Magnetic properties and spin dynamics of 3d-4f molecular complexes

TL;DR

This study investigates the magnetic behaviors and spin dynamics of three new 3d-4f molecular complexes using magnetization and NMR techniques, revealing diverse magnetic interactions and relaxation mechanisms at different temperatures.

Contribution

It provides detailed experimental insights into the magnetic interactions and spin relaxation processes in binuclear 3d-4f complexes, highlighting the effects of antiferromagnetic and ferromagnetic couplings.

Findings

[NiNd] exhibits antiferromagnetic coupling with a low-spin ground state.

[NiGd] shows ferromagnetic coupling with a high-spin ground state.

[ZnGd] behaves as a weakly interacting magnetic assembly with unexplained anomalies.

Abstract

We present the magnetic properties of three recently synthesized binuclear molecular complexes [NiNd], [NiGd] and [ZnGd] investigated by dc magnetization and proton nuclear magnetic resonance (NMR) measurements. The high-temperature magnetic properties are related to the independent paramagnetic behavior of the two magnetic metal ions within the binuclear entities both in [NiNd] and [NiGd]. On lowering the temperature, the formation of a magnetic dimer, with a low-spin ground state due to antiferromagnetic interaction (J/kB = -25 K) between Ni2+ and Nd3+, is found in the case of [NiNd], while in [NiGd] a ferromagnetic interaction (J/kB = 3.31 K) between the magnetic ions leads to a high-spin (S = 9/2) ground state. The temperature dependence of the proton nuclear spin lattice relaxation rate 1/T1 in [NiNd] is driven by the fluctuation of the hyperfine field at the nuclear site due to relaxation of the magnetization. At high temperature the independent Ni2+ and Nd3+ spins fluctuate fast while at low temperature we observe a slowing down of the fluctuation of the total magnetization of the dimer because of the insurgence of antiferromagnetic spin correlations. The relaxation mechanism in [NiNd] at low temperature is interpreted by a single, temperature dependent, correlation frequency wc\simT^3.5, which reflects the life time broadening of the exchange coupled spins via spin-phonon interaction. The proton NMR signal in [NiGd] could be detected just at room temperature, due to the shortening of relaxation times when T is decreased. The magnetic properties of [ZnGd] are the ones expected from a weakly interacting assembly of isolated moments except for anomalies in the susceptibility and NMR results below 15 K which currently cannot be explained.