The dominance of paramagnetic loss in microwave dielectric ceramics at cryogenic temperatures

Abstract

Commercial manufacturers add transition metals and other impurities to high performance microwave ceramic compounds to improve their manufacturability. Measurements of microwave loss tangent were performed on Ba(Zn1/3Ta2/3)O3, ZrTiO4-ZnNb2O6, Ba(Zn1/3Nb2/3)O3, and BaTi4O9-BaZn2Ti4O11 ceramics. A marked increase in the loss at low temperatures is found in materials containing transition metal with unpaired d-electrons as a result of resonant spin excitations in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping). The loss tangent can be drastically reduced by applying static magnetic fields. Our measurements also show that this mechanism significantly contributes to room temperature loss, but does not dominate.

Measurement of dielectric loss tangent at cryogenic temperature using superconducting film resonator

“…Instead, we selected YBa 2 Cu 3 O 7-d (YBCO) superconductor films deposited on sapphire substrates, because YBCO has higher critical temperature (93 K) compared to Nb films (9.3 K), so can easily measure a larger temperature range up to 80 K. As shown by the sapphire spacer baseline reference data (red dot line), the measurement should have enough sensitivity and precision up to 70 K. The most interesting phenomenon one can observe is that, at 4.2 K, with Ni doping increases from 0 % for undoped BZT to 12 % Ni-doped BZT, the Q factor monotonically decreases and loss tangent goes up. The mechanisms for this phenomenon were well studied in [11][12][13][14] which claimed that the reason is due to resonant spin excitations of unpaired transition-metal d electrons by microwave energy in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping). The reason of high loss of undoped and 3 % Ni-doped BZT at 30-70 K temperature range is not clear so far, but probably due to some extrinsic defect-induced absorption mechanisms in these sintered samples, such as polarons, or shallow level point defects.…”

“…The resonator is then placed inside a small cavity made of oxygen free high-conductivity copper (OFHC) which is gold (purity [99.9 %) plated to minimize the radiation loss. The details of this device are well described in some references [15][16][17][18][19][20] [10][11][12][13][14]. The ceramic was sintered by traditional powder processing method [1][2][3].…”

“…The ceramic was sintered by traditional powder processing method [1][2][3]. The Ni-doped BZT sample was processed as described in [10][11][12][13][14]. First, the pure BZT and Ba(Ni 1/3 Ta 2/3 )O 3 (BNT) powder were synthesized separately.…”

“…In these three parameters, the mechanism of dielectric loss tangent is the one that has been studied by many researchers but still not understood completely [1][2][3]. Recently, abnormal behavior of microwave loss tangent of dielectric materials at cryogenic temperature range, including high-k, low-loss commercial Ba(Zn 1/3 Ta 2/3 )O 3 , (Zr, Sn)TiO 4 , Ba(Zn 1/3 Nb 2/3 )O 3 with different dopants, has been studied [10][11][12][13][14]. Absorption of microwave energy by unpaired spins in d-shell electrons in transition-metal dopant ions has been attributed to the root cause for the high microwave loss of a series of microwave ceramic materials at cryogenic temperatures.…”

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

“…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.…”