Metamagnetic Transition in Low-Dimensional Site-Decorated Quantum Heisenberg Ferrimagnets

TL;DR

This paper investigates phase transitions in low-dimensional decorated quantum Heisenberg ferrimagnets, revealing exact solutions and the potential for ultranarrow phase crossover phenomena at finite temperatures, with implications for magnetic material applications.

Contribution

It provides an exact solution for a decorated quantum ferrimagnet model in the large exchange limit and predicts finite-temperature phase transitions and ultranarrow phase crossover phenomena.

Findings

Identified two finite-temperature second-order phase transitions.

Discovered a 'half-ice, half-fire' regime above the lower transition.

Suggested the possibility of ultranarrow phase crossover at finite temperature.

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 $T_0$ 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 $\mu_aS_a<\mu_bS_b$. 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 $T_0$ in 2D square lattices thanks to its exponentially strong initial magnetic susceptibility $\chi_0\propto e^{4\pi S_a^2 J/T_0}$, though less likely in 1D chains where $\chi_0\propto J/T_0$. These results may shed light on new technological applications of low-dimensional quantum spin systems and attract experimental and computational tests.