High-temperature metal–organic magnets

Abstract

For over two decades there have been intense efforts aimed at the development of alternatives to conventional magnets, particularly materials comprised in part or wholly of molecular components. Such alternatives offer the prospect of realizing magnets fabricated through controlled, low-temperature, solution-based chemistry, as opposed to high-temperature metallurgical routes, and also the possibility of tuning magnetic properties through synthesis. However, examples of magnetically ordered molecular materials at or near room temperature are extremely rare, and the properties of these materials are often capricious and difficult to reproduce. Here we present a versatile solution-based route to a new class of metal-organic materials exhibiting magnetic order well above room temperature. Reactions of the metal (M) precursor complex bis(1,5-cyclooctadiene)nickel with three different organics A-TCNE (tetracyanoethylene), TCNQ (7,7,8,8-tetracyanoquinodimethane) or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)--proceed via electron transfer from nickel to A and lead to materials containing Ni(II) ions and reduced forms of A in a 2:1 Ni:A ratio--that is, opposite to that of conventional (low Curie temperature) MA(2)-type magnets. These materials also contain oxygen-based species within their architectures. Magnetic characterization of the three compounds reveals spontaneous field-dependent magnetization and hysteresis at room temperature, with ordering temperatures well above ambient. The unusual stoichiometry and striking magnetic properties highlight these three compounds as members of a class of stable magnets that are at the interface between conventional inorganic magnets and genuine molecule-based magnets.

Controlling nickel nanoparticle size in an organic/metal–organic matrix through the use of different solvents

“…However, in our samples we attribute the dominant magnetic component at low fields to be a result of bulk 5 The dielectric constant is a good indicator of the polarisability and polar nature of the solvent molecule Ni or Ni nanoparticles/clusters which are most prominent in samples 1, 2 & 4. When looking at the values for the magnetization at B app = 5 T for all the samples, except sample 1, the magnetization is of the same order of magnitude and similar values, especially once the Langevinstep function has been subtracted suggesting that despite the overall nickel nanoparticles mass being different the matrix is largely very similar with regards to elemental content.…”

“…Using only DCM as the solvent (sample 1) similar to Jain et al [5] creates a material that has two clear magnetic components; one that is superparamagentic Ni nanoparticles and a metal/organic based matrix. By initially dissolving the Ni(COD) 2 in THF, the zero-valent Ni atoms are stabilized and using halocarbon solvents (samples 2 and 4) also produces Ni nanoparticles but the blocking temperature can be controlled by varying the polarity of the solvent.…”

“…Opening the reaction mixture to air, as was done by Jain et al [5] in Ni 2 TCNQ, may result in the decomposition of these stable Ni(0)-isocyano compounds or atomic clusters. It may be that TCNQF 4 may also be reduced by the surface of the Ni atomic clusters rather than individual Ni(0) atoms and the cyano-group on the TCNQF 4 may act as an effective stabilising surface ligand.…”

“…More recently, it was reported that a novel high-temperature metal-organic ferromagnet (T C ≥400 K) could be synthesised by reacting bis-(1,5-cyclooctdiene) nickel (Ni(COD) 2 ) with the organic acceptors (A); 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), TCNE and 7,7,8,8-tetracyanoquinodimethane (TCNQ) to produce charge transfer salts Ni 2 A, via a redox reaction, using DCM as the solvent [5]. The Ni 2 A systems were reported to be amorphous yet still show bulk ferromagnetic behaviour at room temperature similar to V(TCNE) x ·y(DCM), where x ≈ 2 and y ≈ 0.5 [1].…”

“…The precursor used by Jain et al [5] and Miller et al [6] in their metal-organic redox reactions was Ni(COD) 2 , which is a widely used material with many applications [7] such as in the creation of Ni nanoparticles. Such a process is effective due to the material's labile nature and its autocatalytic decomposition from Ni(0) to bulk Ni.…”