Probing the strongly correlated magnetic state of Co 2 C nanoparticles at low temperatures using μSR.

Co 2 C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. A recent study by Nirmal Roy et al. [1] showed that a collection of Co 2 C NPs exhibit exchange bias (EB) effect below T EB = 50 K and also a spin glass (SG) state below TSG = 5 K. We use magnetic, electrical transport, specific heat, and muon spin rotation (μSR) measurements to explore further the magnetic properties of a pellet made with 40 nm diameter pure Co 2 C NPs. We uncover the onset of Kondo localization at Kondo temperature T K (= 40.1 K), which is close to the onset temperature (T EB ) of the EB effect. A crossover from the Kondo-screened scenario to an RKKY interaction-dominated regime is also observed for T < T K . Temperature-dependent specific heat measurement further supports the Kondo localization scenario in the pellet and shows the heavy fermionic nature of the strongly correlated electronic state in Co 2 C. The zero field µSR asymmetry spectra in the low-temperature regime are characterized by two distinct fast and slow relaxation rates. The spectra show the absence of long-range magnetic order in the sample. However, our analysis suggests the NPs-pellet shows the presence of a dominant magnetically disordered fraction and a smaller fraction with short-range order. Muons in the disordered fraction exhibit a slower relaxation rate, while muons in the smaller fraction with short-range order exhibit a faster relaxation rate. We observe an increase in this fast relaxation rate between T EB and T SG . This increase below T EB ⁓ 50 K suggests a slowing down of the fluctuating local magnetic environment around muons. Transverse field (TF) - µSR asymmetry spectra show the emergence of a stable, multi-peaked local magnetic field distribution in the pellet below T EB . Longitudinal field (LF) µSR spectra shows distinct changes in the dynamics of fluctuations suggesting the presence of a frozen glassy like state below 6 K. Based on our results, we suggest that below T EB , the pellet of Co 2 C NPs develops a magnetic interface that separates the two magnetic fractions; one is a disordered fraction, and the other is a fraction with short-range order. The exchange interaction that sets in below T EB at the interface couples the two fractions, leading to a suppression of the fluctuations. With the suppression of magnetic fluctuations below T EB , strong correlation effects in the electronic state of Co 2 C lead to Kondo localization.&#xD.