  The insulating pyrochlore compound Nd2Sn2O7 has been shown to undergo a
second order magnetic phase transition at Tc ~ 0.91 K to a noncoplanar
all-in--all-out magnetic structure of the Nd3+ magnetic moments. An anomalously
slow paramagnetic spin dynamics has been evidenced from neutron backscattering
and muon spin relaxation (muSR). In the case of muSR this has been revealed
through the strong effect of a 50 mT longitudinal field on the spin-lattice
relaxation rate. Here, motivated by a recent successful work performed for
Yb2Ti2O7 and Yb2Sn2O7, analyzing the shape of the muSR longitudinal
polarization function, we substantiate the existence of extremely slow
paramagnetic spin dynamics in the microsecond time range for Nd2Sn2O7. Between
1.7 and 7 K, this time scale is temperature independent. This suggests a double
spin-flip tunneling relaxation mechanism to be at play, probably involving spin
substructures such as tetrahedra. Unexpectedly, the standard deviation of the
field distribution at the muon site increases as the system is cooled. This
exotic spin dynamics is in sharp contrast with the dynamics above 100 K which
is driven by the Orbach relaxation mechanism involving single Nd3+ magnetic
moments.
