Non-transverse factorizing fields and entanglement in finite spin systems

We determine the conditions for the existence of non-transverse factorizing magnetic fields in general spin arrays with anisotropic XYZ couplings of arbitrary range. It is first shown that a uniform maximally aligned completely separable eigenstate can exist just for fields {h}_s parallel to a principal plane and forming four straight lines in field space, with the alignment direction different from that of {h}_s and determined by the anisotropy. Such state always becomes a non-degenerate ground state (GS) for sufficiently strong (yet finite) fields along these lines, in both ferromagnetic (FM) and antiferromagnetic (AFM) type systems. In AFM chains, this field coexists with the non-transverse factorizing field {h}'_s associated with a degenerate Néel-type separable GS, which is shown to arise at a level crossing in a finite chain. It is also demonstrated for arbitrary spin that pairwise entanglement reaches full range in the vicinity of both {h}_s and {h}'_s, vanishing at {h}_s but approaching small yet finite side-limits at {h}'_s, which are analytically determined. The behavior of the block entropy and entanglement spectrum in their vicinity is also analyzed.