μ+SR measurements on pure and copper doped samples of the 1D antiferromagnet Ni(C2H8N2)2NO2(ClO4) (NENP)

Relaxation reflecting the freezing/slowing down of spins is seen in pure and Cu-doped NENP below ≈ 30 K. This relaxation increases sharply for both materials near 2.5 K and saturates below ≈ 1 K. The applied field dependence of the μ+SR spectra at 20 mK indicates that the internal fields corresponding to these spins are still fluctuating and are probably too large to be attributed to loose chain ends or impurities.