for higher fractions of the drive material, CoZr-based spin valves show promise in combining soft properties and high GMR ratios. We have demonstrated the combination of magnetic softness and high GMR ratio in our own CoZr-based spin valves. A free layer coercivity of 7.2 Oe and GMR ratio of 6.4% were achieved in a nominal structure of NiO(400)/Co(50)/Cu(20)/Co(25)/CoZr(200AA). Differential coercivity structures were deposited as well, using Cr underlayers to induce hardness of the bottom Co. An identical “active layer” structure of the form SiO2/Cr/Co(50)/Cu(20)/Co(25)/CoZr(200AA) demonstrates GMR ratios as high as 4.9%; removal of the interfacial Co layer reduces the GMR ratio to 2.8%, in accordance with our expectations. Discussion of optimization and detailed structural analysis will be presented in the full paper.