Room temperature giant magnetoresistance (GMR) and magnetic properties of (Ni–Fe/Cu)n and (Ni–Fe–Co/Cu)n multilayers were investigated. Alternating layers of Ni–Fe–(Co) and Cu were electron-beam evaporated in a computer-controlled high-vacuum system at base pressure of ⩽4×10−8 Torr and deposition rates of ⩽2 Å/s. To complement and expand our previous investigation,1 GMR properties were additionally studied here as a function of cobalt content of Ni–Fe–Co films, the number (n) of bilayers, deposition temperature, and type of buffer layer. The Co content was varied from 7 to 17 at. %, and the number of bilayers ranged from n=8 to 20. No significant GMR was observed in the as-deposited multilayers. To produce tangible GMR, these multilayers were annealed between 300 and 360 °C for 2 h in a 150 Oe magnetic field in an argon atmosphere. The GMR effect (ΔR/R) was essentially independent of copper spacer thickness, which varied between 25 and 30 Å. For Co containing multilayers the highest ΔR/R=7.6% was obtained for 17 at. % Co deposited at 100 °C. The ΔR/R in all Ni–Fe–Co/Cu multilayers was sensitive to the deposition temperature, and R–H loops always showed significant hysteresis independent of the type of buffer layer. For application of these materials to very high density reproduce heads,2 the best results were obtained for (27 Å NiFe/25 Å Cu)14–18 multilayers deposited at 160 °C on 70 Å Ta buffer layer. For example, n=17 multilayers annealed at 350 °C exhibited ΔR/R=7.5%, half-width at half-maximum of ∼50 Oe, essentially no anisotropy, and virtually zero hysteresis (Fig. 1). Frequency dependent permeability measurements showed constant permeability between 10 and 200 MHz. Low- and high-angle x-ray diffraction as well as atomic force microscopy were used to investigate the effect of different geometries of multilayers on structure and roughness and to correlate them with GMR properties.

The magnetic and transport properties of electron-beam evaporated (Ni83Fe17/Cu)10 and (Ni66Fe16Co18/Cu)10 multilayers were studied as a function of the Cu spacer, magnetic layer and Ta buffer layer thicknesses, as well as annealing conditions. All multilayers exhibited very small giant magnetoresistance (GMR) effect (<0.3%) in the as-deposited state, however, after magnetic post-annealing at 300–325 °C, GMR increased up to 4.5%–7%, depending on the multilayer type. In contrast to sputtered Ni-Fe-(Co)/Cu multilayers, GMR showed no oscillatory behavior as a function of Cu thickness. Similar to that reported in sputtered ‘‘discontinuous’’ Ni-Fe/Ag multilayers, it is believed that Cu diffusion along the Ni-Fe-(Co) grain boundaries creates intra-layer magnetic discontinuities in Ni-Fe-(Co) layers which promote inter-layer antiferromagnetic coupling. The evaporated Ni-Fe/Cu multilayers exhibited very low remanence, exceptionally low hysteresis, and quite uniform GMR properties through the thickness of the multilayer. All of these makes them potentially attractive for application to future magnetoresistive reproduce heads for very high areal density magnetic storage systems.

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.

A micromagnetic model for NiFe-TbCo exchange-coupled bilayers which can quantitatively predict and explain the major macroscopic features observed in measured M-H characteristics is presented. Comparison of theoretical and experimental results shows conclusively that the strong interfacial exchange coupling in NiFe-TbCo is essentially indistinguishable from that of a perfect, homogeneous interface. Commonly invoked assumptions concerning the existence or origin of a substantially weakened exchange coupling at a highly imperfect interface are neither necessary, nor even consistant with experimental measurements. The mechanism of the unidirectional exchange anisotropy is the formation of Bloch-type domain walls in an ≂0.08-μm-thick TbCo sublayer of uniaxial, in-plane anisotropy adjacent to the NiFe interface. The manner in which the observable magnetic behavior of NiFe-TbCo bilayers depends on film thicknesses, TbCo anisotropy, interfacial exchange coupling strength, as well as the previously unconsidered large hysteretic effects due to the small net magnetization in ferrimagnetic TbCo, are discussed in detail. It is demonstrated, for a strongly coupled system such as NiFe-TbCo, that the often used single parameter ‘‘exchange field’’ description of a shifted NiFe M-H loop is inadequate. A quantitatively accurate description requires that one take into account the spatial variations in the micromagnetic magnetization distributions of both layers.