Magnetically trapped erbium and thulium: rapid spin relaxation

Spin relaxation due to atom--atom collisions is measured for buffer-gas cooled and magnetically trapped erbium and thulium atoms at a temperature near 500~mK. The rate constants for Er--Er and Tm--Tm collisions are $3.0 \times 10^{-10}$ and $1.1 \times 10^{-10}$ cm$^3$ s$^{-1}$, respectively, 2--3 orders of magnitude larger than those observed for highly magnetic $S$-state atoms. This is strong evidence for an additional, dominant spin relaxation mechanism, electronic interaction anisotropy, in collisions between these ``submerged-shell," $L \neq 0$ atoms. This result is in contrast to the dramatic suppression of electronic interaction anisotropy in collisions between these atoms and helium. The large spin relaxation rates observed here imply that evaporative cooling of these atoms in a magnetic trap will be highly inefficient.