Superexchange coupling between a dilute Gd3+ probe and orphan spins in a valence-bond glass

The valence-bond-glass state of the 4d 1 double perovskite Ba2YMoO6 combines Mo 5+ singlets with residual paramagnetic "orphan" spins whose coupling is hard to reach with bulk probes. We probe them by electron paramagnetic resonance of a dilute Gd 3+ ion on the Y 3+ site via broadband coplanar-waveguide transmission at 5 K. The isostructural nonmagnetic niobate Ba2YNbO6, doped with the same Gd 3+ probe, is a fixed-lattice control. The Gd 3+ linewidth nearly doubles in the magnetic host. Dipolar coupling to the sparse orphans is nearly an order of magnitude too weak; the excess is instead reproduced by a transferred f-d superexchange of J 0.2 K, so the width is a static measure of orphan-spin coupling. The half-field (Delta m S = +/-2) satellite collapses in the magnetic host, a dynamical dephasing channel, while the unchanged g-factor marks a zero-mean random internal field. Together the two channels probe the coupling and dynamics of valence-bond-glass orphan spins.