Exchange coupling and energy-level crossing in a magnetic chain [Dy ₂Cu₂]_n evaluated by high-frequency electron paramagnetic resonance

A 4f-3d heterometallic polymeric coordination compound [{Dy(hfac) ₂(CH₃OH)}₂{Cu(dmg)(Hdmg)}₂] _n ([Dy₂Cu₂]_n; H₂dmg = dimethylglyoxime; Hhfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) was synthesized, and the X-ray crystallographic analysis shows that the structure is isomorphous to the known ferrimagnetic [Gd₂Cu₂] _n polymer. A centrosymmetric diamond-arrayed ferrimagnetic unit involving the oximate bridge, Dy-O-N-Cu, is repeated to form a discrete chain. An onset of the frequency dependence was found in the ac magnetic susceptibility measurements down to 1.8 K. Low-temperature magnetization measurements on [Dy₂Cu₂]_n exhibited magnetic hysteresis with magnetization steps. To examine the energy level structure and the exchange coupling between the Dy and Cu ions, high-frequency EPR (HF-EPR) spectra of a polycrystalline sample of [Dy₂Cu₂]_n were recorded at various frequencies (34.7-525.4 GHz) and temperatures. We analyzed the spectra by treating the Dy moments as Ising spins and built an exchange-coupling model for a diamond-arrayed tetranuclear macrocycle [Dy ₂Cu₂]. The Dy-Cu exchange couplings were precisely evaluated, owing to the high resolution of HF-EPR; J/k_B = -0.895(8) and -0.061(8) K for two independent Dy-Cu relations, where the exchange parameter is defined as - J. The Cu⋯Cu coupling among the macrocycles was estimated to be ferromagnetic with approximately 1 K. The present study has established the definitive methodology to examine the energy level in 4f-3d heterometallic systems by using HF-EPR. The magnetic properties of the [Dy ₂Cu₂]_n chain were plausibly described as a perturbed system of a single-molecule magnet. The quantum tunneling of the magnetization is based on a different principle compared with those of the 3d-based single-molecule magnets, since the energy level structure of [Dy ₂Cu₂]_n is regulated by the exchange couplings.