Giant magnetoresistance in evaporated NiFe/Cu and NiFeCo/Cu multilayers (abstract)

Abstract

The magnetic and transport properties of electron beam evaporated (Ni83Fe17/Cu)10 and (Ni66Fe16Co18/Cu)10 multilayers (ML) were studied as a function of the Cu spacer and magnetic layer thicknesses (tCu and tNiFe), annealing conditions and Ta buffer layer thickness. The ML were evaporated in a magnetic field at deposition rates ∼ 2 Å/s and background pressure <5×10−8 mbar on Si/SiO2 substrates at Ts=200 °C. These ML exhibited two unique features: (1) ΔR/R and the interlayer coupling did not show oscillatory behavior as a function of tCu; and (2) after magnetic post annealing, ΔR/R increased from <0.3% in the as-deposited state, to up to ∼6% and 7% in Ta/(NiFe/Cu) and (NiFeCo/Cu), respectively. The coupling between the NiFe layers changed from ferromagnetic in the as-deposited state Mr/Ms∼0.9k;20 to essentially antiferromagnetic Mr/Ms<0.2) after appropriate annealing, and the ML became virtually isotropic in-plane. This is quite different from strong oscillatory behavior of giant magnetoresistance (GMR) previously reported in (NiFe/Cu) as-deposited ML made by ion-beam sputtering. After annealing at 300° and 325 °C for 2 h, the ΔR/R became ∼4.5% and ∼6.5% in (NiFe/Cu) and (NiFeCo/Cu) ML, respectively, and remained approximately constant for tCu=20 to 40 Å. The coupling field generally decreased with an increase in Cu and NiFe and after annealing at 300 °C dropped to as low as ∼25 and 45 Oe in (NiFe/Cu) and (NiFeCo/Cu) ML, respectively. The of ΔR/R Ta/(NiFe/Cu) ML increased with the thickness of Ta buffer layer from 30 to 70 Å. The high-angle θ–2θ x-ray scans of (NiFe/Cu) ML showed (111) texture, essentially independent of annealing temperature. The low-angle x-ray diffraction did not reveal roughening of the Cu–NiFe interfaces as a result of annealing. In many respects the GMR behavior of these ML is similar to that reported in sputtered ‘‘discontinuous’’ NiFe/Ag. However, in contrast to the latter, the resistivity of NiFe/Cu monotonically increases with annealing temperature. This suggests that lattice interdiffusion is more prominent in the NiFe– system, consistent with a greater equilibrium compared solubility of Cu in the NiFe matrix compared to that of Ag. It is believed that Cu diffusion along the NiFe grain boundaries creates intra-layer magnetic discontinuity in NiFe and promotes inter-layer antiferromagnetic coupling between adjacent NiFe layers, which then gives rise to the observed GMR. Evaporated NiFe/Cu ML showed very small hysteresis and uniform GMR properties throughout the thickness, which makes them good candidates for GMR–DMR heads.