The effect of nanosized CoFe 2 O 4 addition on the magnetic properties of GdBa 2 Cu 3 O 7δ using AC magnetic susceptibility measurements /R. AWAD a,b,* , N. H. MOHAMMED a , A. I. ABOU ALY a , S. ISBER c , H. A. MOTAWEH d , D. EL-SAID BAKEER d , M. ROUMIÉ e - AWAD,R.