Exchange Interactions and High-Energy Spin States in Mn_12-acetate

TL;DR

This study uses inelastic neutron scattering to analyze the magnetic excitation spectrum of Mn_12-acetate, revealing detailed spin states and exchange interactions, and constraining the magnetic coupling parameters.

Contribution

It provides a comprehensive experimental and theoretical analysis of exchange interactions and high-energy spin states in Mn_12-acetate, identifying the dominant exchange couplings.

Findings

Low-energy modes are S=9 excitations above the S=10 ground state.

The first S=11 excitation appears at 27meV (310K).

Only a model with specific exchange couplings matches experimental data.

Abstract

We perform inelastic neutron scattering measurements on the molecular nanomagnet Mn_12-acetate to measure the excitation spectrum up to 45meV (500K). We isolate magnetic excitations in two groups at 5-6.5meV (60-75K) and 8-10.5meV (95-120K), with higher levels appearing only at 27meV (310K) and 31meV (360K). From a detailed characterization of the transition peaks we show that all of the low-energy modes appear to be separate S = 9 excitations above the S = 10 ground state, with the peak at 27meV (310K) corresponding to the first S = 11 excitation. We consider a general model for the four exchange interaction parameters of the molecule. The static susceptibility is computed by high-temperature series expansion and the energy spectrum, matrix elements and ground-state spin configuration by exact diagonalization. The theoretical results are matched with experimental observation by inclusion of cluster anisotropy parameters, revealing strong constraints on possible parameter sets. We conclude that only a model with dominant exchange couplings J_1 ~ J_2 ~ 5.5meV (65K) and small couplings J_3 ~ J_4 ~ 0.6meV (7K) is consistent with the experimental data.