Abstract
The prohibition of finite-temperature phase transition in one-dimensional (1D) Ising models and 1D/2D quantum Heisenberg models with short-range interactions fundamentally constrains the application potentials of low-dimensional magnetic materials. Recently, ultranarrow phase crossover (UNPC), which can approach a transition at a desirable finite temperature T₀ arbitrarily closely, was discovered in 1D decorated Ising chains and ladders. Here we present a theoretical study of similarly decorated, yet much more challenging, quantum Heisenberg ferrimagnets in a magnetic field, which features ferromagnetic backbone exchange J, antiferromagnetic site-decoration coupling J_AF, and different magnetic moments for the backbone and decorating spins μ_aS_a<μ_bS_b. We exactly solved the model in the large J limit -- as a central-macrospin model -- and found two finite-temperature second-order transitions; just above T_c2 a ``half-ice, half-fire'' regime appears. Finite-J weak-field results follow from an effective-field mapping, suggesting the emergence of UNPC at finite T₀ in 2D square lattices thanks to its exponentially strong initial magnetic susceptibility χ₀∝ e^4π S_a² J/T₀, though less likely in 1D chains where χ₀∝ J/T₀. These results may shed light on new technological applications of low-dimensional quantum spin systems and attract experimental and computational tests.