Origin of spontaneous exchange bias in Co/NiMn bilayer structure

Spontaneous exchange bias (EB) is reported for as deposited Si/Pt(t(Pt))/Ni45Mn55(t(AFM)/Co(t(FM))/Pt(30 angstrom) thin film system without requiring any post annealing, deposition with field or field cooling procedures. Magnetic properties of this system were investigated with respect to thicknesses of buffer Pt layer (t(Pt)), antiferromagnetic NiMn layer (t(AFM)) and ferromagnetic Co layer (t(FM)). Exchange coupling between NiMn and Co layers enhanced considerably by increasing t(Pt). In order to observe a spontaneous EB in the system, Pt buffer layer must be thicker than a certain thickness, and NiMn layer must be grown directly on the buffer layer. On the other hand, significant increments in the coercive fields (H-C) were reported for thinner Pt buffer layers. The thickness ranges for Co and NiMn layers were also determined to obtain spontaneous EB. This spontaneous EB is discussed to be a result of NiMn (111) texture which is induced by Pt buffer layer. Greater EB fields (H-EB) are measured for the samples in the negative field direction by the effect of annealing and field cooling (from 400 K to 300 K at 2 kOe). (C) 2016 Elsevier B.V. All rights reserved.