Growth of giant magnetoresistance multilayers: Effects of processing conditions during radio-frequency diode deposition

Abstract

The magnetotransport properties of giant magnetoresistance multilayers are significantly effected by the atomic-scale structure of the interfaces between the nonferromagnetic conducting and ferromagnetic (FM) metal layers. The interfacial roughness and the extent of intermixing at these interfaces are both known to be important. A combination of experimental and multiscale modeling studies have been used to investigate control of interface structure during multilayer growth by rf diode deposition and the consequences of such control for magnetotransport. Experiments were conducted to evaluate the dependence of the magnetotransport properties of NiFeCo/CoFe/CuAgAu multilayers upon the growth conditions (background pressure, input power), and to link the roughness of vapor-deposited copper layers to the same process parameters. These experimental studies reveal the existence of intermediate background pressure (20 mTorr) and plasma power (175 W) that resulted in the highest magnetoresistance and a strong sensitivity of copper layer surface roughness to both the power and pressure at which deposition was conducted. By using a combination of modeling technologies, the deposition process conditions have been linked to the atomic fluxes incident upon the sample surface. This was then used to determine the atomic-scale roughness of the film. Energetic metal atoms (and inert gas ions) were found to have very strong effects upon interfacial structure. The models revealed an increase in interfacial roughness when metal (or inert gas ion) translational energy was decreased by either reducing the plasma power and/or increasing the background pressure. Because high-energy metal impacts activated atomic jumping near the impact sites, high plasma power, low background pressure process conditions resulted in the smoothest interface films. However, these conditions were also conducive to more energetic Ar+ ion bombardment, which was shown by molecular dynamics modeling to induce mixing of the FM on the copper interface. Intermediate plasma powers/background pressures result in the most perfect interfaces and best magnetotransport. The insights gained by the modeling approach indicate a need to avoid any energetic ion bombardment during the early growth stages of each new layer. This could be accomplished by operating at low power and/or high pressure for the first few monolayers of each layer growth and may provide a growth strategy for further improvement in magnetotransport performance.