Javier Luzon

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*/kB ≈ −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 EA,SCM = 2.9(3) K and EB,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.

The very low-temperature relaxation processes of the magnetic moment in the “butterfly” molecule [Fe3Tb(μ3–O)2(CCl3COO)8(H2O)(THF)3], {Fe3TbO2}, have been investigated by a combination of heat capacity and ac susceptibility measurements, both at zero magnetic field and under an applied magnetic field. The heat capacity has been interpreted within an approximation in which the Fe3 subcluster has a total spin SFe3 = 5/2, the Tb3+ (J = 6) ligand field interaction is described in terms of Stevens operators, and an Fe3–Tb antiferromagnetic Heisenberg interaction is considered, with JTbFe3/kB = –0.23 K. The coupled Fe3 (Kramers) and Tb (non-Kramers) yield to the cluster to behave as a Kramers electronic spin system. At zero applied field (H = 0), the uniaxial anisotropy barrier introduced by the magnetic lanthanide ion (Tb3+) gives rise to two thermally activated quantum tunneling of magnetization (TAQTM) Orbach processes. The first relaxation pathway predominates at the lowest range of temperatures with a TAQTM1 process with an activation energy of E1/kB = 1.35(1) K. Since the Tb3+ ion consists entirely of isotopes with non-zero nuclear spin (I = 3/2), the tunneling processes are assisted via electron–nuclear hyperfine pathways. As the system is cooled below T = 0.17 K, a slow direct acoustic phonon assisted relaxation process between the nuclear split ground states is dominant. This latter process is affected by phonon-bottleneck effect. A second, TAQTM2 faster process is observed at the highest measured temperatures, with activation energy of E2/kB = 3.70(1) K. Both activations may be related to the first and second excited energy levels of the magnetic cluster. Upon cooling, the relaxation time crosses over to a temperature-independent quantum tunneling of magnetization regime via electron–nuclear hyperfine pathways. At temperatures above 2 K, even under the application of an external magnetic field, the relaxation time decreases below the experimental time window, resulting in too fast relaxation processes. Consequently, the {Fe3TbO2} molecule can be classified as a nanomagnet exhibiting single-molecule magnet behavior at sub-K temperatures.

[Tb(coum) ₃ (batho)]·[0.7EtOH] is a versatile multifunctional molecular material combining single-ion magnet behavior, high quantum yield luminescence and light-induced magnetic switching.

We report the deposition of monolayers and multilayers of {Cr₁₀(OMe)₂₀(O₂CCMe₃)₁₀} wheels, hereafter {Cr₁₀}, onto Au(111) and Cu(111) single‐crystal substrates, and their characterization combining scanning tunneling microscopy, X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, X‐ray natural linear dichroism, and X‐ray magnetic circular dichroism (XMCD). {Cr₁₀} in bulk shows axial magnetic anisotropy and a cluster total spin S = 9 ground state, stemming from an interaction scheme of two semi‐crowns containing four Cr³⁺ ions interacting ferromagnetically, separated by two Cr³⁺ antiferromagnetically coupled. The one‐monolayer (1ML) samples of {Cr₁₀} sublimated on Ag(111) and Cu(111) show slightly different applied magnetic field dependence of XMCD signal. The field‐dependence of the magnetization evolves from a lower curve for the 1ML {Cr₁₀} samples to a curve resembling the bulk one as the number of layers is increased, as shown in a 14ML sample. Monte Carlo simulations allow rationalizing the magnetization curves of the 1ML samples in terms of a reduction of the cluster ground‐state total spin, S = 3 or S = 6, as a consequence of variations in the intra‐wheel coupling interactions induced by the on‐surface deposition. The sensitivity of the magnetic configuration of {Cr₁₀} to minor distortions of the intramolecular conformation might be exploited to control magnetism by external stimuli for applications.

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H₂O}_n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn_0.8Co_0.2CxAPy]·1.5 CH₃OH}_n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C_2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a t_lf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/k_B = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

Two new neodymium molecular magnets of formula {[Nd(α-fur) ₃ (H ₂ O) ₂ ]·DMF} _n ( 1 ) and {[Nd _0.065 La _0.935 (α-fur) ₃ (H ₂ O) ₂ ]} _n ( 2 ), α-fur = C ₄ H ₃ OCOO, have been synthesized and characterized.

The new polymeric magnet {[Tb ₂ Ba(α-fur) ₈ (H ₂ O) ₄ ]·2H ₂ O} _n constitutes a rare example of AF coupled dimers of non-Kramers ions displaying slow relaxation dynamics.

The characterization of the crystal structure, phase transitions, magnetic structure and dielectric properties has been carried out on [CH₃NH₃][Co(COOH)₃] (1) perovskite‐like metal–organic compound through variable‐temperature single‐crystal and powder neutron and X‐ray diffraction and relative permittivity measurements. The paraelectric to antiferroelectric‐like phase transition observed at around 90 K is triggered by a structural phase transition; the structural studies show a change from Pnma space group at RT (1A) to P2₁/n space group at low temperature (1B). This phase transition involves the occurrence of small distortions in the framework and counterions. Neutron diffraction studies have shown a magnetic order showing spontaneous magnetization below 15 K, due to the occurrence of a non‐collinear antiferromagnetic structure with a weak ferromagnetic component, mainly due to the single‐ion anisotropy of the Co^II ions.

A {Tb(α-fur) ₃ } _n one-dimensional complex shows Single-Chain-Magnet (SCM) behavior at H = 0 in two different types of antiferromagnetic transverse chains (A and B), triggered by the existence of defects breaking the chains into segments with short-range order.