Tuning the magnetocrystalline anisotropy in $R$CoPO by means of $R$   substitution: a ferromagnetic resonance study
  We report on broad-band electron spin resonance measurements performed within the itinerant ferromagnetic phase of $R$CoPO ($R$ = La, Pr, Nd and Sm). We reveal that the $R$ substitution is highly effective in gradually introducing a sizeable easy-plane magnetocrystalline anisotropy within the Co sublattice. We explain our results in terms of a subtle interplay of structural effects and of indirect interactions between the $f$ and $d$ orbitals from $R$ and Co, respectively. 
G. Prando, A. Alfonsov, A. Pal, V. P. S. Awana, B. B\"uchner, V.   Kataev
cond-mat.str-el