Magnetization relaxation in the single-ion magnet DySc2N@C80: quantum tunneling, magnetic dilution, and unconventional temperature dependence.

D S Krylov,B Büchner,A A Popov,S M Avdoshenko,A U B Wolter,V Bon,S Kaskel,L Spree,A Brandenburg,F Liu

doi:10.1039/c8cp01608a

Abstract

Relaxation of magnetization in endohedral metallofullerenes DySc2N@C80 is studied at different temperatures, in different magnetic fields, and in different molecular arrangements. Magnetization behavior and relaxation are analyzed for powder sample, and for DySc2N@C80 diluted in non-magnetic fullerene Lu3N@C80, adsorbed in voids of a metal-organic framework, and dispersed in a polymer. The magnetic field dependence and zero-field relaxation are also studied for single-crystals of DySc2N@C80 co-crystallized with Ni(ii) octaethylporphyrin, as well as for the single crystal diluted with Lu3N@C80. Landau-Zener theory is applied to analyze quantum tunneling of magnetization in the crystals. The field dependence of relaxation rates revealed a dramatic dependence of the zero-field tunneling resonance width on the dilution and is explained with the help of an analysis of dipolar field distributions. AC magnetometry is used then to get access to the relaxation of magnetization in a broader temperature range, from 2 to 87 K. Finally, a theoretical framework describing the spin dynamics with dissipation is proposed to study magnetization relaxation phenomena in single molecule magnets.

Highlights

DySc2N@C80 with an icosahedral carbon cage was the first endohedral metallofullerenes (EMFs) proven to be a single molecule magnets (SMMs).[28]
DySc2N@C80 with an icosahedral carbon cage was the first EMF proven to be a SMM.[28]. Both SQUID magnetometry and X-ray magnetic circular dichroism indicated that at low temperature the molecule exhibits magnetic hysteresis with an abrupt drop of the magnetization in zero magnetic field ascribed to the quantum tunneling of magnetization (QTM)
Similar to what was observed in the measurements on powder samples, the single crystal of DySc2N@C80 diluted with Lu3N@C80 showed a considerable decrease of the QTM-induced drop of magnetization in zero magnetic field (Fig. 5)

Summary

Introduction

DySc2N@C80 with an icosahedral carbon cage was the first EMF proven to be a SMM.[28]. Similar to what was observed in the measurements on powder samples, the single crystal of DySc2N@C80 diluted with Lu3N@C80 showed a considerable decrease of the QTM-induced drop of magnetization in zero magnetic field (Fig. 5). Illustrative is the difference between DySc2N@C80 and Dy2ScN@C80 The latter does not show fast QTM relaxation in zero field, which is explained by the ferromagnetic exchange and dipolar coupling between the magnetic moments of two Dy ions in the Dy2ScN cluster, creating an additional barrier and preventing QTM.[16] This work on DySc2N@C80 and our recent study of Dy2ScN@C8029 provide comprehensive information on the relaxation times of the two EMFs in a broad temperature range and allow a more detailed comparison of these SMMs. Fig. It is relatively straightforward to add microscopic parameters of the spin– phonon coupling from ab initio calculations into the temperaturedependent ge(T) term or increase the number of the spin states involved in the consideration, but this modelling goes beyond of scope of this work and will be reported elsewhere

Conclusions

Endohedral Fullerenes

Findings

18 Introduction to Molecular Magnetism