Magnetic ground states of RMn4Al8, R=La, Pr, Y

Yamasaki et al. (Solid State Commun. 119 (2001) 415) have suggested that LaMn4Al8 is a nearly antiferromagnetic metal with linear chains of Mn atoms. We present evidence from ZF-μSR measurements that both LaMn4Al8 and the related compound PrMn4Al8 display antiferromagnetic order at low temperatures. In LaMn4Al8 the μSR relaxation rate rises to a peak at 4.5 K; below this temperature the asymmetry decreases smoothly. In PrMn4Al8 well-defined oscillations appear below 35 K. Neutron crystal field spectroscopy suggests a singlet ground state for the Pr3+ ion, so it is likely that the internal field seen by the muons arises from ordered Mn moments. YMn4Al8 shows no sign of magnetic correlations down to 1.2 K