Abstract
Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh3)2X2 (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.
| Original language | English |
|---|---|
| Pages (from-to) | 11110-11125 |
| Number of pages | 16 |
| Journal | Chemistry - A European Journal |
| Volume | 27 |
| Issue number | 43 |
| DOIs | |
| State | Published - Aug 2 2021 |
Funding
US National Science Foundation (NSF, CHE-1900296 to Z.-L.X. and DMR-2004732 to S.H.) and a Shull Wollan Center Graduate Research Fellowship (Z.L.) are acknowledged for partial support of the research. Part of this work was performed at the National High Magnetic Field Laboratory which is supported by NSF Cooperative Agreement No. DMR-1644779 and the State of Florida. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the Disk Chopper Spectrometer (DCS) used in this work and HZB, Germany, for the allocation of the FD-FT THz-EPR instrument time. Additional neutron scattering experiments were conducted at the VISION beamline at ORNL's Spallation Neutron Source, which is supported by the Scientific User Facilities Division, Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE), under Contract No. DE-AC0500OR22725 with UT Battelle, LLC. The computing resources were made available through the VirtuES and the ICEMAN projects, funded by the Laboratory Directed Research and Development program and the Compute and Data Environment for Science (CADES) facility at ORNL. We thank Dr. Wei Zhou of the NIST Center for Neutron Research (NCNR) for help with the DCS experiment on Co-Cl, Dr. Thomas Lohmiller (HZB) for help with simulation of the FD-FT THz-EPR data, Dr. Andrew Ozarowski (NHMFL) for his EPR simulation and fit program SPIN, Prof. Jesper Bendix, Copenhagen University, Denmark, for the program Ligfield, and Prof. Michael A. Hitchman, University of Tasmania, Australia, for helpful discussions at the time of the original HFEPR study on Co-Cl, which are now relevant. US National Science Foundation (NSF, CHE‐1900296 to Z.‐L.X. and DMR‐2004732 to S.H.) and a Shull Wollan Center Graduate Research Fellowship (Z.L.) are acknowledged for partial support of the research. Part of this work was performed at the National High Magnetic Field Laboratory which is supported by NSF Cooperative Agreement No. DMR‐1644779 and the State of Florida. We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the Disk Chopper Spectrometer (DCS) used in this work and HZB, Germany, for the allocation of the FD‐FT THz‐EPR instrument time. Additional neutron scattering experiments were conducted at the VISION beamline at ORNL's Spallation Neutron Source, which is supported by the Scientific User Facilities Division, Office of Basic Energy Sciences (BES), U.S. Department of Energy (DOE), under Contract No. DE‐AC0500OR22725 with UT Battelle, LLC. The computing resources were made available through the VirtuES and the ICEMAN projects, funded by the Laboratory Directed Research and Development program and the Compute and Data Environment for Science (CADES) facility at ORNL. We thank Dr. Wei Zhou of the NIST Center for Neutron Research (NCNR) for help with the DCS experiment on , Dr. Thomas Lohmiller (HZB) for help with simulation of the FD‐FT THz‐EPR data, Dr. Andrew Ozarowski (NHMFL) for his EPR simulation and fit program SPIN, Prof. Jesper Bendix, Copenhagen University, Denmark, for the program Ligfield, and Prof. Michael A. Hitchman, University of Tasmania, Australia, for helpful discussions at the time of the original HFEPR study on , which are now relevant. Co‐Cl Co‐Cl
| Funders | Funder number |
|---|---|
| CADES | |
| Data Environment for Science | |
| Scientific User Facilities Division | |
| Shull Wollan Center | DMR‐1644779 |
| State of Florida | |
| National Science Foundation | DMR‐2004732, CHE‐1900296 |
| U.S. Department of Energy | DE-AC0500OR22725 |
| National Institute of Standards and Technology | |
| U.S. Department of Commerce | |
| Basic Energy Sciences | |
| Oak Ridge National Laboratory | |
| Laboratory Directed Research and Development | |
| NIST Center for Neutron Research | |
| University of Tasmania | |
| Københavns Universitet |
Keywords
- avoided crossings
- magnetic relaxation
- single-molecule magnets (SMMs)
- spin-phonon coupling
- zero-field splitting (ZFS)