Tuning exchange-bias properties by thermal effects in a hard/soft bilayer

Abstract : The effect of temperature on exchange-bias properties of an antiferromagnetically coupled hard/soft bilayer ͑Tb 12 Fe 88 /Gd 40 Fe 60 ͒ is studied. In a similar manner to its cooling field dependence, a continuous transition from a negative to a positive exchange-bias field is observed with increasing temperature. The changes of magnetic configuration responsible for this effect are studied, combining both magnetization and polarized neutron reflectivity measurements. The temperature is found to enhance the exchange-bias training effect as a result of the relaxation of an interface domain wall. The present study demonstrates that both temperature and cooling field may be used to tune the exchange field. For the last 15 years, the exchange-bias effect in antiferromagnetic/ferromagnetic ͑AF/FM͒ bilayers have been widely studied due to its technological applications and the specific physical mechanisms involved. 1 When an exchange-biased system is cooled below a certain temperature known as the blocking temperature ͑T B ͒, the hysteresis loop of the FM layer is shifted toward a field H E , named the exchange-bias field. Bilayers showing an antiferromagnetic interfacial exchange coupling are particularly interesting since H E shows a continuous transition from negative to positive with increasing cooling field ͑H cf ͒. 2,3 Recently, a similar transition has also been observed as a function of temperature in an AF/FM system FeZnF 2 /Co ͑Ref. 4͒ and in an AF/ferrimagnetic bilayer NiCoO / GdFe. 5 In both cases, the given explanation involves a reorientation of the magnetic configuration inside the AF layer as the temperature changes. However, the type of reorientation mechanism proposed is quite different in the two cases. For the first one, it was assumed that an unstable domain structure is formed in the AF, which then relaxes. For NiCoO / GdFe, a memory effect is proposed, 6 which takes into account the evolution of an in-plane domain wall localized in the AF. The main difficulty in validating either one of these theories is that the interface magnetic configurations in AF layers are very difficult to probe mainly because the antiferromagnetic layer has almost no net magnetization. To go beyond this problem, we have studied a ferrimagnetic/ferrimagnetic, hard/soft bi-layer, namely, TbFe/ GdFe, which mimics antiferromagneti-cally coupled AF/FM systems. Such a system is a model as its magnetic configuration is easy to probe with conventional means. Recently, we were able to prove that during field cooling, a partial interfacial domain wall is frozen in the TbFe layer. The resulting exchange-bias field is then determined only by the orientation of the quenched TbFe magne-tization at the interface, which rotates continuously from an-tiparallel to the cooling field direction to parallel as H cf increases. 7,8 We have used this system to study the exchange-bias training effect. This effect is characterized by the evolution of the hysteresis loop with the number of field cycles. 9,10 Using polarized neutron reflectivity ͑PNR͒, we demonstrated that these changes in H E are caused by irreversible modifications of the magnetic configuration in TbFe.
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Thomas Hauet, S Mangin, F Montaigne, J. A. Borchers, Y Henry. Tuning exchange-bias properties by thermal effects in a hard/soft bilayer. Applied Physics Letters, American Institute of Physics, 2007, ⟨10.1063/1.2753108͔⟩. ⟨hal-01345235⟩

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