Temperature and set field dependence of exchange bias training effects in Co/NiO/[Co/Pt] heterostructures with orthogonal easy axes A. Baruth n , S. Adenwalla Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588-0111, USA article info Article history: Received 14 November 2009 Received in revised form 13 January 2010 Available online 28 January 2010 Keywords: Exchange bias Training effect NiO abstract Training effects in a new class of exchange biased ferromagnet/antiferromagnet/ferromagnet trilayers (Co/NiO/[Co/Pt] 3 ) with mutually orthogonal easy axes have been measured and successfully modeled. Previous experiments have demonstrated an enhanced blocking temperature as well as the ability to isothermally field tune the magnitude of the room temperature in-plane exchange bias. These effects have been attributed to the presence of the [Co/Pt] multilayer with perpendicular magnetic anisotropy, which variably pins the backside NiO domains. Here we show that the tuning of the exchange bias and the blocking temperature enhancement are highly dependent on both the temperature and the in-plane remanence of the normally out-of-plane [Co/Pt] multilayer, achieved using modest in-plane set fields. Training effects and their dependence on temperature and in-plane remanence are modeled using a thermodynamic approach. The in-plane remanence of the [Co/Pt] acts only to set the equilibrium exchange bias value and sets the scale for the blocking temperature; it has no effect on the training. We conclude that training effects occur only at the Co/NiO interface and that the relaxation towards equilibrium is confined to this interface. The field enhanced blocking temperature and isothermal tuning of exchange bias in these magnetic heterostructures with mutually orthogonal easy axes could play a role in the enhancement of exchange bias effects in future spin-valve devices. A thorough knowledge of the training effects is essential to account for the fundamental relaxation mechanisms that occur with repeated field cycling. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Interfacial coupling at the interface between a ferromagnet (FM)/antiferromagnet (AFM) leads to a symmetry breaking and a subsequent shift of the hysteresis loop (the exchange bias), among other phenomena. Although discovered over 50 years ago [1], exchange bias continues to pose intriguing questions [2, and references therein], one of which is the training effect, in which the exchange bias field is progressively reduced on repeated magnetic field cycling [2,3]. Exchange bias has important technological applications in magnetic memory devices [4–6] and a clear understanding of the training effect could lead to technological advances by increasing the magnitude of the exchange bias. Exchange bias has been modeled by allowing for the formation of multiple domains, usually within the antiferro- magnet. A net interfacial magnetization within the antiferro- magnet, S AFM , exchange couples to the FM. The training effect is commonly ascribed to the rearrangement of these domains towards equilibrium on repeated field cycling, thereby altering S AFM . Numerous models based on experimental observations of the training effect have been proposed [7–13]. Much of the data on the training effect fit a 1= ffiffiffi n p dependence [2,14–17], although an understanding of this dependence has been lacking. The addition of non-magnetic impurities to the AFM [18] leads to an increase in the exchange bias, and has been ascribed to the lower energy cost associated with the formation of a domain wall that passes through a non-magnetic impurity. Monte Carlo modeling of the interface magnetization of these diluted AFM films displays hysteretic behavior where the hysteresis loop does not close at positive saturation [18], implying a decrease in interfacial magnetization with subsequent loops leading to a decrease in the exchange bias. This effect has also been studied extensively by C. Binek in the framework of non-equilibrium thermodynamics, in which consecutive magnetization cycles rearrange the interface spins of the AFM towards equilibrium [11]. This more contem- porary approach provides better insight and predictive power into the temperature dependence [19] of the training effect as well as a physical basis for the phenomenological 1= ffiffiffi n p dependence, and was recently used to successfully model tunable training effects in a Ni/NiO bilayer structure [20]. In this paper, we measure and model the training effect in a new class of exchange biased magnetic heterostructures, consisting of ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.01.033 n Corresponding author. Current address: Department of Chemical Engineering and Material Science, University of Minnesota—Twin Cities, Minneapolis, MN 55455-0132, USA. Tel.: + 1 612 626 2535. E-mail address: agbaruth@umn.edu (A. Baruth). Journal of Magnetism and Magnetic Materials 322 (2010) 2051–2057