Magnetic transition in 122 pnictides XFe2As2 (X=Ca, Sr) probed by hyperfine interaction of 66Ga

• We provide detailed measurement of 66Ga hyperfine field in XFe2As2 (X = Ca, Sr) employing TDPAD technique.
• Hyperfine field data shows second order type magnetic transition.
• The critical exponent near phase transition suggests moments are out of ab plane.
• Fe possesses small local moment showing itinerant behaviour.

Employing the time differential perturbed angular distribution (TDPAD) method we have studied the temperature dependence of the magnetic hyperfine field, Bhf for 66Ga in 122 type pnictides, XFe2As2 (X = Ca and Sr). The saturation hyperfine field, Bhf (0) of Ga in Ca(Sr)Fe2As2 have been observed to be 16.2 kG and 22 kG respectively. Scaled by the hyperfine field of 66Ga in ferromagnetic elements Fe, Co and Ni, our Bhf (0) data indicate small local moments on Fe, consistent with itinerant type magnetism of these materials. The observed temperature dependence of the hyperfine field data suggests second order type magnetic phase transition with a critical exponent β≈0.22(3)β≈0.22(3) indicating quasi-two dimensional magnetic ordering, possibly due to canting of Fe moments out of the ab plane.