IEEE MAGNETICS LETTERS, Volume 3 (2012) 4000204 Nanomagnetics Large Exchange Bias Field in (Pt/Co) 3 /IrMn/Co Trilayers With Ultrathin IrMn Layers J. Moritz, G. Vinai, and B. Dieny ∗ SPINTEC, CEA/CNRS/UJF/Grenoble-INP; CEA/INAC, 17, rue des Martyrs, 38054, Grenoble, France. *Fellow, IEEE Received 19 October 2011, revised 21 December 2011, accepted 6 January 2012, published 28 February 2012. Abstract—The field shift of the hysteresis loop, called exchange bias field, in ferromagnetic/antiferromagnetic (F/AF) Co/IrMn bilayers, or (Pt/Co) n /IrMn structures, appears only above a critical thickness of IrMn, which is related to the IrMn magnetocrystalline anisotropy and to the interfacial coupling between the F and AF layers. In this letter, we show that this critical IrMn thickness can be considerably reduced by sandwiching the IrMn layer between two F layers or multilayers having parallel or orthogonal anisotropy, i.e., the first being magnetized in plane and the second out of plane. An in-plane exchange bias field of 20 mT could be measured in structures of the form (Pt/Co) 3 /IrMn/Co (5 nm) having an IrMn layer as thin as 3 nm at room temperature. Several explanations are proposed and discussed. The first is associated with structural variations of texture resulting in changes in the antiferromagnet grains’ anisotropy energy. The second is based on an enhancement of the stability of the antiferromagnet spin lattice resulting from an indirect intergrain coupling. The third relies on a decrease of the Co/IrMn interfacial coupling due to an out-of-plane canting of the interfacial spin. Index Terms—Nanomagnetics, exchange bias, IrMn, crossed anisotropy, perpendicular-to-plane anisotropy, intergrain coupling . I. INTRODUCTION Exchange anisotropy that arises from interfacial exchange in- teractions in AF/F bilayers has been extensively studied for sev- eral decades (see Ali [2003], Ambrose [1998], Anderson [2000], Sang [1999] for topical reviews). It is currently used in spintronic devices such as magnetic READ heads or magnetic random access memories. When the bilayers are annealed above their blocking temperature and field is cooled down to room temper- ature, the hysteresis loop of the F layer is shifted in the direction of the cooling field [Meiklejohn 1956]. However, the field shift, called exchange bias field ( H EX ), appears only above a critical thickness of the AF layer, denoted t C in the following of this paper. The variation of H EX versus AF thickness (t AF ) has been ex- perimentally studied by several authors for many F/AF systems. Generally, H EX rapidly increases above t C and then reaches a maximum value for t AF = t MAX . Its behavior for larger t AF is system dependent. Many authors have reported a gradual de- crease of H EX in Mn alloys (FeMn [Sang 1999], IrMn [van Driel 2000], PtMn [Xi 2000], NiMn [Lin 1995]) for t AF > t MAX , as well as in AF oxides (CoO [Ambrose 1998]). However, other authors have reported the observation of a continuous increase of H EX versus t AF in all range of AF thick- ness studied in the same type of F/AF bilayers [Mauri 1987, Ali 2003]. By using a random field theory, Malozemoff calculated the hysteresis field shift with respect to the AF thickness and attributed the decrease of H EX for t AF > t MAX to the presence of domains in the AF layer [Malozemoff 1988]. He pointed out also that the exchange bias field vanishes below an AF critical thick- ness due to too weak anisotropy energy. Mauri inferred that t C depends on the AF layer anisotropy ( K AF ) and interfacial ex- change energy ( J K ) [Mauri 1987]. Within the Meiklejohn model Corresponding author: J. Moritz (jerome.moritz@cea.fr). Digital Object Identifier: 10.1109/LMAG.2012.2184794 [Meiklejohn 1956], the authors explained that when K AF × t AF > J K , there are no irreversible changes in the AF layer and the hysteresis field shift occurs. According to their hypothesis, the critical thickness can be calculated as t C = J K / K AF . In this letter, we show that it is possible to significantly re- duce the AF critical thickness characterizing the onset of ex- change bias by using trilayer structures of the form F 1 /AF/F 2 rather than F/AF bilayers. Here, F 1 and F 2 represent two in- plane magnetized layers or two F layers having, respectively, perpendicular-to-plane and in-plane anisotropy. In the second part of this paper, we discuss the origin of this effect in term of magnetic anisotropy, interfacial coupling energy, intergrain coupling and grain-size distribution. II. EXPERIMENTS All samples were deposited onto Si/SiO 2 substrates by magnetron-sputtering under 0.25 Pa Ar pressure, with depo- sition rates of 0.6, 0.72, 2.07, 0.54, and 0.56 ˚ A/s for Ta, Cu, IrMn, Co, and Pt, respectively. The trilayer systems consist of Ta 3 /(Pt 1.8 /Co 0.6 ) 3 /IrMn(t AF )/Co 5 and Ta 3 /Co 5 /IrMn(t AF )/Co 5 , capped with 1.8 nm of Pt to prevent the samples from oxidation (all thicknesses are in nanometer). The hysteresis loops were acquired by the extraordinary Hall effect for the perpendicular- to-plane magnetized (Pt/Co) multilayers and by the magneto- optical Kerr effect for the Co layers, after an annealing at 200 ◦ C during 30 min under 10 −3 Pa to set the easy axis direction. The annealing was performed under a 0.3 T field, applied once in the sample plane direction, and then per- pendicular to the sample plane for the samples comprising (Pt/Co) multilayers. As a reference, two other series of bilay- ers were also prepared under the same conditions consisting of Ta 3 /(Pt 1.8 /Co 0.6 ) 3 /IrMn(t AF )/Pt 1.8 and Ta 3 /Cu 3 /IrMn(t AF )/Co 5 /Pt 1.8 multilayers. In this study, we are mostly interested in the varia- tion of the in-plane exchange bias field of the top Co 5 electrode and the out-of-plane exchange bias field of the bottom (Pt/Co) multilayer for different t AF in the bilayers and trilayers. 1949-307X/$31.00 C  2012 IEEE