Microwave loss in the high-performance dielectric Ba(Zn1/3Ta2/3)O3 at 4.2 K.

Temperature- and magnetic-field-dependent measurements of the loss tangent in Ba(Zn(1/3)Ta(2/3))O(3) doped with transition metals (Mn, Ni) are compared to those from samples doped with other impurities (Cd, Ga, Mg, and Zr). These results, combined with pulsed electron paramagnetic resonance measurements, show conclusively that microwave loss in transition-metal-doped Ba(Zn(1/3)Ta(2/3))O(3) at cryogenic temperatures is attributable to resonant spin excitations of unpaired transition-metal d electrons in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping), a mechanism that differs from the usual suspects.