Effects of spin-lattice coupling and a magnetic field in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

TL;DR

This study uses Monte Carlo simulations to explore how spin-lattice coupling influences magnetic ordering in classical Heisenberg antiferromagnets on breathing pyrochlore lattices, revealing robust magnetization plateaus and unconventional phases linked to lattice distortions.

Contribution

It introduces and compares bond-phonon and site-phonon models to show how breathing bond-alternation affects magnetic phases and long-range order in pyrochlore antiferromagnets.

Findings

The 1/2 magnetization plateau is robust against bond-alternation.

Long-range magnetic order appears only in the site-phonon model.

Breathing bond-alternation induces unconventional tetrahedron-based orders.

Abstract

We theoretically investigate spin-lattice coupling (SLC) effects on the in-field ordering properties of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. Here, we use the two possible simplified models describing the effect of local lattice distortions on the spin ordering via the SLC, the bond-phonon and site-phonon models. It is found by means of Monte Carlo simulations that in both models, the $\frac{1}{2}$ plateau shows up in the magnetization curve being relatively robust against the breathing bond-alternation, although magnetic long-range orders (LRO's) are realized only in the site-phonon model. In the bond-phonon model, additional further neighbor interactions are necessary to induce a magnetic LRO. In the site-phonon model, it is also found that in addition to the low-field, middle-field 1/2-plateau, and high-field phases appearing on both the uniform and breathing pyrochlore lattices, various types of unconventional phases which can be viewed as LRO's in units of tetrahedron are induced by the breathing bond-alternation just below the 1/2 plateau and the saturation field. The occurrence of these tetrahedron-based orders could be attributed to the nature characteristic of the breathing pyrochlore lattice, i.e., the existence of the nonequivalent small and large tetrahedra. Experimental implications of our result are also discussed.