Abstract
We report on a comprehensive characterization of the newly synthesized Cu$^{2+}$-based molecular magnet [Cu(pz)$_2$(2-HOpy)$_2$](PF$_6$)$_2$ (CuPOF), where pz = C$_4$H$_4$N$_2$ and 2-HOpy = C$_5$H$_4$NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, $\mu^+$SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the 2D square-lattice $S=1/2$ antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of $J/k_\mathrm{B} = 6.80(5)$ K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to $XY$ type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature $XY$ anisotropy under the influence of the interlayer coupling, occurs at $T_\mathrm{N} = 1.38(2)$ K, as revealed by $\mu^+$SR. In applied magnetic fields, our $^1$H-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at $T_\mathrm{N} =2.8$ K and 7 T.
Highlights
The study of critical phenomena, related to phase transitions between exotic ground states that emerge from complex underlying electronic correlations, is a subject of high importance in the research of low-dimensional magnetism
Room-temperature electron-spin resonance (ESR) studies were performed on polycrystalline material and single crystals of CuPOF, using a commercially available X-Band Bruker ESR spectrometer operating at 9.8 GHz at Clark University
High-frequency ESR measurements along the crystallographic c axis at 1.5 K and fields up to 16 T were performed at the High Magnetic Field Laboratory (HLD) by using a home-built transmission-type tunable-frequency ESR spectrometer, similar to that described in Ref. [55], with a probe in Faraday configuration
Summary
The study of critical phenomena, related to phase transitions between exotic ground states that emerge from complex underlying electronic correlations, is a subject of high importance in the research of low-dimensional magnetism. For the case of crystalline magnetic lattices of Cu2+ ions, the exchange coupling between spin moments as well as the single-ion properties are almost isotropic Another approach to realize a well-defined investigation of the magnetic correlations in low-dimensional spin systems is by tuning the Zeeman terms of the effective Hamiltonian with the application of an external magnetic field. A weak intrinsic easy-plane anisotropy, revealed by bulk magnetometry, yields a temperature-driven crossover of the spin-correlation anisotropy from isotropic Heisenberg to anisotropic XY -type behavior, which, under the influence of a finite interlayer coupling J , constitutes a driving mechanism for a transition to long-range commensurate order. A strong increase of the transition temperature upon application of a magnetic field from 1.38(2) K at zero field to 2.8 K at 7 T is caused by the field-driven increase of the anisotropy of spin correlations
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