Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session D3: DMP Prize Symposium |
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Sponsoring Units: DMP Chair: David Vanderbilt, Rutgers University Room: Colorado Convention Center Korbel 2A-3A |
Monday, March 5, 2007 2:30PM - 3:06PM |
D3.00001: Organic-based Magnets - New Materials for New Physics Invited Speaker: Organic-based materials exhibiting the technologically important property of bulk magnetism, including ferro-, ferri-, and metamagnetism, have been prepared. These magnets are prepared via conventional organic chemistry methodologies, but unlike classical inorganic-based magnets do not require metallurgical processing, and are frequently soluble in conventional solvents (e.g., toluene, dichloromethane, acetonitrile, THF). They have saturation magnetizations that in some cases exceed twice that of iron metal on a mole basis as well as have coercive fields exceeding that of Co5Sm. Also magnets with critical temperatures exceeding room temperature have been prepared. In addition to an overview of the observed magnetic behaviors, numerous examples of magnets made from molecules will be discussed. These will include [M(III)(C5Me5)2][A], [Mn(III)(porphyrin)][A] (A = cyanocarbon etc. electron acceptors) as well as M[TCNE]x (M = V, Mn, Fe, Co, Ni) which for M = V is a room temperature magnet, which can be fabricated as a thin film magnet. Another new class of magnets of [Ru(II/III)2(O2CR)4]3[M(III)(CN)6] (M = Cr, Fe; R = Me, t-Bu) composition will also be discussed. This broad new family of materials have examples that exhibit most of known magnetic phenomena observed for classical inorganic transition/rare earth metal based magnets, as well as some new, unexpected phenomena and combinations of properties not previously reported. The design of examples of these organic-based magnets will be discussed setting the stage for an overview of their unusual magnetic properties and new physics that will be discussed by Arthur J. Epstein. [Preview Abstract] |
Monday, March 5, 2007 3:06PM - 3:42PM |
D3.00002: Predictable and New Physics and Potential for Applications of Organic-based Magnets Invited Speaker: 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. [Preview Abstract] |
Monday, March 5, 2007 3:42PM - 4:18PM |
D3.00003: A Brief History of the Harris Criterion Invited Speaker: In this talk I will briefly review the ``Harris criterion,'' which was given in a 1974 paper in J. Phys. C. This criterion indicates whether the critical exponents of a system at a phase transition are modified by the presence of locally random impurities. To frame the discussion and since the argument for the criterion is so simple, I will repeat its derivation here. Since some of those who quote the paper may not have actually read it in detail, I will discuss some of the applications given there to systems with randomness which have longer-range correlations and I will emphasize those aspects which are perhaps less well-known. Also, with the benefit of hindsight, I will slightly reinterpret some of the conclusions of the 1974 paper. To further put this work in context, I will discuss how the renormalization group indicated that this argument indeed captured the essential role of local randomneess. Later work on longer range models fits in nicely with the criterion. Finally, I will briefly mention experimental studies of this criterion. Perhaps the appropriate general conclusion from all of this is that a sound qualitative argument can have an honored place along side technically exact solutions. [Preview Abstract] |
Monday, March 5, 2007 4:18PM - 4:54PM |
D3.00004: Opportunities in Nanomagnetism Invited Speaker: This talk addresses the challenges and scientific problems in the emerging area of nanomagnetism. [1] Included are fabrication strategies, and experiments that explore new spin-related behavior in metallic systems, as well as efforts to understand the observed phenomena. As a subfield of nanoscience, nanomagnetism shares many of the same basic organizing principles, such as geometric confinement, physical proximity, and chemical self-organization. These principles are illustrated by means of examples drawn from the quests for ultra-strong permanent magnets, ultra-high-density magnetic recording media, and nanobiomagnetic sensing strategies. As a final example showing the synergetic relationship to other fields of science, the manipulation of viruses to fabricate magnetic nanoparticles is presented. \newline \newline [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006). [Preview Abstract] |
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