Spin coherence and magnetization dynamics of TMA₂[KCo_1-Fe(CN)₆] toward coordination-framework spin qubits

Metal compounds with S = 1/2 coordination-frameworks have been emerging as new powerful qubit candidates. In this study, we have reported the CN-based coordination framework TMA₂[KCo_1-Fe(CN)₆] to be a qubit. We explored the magnetization dynamics and spin coherence of the magnetic dilution of the S = 1/2 Fe(III) complex TMA₂[KFe(CN)₆] (TMA = tetramethylammonium) in its Co(III)-based diamagnetic analogue TMA₂[KCo(CN)₆]. Alternating-current (AC) susceptibility data illustrate a slow magnetic relaxation upon applying a field of 0.1 T, which follows the phonon-bottleneck relaxation mechanism along with the Raman process. A magnetic relaxation time (τ) of 0.3 s (2% Fe) was realized at 1.8 K. Moreover, pulsed EPR data reveal a coherence duration of 1 μs (0.1% Fe) at 4 K with successful observation of Rabi oscillation at 4 K and 13 K (2% Fe) using MW pulses with variable irradiation-field strengths. The overall results indicate that TMA₂[KCo_1-Fe(CN)₆] represents a promising qubit candidate, as it is capable of being placed in any superposition of the two distinct M_s states (M_s = +1/2 and M_s = -1/2).