The competition between interlayer and interfacial exchange couplings is found to be temperature dependent in Co(3 nm)/AF/Fe(10 nm) trilayers with AF≡antiferromagnetic NiO, Cr2O3 or Cr. The temperature dependence in trilayers with AF insulating NiO or Cr2O3 spacer layer differs from that with AF metallic Cr. In the insulator case, the enhancement in the interlayer exchange coupling and the reduction in interfacial exchange coupling with increasing temperature results in dominating interlayer exchange coupling at high temperature. In the metallic spacer case, both the couplings decrease with increasing temperature, resulting in decoupling at high temperatures.
Unconventional exchange bias (EB) has been studied in CoCr2O4/Cr2O3 nanocomposites, in which the Curie temperature of the ferrimagnetic CoCr2O4 is much lower than the Néel temperature of the antiferromagnetic Cr2O3. A negative EB field of about 2.5 kOe at 5 K is achieved upon cooling in a field of 30 kOe. Meanwhile, the coercivity of the CoCr2O4 nanoparticles has been enhanced significantly by coupling with Cr2O3. The effect of the cooling field on the EB field and coercivity at 10 K has also been investigated. The domain-state model is used to interpret the unconventional EB. Cooling field may play a decisive role in the creation of the interfacial spin configuration for the unconventional EB, not only by exchange interaction between the induced magnetization of a polarized paramagnet and interfacial spins of an antiferromagnet but also by Zeeman interaction between the domain-state surplus magnetization and the external field.
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