Spontaneous exchange bias formation driven by a structural phase transition in the antiferromagnetic material

Dr Gonzalo Vallejo Fernandez, University of York. Part of the condensed matter seminar series.

Dr Gonzalo Vallejo Fernandez, of the Department of Physics, University of York, will be presenting a seminar on his research.  All are welcome to attend.

Many magnetic devices in advanced electronics rely on the coupling of a soft ferromagnetic layer to an antiferromagnetic material. Setting and optimising this interaction, known as the exchange bias effect, involves a thermal treatment in the presence of an external magnetic field. This affects the degree of magnetic order at the interface between both materials. In this talk I will present recent results on IrMn/CoFe bilayers where we have found deposition conditions which leave the IrMn layer in a metastable amorphous/nanocrystalline phase. During the days following 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 leading to a highly crystalline, chemically-disordered γ-phase. The magnetic characterisation 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 is progressively exchange-biased in the direction of its remanent magnetisation. This allows the interesting possibility of controlling the direction of the exchange bias, simply by switching the magnetisation of the FeCo layer as the phase transition propagates in the IrMn. The study of this mechanism for establishing and tailoring the exchange bias in IrMn/FeCo bilayers can contribute towards the clarification of fundamental aspects of this phenomenon, as well as the development of innovative spintronic devices. For further details please contact Dr Satoshi Sasaki (s.sasaki@leeds.ac.uk)