Main Source of Microwave Loss in Transition‐Metal‐Doped Ba(Zn_1/3Ta_2/3)O₃ and Ba(Zn_1/3Nb_2/3)O₃ at Cryogenic Temperatures

Abstract

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.

In-situ electron paramagnetic resonance studies of paramagnetic point defects in superconducting microwave resonators

“…A g-factor of $4.2 EPR peak was observed at 800 G, characteristic of Co 2þ in a Ba(Zn 1/3 Nb 2/3 )O 3 lattice. 20 The EPR peak extends all the way to zero applied magnetic field due to dipole broadening of the EPR line, indicating that the dominant loss mechanism in Co-doped Ba(Zn 1/3 Nb 2/3 )O 3 is electron paramagnet resonant losses in exchange-coupled Co 2þ point defects, even without an applied field. This loss mechanism was first identified in our earlier dielectric resonator studies of dopants used in commercial high performance microwave materials.…”

“…[20][21][22]30 We chose to study these materials to further optimize the technique and verify the accuracy of our quantitative in-situ EPR superconductor microwave resonator technique. Mn-doping was chosen for this study because it exhibits classic EPR resonances with easily identified hyperfine peaks.…”

“…From 80 K to 40 K, we first observed an EPR absorption resonance from 3000 G to 5000 G, with a minimum at $4210 G. This corresponds to a g-factor of $2.01. 20,[31][32][33] The inset of Figure 2 shows that the data from the 50 K measurement exhibit the six signature Mn 2þ (3d 5 ) hyperfine EPR peaks. 20,[31][32][33] When the temperature is less than 50 K, an absorption peak (i.e., Q minimum) centered at $2050 G is observed, which corresponds to a g-factor of $4.1.…”

“…19 However, this conclusion has been shown to be incorrect for high-performance commercial microwave dielectrics (e.g., Co 2þ -doped Ba(Zn 1/3 Nb 2/3 )O 3 and Ni 2þ -doped Ba(Zn 1/3 Ta 2/3 )O 3 ), at least at low temperatures where the losses are dominated by electron paramagnetic loss (i.e., spin flips) in the d-electrons of exchange-coupled transition-metal pointdefect clusters. [20][21][22] Furthermore, losses in many dielectric materials at ultra-low temperature and low microwave power levels are dominated by two-level systems (TLSs) 23,24 that can arise from low-energy atomic motion. 25 From these facts, we can conclude there is much to be learned by determining the nature and concentration of the defects that affect the performance of microwave resonators.…”

“…% Mn-doped Ba(Zn 1/3 Ta 2/3 )O 3 and 300 lm 0.4 at. % Co-doped Ba(Zn 1/3 Nb 2/3 )O 3 were prepared by the conventional ceramic powder oxide mixing method 20 The 450 nm Si thin-films were deposited using 1500 W DC sputtering at 4.5 kA current in Ar sputter gas. Annealing was performed in an Ar environment at a temperature of 450 C for 60 min.…”