Predictable and New Physics and Potential for Applications of Organic-based Magnets*

As discussed by Joel S. Miller in the previous talk, magnets utilizing organic groups with essential spin have been reported since the mid-1980's. Though initial organic-based magnets had magnetic ordering temperatures (T$_{c}$'s) below 5K, organic-based magnets now have T$_{c}$'s to above 400K. In addition to magnetic phenomena already known for conventional transition metal and rare earth magnets, organic-based magnets feature unique phenomena enabled by the shape and internal electronic structure of the organic molecules. Examples are illustrated with experimental results for magnets based on tetracyanethylene, [TCNE], which as an anion has spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. For example, chains with spin containing molecules having relatively strong exchange within a chain and weak dipolar interaction with neighboring chains can have an unusual fractal ground state with unusual dynamics leading to `coercive fields' approaching 3 tesla. In contrast to conventional magnets, the internal electronic structure of the molecules that make up a molecule-based magnet can be excited by light of the appropriate wavelength. This leads to changes of the spin state of the molecule and/or changes in the exchange interaction between molecules, opening up the concept of reversible light control of magnetism. Examples will be given from the M$^{++}$[TCNE]$^{-}_{x}$ (x$\sim $2) (M = Mn, V) materials systems. Finally, we explore the new phenomena enabled by V$^{++}$[TCNE]$^{-}_{x}$ (x$\sim $2), a material with T$_{c}$ up to 400K and for which films may be prepared using low temperature CVD. It is a semiconductor (room temperature resistivity and activation energy similar to silicon) and magnetization M(H,T) and coercive field are controlled by chemical composition. Magnetoresistance to 32 tesla supports that V[TCNE]$_{2}$ is a ``half-semiconductor'' with fully spin polarized valence and conduction bands of interest for spintronics applications.