We analyze the entanglement and phase diagram of the exact ground state of finite spin-s arrays with ferromagnetic XXZ couplings under non-uniform fields. It is first shown that these systems can exhibit an exceptionally degenerate multicritical point where all magnetizations plateaus merge and entanglement reaches full range [1], which corresponds to a factorization point. It can arise for a wide variety of field configurations, including standard and sparse alternating fields. It is also shown that under such fields, these systems can exhibit a non-trivial magnetic behavior, with significant magnetization plateaus persistent for large sizes and exhibiting peculiar entanglement properties [1,2]. Finally, it is demonstrated that in these systems the fully aligned phase becomes reachable only above a threshold value of the coupling anisotropy, which depends on the field configuration and can be determined analytically, and below which the maximum attainable magnetization becomes much smaller [2].

[1] M. Cerezo, R. Rossignoli, N. Canosa, E. Ríos, Phys. Rev. Lett. 119, 220605 (2017).

[2] M. Cerezo, R. Rossignoli, N. Canosa, C. Lamas, Phys. Rev. B 99, 014409 (2019).