Abstract
The use of magnetisation decay measurements to characterise very slow relaxation of the magnetisation in single-molecule magnets is becoming increasingly prevalent as relaxation times move to longer timescales outside of the AC susceptibility range. However, experimental limitations and a poor understanding of the distribution underlying the stretched exponential function, commonly used to model the data, may be leading to misinterpretation of the results. Herein we develop guidelines on the experimental design, data fitting, and analysis required to accurately interpret magnetisation decay measurements. Various measures of the magnetic relaxation rate extracted from magnetisation decay measurements of [Dy(Dtp)2][Al{OC(CF3)3}4] previously characterised by Evans et al., fitted using combinations of fixing or freely fitting different parameters, are compared to those obtained using the innovative square-wave “waveform” technique of Hilgar et al. The waveform technique is comparable to AC susceptometry for measurement of relaxation rates on long timescales. The most reliable measure of the relaxation time for magnetisation decays is found to be the average logarithmic relaxation time, e⟨ln [τ]⟩, obtained via a fit of the decay trace using a stretched exponential function, where the initial and equilibrium magnetisation are fixed to first measured point and target values respectively. This new definition causes the largest differences to traditional approaches in the presence of large distributions or relaxation rates, with differences up to 50% with β = 0.45, and hence could have a significant impact on the chemical interpretation of magnetic relaxation rates. A necessary step in progressing towards chemical control of magnetic relaxation is the accurate determination of relaxation times, and such large variations in experimental measures stress the need for consistency in fitting and interpretation of magnetisation decays.
Original language | English |
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Pages (from-to) | 16735-16744 |
Journal | Physical Chemistry Chemical Physics |
Issue number | 25 |
DOIs | |
Publication status | Published - 2 Jun 2023 |
Acknowledgements
We acknowledge Dr Sophie Corner for sample preparation, Mr Adam Brookfield for technical support and Dr Daniel Reta for updating the CC-FIT2 code. We thank Prof. David Collison, Dr Andrea Mattioni, Mr Jakob Staab and Ms Lucia Corti for useful discussions.Funding
We thank The University of Manchester, the European Research Council (ERC-2019-STG-851504 to NFC and ERC-2018-CoG-816268 to DPM) and The Royal Society (URF191320 to NFC) for funding. We acknowledge the EPSRC UK National Electron Paramagnetic Resonance Service for access to the SQUID magnetometer (EP/S033181/1)