1 © 2014 IOP Publishing Ltd Printed in the UK 1. Introduction The exchange bias (EB) phenomenon [1, 2] results from mag- netic exchange coupling between a ferromagnet (FM) and uncompensated spins (UCSs) at the interface with an adjacent antiferromagnet (AF) [3–5]. Its most known characteristics, the magnetization curve shift along the magnetic ield axis called the exchange-bias ield, H EB , is frequently accompa- nied by an increase of the coercivity H C , i.e. the half width of the magnetization hysteresis loop. Although EB has been one of the most extensively studied subjects in both theoretical and applied physics during the last decade and is currently used in integral components of modern spintronic devices, the speciic mechanism which establishes the interfacial coupling of EB systems is still a rather controversial topic. It is currently accepted [6–9] that at the AF/FM interfaces of polycrystalline ilms there might exist regions with UCSs which behave as single FM domains and interact with the adja- cent FM and/or AF grains. In IrMn/CoFe systems, O'Grady et al [6] inferred that the UCSs are spontaneously-frozen AF spins. Berkowitz et al [7] have found strong indications that at the FM/AF interfaces of their Permalloy-CoO system there are hard Co-ferrite-like particles resulting from chem- ical reactions, which are exchange-coupled to UCSs of CoO on the interfacial {1 1 1} planes of the (1 1 1)-textured CoO. While stable UCSs add to the bias (in what follows, the corre- sponding quantities will be indexed by set), less-stable UCSs (these will be referred to as rot) rotate simultaneously with the FM’s magnetization M FM and are responsible for the H C enhancement and for the rotatable anisotropy (RA) sensed by the FM. In order to explain the frequently observed isotropic shift of the angular variation of the ferromagnetic resonance ield in EB systems, a unidirectional RA term has been used [10]. However, when irreversible magnetization processes are involved, a uniaxial RA term proportional to −(M FM · H) 2 has been employed [11] since coupling with the rot-type UCSs is sensed by the FM as an additional uniaxial anisotropy with a symmetry axis parallel to that of the applied magnetic ield H. Although it is usually assumed that the anisotropy of rotatable UCSs is lower than that of the set-type ones, it has been pre- dicted [9] and recently conirmed [12] that a highly anisotropic Journal of Physics D: Applied Physics Strongly enhanced exchange bias of top-pinned Co/IrMn ilms with Py spacers M Gamino, A M H de Andrade, J E Schmidt and J Geshev Instituto de Física, URFGS, Porto Alegre, 91501-970 Rio Grande do Sul, Brazil E-mail: julian@if.ufrgs.br Received 27 June 2014, revised 16 September 2014 Accepted for publication 29 September 2014 Published 5 November 2014 Abstract This work investigates the effects of inserting an ultrathin ferromagnetic Py or Co spacer layer (SL) at the interface of ferromagnet/antiferromagnet ilms, namely Co/SL(Py)/IrMn and Py/ SL(Co)/IrMn. The exchange bias ield, H EB , of the two series of ilms changes with the spacer thickness in very different manners. Whereas H EB of the Py/SL(Co)/IrMn series decreases monotonously with the Co layer thickness, a fourfold initial bias enhancement is obtained for Py spacers with thicknesses up to 1 nm. Moreover, the Co/SL(Py)/IrMn series shows a maximum H EB more than 50% higher than that of the Py/IrMn ilm which, in turn, is almost three times higher than the H EB of the control Co/IrMn ilm. Simulations of experimental magnetization curves through a polycrystalline model for exchange bias showed that these variations, which could yield improvement in the performance of modern spintronic devices, are predominantly governed by modiications of the interfacial exchange interactions. Several mechanisms that could be responsible for the observed behavior are discussed. Keywords: exchange bias, magnetic properties of exchange interactions, magnetic anisotropy (Some igures may appear in colour only in the online journal) 0022-3727/14/475001+6$33.00 doi:10.1088/0022-3727/47/47/475001 J. Phys. D: Appl. Phys. 47 (2014) 475001 (6pp)