High-field magnetization of the distorted spin frustration system Ni ₅(TeO ₃) ₄Br ₂

We performed high-field magnetization measurements, M(H), on the Ni ₅(TeO ₃) ₄Br ₂ by using a pulse magnet. The Ni ²⁺ ions (S = 1) of this compound form a two dimensional distorted spin frustrated system (distorted kagome lattice), and undergo a Neel transition at T _N ∼29 K, due to the anisotropy. Below T _N, a complex spin arrangement which contains ten spin sub-lattices was suggested by neutron scattering and electron spin resonance studies. We observed a step-like transition at H _c ∼11 T when T<T _N . By contrast, at T>T _N, the field-dependent magnetization curves behaved like a straight line. The H _c is close to those obtained by previous spin resonance studies of Ni ₅(TeO ₃) ₄Br ₂ and Ni ₅(TeO ₃) ₄Cl ₂ in which a spin-flop-like transition was proposed to explain the field-dependent resonance spectra. Interestingly, the observed change in magnetization (ΔM) at H _c corresponds to nearly one-half of the magnetic moment of Ni ²⁺ ion, suggesting that the observed step-like behavior possibly corresponds to a spin-flip-like transition (i.e. from antiferromagnetic state to ferrimagnetic state). In addition, in the high-field region (H>H _c ), the M(H) curves did not show plateau behavior, but exhibited the straight lines having finite slopes without any sign of saturation up to 55 T.