We examine the general features of the noncommutativity of the magnetization operator and Hamiltonian for small quantum spin clusters. The source of this noncommutativity can be a difference in the Land\'e
g factors for different spins in the cluster,
XY anisotropy in the exchange interaction, and the presence of the Dzyaloshinskii-Moriya term in a direction different from the direction of the magnetic field. As a result, zero-temperature magnetization curves for small spin clusters mimic those for the macroscopic systems with the band(s) of magnetic excitations, i.e., for the given eigenstate of the spin cluster the corresponding magnetic moment can be an explicit function of the external magnetic field yielding the nonconstant (nonplateau) form of the magnetization curve within the given eigenstate. In addition, the
XY anisotropy makes the saturated magnetization (the eigenstate when all spins in cluster are aligned along the magnetic field) inaccessible for finite magnetic field magnitude (asymptotical saturation). We demonstrate all these features on three examples: a spin-1/2 dimer, mixed spin-(1/2,1) dimer, and a spin-1/2 ring trimer. We consider also the simplest Ising-Heisenberg chain, the Ising-
XYZ diamond chain, with four different
g factors. In the chain model the magnetization curve has a more complicated and nontrivial structure than that for clusters.
The integrability aspects of the continuum limit of the one-dimensional anisotropic Heisenberg spin chain in a transverse magnetic field are studied by carrying out a Painlevé singularity structure analysis. The analytic structure of the system exhibits some unusual features and requires expansions that start like a Taylor series. A careful analysis exhibits the presence of a movable logarithmic critical singular manifold in the combined presence of the anisotropy and transverse field, thereby elucidating the nonintegrable nature. Numerical analysis on the static spin configurations of the nonintegrable case shows typically chaotic spatial structures.
We consider the anisotropic Heisenberg spin-1/2 chain in a transverse magnetic field at zero temperature. We first determine all components of the dynamical structure factor by combining exact results with a mean-field approximation recently proposed by Dmitriev et al. [JETP 95, 538 (2002)]. We then turn to the small anisotropy limit, in which we use field theory methods to obtain exact results. We discuss the relevance of our results to neutron scattering experiments on the one-dimensional Heisenberg chain compound
Cs2CoCl4.