Hydrothermally synthesized oxalate and phenanthroline based ferrimagnetic one-dimensional spin chain molecular magnets [{Fe(Δ)Fe(Λ)}_1-x{Cr(Δ)Cr(Λ)}_x(ox)₂(phen)₂]_n (x = 0, 0.1 and 0.5) with giant coercivity of 3.2 Tesla

Abstract

Oxalate (ox) and phenanthroline (phen) ligands based one dimensional spin chain molecular magnets, [{Fe(Δ)Fe(Λ)}_1-x{Cr(Δ)Cr(Λ)}_x(ox)₂(phen)₂]_n (x = 0, 0.1 and 0.5) have been designed, and synthesized using a hydrothermal synthesis method. The Rietveld refinement of the powder X-ray and neutron diffraction patterns at room temperature confirms the single-phase formation of the compounds in the monoclinic structure with a space group P2₁. The compounds consist of two ligands, the oxalate (C₂O₄^2-) as a coordination acceptor building block and the neutral phen (C₁₂H₈N₂) as a coordination donor building block. Both ligands are connected to Fe ions of different symmetry {Fe(Δ) and Fe(Λ)}, thus forming an alternating zigzag chain like crystal structure having the repeating unit of [phen-Fe(Δ)-C₂O₄-Fe(Λ)-phen]_n. The chain is infinite in length and lies in the crystallographic ac plane. The interchain is well separated with an intermetallic distance of ∼8.8 Å and the absence of an interchain π–π overlap between the organic ligands, resulting in a magnetic isolation between the interchains. The Mössbauer spectroscopy reveals the presence of high spin states of the Fe²⁺ ions of the compound for x = 0 whereas, both high-spin Fe²⁺ (t_2g⁴e_g², S = 2) as well as low spin Fe²⁺ (t_2g⁶e_g⁰, S = 0) states are present for the compounds x = 0.1 and 0.5. The dc magnetization measurements show that the compounds exhibit spontaneous magnetization below ∼9 K. The transition temperature is found to be ∼8.7, 8.2 and 4.0 K for x = 0, 0.1 and 0.5 compounds, respectively. Moreover, a short range antiferromagnetic spin–spin correlation around 18–45 K has been observed for the compounds x = 0 and 0.1. An application of the Ising chain model to the dc magnetization data reveals the presence of a one-dimensional magnetic nature of all compounds with alternately spaced magnetic Fe sites. It is observed that the different Landé g factors (3.4 and 2.8) and exchange coupling constant values (-86 and -54 K) for x = 0 at two alternating Fe sites give rise to a ferrimagnetic-like behavior of the chains. The ferrimagnetic chain like structure transforms toward antiferromagnetic with Cr doping i.e. for x = 0.1 and 0.5. A hysteresis loop with a giant coercivity (3.2 T for x = 0) has been observed at 1.6 K, indicating a hard magnet-type behavior. The frequency dependence of the peak temperature in ac susceptibility vs. temperature curves for the x = 0 compound has been fitted and analyzed using the Arrhenius law as well as the power law, which exclude the possibility of a spin glass behavior. The fitted parameters (Δ/k_B = 208 K and τ₀ = 2.9 × 10^-14 s obtained from the Arrhenius law, and τ₀ = 6.1 × 10^-8 s, and zυ = 2.6 from the power law) show that the compound obeys the Glauber dynamics and is a real ferrimagnetic one-dimensional single chain magnet. In addition, the high pressure magnetization measurements for the x = 0 compound show an enhancement in the transition temperature from ∼8.7 to 10.7 K with increasing pressure. The observation of both a one-dimensional spin chain nature and giant coercivity (3.2 Tesla) in such compounds opens up new opportunities to design and develop low dimensional molecular chain magnets through the appropriate choice of ligands using the hydrothermal synthesis method, because the observation of magnetic hysteresis of molecular origin in single-molecule magnets is considered one of the most relevant achievements in molecular magnetism.