Detection of Pinholes in Ultrathin Films by Magnetic Coupling
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Detection of Pinholes in Ultrathin Films by Magnetic Coupling W. F. Egelhoff, Jr., L. Gan, P. J. Chen, C. J. Powell, R. D. McMichael, and R. A. Fry, National Institute of Standards and Technology Gaithersburg, MD 20899 G. Beach, D. Martien, and A. E. Berkowitz Center for Magnetic Recording Research University of California at San Diego La Jolla, CA 92093 Abstract When two magnetic films are separated by a nonmagnetic film, pinholes in the nonmagnetic film can allow direct contact and, thereby, direct magnetic exchange coupling between the two magnetic films. We have studied this coupling by having one of the magnetic films pinned and leaving the other free to switch at low field. The pinning is accomplished with test structures based on exchange bias and synthetic antiferromagnetic layers. Since the pinning strength increases sharply at low temperatures but orange-peel coupling does not, lowtemperature (77 K) measurements appear to identify whether an observed coupling arises primarily from magnetic coupling through pinholes or primarily from orange-peel roughness. Our measurements appear to indicate that the observed coupling arises primarily from magnetic coupling through pinholes for Cu films less than 2.1 nm thick and for Al2O3 films less than 0.6 nm thick but primarily from roughness-induced (orange-peel) magnetostatic coupling for larger thicknesses. Introduction Pinholes are widely believed to play a key role in limiting the performance of both giant magnetoresistance (GMR) spin valves and magnetic tunnel junctions (MTJs).1 It is generally believed that as the spacer layer, Cu in the case of spin valves and Al2O3 in the case of MTJs, is made thinner the value of the magnetoresistance (MR) increases until pinholes occur. Pinholes couple the two magnetic layers ferromagnetically, making it difficult to achieve antiparallel alignment, and thereby limiting the MR. Pinholes are not easy to observe. There is some evidence from transmission electron microscopy (TEM) for the existence of pinholes, but in systems such as Co/Cu/Co the low electron-scattering contrast between elements of similar atomic number makes conclusive identification of pinholes difficult.1 Another problem is that the thickness of the Cu layer is typically much smaller (. 2 nm) than the depth of the TEM sample in the beam direction (. 20 nm). If the diameter of a pinhole in the Cu film is similar to the thickness of the Cu film, it would be only . 10 % of the sample depth thus exacerbating the contrast problem. In systems such as Al2O3/Co, there is some evidence that electrochemical deposition of Cu clusters can identify
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pinholes, although the applied potential may also create pinholes .1 Two groups have recently reported the use of magnetic hysteresis loops to study coupling between magnetic films of different coercivity separated by an insulating film.2 The method appears to have much promise, and the present work is an extension of this concept. The present work has two aims. One is to develop an improved method for observing the ons
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