High frequency EPR spectra of a molecular nanomagnet provide a key to understand Quantum Tunneling of the Magnetization

TL;DR

High-frequency EPR spectra of Mn12ac reveal the role of transverse magnetic anisotropy in quantum tunneling of magnetization, providing detailed parameters of the spin Hamiltonian and insights into relaxation mechanisms.

Contribution

First high-frequency EPR spectra of a molecular nanomagnet that quantify the transverse anisotropy term responsible for quantum tunneling.

Findings

Identification of the fourth order anisotropy term in the spin Hamiltonian.

Quantitative analysis of the transverse magnetic anisotropy.

Correlation of spectral irregularities with quantum tunneling phenomena.

Abstract

EPR spectra have been recorded in very high field, up to 25T, and at high frequency, up to 525 GHz, on a polycristalline sample of Mn12ac (see paper for detailed formula), the first example of molecular cluster behaving like a nanomagnet. The simulation of the spectra has provided an accurate determination of the parameters of the spin hamiltonian (see paper for formula and values of the various parameters). The presence of the fourth order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to Quantum Tunneling between degenerate M states of the ground S=10 multiplet of the cluster. The term in (S_+^4 + S_-^4) is responsible of the transverse magnetic anisotropy and plays a crucial role in the mechanism of Quantum Tunneling. The HF-EPR spectra have for the first time evidenced its presence and quantified it.