Atomic-clock transition behavior in a Cr₇Mn molecular nanomagnet

A clock transition (CT) occurs at an avoided level crossing where the transition frequency is independent on the magnetic field. Enhancement of T₂ has been observed at clock transitions in some molecular nanomagnets, suggesting these systems as viable spin qubits [1,2]. The study of such systems can elucidate the mechanisms of decoherence since spin-spin interactions are suppressed at CT. A CT is observed in the S = 1 molecular magnet Cr₇Mn with pulsed electron-spin resonance. The Carr-Purcell Meiboom-Gill (CPMG) pulse sequence in dilute samples of Cr₇Mn increases T₂ more than an order of magnitude over that obtained with a Hahn echo sequence, indicating that a significant decoherence mechanism arises from fluctuations that are slower than the delay between the CPMG pulses (~1 us). We also observe electron spin echo envelope modulation (ESEEM) at fields slightly away from the zero-field avoided crossing. Remarkably, CPMG provides a significant enhancement to T₂ when the pulse period matches the ESEEM period, suggesting that CPMG dynamically decouples the molecular and the nuclear spins.
[1] M. Shiddiq et al., Nature 531, 348 (2016).
[2] C. Collett et al., Magnetochemistry, 5, 4 (2019).