Thickness dependence of chemical ordering and fourfold exchange anisotropy in FeRh/CoFe bilayers

Fourfold exchange anisotropy has recently been discovered in bilayers consisting of a ferromagnetic (FM) layer exchange-coupling with an epitaxial antiferromagnetic (AF) layer. The chemical ordering of the AF layer plays an important role in the interfacial exchange coupling of AF/FM bilayers. Herein, we studied the thickness dependence of the chemical ordering and fourfold exchange anisotropy of FeRh/CoFe bilayers before and after the AF-FM phase transition of FeRh. The chemical ordering parameter of FeRh obtained by x-ray diffraction increases with thickness due to the decrease in the proportion of low-order interfaces, which results in an increase in the magnetic phase transition temperature and a decrease in the phase transition width, residual magnetization in the AF state, and lattice constant. After the occurrence of the AF-FM phase transition, the fourfold exchange anisotropy observed in the CoFe layer by magneto-optical Kerr effect changes from the FeRh〈110〉 to 〈100〉 directions, indicating the orientation change in the cubic magnetocrystalline anisotropy of FeRh. The fourfold exchange anisotropy measured by ferromagnetic resonance continues to increase with the FeRh thickness, indicating an effective thickness by far larger than that of chemically disordered AF systems. The FeRh/FM exchange coupling is highly dependent on chemical ordering, not only on the low-order surface of a few nanometers but also on the high-order interior extending to a depth of tens of nanometers.