The search for magnetic order in δ-Pu metal using muon spin relaxation

We review results from previous muon spin relaxation (μ SR) measurements in applied fields of H0=0H0=0 and 0.25 T which established an upper limit for the ordered or disordered frozen spin moment above T=4T=4  K in δ-Pu (4.3 at.% Ga) of μord≤10−3μBμord≤10−3μB. In addition, we present new data in H0=0.25H0=0.25 and 2 T applied field on a highly annealed δ-Pu (4.3 at.% Ga) sample. Neither the muon Knight shift (H0=2H0=2  T) nor the inhomogeneous linewidths in the new sample show appreciable temperature dependence below about T=60T=60  K, also consistent with no spin freezing. Recent theoretical arguments advanced to explain these results are mentioned.