Landau-Zener tunneling in the presence of weak intermolecular interactions in a crystal of Mn4 single-molecule magnets

TL;DR

This study investigates Landau-Zener tunneling in Mn4 single-molecule magnets, revealing how weak intermolecular interactions and external fields influence tunneling rates and magnetic ordering.

Contribution

It demonstrates the role of weak dipolar and exchange interactions in Landau-Zener tunneling within a well-characterized SMM system, highlighting different regimes of tunneling behavior.

Findings

Weak interactions significantly affect tunneling rates.

Magnetic ordering occurs without quenching tunneling.

Transverse fields modulate internal field reshuffling and ordering.

Abstract

A Mn4 single-molecule magnet (SMM), with a well isolated spin ground state of S = 9/2, is used as a model system to study Landau-Zener (LZ) tunneling in the presence of weak intermolecular dipolar and exchange interactions. The anisotropy constants D and B are measured with minor hysteresis loops. A transverse field is used to tune the tunnel splitting over a large range. Using the LZ and inverse LZ method, it is shown that these interactions play an important role in the tunnel rates. Three regions are identified: (i) at small transverse fields, tunneling is dominated by single tunnel transitions; (ii) at intermediate transverse fields, the measured tunnel rates are governed by reshuffling of internal fields, (iii) at larger transverse fields, the magnetization reversal starts to be influenced by the direct relaxation process, and many-body tunnel events may occur. The hole digging method is used to study the next-nearest neighbor interactions. At small external fields, it is shown that magnetic ordering occurs which does not quench tunneling. An applied transverse field can increase the ordering rate. Spin-spin cross-relaxations, mediated by dipolar and weak exchange interactions, are proposed to explain additional quantum steps.