Main Source of Microwave Loss in Transition‐Metal‐Doped Ba(Zn 1/3 Ta 2/3 )O 3 and Ba(Zn 1/3 Nb 2/3 )O 3 at Cryogenic Temperatures

Microwave resonator measurements were performed on high‐performance microwave ceramics Ba(Zn 1/3 Ta 2/3 )O 3 ( BZT ) and Ba(Zn 1/3 Nb 2/3 )O 3 ( BZN ) containing additives commonly used by commercial manufacturers (i.e., Co, Mn, and Ni). We find that the loss tangent, even in ambient magnetic fields, is dominated by electron paramagnetic resonance ( EPR ) absorption by exchange‐coupled 3d electrons in transition metal clusters at cryogenic temperatures. The large orbital angular momentum in Co 2+ and Ni 2+ ions of L  = 3 causes strong anisotropic‐broadened dipolar interactions that extend EPR losses to zero applied field. This effect is greatest in BZN with Co concentrations greater than 0.5 mol%, dominating the losses at liquid nitrogen temperatures (77 K) and below. In samples containing Mn 2+ ions with L  = 0, the dipolar interactions and associated EPR losses in ambient fields are smaller. We show the magnetic‐field‐dependent changes in the EPR losses (i.e., tan δ) and magnetic reactive response (i.e., μ r ) are from the same mechanism, as they follow the Kramers–Kronig relation. Finally, we note that these materials can make ultra‐high Q passive microwave devices with externally controlled transfer functions, as the quality factor ( Q ) of the composition Ba(Co 1/15 Zn 4/15 Nb 2/3 )O 3 at 77 K can be tuned from 1 100 to 12 000 at 10 GHz by applying practical magnetic fields.