Critical exponents and intrinsic broadening of the field-induced transition in NiCl₂ · 4SC(NH₂)₂

The field-induced ordering transition in the quantum spin system NiCl₂ · 4SC(NH₂)₂ is studied by means of neutron diffraction, ac magnetometry, and relaxation calorimetry. The interpretation of the data is strongly influenced by a finite distribution of transition fields in the samples, which was present but disregarded in previous studies. Taking this effect into account, we find that the order-parameter critical exponent is inconsistent with the BEC universality class even at temperatures below 100 mK. All results are discussed in comparison with previous measurements and with recent similar studies of disordered Ni(Cl_1−xBr_x)₂ · 4SC(NH₂)₂.