Exchange coupling in as-deposited and annealed magnetic trilayers Co80Fe20 (CoFe)/MgO/Co80Fe20 (CoFe) for MgO thickness spanning 0.45 to 4.5 nm has been studied. Unlike relevant existing results and theories, antiferromagnetic exchange coupling behavior was observed in annealed CoFe/MgO/CoFe with 2.7-nm MgO insulating barrier. Figure 1 shows in-plane minor hysteresis loops of the as-deposited and annealed CoFe/MgO/CoFe trilayer for MgO thickness spanning 1.8 to 4.5 nm, respectively. Interestingly, the shift of the hysteresis loop toward positive or negative field depends on the temperature. For the temperature range of 25 to 500 deg. C, the hysteresis loop is shifted to the negative and to the positive field for 600 deg. C as shown in Fig. 2(a) 1. Figure 2(b) a transition from antiferromagnetic to nearly paramagnetic response to applied magnetic field was found around 520 deg. C—conceptually called as Neel temperature. Based on grazing incidence x-ray diffraction study, formation of CoO and CoFe2O4 were found at annealed CoFe/MgO interfaces 2, which indicates they act as antiferromagnetic sources giving rise to the exchange-coupled antiferromagnetic/ferromagnetic interface and then antiferromagnetic coupling across the magnetic trilayer 3.
References:
1 T. Ambrose et al., Phys. Rev. B 56, 83 (1997).
2 S. Grytsyuk et al., Eur. Phys. J. B. 85, 254. (2012).
3 J. Spray et al., J. Phys. D: Appl. Phys. 39, 4536 (2006).

Fig. 1 Magnetic hystereis loops of (a) as-deposited and (b) annealed CoFe/MgO/CoFe films for MgO thickness spanning 1.8 to 4.5 nm.

Fig. 2 (a) Temperature dependence of magnetic hysteresis loops of 4-nm CoFe/2.7-nm MgO/8-nm CoFe film annealed at 250 deg. C for 1 hour at the rotation angle 75 deg. (b) exchange coupling field of the same film as mentioned in (a).