Abstract
Both spin-liquid and magnetically ordered phases of both half-integer and integer low-spin quantum magnets are of interest, since the magnetic structures found in the latter case usually have no classical counterparts. Such a magnetic structure was found in a combined experimental and theoretical study of the integer spin system Ni(NO3)(2). Our thermodynamic measurements have revealed a magnetically ordered phase with small spontaneous magnetization at T-C = 5.5 K. The magnetization saturation of about 2 mu(B) at low temperatures corresponds to the high-spin state (S = 1) of Ni2+ ions evidenced in L-2,L-3 edges in x-ray absorption spectroscopy spectra. We show that a consistent description of the available data is possible within a noncollinear umbrella-type ferrimagnetic ground state model for which both intra-and interlayer magnetic interactions should be antiferromagnetic. Such a scenario is suggested by the first-principles and model calculations.