Microwave resonator measurements were performed on high‐performance microwave ceramics Ba(Zn1/3Ta2/3)O3 (BZT) and Ba(Zn1/3Nb2/3)O3 (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 Co2+ and Ni2+ 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 Mn2+ 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., μr) 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(Co1/15Zn4/15Nb2/3)O3 at 77 K can be tuned from 1 100 to 12 000 at 10 GHz by applying practical magnetic fields.

The temperature coefficient of resonant frequency (s f ) of a microwave resonator is determined by three materials parameters according to the following equation: s f =À(½ s e + ½ s l + a L ), where a L , s e , and s l are defined as the linear temperature

Low-frequency electron paramagnetic resonance (EPR) is used to extract the EPR parameter A-mid and support the approximate X-band value of g-mid for Ba(CoyZn1/3−yTa2/3)O3. Although the cobalt hyperfine structure for the |±1/2〉 state is often unresolved at X-band or S-band, it is resolved in measurements on this compound. This allows for detailed analysis of the molecular orbital for the |±1/2〉 state, which is often the ground state. Moreover, this work shows that the EPR parameters for Co substituted into Zn compounds give important insight into the properties of zinc binding sites.

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The physical nature and concentration of paramagnetic point defects in the dielectrics of superconducting planar microwave resonators have been determined using in-situ electron paramagnetic resonance spectroscopy. To perform this work, the quality factor of parallel plate and stripline resonators was measured as a function of the magnitude of a magnetic-field applied parallel to the electrode surfaces. YBa2Cu3O7−δ thin film electrodes proved to be a preferred choice over Nb and MgB2 because they are readily available and have a small surface resistance (Rs) up to high temperatures (∼77 K) and magnetic fields (i.e., <1 T). Stripline resonators with a widely used high performance microwave dielectric, Co2+-doped Ba(Zn1/3Nb2/3)O3, are shown to have losses dominated by d-electron spin-excitations in exchange-coupled Co2+ point-defect clusters, even in the absence of an applied magnetic field. A significant enhanced microwave loss in stripline and parallel plate resonators is found to correlate with the presence of paramagnetic Mn2+ dopants in Ba(Zn1/3Ta2/3)O3 ceramics and dangling bond states in amorphous Si thin films, although the identification of the dominant loss mechanism(s) in these dielectrics requires further investigation.