Two-dimensional magnetism in the pnictide superconductor parent material SrFeAsF probed by muon-spin relaxation

We report muon-spin relaxation measurements on SrFeAsF, which is the parent compound of an iron-arsenic- fluoride based series of superconducting materials. We find that this material has very similar magnetic properties to LaFeAsO, such as separated magnetic and structural transitions (T-N=122 K,T-s=175 K), contrasting with SrFe2As2 where they are coincident. The muon oscillation frequencies fall away very sharply at T-N [beta=0.22(3)], which suggests that the magnetic exchange between the layers is weaker than in comparable oxypnictide compounds. Our specific-heat measurements show that a small but distinct feature occurs at T-s. There is no anomaly evident at T-N as is typical for quasi-two-dimensional magnets. The magnetic entropy change between 100 and 185 K is Delta S=0.5 J mol(-1) K-1.