The CO/NH3 plasma chemistry operated under conventional reactive ion etching conditions does not,etch NiFe or NiFeCo. However, under high density plasma conditions, etch rates up to similar to 500 Angstrom min(-1) are obtained for both materials provided optimized ratios of CO:NH3 and values of ion flux and ion energy are-employed. The etch mechanism still has a strong physical component and appears to depend on having sufficient CO to form carbonyl etch products, and to avoid formation of a carbide-like surface layer. Under nonoptimized conditions, the latter can lead to net deposition rather than etching.