2990 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006 Exchange Bias in Annealed [Pt/Co]/NiFe Systems A. Bollero , L. D. Buda-Prejbeanu , V. Baltz , B. Rodmacq , and B. Dieny SPINTEC, C.E.A/C.N.R.S URA 2512 CEA Bât 1005, 38054 Grenoble Cedex 9, France Condensed Matter Group School of Physics and Astronomy, E. C. Stoner Laboratory, University of Leeds, Leeds LS2 9JT, U.K. Exchange bias without antiferromagnetic layer was observed in systems consisting of two exchange coupled ferromagnetic materials: (Pt/Co) multilayer (with out-of-plane anisotropy) and permalloy, NiFe, (with in-plane anisotropy). We investigated the influence of various parameters on this phenomenon: Pt buffer layer thickness, number of repeats in the multilayer, thickness of the NiFe layer. Annealing yields a doubling in the magnitude of the loop shift as a result of an improved texture and mosaicity of the films. Index Terms—Coercive field, exchange bias, perpendicular anisotropy. I. INTRODUCTION E XCHANGE BIAS (EB) usually results from interfacial in- teractions between a ferromagnet (FM) and an antiferro- magnet (AFM) and is associated with a shift of the hysteresis loop along the magnetic field axis by a quantity termed ex- change bias field, [1]. This shift is usually accompanied by an increased coercivity. This phenomenon is frequently used in magnetic random access memories and magnetoresitive read heads to pin the magnetization of a reference layer [2]. Induce- ment of is usually achieved by cooling the FM-AFM bilayer under an applied field through the blocking temperature of the system, . The possibility of inducing exchange bias like effects has been recently demonstrated in systems consisting of two coupled FM materials: Pt/Co multilayer (with out-of-plane anisotropy)-NiFe (with in-plane anisotropy) [3], [4]. A shift along the field axis, , in the in-plane hysteresis loop of the NiFe is obtained without necessity of a field cooling procedure. This is achieved by applying an in-plane magnetic field strong enough to saturate the magnetization of all layers, prior to the measurement of the hysteresis loop [3]. Magnetic force microscopy (MFM) studies along with 2D micromagnetic simulations allowed understanding the magnetic configuration at the origin of ; it consists of asymmetric closure domains created at the interface [4]. In this work we investigate the influence of various parame- ters on the exchange bias field: Pt buffer layer thickness, number of repeats in the (Pt/Co) multilayer, thickness of the NiFe layer. Furthermore, annealing of the [Pt/Co]-NiFe system is shown to be an efficient way to increase . II. EXPERIMENT Three series of multilayers were prepared of composition: Pt( /(Co /Pt ) , Pt( /(Co /Pt ) / Co /NiFe /Cu /Pt and Pt /(Co / Pt ) /Co /NiFe(t )/Cu /Pt where is the thickness of the Pt underlayer, n the number of (Pt/Co) repeats, and the NiFe thickness. The films were deposited at room temperature onto naturally oxidized Si substrates by Digital Object Identifier 10.1109/TMAG.2006.879758 dc magnetron sputtering. The base pressure was mbar and the Ar gas pressure was maintained at mbar during deposition. After a first set of measurements “as-deposited”, the samples were annealed without applying a magnetic field at 250 C for 30 min, under vacuum, at pressures lower than mbar. The shift in the hysteresis loop of the [Pt/Co]-NiFe films, before and after annealing, was induced by first fully saturating the magnetization of the system with an in-plane magnetic field of 15 kOe; subsequently, the in-plane hysteresis loop was measured with a maximum applied mag- netic field ( kOe) large enough to saturate the NiFe but insufficient to saturate the [Pt/Co] multilayer. In-plane hysteresis loops were measured at room temperature with a vibrating sample magnetometer (VSM). Hysteresis loops along the perpendicular to the film direction were measured by using extraordinary Hall effect (EHE). This technique is sensi- tive to the perpendicular-to-film plane component of the mag- netization [5]. The structure of the multilayers was analyzed by x-ray diffraction (XRD) ( scans and rocking curves). III. RESULTS AND DISCUSSION In order to study the effect of and the effect of annealing on the out-of-plane coercivity, , a Pt /(Co /Pt ) series with , 1.8 and 20 nm was prepared. In good agreement with previous studies, Fig. 1 shows an increase in with increasing [6]. XRD data showed in all cases a strongly developed orientation for all films. However, the full width at half-maximum of the Bragg peaks increased with decreasing . This can be at- tributed to a poorer (111) texture for the films with thinner . Consequently, the enhanced observed with increasing is probably due to an increase of the perpendicular anisotropy resulting from a better multilayer structure [6]. Interestingly, annealing of the samples yields a further enhancement of , with the exception of the multilayer with no Pt underlayer. This latter sample exhibits the lowest coercivity among the series, before and after annealing, with an approximately unchanged value of Oe. increases however dramatically after annealing for the films containing a Pt underlayer, remarkably in the case nm (Fig. 1). After annealing, this sample exhibits an enhanced coercivity of about 1.1 kOe which is about twice the value of the virgin sample. values of 130 and 230 Oe were obtained for the film with nm 0018-9464/$20.00 © 2006 IEEE