Quenched Fe moment in the collapsed tetragonal phase of Ca_1-xPr_xFe₂As₂

We report ⁷⁵As NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca_1-xPr_xFe₂As₂. In both cases of x=0.075, 0.15, a large increase of ν_q upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structural transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/⁷⁵T₁ is enhanced and forms a peak at T≈ 25 K, which may be caused by the magnetic ordering of the Pr³⁺ moments or spin dynamics of mobile domain walls.