Magnetism of the spin-1 tetramer compound A2Ni2Mo3O12 (A = Rb or K)

We measured the temperature dependence of the magnetic susceptibility χ(T ) and the specific heat C(T ) and the magnetic-field dependence of the magnetization M(H) of A2Ni2Mo3O12 (A = Rb or K) powder. We consider that the probable spin model is an interacting spin-1 antiferromagnetic tetramer model. We evaluated values of the intratetramer interactions as J1 = 9 K and J2 = 18 K, and the effective intertetramer interaction as Jeff = 4 K for Rb2Ni2Mo3O12. The susceptibility and magnetization at 1.3 K of K2Ni2Mo3O12 are very close to those of Rb2Ni2Mo3O12. We observed a phase transition to a magnetically ordered state in C(T )/T in magnetic fields above 3 T. The transition temperature increases with magnetic field. Probably, the ordered state appears around 1.8 K even in 0 T. The ordered state in 0 T, however, is not stable enough like an order in the vicinity of a quantum critical point. Longitudinal-mode magnetic excitations may be observable in single crystalline A2Ni2Mo3O12 (A = Rb or K).