Energy-gap driven low-temperature magnetic and transport properties in Cr1/3MS2 (M = Nb, Ta)

The helimagnets Cr1/3MS2 (M = Nb or Ta) have attracted renewed attention due to the discovery of a chiral soliton lattice (CSL) stabilized in an applied magnetic field, but reports of unusual low-temperature transport and magnetic properties in this system lack a unifying explanation. Here, we present electronic structure calculations that demonstrate the materials are half metals. There is also a gaplike feature (width in range 40-100 meV) in the density of states of one spin channel. This electronic structure explains the low-temperature electronic and magnetic properties of Cr1/3MS2 (M = Nb or Ta), with the gaplike feature particularly important for explaining the magnetic behavior. Our magnetometry measurements confirm the existence of this gap. Dynamic spin fluctuations driven by excitations across this gap are seen over a wide range of frequencies (0.1 Hz to MHz) with ac susceptibility and muon-spin relaxation (mu+SR) measurements. We show further how effects due to the CSL in Cr1/3NbS2, as detected with mu+SR, dominate over the gap driven magnetism when the CSL is stabilized as the majority phase.