2628 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 Magnetic Properties of Perpendicularly Magnetized [Co/Pd]/Au/[Co/Pd] Pseudo-Spin-Valve Nano-Ring Structures X. M. Liu , S. Jain , and A. O. Adeyeye Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore- 117576, Singapore Advanced Materials for Micro- and Nano- System, MIT Alliance Singapore We systematically investigate the effects of interlayer, inter-ring dipolar coupling and the number of bilayer repeats on the magnetiza- tion reversal process of pseudo-spin-valve (PSV) nano-rings. We observed that exchange coupling dominates the magnetization reversal process in PSV rings when the Au spacer layer thickness is 1 nm, while magnetostatic coupling plays a more significant role for thicker spacer layer. The magnetization reversal process of the PSV nano-rings as a function of inter-ring spacing provides a way to engineer the switching fields in both the ferromagnetic layers. We observed that the coercivity of both the top and bottom Co/Pd stack increases with increasing inter-ring spacing. Index Terms—Co/Pd multilayer, ferromagnetic nanoring, interlayer coupling, magnetization reversal, perpendicular anisotropy, pseudo-spin-valve. I. INTRODUCTION F ERROMAGNETIC nano-rings have received consid- erable interest recently due to their potential in future magnetoelectronic devices such as magnetic random access memory (MRAM) cells [1], [2] and high sensitivity mag- netic sensors [3]. For a viable MRAM cell design, a device configuration consisting of at least two ferromagnetic (FM) layers separated by a nonmagnetic (NM) spacer layer would be desirable. Compared to pseudo spin valve (PSV) structures with in-plane magnetic anisotropy, spintronic devices with perpendicular magnetic anisotropy (PMA) are predicted to benefit more from improved thermal stability and lower critical current density for spin transfer switching, with significant ad- vantages for high packing density [4], [5]. So far, most research work has focused on the investigation of PSV nano-rings with in-plane anisotropy [1]–[3], [6], [7]. The study of the magnetic properties of perpendicularly magnetized PSV structures is of great interest because both the interlayer coupling between two FM layers and dipolar coupling between magnetic elements are vital in the magnetization reversal process. The interlayer coupling can occur by direct ferromagnetic coupling through pinholes in the spacer layer, indirect exchange coupling and magnetostatic coupling from stray fields, all of which have close relationship with the thickness and material of the spacer layer [8]. In this work, we have conducted a systematic investigation of the effects of interlayer coupling and inter-ring dipolar coupling on the magnetization reversal of nano-rings with PSV structure by varying the Au spacer layer thickness and nano-ring center-to-center spacing, Manuscript received February 20, 2011; accepted May 10, 2011. Date of cur- rent version September 23, 2011. Corresponding author: A. O. Adeyeye (e-mail: eleaao@nus.edu.sg). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2155634 Fig. 1. Schematic of deposited pseudo-spin-valve (PSV) structure; (b) representative SEM image of PSV rings; and (c) XRD patterns of the contin- uous films with PSV structures as a function of Au spacer thickness . respectively. We observed that magnetic properties of the nano-rings are markedly dependent on the Au spacer layer thickness and center-to-center spacing. II. EXPERIMENTAL DETAILS Arrays of perpendicularly magnetized Co/Pd rings with inner diameter of 190 nm, outer diameter of 520 nm and varied center-to-center spacing in the range from 720 nm to 1170 nm were fabricated on silicon substrates using deep ultraviolet lithography at 248 nm wavelength followed by lift-off process. A PSV structure was deposited using magnetron sputtering in a 3 mTorr Ar pressure at room temperature with Au spacer thickness varied from 1 nm to 8 nm. The base pressure of the chamber was better than . In order to obtain perpendicular anisotropy, a 5 nm Pd seed layer was deposited prior to the PSV multilayers. Shown in Fig. 1(a) is a schematic of the deposited Co/Pd PSV structures. A representative scanning electron micrograph (SEM) of an array of the PSV rings with and is shown in Fig. 1(b). The structural properties of the PSV films were studied using X-ray diffractometer (XRD). The collective magnetic switching behavior of Co/Pd multilayer ring arrays and PSV ring arrays was characterized using a focused polar magneto-optic Kerr Ef- fect (MOKE) set up with a spot size of about 5 and external magnetic field applied perpendicular to the plane of the sample. 0018-9464/$26.00 © 2011 IEEE