Oximato-bridged light-lanthanide Ln4Cu complexes showing ferromagnetic Ln-Cu exchange coupling

► Exchange couplings were evaluated by high-frequency EPR and magnetization techniques. ► This method can be broadly applied to 3d-4f heterometallic complexes. ► The coupling is ferromagnetic for all of the light lanthanide Ln4Cu compounds. ► A chemical trend of the coupling parameter is found in the order of the periodic table.