Investigation of Exchange Bias in Bilayer Systems with FORC Diagrams

Recently, it has been shown that an improved characterization of hysteretic processes can be obtained by using a set of First Order Reversal Curves (FORC) that scan uniformly the Major Hysteresis Loop (MHL) of the sample. In this paper we are presenting the results of systematic studies, based on the FORC diagram method, made on a CoFe / NiMn system. Glass / NiFeCr 6.5 / CoFe x / NiMn y / NiFeCr 4.5 multilayers have been deposited in a Nordiko 3600 IBD system, using a 50 Oe field to induce the easy axis. The exchange bias is obtained after 1 hour annealing at 260degC in a field of 3 kOe applied in the direction of the easy axis. Two set of samples have been measured: in one the thickness of the ferromagnetic (F) CoFe layer have been varied from 1 to 6 nm in steps of 1 nm using a 30 nm antiferromagnetic (AF) NiMn layer and in the other 5 nm of CoFe has been coupled with 20, 30 and respectively 40 nm of NiMn. The increase of the AF thickness induces a movement of the entire FORC distribution towards higher values of exchange fields and higher coercive fields.