Single-crystal ac susceptibility measurements on [Co(NH<inf>3</inf>) <inf>6</inf>][CuCl<inf>5</inf>], a 3D, S=1/2 Heisenberg antiferromagnet

Single-crystal ac magnetic susceptibilities of [Co(NH3) 6][CuCl5] along the three crystallographic axes in the temperature range from 1.1 to 90 K are presented. The magnetic behavior is characteristic of a three-dimensional antiferromagnet, its ordering temperature being at Tc=3.8 K. Susceptibilty data can be fit to a Heisenberg S=1/2 simple cubic model using high-temperature series expansions extrapolated with Pade approximants. Good agreement is found for an exchange constant J/kB=-3.13 K and values of g factor ga=2.09, g b=gc =2.04, a,b, and c being the crystallographic axes. This result makes [Co(NH3)6][CuCl5] one of the few examples of a 3D antiferromagnetic Heisenberg S=1/2 model. The magnetic behavior below Tc indicates the existence of crystallographic domains due to the structural transition from cubic to tetragonal symmetry that the system has at about 280 K.