The role of interfaces in CoFe/IrMn exchange biased systems

A trilayer system consisting of an IrMn layer exchanged coupled to two CoFe layers of equal thickness has been studied. A single stage reversal was observed over a wide range of temperatures. Two bilayers with the same thicknesses of the pinning layer but different ferromagnetic thicknesses were also studied. By comparing the magnetic properties of these three stacks the effect of the interfacial area on the exchange field and the coercivity has been determined. We find that the interfacial area has a very minor effect on the exchange field H-ex and the blocking temperature (T-B) but causes a doubling of the coercivity (H-c). This indicates that H-c is dominated by the interface whereas the exchange bias is controlled by volumetric effects

Measurement of the anisotropy constant of antiferromagnets in metallic polycrystalline exchange biased systems

A method for the measurement of the anisotropy constant of the antiferromagnet (AF) KAF in exchange biased systems has been developed. This has been achieved by measurement of the median blocking temperature 〈TB〉 of a CoFe/IrMn bilayer. In thermal activation-free conditions, this is the temperature at which equal volumes of the AF are oriented in opposite senses. Hence, for a grain size dependent model, the critical volume for thermal activation at this point is equal to the median volume of the grain size distribution. A value of (5.5±0.5)×106 erg/cc has been obtained at room temperature for a 4 nm thick IrMn layer

Thermal instabilities in exchange biased materials

The use of antiferromagnetic (AF) materials to pin a soft ferromagnet (FM) in a spin-valve, read head, or MRAM cell, has focussed attention on the exchange bias phenomenon. We report on a study of the thermal stability of exchange bias systems, IrMn/CoFe and FeMn/NiFe, with different thicknesses and grain sizes. For these materials a temperature has been determined at which thermal activation does not affect the state of the antiferromagnet, leading to reproducible data. Grain size studies have shown that the coercivity and exchange bias have different origins. Training effects studied over a range of temperatures from 4.2 to 400 K indicate a range of different contributions to exchange bias from the magnetic state of the antiferromagnet

Bulk and interfacial effects in exchange bias systems

In this work we report on an extensive and detailed study of exchange bias systems consisting of two ferromagnetic layers deposited with an antiferromagnetic layer of IrMn between them. Systems with two different ferromagnetic layers have been studied in which sample 1 had CoFe layers of different thicknesses and sample 2 had not only ferromagnetic layers of different thicknesses but also the composition of the upper ferromagnetic layers was changed from CoFe to NiFe. In both the samples the antiferromagnetic layer thickness was maintained constant at 5 nm, NiCr seed and capping layers (5 nm) were used on all samples. Such a system is of considerable interest as the properties of each ferromagnetic layer are affected by the same antiferromagnet. However differences in behaviour will occur due to the nature of the interfaces between the different layers as well as other parameters such as the ferromagnetic layer thickness. We have conducted a study of thermal activation effects in these systems where both or a single ferromagnetic layer can be reversed whilst the system is heated. We find that we can differentiate between bulk and interface effects indicating that the role of spin disorder at the interface is crucial in determining the final value of the exchange bias