Sub-Kelvin magnetic relaxation in the non-Kramers Ho(III) antiferromagnetic chain {Ho(α-fur)3}n

Анотація

We report the synthesis, structural and magnetothermal characterization down to sub-Kelvin temperatures of the polymeric complex {Ho(α-fur)₃}_n (α-fur = C₄H₃OCOO⁻). The α-furoate ligands bridge Ho(III) ions into one-dimensional zigzag chains running along the crystallographic c-axis. Two slightly distinct coordination environments, denoted Ho(A) and Ho(B), arise from positional disorder of one furoate ligand. Ab initio calculations were performed to estimate the zero-field splittings of both Ho at A and B sites. Magnetic susceptibility and heat capacity measurements show that at low temperatures, the compound behaves as an assembly of antiferromagnetically coupled S* =1/2 Ising-like chains (J*/k_B ≈ −0.4 K) of Ho ions with transverse anisotropy. The low-temperature susceptibility further reveals a small concentration of chain defects. Zero-field ac susceptibility measurements evidence slow relaxation of the magnetization assigned to defect-enabled single-chain magnet (SCM) dynamics in the two chain types, with activation energies E_A,SCM = 2.9(3) K and E_B,SCM = 4(3) K. Below 0.4 K, a crossover to a coherent quantum tunneling of the magnetization regime, assisted by hyperfine interactions, is observed. The relaxation behavior is discussed in comparison with the related compounds {Ln(α-fur)₃}_n (Ln = Dy, Tb) and {Ho₂Ba(α-fur)₈}_n, highlighting the interplay between Kramers versus non-Kramers ion character, hyperfine interactions, and magnetic exchange topology in governing the low-temperature dynamics.