Magnetic characterization of MnPt/CoFe bilayers using the MOKE technique J.M. Teixeira a, * , A.M. Pereira a , J. Ventura a , R.F.A. Silva a , F. Carpinteiro a , J.P. Arau ´ jo a , J.B. Sousa a , M. Rickart b , S. Cardoso b , R. Ferreira b , P.P. Freitas b a DFFCUP and IFIMUP, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal b IST and INESC-MN, Rua Alves Redol, 9-1, 1000-029 Lisbon, Portugal Keywords: MnPt MOKE Exchange bias Domain imaging Magnetization reversal processes Spintronics PACS: 73.40.Gk 73.40.Rw 85.35.p 85.75.d 85.75.Dd abstract The performance of magnetoresistive devices (spin valves, tunnel junctions), made of two ferromagnetic (FM) layers and separated by a non-magnetic spacer, rely on the existence of two well separated re- sistance states. For this to occur, one of the FM layer is deposited just adjacent to an antiferromagnetic (AFM) layer. Due to the exchange interaction at the AFM/FM interface, the reversal of the magnetization (M) of such FM-pinned layer occurs at a high applied magnetic field. The magnetization of the other FM layer reverses almost freely when a small magnetic field is applied. Here we study the exchange bias effect in the MnPt (t)/CoFe (50 Å) system, using the Magneto-Optical Kerr Effect (MOKE) and domain imaging techniques. The exchange (H E ) and coercive (H c ) fields increase with increasing AFM thickness, saturating for t > 200 Å (H E z 670 Oe and H c z 315 Oe). Furthermore, we observe that the value of the exchange field is almost independent of the applied magnetic field sweeping rate (up to z 300 kOe/s). Domain imaging allowed us to conclude that magnetization reversal in the studied system proceeds essentially by coherent magnetic moment rotation. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The highly competitive field of spintronics [1] and magnetic nanostructures is continuously seeking new ways to improve device performance. Two of such devices are constituted by two ferromagnetic (FM) layers separated by a metallic (spin valve, [2]) or insulator (magnetic tunnel junctions, [3]) spacer layer (Fig. 1). The electrical resistance of such device depends on the relative orientation of the magnetizations of the FM layers, which can be altered by the application of an external magnetic field. To obtain well separated states, an antiferromagnetic (AFM) layer is de- posited just adjacently to one of the FM layers. Due to an exchange interaction at the AFM/FM interface, the magnetization of this FM layer is pinned and reverses at much higher magnetic fields than the other FM layer. New AFM and FM materials with improved exchange coupling, better thermal stability, higher spin polariza- tion and lower coercivity are then intensively researched. These materials currently include AFM MnPt [4] and FM CoFe. An effective technique to magnetically characterize such materials is Magneto- Optical Kerr Effect (MOKE) because of its versatility and easy implementation [5,6]. Here we study the magnetic properties of exchange-coupled bilayers Mn 51.5 Pt 48.5 (t)/Co 90 Fe 10 (50 Å) (100 Å < t < 300 Å) using vectorial MOKE magnetometry and imaging. MnPt is antiferro- magnetic after annealing at high temperatures [7]. We show that the exchange field (H E ) starts by increasing with AFM thickness, saturating for t > 200 Å (H E z 670 Oe and H c z 315 Oe). The co- ercive field (H c ) also increases with increasing AFM thickness. We further study the dependence of the exchange and coercive fields on the applied magnetic field frequency, ranging from more than seven decades, up to a maximum sweeping rate (sr) z 300 kOe/s. The results show an almost independent sr behaviour of the char- acteristics fields (H c , H E ) of the magnetic bilayers for all reported thicknesses. The magnetization reversal mechanisms observed for these bilayers are essentially magnetic moment rotation, as con- firmed by both MOKE magnetometry and domain imaging. 2. Experimental details Exchange-coupled bottom-pinned bilayers Ta (70 Å)/Ru (30 Å)/ Mn 51.5 Pt 48.5 (t)/Co 90 Fe 10 (50 Å)/Ta (30 Å) were prepared in an automated NORDIKO 2000 magnetron sputter system (100 Å < t < 300 Å) [4,7,8]. The substrate was glass (CORNING 7059) coated with an Al (600 Å) buffer which was preheated at a set temperature of 150  C for 30 min in vacuum and then ion-beam milled to reduce the surface roughness. The films were sputtered from a composite target with a composition of Mn 63.6 Pt 36.4 where additional Pt pieces were glued on top of the target to obtain a film composition of Mn 51.5 Pt 48.5 . Base pressure of the system was less * Corresponding author. E-mail address: jmteixeira@fc.up.pt (J.M. Teixeira). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.072 Vacuum 82 (2008) 1486–1488