Magnetic domain structure and exchange coupling in epitaxial Fe/FeRh(0 0 1) and NiFe/FeRh(0 0 1) bilayers

Magnetic domain structure related to a coercivity mechanism in Fe/FeRh(0 0 1) exchange coupled ferromagnet/antiferromagnet bilayers was studied as a function of Fe layer thickness, tFe, by magnetic force microscopy (MFM). Magnetooptic magnetometry revealed coercive fields reaching a value of 250 Oe at tFe=5 nm and decreasing down to 80 Oe at tFe=20 nm. In as-grown specimens with tFe>20 nm, the MFM images show micron-scale magnetic domains with rather straight domain walls parallel or perpendicular to [1 1 0] crystallographic direction of the Fe film. For tFe<20 nm, fine and random magnetic patterns composed of small elements with a lateral size of 200–300 nm are observed. For tFe<4 nm these elements are separated. With increasing tFe they start to gather and form complicated elongated island objects. Mechanism to form such very small elements in the Fe layer is thought to be the fine magnetic structure of the underlying FeRh–Ir layer, i.e. to the excessive magnetic charges at terrace edges and screw dislocations or to a related domain structure of the antiferromagnetic layer.