Mechanisms by which oxygen acts as a surfactant in giant magnetoresistance film growth D. J. Larson, 1 A. K. Petford-Long, 2 A. Cerezo, 2 S. P. Bozeman, 1 A. Morrone, 1 Y. Q. Ma, 2 A. Georgalakis, 2 and P. H. Clifton 3 1 Seagate Technology, Minneapolis, Minnesota 55435 2 Department of Materials, University of Oxford, Oxford OX1 3PH, England 3 Seagate Technology, Londonderry BT48 0BF, N. Ireland Received 12 December 2002; published 28 April 2003 The mechanisms by which oxygen acts as a surfactant in giant magnetoresistance multilayers have been elucidated for the first time. Three-dimensional atom probe analysis of Cu/CoFe multilayers reveals the el- emental distributions at the atomic level. Interfacial intermixing and oxygen impurity levels have been quan- tified for the first time. Both with and without oxygen the intermixing is greater at the CoFe-on-Cu interface than at the Cu-on-CoFe one and for both interfaces, oxygen reduced the intermixing. The oxygen largely floats to the growing surface and is incorporated at grain boundaries. The oxygen also reduces conformal roughness and grain boundary grooving, indicating a reduction in long-range surface diffusion. DOI: 10.1103/PhysRevB.67.144420 PACS numbers: 68.35.Ct, 68.37.Lp, 68.65.Ac, 75.47.De Metallic layered thin films that exhibit giant magnetore- sistance GMR, 1 such as the spin-valve structure, 2 are the subject of intensive worldwide research efforts because of the vital role they play in information storage. However, con- trol of the thin-film microstructure, which determines the magnitude of the GMR effect, 3 is a great challenge. There is much controversy over the growth mechanisms of these ma- terials, which makes it difficult to know how to optimize the performance of GMR materials. For example, one factor that can significantly affect the GMR properties of thin-film structures is the presence of impurities in the sputtering chamber during deposition. The exact mechanisms by which impurities affect GMR is not understood, and controversies have arisen over the role that the impurities, and indeed small quantities of alloying elements, play in modifying the microstructure of the films. 4–7 Oxygen is one such impurity, and its effect on the growth of magnetoresistive films has been investigated recently by several researchers. 4,8–12 Egel- hoff et al. found that introducing small amounts of oxygen during the deposition of spin valve structures led to lower resistance and increased GMR from 17% to 25% and from 14% to 18% for symmetric and bottom spin valves, respectively. 4 They proposed that oxygen suppresses inter- mixing between the Cu and Co layers during the growth process, leading to lower resistance and less ferromagnetic coupling between the layers. Residual oxygen levels and the exact position of oxygen atoms trapped in the films during growth were not quantified, although it was suggested that not all of the oxygen floats to the surface during deposition and may be trapped at grain boundaries. 4 In Cu/Co multilay- ers, Kubinski et al. observed an increase in GMR from 10% to 40% with the addition of 1000-appm O 2 during deposition. 8 Transmission electron microscopy TEM stud- ies of these films showed little change in the conformal roughness, and it was suggested that fewer ‘‘pinholes’’ in the Cu layers led to the increase in antiferromagnetic AF cou- pling and higher GMR, although this was not confirmed ex- perimentally. Kagawa et al. 9 investigated the effect of back- ground pressure on magnetoresistance in Co/Cu multilayers and inferred that oxidation of the interfaces weakens AF cou- pling and reduces GMR, although no direct evidence for in- terfacial oxidation was presented. Miura et al., however, pro- posed an increased AF coupling and higher GMR in Cu/Co multilayers as a result of partial oxidation. 10 They also pro- posed a reduction in grain size and a decrease in interface roughness observed by x-ray diffraction and atomic force microscopy as additional mechanisms behind the increased AF coupling and improved GMR. A decrease in grain size for film growth in the presence of oxygen was also observed by Aitchison et al., 11 with a suppression of the formation of the hcp Co phase and reduced texturing, which led to an increase in the degree of bilinear coupling accompanied by an increase in GMR. Marrows et al. observed a large de- crease in antiferromagnetic coupling and GMR for films grown in a small amount of oxygen although no significant microstructural changes were apparent. 12 In this study we have used three-dimensional atom probe 3DAPRef. 13 and TEM analysis to make a detailed study of the mechanism by which oxygen acts as a surfactant in these materials. 3DAP is unique in being able to measure the extent of interdiffusion or interface segregation at the atomic scale, and separate these from nanometer-scale topological features such as layer curvature. The issues that we have set out to address with respect to oxygen-doped multilayer growth, and which have not been analyzed before in a quan- titative way, are the amount and position of residual oxygen trapped in the layered structures and the exact effects of oxy- gen on the nanoscale nature of the interfaces. Our studies have enabled us to gain insights into the underlying thin-film growth phenomena that enable oxygen to improve GMR characteristics in metallic multilayer films and replace specu- lation with quantitative measurement. In the current work, investigation of thin film microstruc- ture resulting from the addition of an oxygen surfactant during growth of certain layers in  Si  NiFeCr 5 nm / Co 90 Fe 103 nm /(Cu 1.8 nm Co 90 Fe 103 nm ) 5/cap 50 nm  films has been performed. The Ni-based alloy seed layer leads to a 111 crystallographic orientation in the films, as confirmed by x-ray diffraction. The multilayers were deposited using dc magnetron sputtering base pressure 1 10 -8 Torr) onto low resistivity silicon 100 substrates held at approximately room temperature, with the substrates used for 3DAP analy- PHYSICAL REVIEW B 67, 144420 2003 0163-1829/2003/6714/1444204/$20.00 ©2003 The American Physical Society 67 144420-1