The relationship between the transformation kinetics and exchange field development in PtMnCr/NiFe bilayers

PtMnCr and other ordered tetragonal alloys used as antiferromagnetic pinning layers are in a nonmagnetic, disordered FCC phase upon deposition. In order to induce the FCT antiferromagnetic L10 ordered phase, a high temperature anneal must be performed. In PtMnCr, the ordered phase can nucleate and grow in either a [111] textured or random orientation. [111] textured growth of the FCT phase results in a shift in the PtMnCr [111] Bragg angle, while random growth results in a decrease in intensity of the peak. In addition, the random growth causes an increase in the intensity of the PtMnCr [111] peak detected off axis using low angle X-ray diffraction. Using this, the relationship between the transformation kinetics and development of the exchange field is studied in detail with X-ray and TEM. It is found that the development of exchange field depends on three stages of the ordering transformation: (1) a nucleation period in which no exchange field develops, (2) growth of the ordered phase, in which the exchange field develops in proportion to the FCT phase formed at the interface, and (3) a period in which the coercivity increases after the bulk transformation has completed. The mechanisms responsible for this behavior are discussed.