Tuning of Anisotropy in IrMn/CoFe Exchange Bias Systems

Abstract

In this paper, we report an enhancement of the anisotropy constant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> of IrMn/CoFe exchange bias systems with the variation of the NiCr seed layer thickness. This is related to an increase in the (111) texture of IrMn parallel to the interface. Silang100rang /NiCr (Xnm)/IrMn (7 nm)/CoFe (3 nm)/Ta (10 nm) with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> = 2, 4, 6, 10, 15, 20 nm were prepared by direct current (dc) sputtering. X-ray diffraction studies revealed a peak in the (111) texture of the IrMn parallel to the interface for the samples with a 6-nm NiCr seed layer which reduced with increasing NiCr thickness. NiCr seed layer thicknesses of 2, 4, and 20 nm had negligible (111) texture. Measurement of the grain size distribution and thermal activation measurements were used to calculate the anisotropy constant of the IrMn. This was found to increase from (0.4 plusmn 0.1) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ergs/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a 20-nm NiCr seed layer to (4.5 plusmn 2) times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ergs/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a 6-nm NiCr seed layer. TEM studies of the seed layers showed the 6-nm NiCr layer was amorphous/nanocrystalline, and the grain size increased with NiCr thickness. This change in the morphology of the seed layer results in a lattice mismatch with the IrMn reducing the (111) texture of the IrMn and hence <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> . This allows tuning of the anisotropy by controlling the lattice mismatch between the seed layer and the IrMn.