Exchange bias setting driven by a spontaneous crystallization of the antiferromagnetic layer

Trabajo presentado en el 21st International Conference on Magnetism (ICM2018), celebrado en San Francisco (California, EE.UU.), del 15 al 20 de julio de 2018
Many magnetic devices in advanced electronics rely on the exchange bias (EB) of a soft ferromagnetic layer coupled to an antiferromagnetic material1-3. Setting and optimizing the exchange bias involves a thermal treatment in presence of an external magnetic field, which affects the degree of magnetic order at the interface between both materials4-5. In this work we demonstrate an alternative process for the generation of exchange bias6. In IrMn/FeCo bilayers, we have found the deposition conditions which leave the IrMn layer in a metastable amorphous/nanocrystalline phase (top inset to fig. 1). During the days after deposition, a structural phase transition in the IrMn layer develops spontaneously at room temperature and spreads according to a two-dimensional nucleation and growth process (fig. 1), leading to a highly crystalline, chemically-disordered γ-phase (bottom inset to fig. 1), which was characterized by XRD and TEM. The magnetic characterization by VSM and MOKE microscopy (fig. 2) reveals that the disordered state of the as-deposited IrMn does not lead to any measurable exchange bias in the adjacent FeCo layer. As the crystalline IrMn phase propagates across the sample, the FeCo gets progressively exchange-biased in the direction of its remanent magnetization. This allows the interesting possibility of controlling the direction of the exchange bias, simply by switching the magnetization of the FeCo layer as the phase transition propagates in the IrMn. The resulting microscopic patterning of the EB direction in a spin valve device is shown in fig. 2. The study of this mechanism for establishing and tailoring the exchange bias in IrMn/FeCo bilayers, can contribute toward the clarification of fundamental aspects of this phenomenon, as well as toward the development of innovative spintronic devices.