Magnetic-field-tuned phase transition of a copper compound from the isolated-spin to the coupled-spin regime probed by EPR

SANTANA, VINICIUS T.; NEUGEBAUER, PETR; NASCIMENTO, OTACIRO R; CALVO, RAFAEL

Glasgow

Congreso; 52nd Annual Meeting of the ESR group of the Royal Society of Chemistry; 2019

Royal Society of Chemistry

Molecular compounds with one or more coordinated metal ionsper molecule may be connected by covalent or non-covalent paths to their neighbours.These so-called coordination polymers are relevant to several fields such asmolecular magnetism, catalysis, biochemistry, and others. Besides, they may displayinteresting phenomena such as quantum phase transitions.  We report electron paramagnetic resonance(EPR) studies at Q and X-bands of powder and oriented single crystal samples ofthe compound [Cu(N',N'-dimethyl-N'-benzoylthiourea)(2,2'-bipyridine)Cl], calledCuBMB.[1] EPR spectra of single crystal samples at Q-band and room temperature display  abrupt mergings and narrowing of the peaks arisingfrom two rotated copper sites with the orientation of an external magneticfield B0. Weak intermolecular exchange interactions |Ji| between neighbour copperspins, responsible for the exchange narrowing processes produce a quantumtransition from an array of quasi-isolated spins to a quantum-entangled spinarray. This transition occurs when the magnitudes of the anisotropic contributionsto the Zeeman couplings, tuned with the direction of B0, approachthese |Ji| and producelevel crossings. We pinpoint the excellent capability of EPR in this kind ofstudies, where weak exchange coupling magnitudes would require extremely lowtemperatures to be detected via thermodynamic techniques such magneticsusceptibility measurements. The effect of an entagled phase in the EPRspectrum directly affects the properties of an absorption profile via themodulation of the linewidth by the exchange narrowing phenomenon even at roomtemperature. Besides, we indicate specific features in the powder spectra thatpoint to the presence of an entangled phase. A similar approach can be followedfor any system where either inter or intra molecular exchange coupling causesthe collapse of the EPR spectra around transition crossings.[2] In conclusion,it is clear that molecules can be engineered to behave according to theexpected phase depending on the application and we intend to further investigatethis phenomenon at higher frequencies and magnetic fields for a wider range ofmaterials and spin-spin interaction magnitudes.________________[1]    V. T. Santana, B. N. Cunha, A. M. Plutín, R.G. Silveira, E. E. Castellano, A. A. Batista, R. Calvo and O. R. Nascimento, Phys. Chem. Chem. Phys., 21, 4394?4407(2019).[2]  R. Calvo, V. T. Santana and O. R. Nascimento, Phys. Rev. B, 96, 064424 (2017).