Nuclear Relaxation Rates in Single-Domain Cobalt

Recent studies indicate that the “intrinsic” nuclear spin-lattice relaxation rate, 1/T1, in the domains of bulk ferromagnetic metals is obtained only when an external field Hex is applied which exceeds the demagnetizing field and no walls remain. This has motivated the present work, on small, single-domain Co particles with the following results: (1) At low fields and at all temperatures between 1.3° and 300°K a very strong field dependence of T1 is found; T1 appears to be constant when Hex ≳ 4 kOe. (2) At all temperatures 1/T1 decreases monotonically with increasing particle size. (3) Studies above 4 kOe show that, unlike in the bulk, T1T is not constant. However, (d/dT) (1/T1) approaches a constant value for T ≳ 60°K which is independent of particle size. This is indicative of a contribution to the relaxation rate, not present in the bulk particles, arising from paramagnetic-like surface atoms that are exchange-coupled either to each other or to the cobalt host. No effects of superparamagnetism were found even for the smallest (∼110 Å) particles.