V. K. Sankaranarayanan

Exchange bias in NiFe∕FeMn∕NiFe trilayers

NiFe∕FeMn∕NiFe trilayer structure forms an integral part of many conventional and tunneling magnetoresistance spin valve structures with FeMn antiferromagnetic layer. A systematic investigation of the exchange bias variations of the seed and top pinned NiFe layers in the NiFe∕FeMn∕NiFe trilayer structure is reported as a function of thickness of all the three constituting layers, in multilayers prepared by rf magnetron sputtering. X-ray diffraction patterns show the (111) texture for the NiFe and FeMn layers, necessary for the development of antiferromagnetic γ-fcc phase. In thickness variation studies of all the three magnetic layers, seed NiFe layer shows greater bias (150Oe) than the top pinned NiFe layer (80Oe only). The exchange bias shows the expected 1∕t behavior for increasing NiFe layer thickness after initial maxima at low thickness. In the FeMn antiferromagnet layer thickness variation on the other hand, the large bias values attained around 5nm thickness is nearly retained up to a thickness of...

Distribution of interlayer‐exchange coupling on MTJ multilayer

Local M-H loops have been measured on the free layer of a magnetic tunnelling junction using a magneto-optical Kerr effect system, with 2-μm spatial resolution, to investigate the distribution of the interlayer exchange-coupling field (HE) and the coercivity (H c ) on the free layer. HE and He increase gradually from a junction edge, reach a maximum at the centre and then fall off gradually at the other edge. Annealing enhances HE and smooths it out at the junction centre but H c decreases after annealing. Two-dimensional plots of HE and H c show symmetric saddle shapes with their axes aligned with the pinned layer due to the shadow-mask effect during deposition. HE measured along the pinned layer in this case shows a ridge curve with peaks at junction edges but H c is nearly constant. Atomic force microscopy measurements show that the roughness variations over the junction are similar to HE with greater roughness at the centre, indicating mutual correlation and confirming the origin of HE to be Neels orange-peel coupling based on surface roughness.

Exchange bias investigations in FeMn based multilayers

We have carried out a systematic investigation of the development of exchange bias in an NiFe/FeMn/NiFe trilayer and in a trilayer with a nonmagnetic interlayer, in order to understand the development of bias in FeMn-based rf magnetron-sputtered multilayers. The trilayer shows two bias loops corresponding to the two NiFe layers where the seed layer shows a greater bias. The bias shows the expected 1/t variation with increasing seed layer thickness. The bias for the top NiFe layer of 5 nm constant thickness increases initially in the 2–6-nm range to reach a maximum value, and thereafter shows a surprising increase in bias at larger seed-layer thicknesses. The introduction of an Al layer of increasing thickness at the top FeMn surface causes the vanishing of the bias for the top NiFe layer and an increase in bias for the seed layer up to 1-nm thickness of Al layer and a decrease thereafter. The seed layer interestingly shows greater bias when an Al layer is present.