1 Magnetic Order in the Pyrochlore Iridates A 2 Ir 2 O 7 (A = Y, Yb) S. M. Disseler 1 , Chetan Dhital 1 , A. Amato 2 , S. R. Giblin 3 , Clarina de la Cruz 4 , Stephen D. Wilson 1 , and M. J. Graf 1 * 1 Department of Physics, Boston College, Chestnut Hill, MA 02467, USA 2 Paul Scherrer Institute, CH 5232 Villigen PSI, Switzerland 3 Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK 4 Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA Abstract We present results from muon spin relaxation/rotation, magnetization, neutron scattering and transport measurements on polycrystalline samples of the pyrochlore iridates Y 2 Ir 2 O 7 (Y-227) and Yb 2 Ir 2 O 7 (Yb-227). Well-defined spontaneous oscillations of the muon asymmetry are observed together with hysteretic behavior in magnetization below 130 K in Yb-227, indicative of commensurate long-range magnetic order. Similar oscillations are observed in Y-227 below 150 K; however the onset of hysteretic magnetization at T = 190 K indicates a transition to an intermediate state lacking long-range order as observed in Nd-227. Our results also show that insulating members of the iridate family have nearly identical magnetic ground states, and that the presence of magnetic A-site species does not play any significant role in altering the ground state properties.