Half-Step magnetization in the polyoxometalate family with {Cu3}-type triangular spin ring

We report on pulsed field magnetization and ESR measurements of the family of copper(II)-substituted polyoxotungstates [Cu3(H2O)3(α-XW9O33)2] 12− (X=As, Sb) where the three Cu (S=1/2) ions form an antiferromagnetically-coupled triangle. The distinct features are the observation of half step magnetization and hysteresis loops as well as asymmetric/symmetric magnetization between a positive and negative field, depending on the diamagnetic heteroatom X. This is attributed to the interplay between an adiabatic magnetization and Dzyaloshinskii-Moriya interactions. Moreover, the more symmetric magnetization curve of X=Sb is discussed in terms of dynamical mixing of a S=1/2 state via a phonon mode. In recent years, molecular clusters consisting of a finite number of interacting paramagnetic ions have attracted considerable interest. As zero-dimensional, nano-sized magnets these systems provide an opportunity for a better understanding of the basic principles of magnetism [1, 2]; for example, the crossover between quantum and classical behavior, the effect of geometrical frustration, quantum spin dynamics and tunneling. A unique advantage of investigating molecular magnets lies in the flexibility of synthesizing compounds with various magnetic parameters; (i) the size of magnetic spin, (ii) the sign and magnitude of exchange interaction, and (iii) the spin topology. Among them S=1/2 antiferromagnetic triangle spin rings belong to the simplest class of a frustrated spin system. In spite of a simple spin topology, however, quantum magnetization behavior is expected to be rich due to the presence of two doublets with a different spin chirality [3, 4, 5]. The prototypical example of an S=1/2 triangle spin ring is found in the copper(II)-substituted polyoxotungstates [Cu3(H2O)3(α-XW9O33)2]12− (X=As, Sb) (hereafter shortened as {Cu3-X}) [6, 7]. The {Cu3-X} compounds possess a sandwich-type structure with D3h symmetry, in which the central belt consisting of three Cu2+ ions is separated by two (α-XW9O33) subunits. A spin triangle is formed by three CuO4(H2O) square pyramids linked by Na+ ions. The spin triangle with Cu · · · Cu distances is sketched in the inset of Figure 1. In the following, we will address a quantum magnetization process in the family of the {Cu3X} compounds using fast-sweeping pulsed field magnetization and ESR [8]. Institute of Physics Publishing Journal of Physics: Conference Series 51 (2006) 95–98 doi:10.1088/1742-6596/51/1/020 Yamada Conference LX on Research in High Magnetic Fields 95 © 2006 Yamada Science Foundation and IOP Publishing Ltd