Magnetic order and spin liquid behavior in [Mo3+]^{11+} triangular magnets

TL;DR

This study investigates the magnetic properties of Mo-based molecular magnets, revealing ferromagnetic order in some compounds and quantum spin liquid behavior in others, highlighting the sensitivity of magnetic ground states to structural variations.

Contribution

The paper provides comprehensive experimental characterization of Mo3+ triangular magnets, identifying different magnetic ground states and proposing a phase diagram based on structural sensitivity.

Findings

ZnScMo3O8 and MgScMo3O8 exhibit ferromagnetic order with low Curie-Weiss temperatures.

Na3Sc2Mo5O16 shows no magnetic order down to 20 mK, indicating potential quantum spin liquid behavior.

Magnetic ground states are highly sensitive to small changes in Mo-Mo distances.

Abstract

Molecular magnets based on [Mo$_3$]$^{11+}$ units with one unpaired electron per trimer have attracted recent interest due to the identification of quantum spin liquid candidacy in some family members. Here, we present comprehensive measurements on polycrystalline samples of ZnScMo$_3$O$_8$, MgScMo$_3$O$_8$, and Na$_3$Sc$_2$Mo$_5$O$_{16}$ with the same Mo$_3$O$_{13}$ magnetic building blocks. The crystal structures are characterized with x-ray or neutron powder diffraction and the magnetic ground states are determined by performing ac and dc susceptibility, specific heat, neutron powder diffraction, and $\mu$SR measurements. Our work indicates that ZnScMo$_3$O$_8$ and MgScMo$_3$O$_8$ have ferromagnetic Curie-Weiss temperatures of 18.5 K and 11.9 K, ordered ground states with net moments (low-moment ferromagnetism or canted antiferromagnetism), and zero field ordering temperatures of $T_c =$ 6 K and $<$ 2 K respectively. On the other hand, Na$_3$Sc$_2$Mo$_5$O$_{16}$ hosts a dynamical magnetic ground state with no evidence for magnetic ordering or spin freezing down to 20 mK despite an antiferromagnetic Curie-Weiss temperature of -36.2 K, and therefore is a candidate for quantum spin liquid behavior. By comparing the present results to past work on the same family of materials, we construct a phase diagram which illustrates that the magnetic ground states of these Mo-based molecular magnets are very sensitive to small changes in the nearest neighbor Mo-Mo distance.