2282 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010 Thickness and Temperature Effects on Magnetic Properties and Roughness of -Ordered FePt Films Chang Soo Kim , Jonathan J. Sapan , Stephanie Moyerman , Kangho Lee , Eric E. Fullerton , and Mark H. Kryder Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213-3890 USA Center for Magnetic Recording Research, University of California, San Diego, La Jolla, CA 92093-0401 USA Advanced Technology, Qualcomm Incorporated, San Diego, CA 92121-1714 USA -ordered FePt films with strong perpendicular magnetic anisotropy have been successfully obtained using Ta and MgO seed layers deposited on thermally oxidized Si wafers. In this paper, we focused on examining the -crystalline ordering, surface roughness, and magnetic properties of the bottom FePt electrode in a perpendicular magnetic tunnel junction device. The influence of varying FePt thickness (2–18 nm) and deposition temperature (380–550 ) on the formation of -ordered FePt films has been studied. In order to investigate the FePt grain growth effects on the magnetic properties and the surface roughness, the morphology of -ordered FePt films was examined through transmission electron microscope plan-view images. Index Terms— -ordered FePt, perpendicular magnetic tunnel junction, surface roughness, Ta and MgO seed layers. I. INTRODUCTION R ECENT research has indicated that perpendicular mag- netic tunnel junctions (pMTJs) offer superior scalability relative to in-plane magnetic tunnel junctions [1], [2]. The mag- netic properties of pMTJs are more strongly determined by the intrinsic material properties rather than by the shape of the de- vice. The performance of devices is less sensitive to lithography variations and is controllable by judicious engineering of mate- rial properties. In addition, pMTJs have potential to reduce the critical current density for spin transfer torque switching. On the other hand, thermal fluctuation of magnetization has emerged as a problem in high-density magnetic recording and is also an issue in nanometer-scale pMTJ devices. In order to solve this problem, -crystalline ordered alloys such as FePt are a very promising class of magnetic materials for use in pMTJs due to their large magnetic anisotropy and the resulting high thermal stability [1]. However, it has proven dif- ficult to grow (001) oriented top and bottom -FePt magnetic layers, separated by an MgO-based tunneling barrier by phys- ical vapor deposition techniques. In addition, the MgO-based tunneling barrier uniformity that is critical to high tunnel mag- netoresistance (TMR) predominantly depends on the roughness of the bottom -FePt electrode [2]. In this work, Ta and MgO seed layers were used on top of thermally oxidized Si (100) wafers in order to promote the per- pendicular orientation of FePt at high temperature, because a sputter deposited MgO film grows with (002) texture on amor- phous Ta due to its favorable surface energy and also provides a good epitaxial template on which to grow the FePt with (001) orientation. In addition, a thin layer of amorphous Ta on Manuscript received October 31, 2009; revised January 19, 2010; accepted March 02, 2010. Current version published May 19, 2010. Corresponding au- thor: M. H. Kryder (e-mail: kryder@ece.cmu.edu). 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.2010.2045485 top of the substrate was found to smooth the surface roughness and improve the (002) texture of the MgO films [3]–[5]. The de- gree of chemical ordering of the bottom FePt electrode can be related to its local surface roughness and its dependence on the FePt thickness and deposition temperature. The degree of ordering is evaluated by the order parameter (S) in a quan- titative way, which is defined by the following formula [6], [7]: where is the integrated intensity, F is the structure factor, L is the Lorentz polarization factor, A is the absorption factor, D is the temperature factor, and the subscripts f and s refer to the fundamental peak and super-lattice peak, respectively. In this paper, we focused on optimizing FePt thickness and deposition temperature of -FePt films to control the -crystalline or- dering and surface roughness of the bottom FePt electrode in our pMTJ devices. II. EXPERIMENTAL PROCEDURES All samples were prepared using a high-vacuum Leybold- Heraeus Z-400 magnetron sputtering system with a base pres- sure of and an Ar pressure of 10 mTorr. An alloy target with the composition of was used to produce the FePt thin films. Ta/MgO seed layers with 10 nm thickness were deposited at room temperature onto a 3 thermally oxi- dized Si (100) wafer, respectively. The substrate was heated in vacuum for 45 min to various temperatures ranging from 380 to 550 . This was followed by FePt deposition with thick- nesses ranging from 2 to 18 nm. Structural properties and mi- crostructures of the films were characterized by a Philips x-ray diffractometer ( radiation, 45 kV, 40 mA) and a JEM 2000 EX II transmission electron microscope (TEM). Magnetic properties were characterized at room temperature by vibrating sample magnetometry in a Quantum Design physical property measurement system (PPMS) at fields up to 9 T. The surface 0018-9464/$26.00 © 2010 IEEE