Cross relaxation by single spin flips in silver at nanokelvin temperatures

The cross-relaxation time τₓ between the Zeeman temperatures of the ¹⁰⁷Ag and ¹⁰⁹Ag spin species in silver metal has been measured over five decades at low nanokelvin temperatures as a function of the external magnetic field B. Besides increasing with B, τₓ decreases with spin polarization. From the data, the strength of the Ruderman-Kittel interaction between the nuclei is determined. These experiments demonstrate that thermal equilibrium is not reached via mutual flips of unlike spins, but in much slower processes involving single spin flips, caused by the dipolar interaction.