Magnetic Materials

Molecule Based Magnets

Magnetic Properties of Materials
A History of Superconductivity
Molecule-Based Magnets

Molecule-Based Magnets

In general, molecule-based magnets have magnetic properties comparable to traditional magnets. However, being molecular, they have many advantages over metal-based magnets in terms of device fabrication. For example, they can be deposited as thin films by lowtemperature (40 ºC) CVD, are low density, and can be transparent. This makes them ideal candidates for such advanced devices utilizing magnetic imaging, data storage, magnetic shielding, or magnetic induction. In addition, molecule-based magnets can have more specialized properties such as photomodulated magnetization.21

Theories likening electron-transfer salts to molecular magnets date back to 1963.22 The phenomenon was not observed until 1985,23 however, when Miller and coworkers identified ferromagnetism in decamethylferrocenium tetracyanoethenide [Fe(Cp*)2][TCNE].24 Since this time many electron-transfer salts of decamethylmetallocenes with TCNE or TCNQ (7,7,8,8-tetracyano-pquinodimethane) (see Figure 5 and Table 5) have been reported.

Figure 5. Structures of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and tetracyanoethenide (TCNE).

Electron-Transfer Salt Curie Temperature (K) Critical Temperature (K)
[CrCp*2]+[TCNE]- 22.2 3.65
[CrCp*2]+[TCNQ]- 12.8 3.5
[FeCp*2]+[TCNE]- 16.8 4.8
[FeCp*2]+[TCNQ]- 12.3 2.55
[MnCp*2]+[TCNE]- 22.6 8.8
[MnCp*2]+[TCNQ]- 10.5 6.5
Table 5. Curie and critical temperatures of some Electron-Transfer salts.24

One aspect of molecule-based magnets that distinguish them from traditional magnetic materials is dimensionality. Molecular magnets are often only magnetic in a single direction or along a single dimension. For example, hexylammonium trichlorocuprate(II) (CuCl3(C6H11NH3) or CHAC) consists of double-bridged chain of CuCl3 units with the hexylamine cations hydrogen bonding parallel chains together (see Figure 6). There is a ferromagnetic interaction along the chain within the orthorhombic crystal structure.

The CuCl3 core of CHAC (chlorine atoms are not labeled).

Figure 6. The CuCl3 core of CHAC (chlorine atoms are not labeled).

The interaction parameter (J) along the c axis in figure 5 (JC) is 100 cm-1 and ferromagnetic. This is about four orders of magnitude larger than the ferromagnetic interaction along the b axis (Jb ≈ 10-1 cm-1) and more than five orders of magnitude larger than the antiferromagnetic interaction along the a axis (Ja = < -10-2 cm-1).26 Many ferromagnetic chain molecules have since been reported among them [MnCu(dto)2(H2O)3·4.5 H2O] (dto = dithiooxalato) was the first characterized.27,28

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