Exchange Coupling and Energy-Level Crossing in a Magnetic Chain [Dy2Cu2]n Evaluated by High-Frequency Electron Paramagnetic Resonance

Abstract

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 [Dy2Cu2]. The Dy−Cu exchange couplings were precisely evaluated, owing to the high resolution of HF-EPR; J/kB = −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 [Dy2Cu2]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 [Dy2Cu2]n is regulated by the exchange couplings.